The displacement measurement of the levitated particle is essential in optical tweezers in vacuum. However, crosstalk between the radial axes often occurs and will deteriorate the measurement precision. Common methods are proposed to align the coordinate systems of the motion and measurement, but few have considered the polarizations of the trapping beam and possible crosstalk control. Here single SiO2 particles with a diameter of 200nm are trapped in the single-beam optical tweezers in vacuum. Balanced detectors and D-shaped mirrors are used to measure the particle's displacements. As expected, the crosstalk coefficient can be periodically changed with the control of the linear polarization of the trapping beam. When the polarization direction is along Y axis, the crosstalk on the displacement x from the other radial axis can reach to infinity. When the polarization direction is along X axis, the crosstalk is eliminated. For comparison, crosstalk elimination is also achieved by an inserted Dove prism to rotate the beam along the propagation axis. The crosstalk elimination by polarization control is simpler, but it needs linear-polarized beam and will slightly change the particle’s resonant frequencies. The crosstalk elimination by beam rotations will need at least two Dove prisms, but it is adapted to most common conditions and does not change the resonant frequencies. The research is useful for the feedback cooling and the precise measurement of the physical quantities in future.
KEYWORDS: Filtering (signal processing), Particles, Electronic filtering, Signal processing, Optical filters, Optical tweezers, Systems modeling, Motion models, Interference (communication), Signal to noise ratio
The cooling and quantum control of the optically trapped particles is a hot topic in quantum frontier research. One of the key steps is using Kalman filter to extract the particle’s motion from noisy signals. Time delays of the Kalman filters are found in the process of signal extraction. Here the particle displacements based on the parameters of actual optical trapping systems are simulated, and the time delays of the Kalman filtering process are observed by changing the oscillation periods and the relaxation time for stabilization. The results indicate that Kalman filtering can effectively compress the noises in the displacement signal and thus improve the signal-to-noise ratio. Furthermore, as smaller the signal frequency is, larger time delays are observed in the process. It shows that the time delays should be noticed and compensated. Meanwhile, it is shown that the consuming time for signal stabilizations in the filtering process and the phase of the original signal, neither of which affects the filtering effect. These simulation results are our initial explorations for the cooling of optically trapped particles in vacuum. It would provide possible help to deal with the delay mismatch resulted from Kalman filtering and for the cooling of the optically trapped particles.
Optical tweezers (OTs) are an important tool for the viscosity measurements in microrheology, and passive techniques have the features of being simple and need no external force generations. Current passive methods using OT always first calibrate the potential stiffness and then do parameter fittings to obtain the viscosity. Here, we introduced and demonstrated a passive viscosity estimation method for low-viscous microfluids using OTs without stiffness calibration and parameter fitting. By Brownian trajectory tracking of single trapped bead, the viscosity coefficients of water and NaCl solutions are quickly obtained with small deviations (typical <10 % ) from the reference values. Besides, we introduce estimations for the commonly used voltage-to-displacement conversion factor, and the consistency check between the estimations and calibrations is used to represent the estimation quality. The whole process is very convenient for automatic processing. Further matrix operations are proposed and tested, which are expected to be integrated with holographic OTs and optical fiber traps for distributed multidimensional measurement.
We present a method for measuring the van der Waals force between two microspheres based on photonic force microscopy. We trapped a microsphere as probe by optical tweezers. The restricted Brownian motion of Gaussian distribution could be found in this system. The vibration center of the probe was affected by the van der Waals force when a target microsphere was closer to the probe. We measured the vibration center of the probe at different separation between the pair of microspheres. Based on this, the measurement of the van der Waals force between the two microspheres was realized with a high precision. Our method can realize the direct measurement of van der Waals force without using the variation rules of it. This method results in a simple structure, would not damage the sample, and can be suitable for the surface of any shape. It is general and has a wide range of applications in other fields of micro-force measurement.
Since microcavity Kerr soliton combs have spectrum which can exceed one octave, high repetition rate and potential for on-chip integration, the dissipative Kerr soliton generation in microresonators has been widely studied in recent years. Although microcavity soliton combs have been demonstrated in microcavities of different materials and shapes, it is still challenging for soliton generation due to positive thermal effects. In this paper, a sol-gel processed SiO2-CaF2 hybrid toroid microresonator is numerically investigated. Based on the calculation and simulation model we developed, this CaF2 coated SiO2 microresonator may avoid thermal effects and thermo-mechanical oscillations. Compared to organic coatings for thermal compensation in previous studies, it is a more promising platform for soliton generation.
We presented an optical system that could measure the viscosity coefficient of liquid in a micro-area. The orbital rotation of a polystyrene microsphere was realized by a dual-beam fiber-optic trap with a transverse offset. The rotation rate increased with the viscosity coefficient of the environmental medium. On this basis, the viscosity coefficients of ethanol solutions with different concentrations were measured successfully. The volume of solution samples was less than 1 μL. This provides a basis for the viscosity measurement of rare liquid or enchylema, which is of great significance for biological applications such as cell characteristics and reaction dynamics.
The angular velocity of a vaterite microsphere spinning in the optical trap is measured using rotational Doppler effect. The perfectly spherical vaterite microspheres are synthesized via coprecipitation in the presence of silk fibroin nanospheres. When trapped by a circularly polarized beam, the vaterite microsphere is uniformly rotated in the trap center. The probe beams containing two Laguerre–Gaussian beams of opposite topological charge l = ± 7, l = ± 8, and l = ± 9 are illuminated on the spinning vaterite. By analyzing the backscattered light, a frequency shift is observed scaling with the rotation rate of the vaterite microsphere. The multiplicative enhancement of the frequency shift proportion to the topological charge has greatly improved the measurement precision. The reliability and practicability of this approach are verified through varying the topological charge of the probe beam and the trapping laser power. In consideration of the excellent measurement precision of the rotation frequency, this technique might be generally applicable in studying the torsional properties of micro-objects.
Optical traps have been widely used in a large variety of applications ranging from biophysics to nano-sciences. More than one microscopic object can be captured in an optical trap. In the practical application, it is always necessary to distinguish and control the number of captured objects in the optical trap. In this paper, a novel method has been presented to distinguish the number of trapped microspheres by measuring the intensity of back signal. Clear descent of the back signal has been observed when a microsphere is captured in the center of optical trap. The relative coupling efficiency of back signal decreases as the number of captured microspheres increases both in experiment and theory. This method contributes to miniaturization and integration of applied systems due to getting rid of the imaging system, and is generally applicable to the area of nanoparticle trapping.
Controllable rotation of the trapped microscopic objects has traditionally been thought of one of the most valuable optical manipulation techniques. The controllable rotation of a microsphere chain was achieved by the dual-beam fiber-optic trap with transverse offset. The experimental device was made up of a PDMS chip housing two counter-propagating fibers across a microfluidic flow channel. Each fiber was coupled with different laser diode source to avoid the generation of coherent interference, both operating at a wavelength of 980 nm. Each fiber was attached to a translation stage to adjust the transverse offset distance. The polystyrene microspheres with diameter of 10 μm were chosen as the trapped particles. The microfluidic flow channel of the device was flushed with the polystyrene microspheres solution by the mechanical fluid pump. At the beginning, the two fibers were strictly aligned to each other. Five microspheres were captured as a chain parallel to the axis of the fibers. When introducing a transverse offset to the counter-propagating fibers by adjusting the translation stages, the microsphere chain was observed to rotating in the trap center. When the offset distance was set as 9 μm, the rotation period is approximately 1.2s. A comprehensive analysis has been presented of the characteristics of the rotation. The functionality of rotated chain could be extended to applications requiring microfluidic mixing or to improving the reaction speed in a localized environment, and is generally applicable to biological and medical research.
We build numerical models of dual-waveguide trap with rough and tilted endfaces using both the finite element method. The optical field distribution of waveguide trapping house with rough and tilt endfaces is simulated and analyzed. The results shows that rough endfaces cause the incident beam scattered and the tilted endfaces make incident beam refracted. According to optical field distribution, axial and transversal optical trapping forces are calculated. When endfaces roughness increase, both the axial and transversal trapping forces decrease, meaning trapping depth decreased. The transversal equilibrium positions move around unpredictably, off center. The stiffness and width of optical trap change little. When endfaces tilt angles increase, both the axial and transversal trapping forces decrease, meaning trapping depth decreased. The transversal equilibrium positions move along minus transversal axis. It is no obvious change in stiffness and width of optical trap.
Design a chip for flexible multifunction optical micro-manipulation based on elastomeric materials-PDMS. We realized the different motion types of microspheres, including stably capture, spiral motion and orbital rotation, by adjusting the input voltage of piezoceramics designed in PDMS Chip. Compared to conventional techniques, this PDMS chip based method does not require special optical properties of the microspheres to be manipulated. In addition, the technique was convenient and precise for dynamical adjustment of motion types without external influences. From these results, we verify that this multifunctional optical micro-manipulation technique of PDMS elastomeric materials can find potential applications for optical manipulation, including cost-effective on-chip diagnostics, optical sorting and optical binding, etc.
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