This PDF file contains the front matter associated with SPIE Proceedings Volume 11083, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Optical forces have revolutionized nanotechnology. In particular, optical forces have been used to measure and exert femtonewton forces on nanoscopic objects. This has provided the essential tools to develop nanothermodynamics, to explore nanoscopic interactions such as critical Casimir forces, and to realize microscopic devices capable of autonomous operation. The future of optical forces now lies in the development of smarter experimental setups and data-analysis algorithms, partially empowered by the machine-learning revolution. This will open unprecedented possibilities, such as the study of the energy and information flows in nanothermodynamics systems, the design of novel forms of interactions between nanoparticles, and the realization of smart microscopic devices.

Holographic Acoustic Tweezers can trap and manipulate multiple particles individually using acoustic radiation forces. This technology is similar to the well-established Holographic Optical Tweezers but presents some notable differences. For instance, acoustic tweezers can operate on particles ranging from the micrometre to the centimetre scales. The particles can be made of a wide range of materials such as gases, liquids, metals, plastics or even living things. The propagation media can be air, water and does not need to be optically transparent, therefore manipulations inside the human body from the exterior are a possibility. These unique features enable multiple applications such as acoustophoretic displays in which several particles or one fast-moving particle form objects in mid-air. On the other hand, tiny quirurgic instruments that are inserted in the body could be controlled not only in position but also in orientation due to the multiple trapping point capabilities.

Driven by the demands for speed and field of view in the holographic photostimulation community, we designed, built, and tested a liquid crystal on silicon (LCoS) spatial light modulator (SLM) with a 1536x1536 square pixel array and high-voltage LC drive. We discuss some of the engineering work that made the MacroSLM possible, including the custom FPGA board for handling huge data rates, the large pixel size for minimizing rolloff and crosstalk, and the temperature control to handle heating effects from the high-voltage controls and high-power laser illumination. We also designed an FPGA implementation of the overdrive method for increasing liquid crystal switching speed, allowing us to overcome the significant data bottlenecks that limit frame rates for large arrays. We demonstrate 500 Hz hologram-tohologram speed at 1064 nm operating wavelength, and discuss the new science that these speeds and array sizes have enabled.

The invention of holographic optical tweezers (HOTs) revolutionized the field of optical tweezers by allowing the simultaneous manipulation of many particles using arrays of scalar beams. Here, we go one step further and produce arrays of digitally controlled Higher-Order Poincaré Sphere (HOPs) beams employing a simple set-up using a spatial light modulator (SLM). Our system enables the on-demand manipulation of each beam in the array to obtain any HOPs state. We demonstrated trapping and tweezing with customized arrays of HOPs beams comprising scalar and cylindrical vector beams simultaneously in the same array. Our approach is general enough to be easily extended to arbitrary vector beams, could be implemented with fast refresh rates, and will be of interest to the structured light and optical manipulation communities alike.

In the last three decades, optical trapping techniques were heavily employed for contactless trapping and manipulation of biological samples. Dual-beam laser traps (DBLT) proved their convenience and became widely used as biophysical tool once a simplified experimental setup was proposed. This simplification was achieved by replacing the two objectives with optical fibers to deliver the two counter-propagating laser beams. However, fiber alignment can be inconvenient, time consuming and requires a lot of practice. Here, we present a novel way to overcome these issues by combining reconfigurable diffractive optical elements (DOE) and two photon lithography (2PL), using a single low NA objective. A single laser beam is divided into several beams by displaying a DOE on a spatial light modulator (SLM). This allows us to dynamically reconfigure the number of the beams, their shape, and relative 3D alignment. Furthermore, we use 3D printed micro-mirrors to direct the laser beams against each other and obtain a DBLT. The micro-mirrors were fabricated on top of a coverslip, by means of 2PL. Our preliminary results show the ability to trap dielectric and biological samples and their full 3D manipulation in a DBLT configuration. The ability to use DOEs to set the number of beams and their shape allow this technique to be coupled with novel forms of microscopy.

The fundamental problem of complex fluid rheology is one of microstructure engineering. The rheology derives from how this microstructure deforms, relaxes, or breaks. Optical tweezers have become exceptionally useful tools for measuring the interactions and mechanics of on microscopic length scales to provide critical insight into the rheology of colloidal suspensions. In this talk, I will discuss how laser tweezer micromanipulation enables us to build, prod, deform, and break microstructures to gain insight into the mechanics and flow behavior of colloidal gels. Our experiments include measurements of the bending mechanics of assembled colloidal aggregates, the measurement of "bond rupture" between sticky particles, and probing the strain propagation in cluster gels.

Acoustic oscillations of metal nanoparticles can be used to study the properties of liquids at GHz frequencies and nanometer length scales. We use time-resolved spectroscopy to probe the dynamics of the metal nanoparticle oscillations utilizing a pump-probe technique. The incident pump laser pulse heats the nanoparticles leading to expansion and impulsive excitation of vibrations of the nanoparticles. The oscillations produce shifts in the plasmon resonance, which are monitored by measuring the change in absorption of a second weak broadband probe pulse. In our experiment, we immersed a sample of highly monodisperse gold bipyramids in water-glycerol mixtures from which we determined the damping resulting from the structure-liquid interactions. Performing these measurements over a range of temperatures provides a means to vary the fluid properties of a given water-glycerol mixture. Viscous damping could account for the measured results at low glycerol concentrations and sufficiently high temperatures but failed to describe the damping for high glycerol concentrations and sufficiently low temperatures. Accounting for the viscoelastic nature of the liquid mixtures mostly resolved the discrepancies, but consistently overestimated the degree of damping. Ultimately, allowing for a finite slip length produced good agreement with the measured damping rates. Our results show that standard assumptions in the fluid mechanics of simple liquids – a purely viscous response and the no-slip boundary condition – must be revisited at short length scales and fast time scales.

Optical trapping is a widely used technique allowing for remote and precise manipulation of particles and measurement of forces acting on them. It also gives possibility of measuring viscosity, by analyzing the Brownian motion, and temperature by analyzing Raman scattering or luminescence of trapped particle. Large variety of nanoparticles including resonant one, like plasmonic and high-index dielectric, and non-resonant, like rare earth ions doped nanocrystals, and their hybrid combination makes them excellent probes for thermo-rheological measurements in microliter volumes and basic thermodynamic studies in nanoscale. Resonant nanoparticles, which strongly interact with light, allow better control of position and orientation, and give possibility of rapid rotation due to large optical forces and torques acting on them. The same property makes their optical trapping in 3D challenging and limited to a narrow size range due to the strong radiation pressure. Here, we show how large plasmonic nanorods can be optically trapped and rapidly rotate in three dimensions using focus splitting in anisotropic crystal phenomenon [1]. We also show that it is possible to optically trap and rotate silicon nanoparticles of anisotropic shape and simultaneously measure their inner temperature from Raman scattering signal and outer one from Rotational Hot Brownian Motion analysis. We use NaYF4:Er,Yb up-converting nanocrystals and their hybrid combination with gold for simultaneous heating, temperature and viscosity measurements in microliter volumes. [1] P. Karpinski, S. Jones, D. Andren, and M. Kall, Laser Photonics Rev. 2018, 1800139.

When trapping a dielectric bead with optical tweezers, light backscattered by the bead interferes in the back focal plane of the objective with light reflected by the microscope cover slip. This well contrasted interference pattern can be used to calibrate precisely the relative position of the bead with respect to the center of the trapping laser, as well as the stiffness of the trap. We compared four calibration methods previously implemented in forward detection, namely step response, Bayesian inference, power spectrum analysis and equipartition. We showed that they agree for different heights and trapping powers. Effect of acoustic noise on all methods was observed, and the step response method was shown to be the least sensitive. In addition to giving better access to the sample, this backscattered interference pattern provides precise location of the bead with respect to the laser, both laterally and axially, even when the bead is not trapped by the laser. We apply this to microrheology of blood clots, where the focused laser exerts a force on a bead confined by the fibrins of the clot but does not actually trap it .

Despite the extensive studies of different types of nanoparticles as potential drug carriers, application of red blood cells (RBC) as natural transport agents for systemic drug delivery, is considered as a new paradigm in modern medicine and possesses great potential. There is a lack of studies on influence of drug carriers of different composition on RBC, especially regarding their potential impact to the human health. Here, we apply conventional microscopy to observe formation of RBC aggregates and the optical tweezers to assess quantitatively mutual interaction of RBC, incubated with inorganic and polymeric nanoparticles. Scanning electron microscopy is utilized for direct observation of nanoparticles localization on RBC membranes. The experiments performed in a platelet-free blood plasma mimicking RBC natural environment. We show that nanodiamonds influence mutual RBC interaction more antagonistically compared to other nanoparticles, resulting in higher aggregation forces and formation of larger cell aggregates. In contrast, polymeric particles do not cause anomalous RBC aggregation. The results emphasize application of optical tweezers for direct quantitative assessment of mutual interaction of RBC influenced by nanomaterials.

Optical trapping of plasmonic nanoparticles for controlled nanoscopic damage of cellular plasma membranes can be used to gain deeper insight into the role of plasma membrane repair proteins. Here we present a synthetic platform of giant unilamellar vesicles (GUVs) in the vicinity of trapped nanoplasmonic particles as a proposed model assay to characterize the permeability of a damaged GUV membrane, i.e. size of an inflicted hole. Water soluble fluorescent molecules with different sizes are used to characterize the extent of the membrane lesion since their differential permeability will provide information about the size of the rupture. We find that trapped gold nanoparticles can create substantial holes, observed via the discriminating influx of various sized molecules across the membrane. The technique, yet unrefined, provides groundwork for future investigations of annexin repair proteins, using nanoscopic heating of plasmonic particles to create quantifiable membrane damage.

Life sciences that focus on improving the quality and standard of life have attracted worldwide attention in fundamental sciences and have promoted the development of novel tools to reveal the mechanism behind biological activities. Advances in single-cell level methods have further deepened the cell biology study, and helped unravel the different structures and function of living cells on a microscopic and molecular level. Among these methods is the Optical Tweezers (OT), a significant achievement of laser physics, which has been widely applied to understand cell interaction dynamics with the ability to non-invasively trap, manipulate and displace a living cell or part of it with highly accurate positioning of the cells. The reversible aggregation of red blood cells (RBC) that strongly influences the hemodynamic mechanisms and blood microcirculation may serve not only as an indicator of disease, but also as a factor affecting the pathological process. In this study, a two-channel OT system combined with a chopper-modulated laser irradiating system was applied to investigate the RBC aggregation mechanism and to reveal the influence of the low-level pulsed He-Ne laser short-time irradiation on this process. A proportional relationship between the interaction area of RBC and the aggregation force was obtained, verified the applicability of the depletion layer model to the RBC aggregation process in plasma. More importantly, a regulating effect of low-level He-Ne laser irradiation on this process was discovered. A statistically significant decrease (p < 0.05) in RBC aggregation forces was observed following 120 s laser light with 225 Hz pulse frequency. This observation brings new insights into conception of the regulating effects occurring during the laser light interaction with blood.

Understanding the deformability and associated biomechanical properties of red blood cells (RBCs) is crucial for many pathological analysis and diagnosis of human diseases. In such endeavors, optical tweezers have played an active role over the past decades. Here, we study the RBC deformability by employing a novel “tug of war” (TOW) optical tweezers consist of a pair of elongated diverging accelerating beams that can stably trap and stretch a single RBC under different osmotic conditions without any tethering or mechanical movement. With a viscous drag method, we compare directly the trapping force at different states of RBCs, and find that even one arm of the TOW tweezers can apply a force of over 18pN with only 100mW laser power, more than 2 times stronger than that from the Gaussian trap at the same condition. Without the need of two independent controls as in a conventional dual trap, the spacing between the two TOW traps can be increased conveniently from 0 to over 9m, resulting in nearly 15% of cell deformation. We obtain the shear modulus of the RBCs in different osmotic conditions, with the largest value of 3.36±0.95pN/μm in the hypertonic case, and compare with those previously reported results. Our work may bring about a new photonic tool for the study of biomechanical properties of living cells, promising for applications such as distinguishing healthy and diseased cells.

Over the years, Ashkin's pioneering work on laser trapping of small particles has been exploited to study many phenomena in liquid suspensions, leading to advances in biophysics and polymer science. Could trapping in vacuum lead to advances on a similar scale in fundamental physics? In this talk I will describe work originally intended to improve our ability to test gravity at sub-100um scale. Towards this lofty goal, many manipulation techniques have been developed, some of which are interesting in their own right. The arsenal of tricks being assembled can indeed be applied to solve very hard experimental problems in fundamental physics.

In high vacuum, optically-levitated dielectric nanospheres achieve excellent decoupling from their environment, making them ideal for precision force sensing. We have shown that 300 nm silica spheres can be used for calibrated zeptonewton force measurements in a standing-wave optical trap. The sensitivity achieved exceeds that of any conventional room-temperature solid-state force sensor by over an order of magnitude, and enables a variety of applications including electric field sensing, inertial sensing, and gravimetry. I will describe our progress towards using these sensors for tests of the Newtonian gravitational inverse square law at micron length scales. Optically levitated dielectric objects also show promise for a variety of other applications, including searches for gravitational waves, and experiments in quantum optomechanics.

In the context of cavity optomechanics alternative techniques without the need of atomic resonances have widened the possibilities towards the cooling of macroscopic objects. Recently the radiation pressure cooling of mechanical oscillators, optomechanically induced transparency and ground state cooling have been demonstrated. Current progress in optomechanics has brought forward multiple experimental platforms of which many platforms necessitate complex cryogenic environments and suffer from clamping losses as major decoherence sources. An alternative approach is the cooling of levitated nanoparticles from room temperature, which have been suggested for probing quantum mechanics on the mesoscopic scale. In levitated systems collisions with residual gas molecules and photon recoil heating are now the remaining decoherence sources paving the way towards low phonon occupations. In the context of cavity optomechanics, resolved sideband cooling of a levitated nanoparticle has recently been realised. Here we demonstrate the resolved sideband cooling of a levitated nanoparticle within a high nesse cavity at high vacuum. Trapping the nanoparticle in an external optical tweezer allows on one hand the free positioning of the particle within the cavity fi eld and on the other hand the additional cooling via parametric feedback cooling. The combination with well-established resolved sideband cooling techniques creates a powerful platform for controlling the centre of mass motion (COM) of a mesoscopic object. By exploiting cavity enhanced Anti-Stokes scattering we all optically cool the COM to minimum temperatures of T ~ 100mK for a silica particle of 235nm diameter. Power dependent laser noise heating is observed, being the main current limitation in reaching lower temperatures. In the future laser noise suppression for resolved side band cooling brings low phonon occupation numbers of mesoscopic systems via passive cooling schemes within the reach of table top experiments at room temperatures.

Nanomechanical resonators are exquisite force sensors and have recently been used to “feel” the vacuum fluctuations of a laser field. I’ll describe a system consisting of a glass nanostring coupled to an optical microcavity and how it has been used to not only sense radiation pressure shot noise, but also squeeze it, to cool a vibration of the string to near its ground state, and to witness its zero-point energy as motional sideband asymmetry—all long-standing goals in the field of optomechanics. Underlying these advances are new insights into dissipation of nanomechanical resonators. Combining strain and mode-shape engineering, we've recently fabricated strings with effective masses of picograms, frequencies of megahertz, and quality factors approaching 1 billion at room temperature. These numbers spark the imagination, inviting speculation about applications ranging from ultrasensitive accelerometry to tests of quantum collapse models.

Cavity optomechanical systems show great promise as force and displacement sensors, with scope to operate across the classical to quantum regimes. I will discuss the commercial development of an optical whispering gallery mode (WGM) accelerometer, which relies on a dispersive and dissipative coupling between the cavity resonance and the motion of the cavity. The accelerometer operates at a sensitivity of micro-g Hz^-1/2 (g=9.81 ms^-2) with plans to approach nano-g Hz^-1/2 through tailoring the mechanical and optical properties. I also describe the first prototype assembly, results from outdoor field-trials, and recent work using micro-electro-mechanical systems engineering to produce a chip-scale device.

Photonic crystals have been an object of interest because of their properties to inhibit specific wavelengths and allow the transmission of others. Using these properties, we have designed a microcavity of Porous Silicon using two one-dimensional photonic crystals with an air defect between them. When we illuminate the microcavity with the appropriate light (laser with a wavelength of 633 nm) allows us to generate electromagnetic forces within the structure. These electromagnetic forces allow the microcavity to oscillate mechanically and we have named such a device Photodyne. Experimentally, we have characterized the maximum displacement of several photodynes by using different driven frequencies and light powers. The displacements were put in evidence using a commercial vibrometer and by interferometry. From these measurements, it is possible to estimate the generated forces. Finally, we induced mechanical self-oscillations. The electromagnetic force generated within the whole photonic structure, by light is enough to overcome energy losses and sustain self- oscillations at two different frequencies. From these mechano-optical measurements, we estimated the stiffness and Young's modulus of porous silicon and compared the results with values reported elsewhere and with values estimated herein by a mechanical method.

Plasmonics has been used to enhance light-matter interaction at the extreme subwavelength scale. Intriguingly, it is possible to achieve multiple plasmonic resonances from a single nanostructure and these can be used in combination to provide cascaded enhanced interactions. Here, we demonstrate three distinct plasmon resonances for enhanced up-conversion emission from a single up-converting nanocrystal trapped by a metal nanoaperture optical tweezer. For apertures where the plasmonic resonances occur at the emission wavelength only, a moderate enhancement of a factor of 4 is seen. However, by tuning the aperture to enhance the excitation laser as well, an additional factor of 100 enhancement in the emission is achieved. Since lanthanide doped nanocrystals are stable quantum emitters, this approach of using multiple subwavelength resonances is promising to achieve better performance for their applications in photovoltaics, photocatalysis, single photon sources and subwavelength imaging.

Optical tweezers have emerged as a powerful technique for non-invasive trapping and manipulation of microscopic objects. The diffraction limit precludes the low power trapping of nanoscale objects with optical tweezers. Plasmonic optical tweezers, which employ resonant plasmonic nanoantenna to confine electromagnetic fields to the nanoscale have been developed to enable the optical trapping of nanoscale objects. A peculiar advantage of plasmonic tweezers is the capability to not only trap nanoscale objects and biomolecules but also to perform spectroscopy on the trapped object. Such new capability would benefit from high throughput trapping and sorting of nanoscale objects on a plasmonic substrate. To meet this need, we present a thermoplasmonic nanohole metasurface platform for high throughput trapping and size-based sorting of nanoscale particles. The thermoplasmonic metasurface comprises of sub-wavelength nanoholes patterned on a gold film. A microfluidic channel is constructed over the nanohole metasurface region and another substrate is placed over it, to create a parallel plate capacitor configuration. The illumination of the thermoplasmonic nanohole metasurface causes photothermal heating and a thermal gradient in the fluid. The application of an AC electric field across the fluid element creates an electrothermoplasmonic microfluidic vortex. This vortex enables the long-range capture of the nanoparticles for rapid trapping and assembly. Additionally, the thermoplasmonic nanohole metasurface structure creates a distortion of the applied AC electric field. The tangential component of the AC electric field induces an AC electro-osmotic flow. By harnessing the interplay of these forces with the optical gradient force, we demonstrate several features including trapping, dynamic manipulation of nanoparticles, and size-dependent sorting of nanoscale particles.

The single-beam gradient force optical tweezers have transformed various fields of scientific research by enabling manipulation and characterization of single molecules. Conventional optical tweezers pose limitations in trapping particles in the sub-Rayleigh regime. These limitations have been overcome with the help of plasmonic nanoapertures like the double-nanohole aperture. A modified colloidal lithography technique has been used in fabrication of double-nanohole apertures achieving dimensions appropriate for trapping single molecules in this regime. This paper demonstrates optical trapping of a single 10 nm enzyme, rubisco, using double-nanohole apertures fabricated using the modified colloidal lithography technique as well as presents the results from transmission characterization of different double-nanohole apertures carried out using the finite-difference time-domain (FDTD) simulations.

Controlled manipulation of nanoscale objects in fluidic media is one of the defining goals of modern nanotechnology. In this respect, optical traps based on highly localized electromagnetic fields around plasmonic nanostructures offer a promising solution in generating strong trapping forces at low levels of optical illumination. However, conventional plasmonic trapping occurs at predefined spots on the surface of a nanopatterned substrate where trapping is limited by the diffusion of colloidal objects into a small trapping volume which renders the process inherently slow. As we discuss here, this limitation can be overcome by integrating plasmonic nanostructures with magnetically driven helical nanoswimmers and maneuvering these mobile nanotweezers under optical illumination. In an alternate strategy, a similar functionality has been obtained in a unique nanophotonic device, where sub-micron colloids could be manipulated using optical forces alone. The strategy with magnetic nanoswimmers provide a working range that matches with state-of-the-art plasmonic tweezers and in-addition allows selective pickup, transport, release, and positioning of submicrometer objects over large areas in standard microfluidic environments with great speed and control. The MNTs can be used to manipulate one or many nano-objects in three dimensions and are applicable to a variety of materials beyond model colloids (e.g. silica, polystyrene) including living bacteria and fluorescent nanodiamonds. A crucial component of these tweezers is the generation of thermofluidic forces which provide an additional handle to trap and sort objects. The alternate strategy with optical forces, as we will explain in detail, works in a regime where optical absorption and therefore generated heat is minimized.

The non-conservative nature of optical forces has been explored many times previously. Non-conservative optical forces occur in many guises and include lateral forces due to shape asymmetry, polarisation dependant optical torques and spin-dependant effects. When considering the design of optical force actuators, one recent approach has been to exploit the periodic structure of 2-dimensional metamaterial surfaces. In some cases, extremely large optical forces have been reported (Zhang et al, Optics Letters, 39, 4883 (2014)). Most recently macroscopic forces and torques have been demonstrated (Magallanes and Brasselet, Nature Photonics, 12, 461 (2018)). Here we consider refractive-index patterning of a substrate, achieved through a photo-lithographic approach. In this paper we will explore the use of different types of patterning, and show how the use of a periodic structure can enhance the optical forces and torques that may be generated. Using computational electromagnetic techniques we will demonstrate how enhancements of two orders of magnitude in the optical force are available in specific cases. We will examine the influence of such surface structuring on the resultant forces and torques, with a view to optimising such 2-dimensional materials for applications as light-driven actuators. Further, we will demonstrate the sensitivity of the forces generated to variations in the local environment, opening up possibilities for optical sensing applications.

A single beam optical trapping system is used to trap and rotate silica and vaterite microspheres in high vacuum. Large vaterite microspheres with diameters up to 15 μm are fabricated with multi-stage precipitation reactions and are rotated in the trap through the transfer of spin angular momentum from the photons in the trapping beam to the spheres. An electro-optic modulator is used to vary the polarization of the trapping beam, allowing for control over the rotation with damping times on the order of a day and with rotation frequencies up to 10 MHz for 10 μm diameter spheres. While highly birefringent spheres are successfully trapped at moderate vacuum pressures (⪆10⁻² mbar), poor reproducibility is observed for trapping spheres in high vacuum. This trapping behavior is found to be independent of the morphology, birefringence, and monodispercity of the spheres.

Temperature at the mesoscale is important in many fields due to its key role in, e.g., cell mechanics, quantum ground state studies and hydrodynamics. In levitated optomechanics, measuring temperature is challenging at the same time as necessary to understand the dynamics of the optically trapped particle. Generally, the particle’s temperature has been directly correlated to its centre-of-mass (CoM) motion (i.e. translational dynamics). This, together with the rotational dynamics, encompasses the particle’s external degree of freedom which is affected by the external temperature. However, the particle presents an internal structure that is at another internal temperature. Generally, the CoM temperature is experimentally measured and compared to a theoretically calculated internal temperature. The rotation rate (i.e. rotational dynamics) has also been correlated to an experimentally measured internal temperature for thermometric studies. Despite its importance, the temperature of these three degrees of freedom had never been simultaneously measured and correlated. We developed a tripartite method able to independently measure both the internal temperature of the particle (through temperature-dependent luminescence) and the external temperature (through the rotational rate and trap stiffness). We found that, even though they are strongly coupled, the external and internal degrees of freedom present distinct temperatures. This study gives new insight into thermometry at the mesoscale where the appropriate parameter should be carefully chosen for an accurate characterisation of temperature. Moreover, experiments attempting to cool levitated particles to the quantum ground state, in which all degrees of freedom must be independently controlled and characterised, will also benefit from this advance.

Optically levitated nanoparticles in vacuum have great potentials in precision measurements, thermodynamics and macroscopic quantum mechanics. We have assembled and levitated silica nanodumbbells in high vacuum. With a circularly polarized laser, we have driven them to rotate beyond 1 GHz [J Ahn, et al. Phys. Rev. Lett., 121, 033603 (2018)]. With a linearly-polarized laser, we have observed its torsional vibration. Based on our experimental results, we proposed that this system can be used to study the coupling between the rotation of a nanoparticle and an electron spin [arXiv:1811.01641], and study the Casimir torque due to the angular momentum of quantum vacuum fluctuations [Phys. Rev. A, 96, 033843 (2017)]. With a levitated nanoparticle under drive, we also tested the differential fluctuation theorem and a generalized Jarzynski equality that is valid for arbitrary initial states [Phys. Rev. Lett. 120, 080602 (2018)]. Recently, we investigated the rotation of a levitated nanocluster to deepen our understanding of light-matter interaction.

I describe the electrically-driven rotation of 2.4-micron-radius, optically levitated dielectric microspheres. Electric fields are used to apply torques to a microsphere's permanent electric dipole moment, while angular displacement is measured by detecting the change in polarization state of light transmitted through the microsphere. This technique enables greater control than previously achieved with purely optical means. We measure the spin-down of a microsphere released from a rotating electric field, the harmonic motion of the dipole relative to the instantaneous direction of the field, and the phase lag between the driving electric field and the dipole moment of the MS due to drag from residual gas. We also observe the gyroscopic precession of the MS when the axis of rotation of the driving field and the angular momentum of the microsphere are orthogonal. These observations are in quantitative agreement with the equation of motion.

Levitated optomechanics have invited growing interest partly due to their capabilities to reach high Q factors, >109, and for studies in force sensing, fluctuation theorems, nanothermodynamics and macroscopic quantum systems, to name a few. A levitated anisotropic particle, untethered from its environment can exhibit a rich spectrum of rotation and translational motion. Rotational motion is acutely dependant upon the size and shape of an object and the proprieties of the light which imparts angular momentum to the particle. It is also highly susceptible to changes to its environment, i.e. gas pressure or external conservative and non-conservative forces. In this talk, I will present the latest efforts in rotational optomechanics, specifically looking at how rotation, libration, nutation and precession motion can arise in levitated systems, as well as the realisation of state control for rotating systems. Finally, I will present a proof-of-principle experimental work on precession motion, which we use for detecting optical torque as small as, $10{^-23}$ Nm, with the potential to reach torque sensitivities of 1$10^{-31}$ Nm/$\sqrt{Hz}$ [Rashid et al Phys. Rev. Lett. 121, 253601].

Levitated optomechanics in vacuum has shown promise for fundamental tests of physics including quantum mechanics and gravity, for sensing weak forces or accelerations, and for precision measurements. While much research has focused on optical trapping of dielectric particles, other approaches, such as magnetic trapping of diamagnetic particles, have been gaining interest. Here we review geometries for both optical and magnetic trapping in vacuum, with an emphasis on the properties of traps for particles with a diameter of at least one micrometer.

Levitated particles are unique among optomechanical systems in that they benefit from the absence of physical contact with the external environment. Recently, a new research direction known as levitated optomechanics has attracted interest in numerous research groups, with a major focus on optically suspended particles. In contrast to optical trapping experiments, we levitate charged silica nanospheres in high vacuum by means of a Paul trap. This method provides a deeper confining potential than that of optical traps and enables trapping of optically opaque objects. A detection system based on back-focal-plane interferometry allows us to observe center-of-mass (CoM) motion of the particle. We introduce an additional laser beam that is focused on the particle and provides optical forces with projections on all three principal axes of the Paul trap. This additional beam is intensity-modulated by an acousto-optic modulator controlled by feedback electronics. In this way, we are able to cool the secular motion of the CoM below 1 K, the effective temperature in all three directions being currently limited only by the detection efficiency. This is the first time, to the best of our knowledge, that laser cooling of mechanical motion of a nanoparticle in a Paul trap potential has been demonstrated. Such cooling acts locally on a single particle, in contrast to feedback provided by auxiliary electric fields, and opens up possibilities for sympathetic cooling of particles levitated in Paul traps when other methods are not suitable, for example, in the case of highly absorptive particles.

Following advances in levitated optomechanics, we explore levitated electromechanics (LE) as a novel alternative method for trapping and controlling micro- and nanoparticles. LE provides an opportunity to circumvent the limitations of traditional optical tweezers, allowing robust trapping of particles with a wide range of sizes and compositions, from metals to biological material. This platform also offers a clear route to miniaturization, force sensing and signal processing. We present the theory of LE, and the latest experimental efforts in realising a levitated electromechanical system with all-electrical detection and state control.

Laser ablation in superfluid helium, having extremely low temperature, negligibly small viscosity, huge thermal conductivity, and high transparency in visible region, provides us a unique opportunity to fabricate novel microstructures and control their motion. We have successfully fabricated nano and micro spheres of semiconductors by the laser ablation in the superfluid helium with a nanosecond Nd:YAG laser. [Scientific Reports 4, 5186 (2014).] Recently, we applied this method to metals, such as indium and rhenium, which show superconductivity at low temperature. To select superconducting particles, we utilized perfect diamagnetism caused by Meissner effect, designing a magnetic trap with two permanent magnets for the superconducting particles. Thus we fabricated and trapped a single or several superconducting particles after the laser ablation in the superfluid helium [Applied Physics Express 10, 022701(2017).] Here we successfully control the positions of the magnetically trapped superconducting particles, by irradiating a laser to them. The particles were pushed away from their original trapped positions and after the irradiation released from the displaced positions, moving along the force of the trapping potential and the viscosity force of the superfluid helium. By tracking the particles motion we can deduce physical properties of the superfluid helium and trapped particles. Thus the optical fabrication and manipulation of the superconducting micro particles provide us a unique opportunity to investigate superfluidity and superconductivity.

The nearly 100 year-old paradigm of navigating the heavens by means of radiation pressure on a reflective solar sail is being challenged by the advent of advanced diffractive films that offer efficient propulsion and navigation. Other advantages include photon recycling when the sail is transmissive, and non-mechanical (e.g., electro-optic) navigation protocols. Unlike optical tweezers, the transverse force on a solar sail is afforded by the angular deviation of light away from the sunline, rather than a gradient force. Whereas a metal-coated film makes use of the law of reflection, the deviation of light from a diffractive film may be described by use of the grating equation. In the latter case, the grating momentum is, in fact, a mechanical phenomenon that we have observed in the laboratory by means of a vacuum torsion oscillator. Following a brief history of solar sailing I will describe how diffractive sails may enable the placement of a constellation of solar polar orbiters for monitoring the entire surface of the Sun. Laser-driven sails provide another opportunity for in-space propulsion, provided a stable “beam rider” can be invented. I will report our progress on the demonstration of a diffractive beam rider (patent pending).

Force spectroscopy on single molecular machines is often performed using optical tweezers. However, the use of common microspheres composed of silica or polystyrene may have limitations in the maximum force, measurement precision, or the degrees of freedom that can be measured. For example, the ultimate precision of the experiment is limited by the drag coefficient, i.e. the size of the microsphere. Thus, ideally, microspheres should be as small as possible. However, if microspheres are too small, maximum trapping forces are smaller than biological motor-generated forces creating a lower practical size limit of about 200 nm for polystyrene. Here, we have developed germanium nanospheres with diameters ranging from 30--200 nm. With a high refractive index of 4.4, their trapping efficiency is more than 10-fold improved compared to silica. Using 70-nm-diameter germanium nanospheres, we measured the stepping behavior of the molecular motor kinesin-1. With an improved precision, we could measure intermediate steps. In the long-term, the development of novel probes enables novel applications.

Active colloidal particles dissipate energies in a fashion different from Brownian particles, in that active particles undergo directed motions characterized by a persistent length. An outstanding question, debated to date, pertains to how a quantitative and well-defined means can be established to quantify the differences between the statistical behavior of an active particle and a passive Brownian particle. To address this question, we set out to investigate the motions of a single, induced-charge electrophoretic (ICEP) metallic Janus particles in a quadratic potential of an optical trap, by experiments and numerical simulations. The positions of the particle under different driving forces were measured by experiments and simulated numerically using a generalized Langevin equation. The 1-D positional histograms of the active particle, distinctively different from that of a Boltzmann distribution, reveal splitting of the positional distribution of a single peak centered at the bottom of the well into two symmetrical peaks, whose centers move away from the center to a distance increasing with the driven force. Kurtosis of the particle’s spatial distribution is used as a way to quantify the deviation from Gaussian distribution and it was found that this deviation is a function of the particle’s rotational relaxation, the stiffness of the trap and the driving force. The temporal fluctuations of the active particle in the well are analyzed by their power spectral density (PSD).

This paper investigates the underdamped motion of a bead as it moves toward the focal point of an optical trap. The model used herein represents a new approach toward addressing singular perturbation problems in dynamics. The experiments involve trapping microbeads at the focal point of an optical tweezer/trap. The optical trap can accurately measure the position of a microbead only in two directions. Given experimental data, the model can be used to estimate the bead’s position in the third direction. This estimate allows an examination of the full position, velocity and acceleration of the bead, which in turn allows and investigation of its particle Reynolds number (Re_p). It is generally believed that a low Re_p implies that a small bead will exhibit overdamped motion. The velocity estimates obtained herein for three bead diameters, 1950nm, 990nm and 500nm, provide new insights into the interpretation of a low Rep in light of the underdamped motion observed in experiments.

The accurate measurement of microscopic force fields is crucial in many branches of science and technology, from biophotonics and mechanobiology to microscopy and optomechanics. These forces are often probed by analysing their influence on the motion of Brownian particles. Here we introduce a powerful algorithm for microscopic force reconstruction via maximum-likelihood-estimator analysis (FORMA) to retrieve the force field acting on a Brownian particle from the analysis of its displacements [1]. FORMA estimates accurately the conservative and non-conservative components of the force field with important advantages over established techniques, being parameter-free, requiring ten-fold less data and executing orders-of-magnitude faster. We demonstrate FORMA performance using optical tweezers, showing how, outperforming other available techniques, it can identify and characterise stable and unstable equilibrium points in generic force fields. Thanks to its high performance, FORMA can accelerate the development of microscopic and nanoscopic force transducers for physics, biology and engineering. [1] García, Laura Pérez, Jaime Donlucas Pérez, Giorgio Volpe, Alejandro V. Arzola, and Giovanni Volpe. "High-performance reconstruction of microscopic force fields from Brownian trajectories." Nature Communications 9, no. 1 (2018): 5166. https://doi.org/10.1038/s41467-018-07437-x

The optical trapping of nanoparticles is important in the assembly of nanostructured materials and in fundamental studies of plasmonics and coupled light-matter interactions. These applications demand an accurate model of the trapping force and the ability to manipulate nanoparticles at high speeds over long distances. The trapping force is most simply modeled using a dipole approximation for particles much smaller than the wavelength, resulting in a force proportional to particle volume. For metallic nanoparticles, it was previously thought to be more accurate to replace the full particle volume with an effective volume based on a spherical shell with thickness equal to the metallic skin depth. The resulting optical trapping force is then proportional to surface area rather than volume. However, experimental studies have generally failed to find forces that scale with surface area, with qualitative explanations such as enhanced radiation pressure displacing particles from the beam focus or the presence of spherical aberration. Here we show through comparison to rigorous Mie theory that the complex permittivity of the metal fully accounts for the skin effect in metallic nanoparticles, and it is more accurate to use the full volume with a radiation reaction correction rather than an effective volume based on the skin depth. We compare these predictions to experiments, where we also show particularly high-speed (>0.1 mm/s) and long-distance (1 mm) manipulation of gold, silver, and polystyrene nanoparticles using a high-powered laser and low-aberration optical tweezer. We hope that that these results will help to enable high-speed nano-assembly.

We introduce a floating-probe force spectroscopy technique with femtonewton resolution and piconewton range, capable of simultaneous measurement of a three-dimensional force field [1]. The technique is uniquely suited to optical force measurements and can be tailored to generate a force map in a plane or a three-dimensional volume. We apply our technique to the measurement of helicity-dependent spin-momentum forces on a non-chiral microsphere in an evanescent optical field. A direct measurement of such spin-dependent forces has thus-far been elusive, as the widely differing force magnitudes in the three spatial dimensions place stringent demands on a measurement’s sensitivity and range. Our results show quantitative agreement with theory and represent the first direct and simultaneous measurement of all components of this polarization-dependent optical force. [1] Liu, Lulu, et al. "Three-Dimensional Measurement of the Helicity-Dependent Forces on a Mie Particle." Physical review letters 120.22 (2018): 223901.

Invoking tools and techniques from elementary theories of classical electrodynamics and special relativity, we analyze some of the thought experiments that have contributed substantively to the conceptual development and understanding of the linear and angular momenta of light.

Photopolymerization, the process of using ultraviolet light to activate polymerization within resins, is a powerful approach to create arbitrary, transparent micro-objects with a resolution below the diffraction limit. Importantly, to date all photopolymerization studies have been performed with incident light fields with planar wavefronts and have solely exploited the intensity profile of the incident beam. We investigate photopolymerization with light fields possessing orbital angular momentum (OAM), characterized by the topological charge “l”. We show that, as a consequence of nonlinear self-focusing of the optical field, photopolymerization creates an annular-shaped vortex-soliton and an associated optical fibre, which exhibits a helical trajectory, with a chirality determined by the sign of “l”. In particular, due to a transverse modulation instability in the nonlinear self-focusing photopolymer, the vortex beam breaks up into the “l” solitons or microfibers, each of which exhibit helical trajectories and together form a bundle of helical microfibers. Our numerical simulations, based on the nonlinear paraxial wave equation for the photopolymer, captures all the experimental observations for a variety of optical vortices characterized by “l”. This therefore represents a new physical manifestation of the use of OAM light fields. This research opens up a new application for light fields with OAM, and our generated microfibers may have applications in optical communications and micromanipulation. In a broader context, our work adds a new facet to the emergent field of helical fibres that have themselves recently come to the fore in the photonic crystal community as a route to generating fields with OAM.

In this contribution we present experiments used to control and characterize single optically trapped aerosol particles. These experiments include a counter-propagating optical tweezer, a feedback control mechanism to stabilize the particle in the trap and a two-angle optical scattering measurement to monitor the time-evolution of the particle size. Experimental setups and results are presented for these experiments.

A prototype sample cell has been designed and tested with the long-term aim of following heterogeneous catalysis chemistry and aerosol synthesis on optically trapped aerosols at temperatures approaching 500°C. Liquid aerosol droplets that contain high molecular weight molecules that gel, and then solidify, at higher temperatures have become of increasing interest to the catalysis community. To address the challenges of performing spectroscopic studies on individual airborne particles at high temperatures a sample cell was designed to localise heating at the optical trapping position whilst maintaining the objective lenses at close to ambient temperature. The heating cell was tested using polystyrene beads (2.0 μm diameter) that were trapped in air between opposed 1064 nm laser beams, and illuminated with a broadband white LED. Backscattered light from the trapped particle was collected to obtain a Mie spectrum over the 450-620 nm wavelength range. Mie spectral fitting was used to determine particle radius and wavelength dependent refractive index. In addition, a 514.5 nm laser beam was used to illuminate the particle to generate a Raman spectral signal. Raman spectroscopy enables the measurement of conformational changes in the polymer sample. The trapped particle was heated within the aperture of a ceramic heating element and retained through the melting point of polystyrene (~240°C). The changes in size and refractive index were measured. Both the glass transition temperature (T_g), melting point and the thermal expansion coefficient of a single bead were determined in comparison to literature values.

Stability of optically bound cluster, in the Lyapunov sense, is governed by the force constant matrix (or stiffness matrix), which is the first order Taylor Series Expansion about equilibrium, and it determined the stability and vibration frequencies. The openness of light matter interaction, i.e. the incident wave propagates, the scattered wave propagates outward, and absorption, caused non-conservative forces, making the force constant matrix non-symmetric (non-Hermitian). This opens up the possibility of having unstable vibrational modes with complex frequencies when the system is driven beyond its exceptional point, i.e., the non-conservative force is strong compare to the “gap” between adjacent vibrational frequencies. Noting that in large scale optical binding, the “gap” goes to zero. The Lorentz force alone will not be sufficient in general to ensure the stability of optically bound structure. Other forces, such as hydrodynamic damping in water, is essential.

When we illuminate gold nanofluids over indium-tin-oxide (ITO)-coated substrates, nanoparticle chains selfassemble via optical binding forces. We speculate that charge transfer between gold and ITO pins nanoparticles to the substrate and reduces the lateral Brownian motion as they attach to the substrate. We correspondingly model the self-assembly with additional stochastic or random forces. Simulations show a nonequilibrium-phase transition: when the stochastic force is small, nanoparticle chains align perpendicular to the light polarization and nanoparticles settle at shallow but stable nodes; when the stochastic force is large, however, the nanoparticle chains align parallel to the light polarization and nanoparticles settle at saddlepoints where the optical binding force is largely zero. Since the presence and strength of Brownian forces influence which state is formed, we reconsider the role that surfaces have—not only in relation to charge transfer but also heat transfer.

A system far from thermodynamic equilibrium is usually disordered yet ordered dissipative structures can still spontaneous form under certain conditions.1-2 The optical field provides a steady energy supply and enables a non-equilibrium dissipative state, where disorder-to-order transition occurs under anisotropic electrodynamic interactions. We find that a large number of Ag nanoparticles illuminated by a linearly polarized laser beam could self-assemble into partially ordered arrays, but they exhibited frequent structure transition between dimer chains and hexagonal nanoparticle lattices.3 In order to selective assembly of ordered lattice structures or dimer chains, a single Ag nanowire is illuminated to create a 3D interferometric optical field.4 The nanowire-guided self-assembly can be controlled by tuning the direction of linear polarization relative to the long-axis of a nanowire. The plasmonic nanowire can enhance the optical binding of nanoparticles both along and perpendicular to the laser polarization when the polarization is aligned at a specific angle. On the other hand, when specific dimer chains are perturbed and destabilized by another laser, their structures can self-heal after the perturbation is removed. Our observations suggest that light-driven self-organization of metal nanoparticles with strong optical binding interactions will provide new opportunities to discover new dissipative structures and build novel reconfigurable artificial nanostructures at mesoscale.

Optical binding was first observed between spherical particles around thirty years ago. Optical binding forces can result in various geometric arrangements of colloidal matter into regular, crystalline arrays and can induce complex, non-conservative and quasi-periodic motions. Although most studies of optical binding have focussed on dielectric spheres, recent departures from this trend have included silver bipyramids, nanowires and chiral particles. With reduction of particle symmetry comes extra complexity: torques can be applied to non-spherical objects by the applied beam, as well as by the scattered field from neighbouring particles. Another way to lower the symmetry of the system is if optical binding itself results in the formation of a lower symmetry cluster. The resulting cluster may then interact with the angular momentum of the beam, generating non-conservative, quasi-cyclic motion. When large numbers of particles are present in the optical trap, the precise natures of the motions become hard to predict. In this paper, we present the results of computer simulations used to explore the dynamical behavior of optically bound clusters of spherical nanoparticles, in beams possessing both spin and orbital angular momentum. While some of the behaviour observed has previously been predicted for low-symmetry shaped and chiral nanoparticles, the use of spheres enables a deeper understanding of the processes underlying the dynamics obtained. The diverse complex motion possible will be explored for a variety of homogeneous and heterogeneous optical fields, with sufficiently large numbers of particles to explore the possibilities of optically driven swarms.

The recently introduced mass-polariton (MP) theory of light describes light in a medium as a coupled state of the field and matter [Phys. Rev. A 95, 063850 (2017)]. In the MP theory, the optical force density drives forward an atomic mass density wave (MDW) that accompanies electromagnetic waves in a medium. The MDW is necessary for the fulfilment of the conservation laws and the Lorentz covariance of light. In silicon at wavelength λ₀ = 1550 nm, the atomic MDW carries 92% of the total momentum and angular momentum of light. The MDW of a light pulse having field energy E propagating in a dielectric also transfers a net mass equal to δM = (n_pn_g − 1)E/c² , where n_p and n_g are the phase and group refractive indices. In this work, we present a schematic experimental setup for the measurement of the MDW in a silicon crystal. This setup overcomes many challenges that have been present in previously introduced setups and that have made the experimental observation of the MDW effect difficult due to its smallness in comparison with other effects, such as the momentum transfer by absorption and reflections. The present setup also overcomes challenges with elastic relaxation effects while extending possible measurement time scales beyond the time scale of sound waves in the setup geometry. For the proposed setup, we also compare the predictions of the MP theory of light to the predictions of the conventional Minkowski theory, where the total momentum of light is carried by the electromagnetic field. We also aim at optimizing experimental studies of the MDW effect using the proposed setup.

The Abraham-Minkowski controversy is theoretical debate originating at the beginning of the 20th century regarding the electrodynamics of moving media. The controversy has become synonymous with a debate over photon momentum in materials, and has implications in optical micromanipulation, which impacts scientific discovery and technology development in chemistry, physics, biology, material engineering, photonics, and medicine. An important, recent advance in the field of electrodynamics identifies a kinetic momentum as the momentum of the fields responsible for center of mass translations and a canonical momentum related to the coupled field and material system. We review the identification of the two subsystems, and show that neither the Abraham nor the Minkowski formulations are physically valid representations of electrodynamics. The Abraham tensor, like the Einstein-Laub tensor, does not represent a valid stress-energy-momentum (SEM) tensor because it is not frame invariance, which is a tenant of relativity. The Minkowski tensor does satisfy relativistic principles, but it violates the principle of causality. In this correspondence, we present the field-kinetic subsystem of macroscopic electrodynamics, which represents the mass-free contributions to the global SEM tensor. We demonstrate that the relation to microscopic electrodynamics by comparing the field-kinetic macroscopic force density with the microscopic force density via a limiting procedure with increasingly small dipoles representation of matter. We also derive the material response contribution to the SEM tensor in causal media demonstrating the canonical momentum of electrodynamics. Through analytical examples, we show how practical optical manipulation experiments are modeled using both the field-kinetic and canonical SEM tensors in electrodynamics.

Several models have been used to calculate optical forces since the beginning of optical trapping in the 1970´s to the Optical Tweezers (OT) in 1986 and are still under discussion these days. These models range from ray tracing geometrical optics to full electromagnetic Maxwell stress tensor formalisms, for very small particles in the Rayleigh regimen to tens of micron size particles in the Mie regime, where Mie resonances also appear. There is also a large effort to distinguish gradient vs scattering forces, which are clearly distinguished the geometrical optics frame, by reflected vs refracted beams, but not so easily distinguished in the full electromagnetic formalism. Moreover, Mie formalism was developed for an incident plane wave, which is far from the high numerical aperture (NA) beams used in OT. Even though there are models for high NA axial beams but most optical tweezers systems today use several OT points with inclined beams. Furthermore, the development of Bessel beams and other beams, also require a much more general calculation of the optical forces, as well as a theory for the forces in waveguides and photonic bandgap fibers in which is impossible to speak in terms of ray optics. One of the main difficult to calculate the optical forces in all these systems is the vector spherical wave decomposition of the incident beam. We develop a very general theory to perform this expansion for any beam which allowed us to calculate the forces for any beam and any particle size, and show a criteria to distinguish gradient from scattering forces, and the role of the Mie resonances in these cases. Dependence of the forces on the size of the particles is an important result for the development of optical chromatography. Experimental results in suspended microspheres allow us to compare the results with theory.

Optical tweezers manipulate microscopic objects using forces arising from the subtle interplay of optical scattering and gradient forces of highly focused laser beams. Unlike conventional backscatter signal for optical tweezers, the twophoton fluorescence (TPF) trapping signal from femtosecond optical tweezers (FOTs) shows a slow counterintuitive decay, when the trapped particles are not entirely within the laser-illuminated volume. A change in the corner frequency of FOT is also noted with the TPF technique. These observations are evident even at low average powers. The high peak powers trap not only single microspheres but also encourage optically directed self-assembly. We use TPF signatures of trapped particles to show the existence of a directed self-assembly process and elucidate the structural dynamics during the process of cluster formation.

Biological samples often have various absorption bands that need to be either targeted or avoided in opto-fluidic micromanipulation or biomedical imaging. With nonlinear optics, it is possible for light to self-induce a waveguide. However, the desired wavelengths may not be suitable to exhibit nonlinear self-guiding due to the absorption bands or the light-bioparticle interaction is not strong enough. Here we study formation of waveguides in red blood cell suspensions for a range of different wavelengths. We utilize nonlinear optical response for self-trapping of a laser beam, forming light guides in RBCs suspended in a phosphate buffer solution. To improve the number of usable light wavelengths over purely self-guided propagation, we use the master-slave relation, in a manner similar to the pump-probe experiment: a master beam creates a waveguide first in a scattering bio-soft-matter suspension over a few centimeters, and then a “slave” beam uses this waveguide to propagate through the medium. The slave beam, injected simultaneously, has no appreciable nonlinear self-action itself but experiences the master waveguide akin to an optical fiber. This new approach can provide a path to guide a wide range of wavelengths, including those in the absorption bands at lower power so as not to damage the sample. The fact that we can guide a wide range of wavelengths may bring about new applications in medicine and biology, for instance, in developing alternative solutions to transmit energy and information through scattering media, as needed in deep-tissue imaging, treatment and diagnostics.

Surface effects are crucial in several mesoscopic phenomena, especially those concerning biological entities. Here we determine the effects of Van der Waals forces at relatively long range ( 80 nm) by optically trapping a probe particle close to a large silica particle and modulating the spatial position of the probe employing oscillating optical tweezers. This method has greater signal-to-noise in the experimentally measured probe-response as compare to that obtained from measurements of Brownian fluctuations. We quantify the H-value experimentally by analyzing the amplitude response of a single trapped particle in comparison to numerically expected results by employing chi-square fitting, and obtain good agreement with the known H-value for the system.

We show that colloidal crystals can be assembled by means of temperature gradients produced by light absorption (λ=1070 nm) in a 21 nm titanium thin film deposited on one of the cell´s walls. Depending on the position of a 100x microscope objective focus within the 20 μm thick cell, three different regimes of crystal formation can be identified: 1) convective currents regime; 2) convective-thermophoresis regime, and 3) thermophoresis regime. We show that defects on the crystal can be modified dynamically by switching on and off the laser beam. In addition, the crystal can be 2D manipulated along the substrate. This technique could lead to the formation of large area colloidal crystals for photonics applications.

Chirality is a ubiquitous phenomenon in nature, which describes an object that is non-superimposable to its mirror image. A lot of organic molecules, such as amino acids and sugars, are chiral. Mimicking atoms with colloidal nanoparticles owning intriguing optical properties (termed as meta-atoms), the manipulation and organization of colloidal nanoparticles into artificial chiral metamaterials allows the understanding of the origin of chirality at colloidal scale as well as the exploration of new functionality in photonic devices. Herein, we develop an all-optical technique to assemble chiral meta-molecules into arbitrary two-dimensional geometries and to detect their optical chirality in-situ. Taking advantage of the thermophoretic migration of ions under the external temperature field, we generate a light-controlled thermoelectric field to capture and confine colloidal particles at the laser spots, which is termed opto-thermoelectric nanotweezers. Exploring depletion attraction interaction as the interparticle bonding force, we further demonstrate the assembly of colloidal particles of different materials and sizes into chiral meta-molecules. Dark-field spectroscopy is incorporated into the system to detect the scattering intensity of the meta-molecules at different circular polarizations and characterization of the circular dichroism. Specifically, the optical control of the thermoelectric field allows the dis-assembly and re-organization of the chiral meta-molecules into their enantiomers and diastereomers. With its all-optical control, reconfigurability, and versatility, the chiral-metamolecules will finds applications in both optofluidic and nanophotonic devices.

Optical trapping and manipulation of colloidal particles and biological objects have demonstrated essential applications in the fields of physical chemistry, biology and condensed matter physics. Currently, most optical tweezers and manipulation techniques are operated in solutions, where undesired Brownian motion will limit the working performance. Herein, we develop an all-optical technique for dynamic manipulation of colloidal particles and nanowires on a solid-state substrate under ambient conditions. Specifically, particles and nanowires are dispersed onto a thin layer of polymer surfactant spin-coated on the glass substrate. The surfactant layer acts as a gate to effectively trigger the particle motion. When the laser is off, particles are firmly bonded on the substrate by van der Waals interactions; under the laser illumination, the photothermal effects of particles induce the transition of polymer layer into a fluid-like phase, which reduces the van der Waals bonding and activates the movement of nanoparticles by optical scattering forces. This optothermally gated photon nudging strategy supports on-demand manipulation and reconfigurable patterning of colloidal particles at nanoscale accuracy with simple optics and low operation power. Our method applies to particles with different materials (e.g., gold, silver, silicon) and variable sizes (from 40 nm to 1.5 mm). Furthermore, integrated with dark-field imaging and spectroscopy, the reported platform enables in-situ detection and characterizations of the colloidal structures. With its versatile capabilities, this novel nanomanipulation technique will have promising applications in optical nanomanufacturing, nanophotonics, and colloidal nanodevices.

In light absorbing liquids optical trapping of solid micro-objects but also gas bubbles can be achieved and explained by the mechanisms involving the hydrodynamic whirls formation. The various forms of these whirls, that arise due to optothermal Marangoni effect induced by laser light beam, are able to accelerate the objects movement, transport them and subsequently trap at the laser beam center but also close to it. The usual light gradient field force and scattering force solely are insufficient and even not adequate to properly describe the mentioned by us particle trapping effects as the trap potential extends to much larger distances that the beam waist. We will demonstrate the mechanism of optical trapping and transporting of gas bubbles and will discuss the physics of whirls formation in this case. The numerical modelling of Marangoni flows at the liquid-gas interface confirms the experimental findings. We also demonstrate a novel type of trapping of micro-objects that occurs inside a toroidal whirl induced by laser in dye-doped oil. This type of trapping is quite unusual but allows to transport objects immobilized far from the beam waist just avoiding their excessive heating.

Förster Resonance Energy Transfer (FRET) is a radiationless distance-dependent transfer of energy from an excited donor fluorophore to an acceptor fluorophore. This radiationless interaction of a donor-acceptor pair through resonance is observed by an increase/decrease in the acceptor/donor fluorescence intensity, respectively. Here we present preliminary results on the fluorescence spectra of optically levitated micro-droplets doped with two different dyes that works as FRET pair. The laser light used for levitation (λ=660 nm) passes through a telecentric system of lenses to form a controllable double optical trap system. Micrometer sized droplets are produced using two on-demand piezo-driven dispensers. This allows independent trapping of differently dyed droplets in two traps where a collision between the droplets can be induced by moving the trap positions. The dye molecules mix when two droplets collide and coalesce. The emission spectrum obtained when the droplets are illuminated with laser having a wavelength of 532 nm is observed with a spectrometer which can record up to 26,000 spectra per second. We compare the results with the spectra taken from the same solutions in a cuvette. The results indicate that we are able to observe the FRET effect in single droplets with an exposure time as short as 100 µs. This spectroscopic investigation is an ongoing research project with the long-term goal to investigate environmental effects of aerosols in the atmosphere.

We present the generation and 3D manipulation of microbubbles by thermal gradients, induced by low power nanosecond pulsed laser in non-absorbent liquids. Light absorption at photodeposited silver nanoparticles on the optical fiber tip heat up the surrounding liquid, which leads to optothermal effects. With each laser pulse a microbubble is detached from the optical fiber end, creating a microbubbles-stream. The microbubbles move away from the optical fiber end driven by non-spherical cavitation until they coalesce creating a main-bubble which is attracted towards the optical fiber end by Marangoni force. In addition, the main-bubbles are under the influence of buoyancy and gravity forces, which act upwards and downwards, respectively. The balance of these forces allows the 3D manipulation of the main-bubble. The main-bubble position can be controlled by careful control of the pulse energy. To our knowledge this is the first time that 3D manipulation of microbubbles using pulsed lasers is demonstrated.

Beams carrying orbital angular momentum (OAM) are attractive as they offer a theoretically unbounded number of discrete states and have therefore been a subject of great interest for a variety of fundamental and applied research including optical communications, optical trapping, super-resolution microscopy, remote sensing and quantum information. In this work, we present a study on the use of fused silica capillary optical fibers for OAM beam propagation by means of antiresonant reflecting waveguiding. In particular, we propose the application of these simple and commercially available fibers for probing the light–matter interactions within the hollow core. We show that OAM beams (topological charge |L| = 1) of high mode purity (>90%) can be achieved in such capillary fibers. The stability of the OAM beam propagation was theoretically and experimentally demonstrated in the visible range. A numerical study based on full-vector finite-element method is conducted to characterize the change of OAM mode purity and loss as a function of the refractive index (RI) of the material filling the core. The propagation loss remains in the range of a few dB/m throughout the range of RI values varying from 1 to 1.39 that encompasses many analytes in the vapor or liquid phase. Finally, we propose that the simple capillary fiber can be used as a cost-effective optofluidic platform to study OAM light-matter interactions and new optical phenomena involving biochemical analytes.

Two-photon absorption polymerization (2PP) is a versatile lithographic method for building three-dimensional microdevices with sub-diffraction resolution and tunable elasticity. Using 2PP of organo-ceramic hybrid OrmoComp (trimethylolpropane triacrylate), we prepared microsized polymeric cantilevers consisting of the surface-anchored micropillar, long elastic neck and spherical head. A typical microcantilever has the length of 30 micrometers and thickness of 1-2 micrometers. To investigate the mechanical properties of our elastic microcantilever, we applied a laser optical trapping force on the head of the cantilever and bent the head from its equilibrium position. After switching off the laser trap, the head returns to the equilibrium position. The time-dependent restoring trajectory of the head was recorded by a fast video-tracking technique (500 fps) and subjected to the custom-made drift-correcting image analysis. We find that the microcantilever relaxation process is well-described by a single-exponential relaxation curve with a time constant of 16.5 ± 1.2 ms. Assuming a highly overdamped regime, theoretical calculations yielded an apparent Young´s modulus for our OrmoComp microstructure of 1 MPa, which is 3-orders of magnitude smaller than the reported value for the bulk material (~1 GPa). The possible reasons for such discrepancy are discussed.

The field of levitated optomechanics studies the interaction between light and the mechanical motion of mesoscopic objects that are suspended by means of magnetic, optical, or electrodynamic traps. The lack of a clamping structure drastically reduces mechanical and thermal coupling with the environment, making these physical systems particularly suitable as ultrasensitive force detectors and as a test bench for quantum mechanics in new regimes. In our experiment, we use a Paul trap to levitate a charged glass sphere that is 300 nm in diameter. We use an ultra-high-vacuum compatible technique to load a nanosphere into the trap. Furthermore, we have developed a method to control the electric charge of the trapped particle, which allows us to tune its oscillation frequency as well as to measure its mass precisely. Here we also report on the observation of cooling of the particle’s secular motion by means of feedback cooling, reaching a few tens of mK starting from room temperature. In future work, in order to reach quantum regimes, we plan to couple the center-of-mass motion of the nanoparticle and a single Ca⁺ ion to an optical cavity. Such a system offers new opportunities for levitated optomechanics, including new cooling schemes in the unresolved sideband regime and protocols for nonclassical motional state preparation.

Elastic micro-cantilever of 30-micrometer size is repeatedly deflected/released by optical tweezers trap, recorded by a high-speed camera (500 frames/sec) and subsequently processed off-line. This paper evaluates the position detection methods of the cantilever head, which is distorted by a diffraction pattern. We developed and tested four methods in our VideoAnalyser software - radial extremes, Hough transform, local corner tracking, and voting normal lines. The time dependence of the head position contains the information about the properties of both: cantilever material and the surrounding environment. Averaging of aligned graphs corresponding to individual cycles significantly improves the signal-to-noise ratio.

We demonstrate a simple scheme for generation phase singularities are coupled with the vortex-like orbital flows based on a biaxial crystal. Using a shear interferometer, we were able to create a self-converging trap from the phase singularity, which allows converging and diverging two absorbing particles. Moreover, in a certain interferometer shear mode, we were able to multiplex the trap.

Green biflagellated microalgae have proven to be of interest in biotechnology and biomedicine due to the production of lipids, carotenoids, and other components that have an environment dependent yield. In this work, we use back focal plane interferometry to obtain information about the behavior of microalgae held by an optical trap under different conditions. It has been observed that the elongated body of a microalga entering an optical trap will align along the beam axis and rotate counter-clockwise. The rotation is produced by the beating flagella, as we conclude from our observation of non-rotation of deflagellated or photodamaged cells. The dependence of rotation frequency on growth phase of the microalgae and on optical trapping power is investigated. To study these effects, each cell is held in the optical trap, and the laser light transmitted by the sample is collected with a microscope objective. Then, the back focal plane of the collection objective is imaged onto a quadrant photodiode. The voltage outputs of the photodiode are then recorded with a computer through the use of a custom Arduino circuit, and written to a text file for post-processing.

We introduce a subwavelength thick (~ 200 nm) plasmofluidic microlens that effortlessly achieves objective-free focusing and self-alignment of opposing optical scattering and fluidic drag forces for selective separation of exosome size bioparticles. Our optofluidic microlens provides a self-collimating mechanism for particle trajectories with a spatial dispersion that is inherently minimized by the optical gradient and radial fluidic drag forces. We demonstrate that this facile platform facilitates complete separation of small size bioparticles (i.e., exosomes) from a heterogenous mixture through negative depletion and provides a robust selective separation capability based on differences in chemical composition (refractive index). Unlike existing optical chromatography techniques that require complicated instrumentation (lasers, objectives and precise alignment stages), our platform open up the possibility of multiplexed and high-throughput sorting of nanoparticles on a chip.

We develop single fiber based optical tweezers employing photophoretic forces to trap absorbing particles exploiting the rotational motion displayed by the particles to be stably trapped radially. This implies that optical beams with a large off-axis intensity would prove to be more efficacious in trapping. Thus, we generate a pure Gaussian as well as a superposition of Gaussian and Hermite-Gaussian beam modes from a single optical fiber which is dual mode at our operating wavelength, and show that the latter is more efficacious in trapping by about 1.8 times than the former. Finally, we show that multimode fiber traps - which have even larger off-axis intensity distribution - are the most effective in trapping and manipulating absorbing particles.

Non-diffracting optical quasi-Bessel beams provide an opportunity to construct optical fields of complex architecture. The constructed beams may have a bright central peak or zero intensity on the beam axis and have the beam size of only a few microns propagating over a long-defined distance, which is not possible with conventional Gaussian or high-order Laguerre- Gaussian beams. In this work we demonstrate the possibility of constructing a needle-like diverging optical funnel with zero intensity on the axis. The primary aim is to numerically construct and optimize the optical field, which could transversely compress and focus a stream of µm- and sub-µm size particles injected into vacuum or gaseous environment by applying light pressure and photophoretic forces pushing particles into the area with lower intensity. We present the results of numerical modelling of an “optical funnel” based on re-imaging a non-zero-order quasi-Bessel beam, formed by an axicon and a phase plate or using an SLM, with a collimator. The funnel geometry, namely, the μm-size of the beam cross-section, several-mm long propagation length and its divergence, all is controlled and optimized by changing the topological charge at a fixed collimation of the re-imaging optics, or/and by varying the collimation with fixed topological charge of the beam. The simulated profiles will have an application for optical guiding and focusing of aerosolised beam of particles, large biomolecules and viruses to the micron-size focus of x-ray Free Electron Lasers in order to increase the delivery efficiency of isolated single particles in coherent diffractive imaging experiments.

The theoretical and experimental confirmation of the existence of a transverse spin momentum in an evanescent wave excited above the surface of a birefringent biological section is suggested in this research work. The possibility of controlling gold nanoparticles by the vertical spin of an evanescent wave in a surrounding fluid of tissue near the surface layer of the section is demonstrated.

New approaches of red blood cell (erythrocyte) controlling by the action of evanescent wave is proposed in the given research work. Theoretical and experimental models for describing the conditions of the erythrocyte transverse motion and the vertical spin realization have been analyzed in the special selected schemes. The use of a linearly polarized plane wave with azimuth of ±45⁰ in a model experiment, specially suggested in this work, allows visualizing the transverse controlled motion of the erythrocyte, which enables to claim about new possibilities for controlling microobjects in biology and medicine.