Low-power and non-invasive optical manipulation of tiny objects is important in both material science and life science. Harnessing the high-efficiency light-to-heat conversion and the thermophoretic migration under a temperature gradient, we explore the opto-thermo-mechanic coupling during light-matter interaction and develop an indirect optical manipulation technique termed opto-thermophoresis, which features low operation power and simple optics. Based on this concept, we development new tools such as opto-thermophoretic tweezers, printers, tractors, and motors. Further, we demonstrate these optical tools in different fields, such as biomedical manipulation, bio-sensing, and reconfigurable optics.
Several studies have been proposed to control particle trajectory in liquid solutions using optically induced thermal gradient. Upon introducing different solutes such as salts and surfactants along with microparticles in these solutions, an additional optically induced thermoelectric trapping force is generated due to the differential motion of ions in the solution under thermal field. As the complexity of the solution increases, it becomes increasing difficult to understand particle response towards laser irradiance. More importantly, the existing models to study the thermoelectric behavior of the particle assumes a constant temperature gradient across the particles, which becomes obsolete in the micro-regime due to discontinuity of thermal conductivity at the particle-solution interface. For a better understanding of trapping and manipulation behavior of particles under light induced thermoelectric field, the temperature gradient distortion must be considered. In this work, full-scale finiteelement solver model has been proposed to determine the temperature variation around a microparticle under laser heating. The resultant temperature distribution is utilized to numerically evaluate the thermoelectric field and the trapping potential of the laser induced opto-thermoelectric trap. To experimentally validate this methodology, polystyrene micro-particles are trapped opto-thermoelectric-ally in CTAC solution and compared the experimental trapping stiffness to theoretical estimates obtained from the model. It is observed that trapping stiffness saturates as surfactant concentration increases which can be optimized by choosing the lowest CTAC concentration at the onset of saturation. The model implemented here can be easily extended to arbitrarily shaped particles, particles with non-uniform surface morphology, different combinations of core-shell particles and electrolyte solutions, which can be implemented to study different phenomenon such as optical pulling, rotation and translation.
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.
Lattice plasmon resonances (LPRs), which originate from the plasmonic-photonic coupling in gold or silver nanoparticle arrays, possess ultra-narrow linewidth by suppressing the radiative damping and provide the possibility to develop the plasmonic sensors with high figure of merit (FOM). However, the plasmonic-photonic coupling is greatly suppressed when the nanoparticles are immobilized on substrates because the diffraction orders are cut off at the nanoparticle-substrate interfaces. Here, we develop the rational design of LPR structures for the high-performance, on-chip plasmonic sensors based on both orthogonal and parallel coupling. Our finite-difference time-domain simulations in the core/shell SiO2/Au nanocylinder arrays (NCAs) reveal that new modes of localized surface plasmon resonances (LSPRs) show up when the aspect ratio of the NCAs is increased. The height-induced LSPRs couple with the superstrate diffraction orders to generate the robust LPRs in asymmetric environment. The high wavelength sensitivity and narrow linewidth in these LPRs lead to the plasmonic sensors with high FOM and high signal-to-noise ratio (SNR). Wide working wavelengths from visible to near-infrared are also achieved by tuning the parameters of the NCAs. Moreover, the wide detection range of refractive index is obtained in the parallel LPR structure. The electromagnetic field distributions in the NCAs demonstrate the height-enabled tunability of the plasmonic “hot spots” at the sub-nanoparticles resolution and the coupling between these “hot spots” with the superstrate diffraction waves, which are responsible for the high performance LPRs-based on-chip refractive index sensors.
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