Mass measurement under medium vacuum in optically levitated nanoparticles based on Maxwell speed distribution law.

As one of the directions of optical levitation technology, the mass measurement of micro-nano particles has always been a research hotspot in extremely weak mechanical measurements. When nanoscale particles are trapped in an optical trap, parameters such as density, diameter, and shape are unknown. Here we propose what we believe to be a new method to measure mass by fitting particle motion information to the Maxwell speed distribution law, with an accuracy better than 7% at 10 mbar. This method has the characteristics of requiring no external driving force, no precise natural frequency, no prior information such as density, and non-destructive testing within the medium vacuum range. With the increasing iterations, the uncertainty of mass measurement is reduced, and the accuracy of mass measurement of levitated particles is verified under multiple air pressures. It provides what we believe is a new method for the future non-destructive testing of nanoscale particles, and provides an apparently new way for the sensing measurement and metrology application fields of levitation dynamics systems.

Suppression of damping in a diamagnetically levitated dielectric sphere via eddy currents and static charge reduction.

Diamagnetically levitated micro-nano oscillators play a crucial role in fundamental physics research and the advancement of high-precision sensors. Achieving high sensitivity in acceleration or force sensing is a fundamental requirement within these research domains. The primary limitation in achieving such sensitivity is thermal noise, which is directly proportional to the motion damping of the oscillator. Theoretical modeling suggests the presence of significant damping mechanisms induced by eddy currents. In this study, we validated the theoretical model by optimizing the structure of the magnet trap, confirming the impact of eddy currents on the damping of the oscillators. Additionally, we observed another type of damping caused by static charge in moving levitated dielectrics. Subsequently, we proposed an innovative theoretical model to explain this phenomenon and verified its validity during the charge neutralization process. Through these efforts, we successfully reduced the total damping from 1.6 mHz to 0.15 mHz, resulting in an order of magnitude improvement in performance. Our sensing system achieved the highest sensitivity of acceleration sensing in diamagnetically levitated submillimeter-scale dielectric to date, measuring 7.6±0.8)×10-10g/Hz. The exploration conducted in this study regarding the analysis and suppression of electromagnetic damping, along with associated thermal noise, holds significant promise for frontier research involving sensing with levitating dielectrics.

Revolution of a trapped particle in counter-propagating dual-beam optical tweezers under low pressure.

We presented faster and more accurate simulations and experiments describing the revolution of a suspended particle in optical tweezers under a low pressure. Instead of the state-of-the-art offline method of pinhole alignment, we proposed an in situ method of revolution suppression by adjusting the laser beam while observing the power spectral density and time-domain plot of the particle centroid displacement. The experimental results under different air pressures show that our method is more effective at low pressures. We observed that "revolution occurs when radial alignment error is below the threshold" and uncovered the mechanism behind this phenomenon. The rapidly growing Q value of the revolution indicates a high-precision resonance measurement method under lower air pressure compared with random translation measurements.

3D calibration of microsphere position in optical tweezers using the back-focal-plane interferometry method.

This paper presents a method to directly calibrate the position of a trapped micro-sphere in optical tweezers utilizing its interference pattern formed at the back focal plane (BFP). Through finite difference time domain (FDTD) and scalar diffraction theorem, the scattering field complex amplitude of the near and far fields can be simulated after interference between the trapped sphere and focus Gaussian beam. The position of the trapped sphere can be recovered and calibrated based on a back focal plane interferometry (BFPI) algorithm. Theoretical results demonstrate that optical tweezers with a larger numerical aperture (NA) Gaussian beam will yield a better detection sensitivity but with a smaller linear range. These results were experimentally validated by trapping a microsphere in a single beam optical tweezer. We used an extra focused laser to manipulate the trapped sphere and then compared its position in the images and that obtained using the BFP method. The interference pattern from simulation and experiments showed good agreement, implying that the calibration factor can be deduced from simulation and requires no intermediate calculation process. These results provide a pathway to obtain the calibration factor, enable a faster and direct measurement of the sphere position, and show possibilities for adjusting the crosstalk and nonlinearity inside an optical trap.

Fast size estimation of single-levitated nanoparticles in a vacuum optomechanical system.

Optical trapping of single nanoparticles in vacuum has various applications in both precise measurements and fundamental physics. However, to date, the number and size of randomly loaded nanoparticles in an optical trap is difficult to determine unless in vacuum. In this Letter, an efficient method for nanoparticle size estimation in an optical tweezer system before the evacuation of air was proposed and demonstrated experimentally, using scattering light from levitated particles. The particle radii deduced from the scattering light power in our proposal and from the kinetic theory of particles in gas match well (with the differences of less than 10%). For sample particles with radii ranging within 50-100 nm, we also provide a preselection rule based on this method, where over half of the trapped particles are verified as single particles. Such a particle analysis method is applicable also for the size estimation of levitated diamond particles, gold particles, and other plasmonic particles and can be applied to discovering novel scattering effects.

Displacement Detection Decoupling in Counter-Propagating Dual-Beams Optical Tweezers with Large-Sized Particle.

As a kind of ultra-sensitive acceleration sensing platform, optical tweezers show a minimum measurable value inversely proportional to the square of the diameter of the levitated spherical particle. However, with increasing diameter, the coupling of the displacement measurement between the axes becomes noticeable. This paper analyzes the source of coupling in a forward-scattering far-field detection regime and proposes a novel method of suppression. We theoretically and experimentally demonstrated that when three variable irises are added into the detection optics without changing other parts of optical structures, the decoupling of triaxial displacement signals mixed with each other show significant improvement. A coupling detection ratio reduction of 49.1 dB and 22.9 dB was realized in radial and axial directions, respectively, which is principally in accord with the simulations. This low-cost and robust approach makes it possible to accurately measure three-dimensional mechanical quantities simultaneously and may be helpful to actively cool the particle motion in optical tweezers even to the quantum ground state in the future.

Radiation forces on a Rayleigh particle produced by partially coherent circular Airy beams.

In this work, the radiation force on a Rayleigh dielectric particle induced by the partially coherent circular Airy beam (PCCAB) is investigated. Our numerical results show that the PCCAB can be used to trap and manipulate particles. The radiation force distribution and trapping stability have been analyzed under different coherent lengths. It is found that, with the increase of the spatial coherent length, the radiation force is increased and the particle can be stably trapped at more points. Therefore, the radiation force as well as the depth of potential well can be effectively modulated by controlling the spatial coherent length in optical micromanipulation. The trapping properties of PCCAB have also been studied under other different parameters, including the scale factor and initial radius.

Dynamic analysis and rotation experiment of an optical-trapped microsphere in air.

A dual-fiber optical trap system to trap and rotate a borosilicate microsphere has been proposed and experimentally demonstrated. The trapping system can be used as a probe to measure environmental parameters, such as torque, force, and viscosity of the surrounding medium. Under various conditions with different fiber misalignments, optical power, and fiber separation, the trapped sphere will exhibit three motion profiles including random oscillation, round rotation, and abnormal rotation. The power spectrum analysis method is used to measure rotation rates up to 385 Hz, which can be further increased by increasing laser power. In addition, simulation and experiment show consistent results in rotation rates and motion trajectory, which verifies the validity and accuracy of dynamic analysis.

Elliptical Airy beam.

We found a new type of noncircular symmetrical Airy beam called an elliptical Airy beam (EAB). Using a simple single-pixel checkerboard hologram method, we achieved the EAB in an experiment. We observed its unique property of double focusing and the ability of the energy to flow towards the endpoints of the long axis during propagation. These particular properties will have some potential applications.

Theoretical Analysis and Experimental Verification of the Influence of Polarization on Counter-Propagating Optical Tweezers.

Counter-propagating optical tweezers are experimental platforms for the frontier exploration of science and precision measurement. The polarization of the trapping beams significantly affects the trapping status. Using the T-matrix method, we numerically analyzed the optical force distribution and the resonant frequency of counter-propagating optical tweezers in different polarizations. We also verified the theoretical result by comparing it with the experimentally observed resonant frequency. Our analysis shows that polarization has little influence on the radial axis motion, while the axial axis force distribution and the resonant frequency are sensitive to polarization change. Our work can be used in designing harmonic oscillators which can change their stiffness conveniently, and monitoring polarization in counter-propagating optical tweezers.

Yoctonewton force detection based on optically levitated oscillator.

Optically levitated oscillators in high vacuum have excellent environmental isolation and low mass compared with conventional solid-state sensors, which makes them suitable for ultrasensitive force detection. The force resolution usually scales with the measurement bandwidth, which represents the ultimate detection capability of the system under ideal conditions if sufficient time is provided for measurement. However, considering the stability of a real system, a method based on the Allan variance is more reliable to evaluate the actual force detection performance. In this study, a levitated optomechanical system with a force detection sensitivity of 6.33 ± 1.62 zN/Hz1/2 was demonstrated. And for the first time, the Allan variance was introduced to evaluate the system stability due to the force sensitivity fluctuations. The force detection resolution of 166.40 ± 55.48 yN was reached at the optimal measurement time of 2751 s. The system demonstrated in this work has the best force detection performance in both sensitivity and resolution that have been reported so far for optically levitated particles. The reported high-sensitivity force detection system is an excellent candidate for the exploration of new physics such as fifth force searching, high-frequency gravitational waves detection, dark matter research and so on.

Application of optical tweezers in cardiovascular research: More than just a measuring tool.

Recent advances in the field of optical tweezer technology have shown intriguing potential for applications in cardiovascular medicine, bringing this laboratory nanomechanical instrument into the spotlight of translational medicine. This article summarizes cardiovascular system findings generated using optical tweezers, including not only rigorous nanomechanical measurements but also multifunctional manipulation of biologically active molecules such as myosin and actin, of cells such as red blood cells and cardiomyocytes, of subcellular organelles, and of microvessels in vivo. The implications of these findings in the diagnosis and treatment of diseases, as well as potential perspectives that could also benefit from this tool, are also discussed.

Corrigendum: Application of optical tweezers in cardiovascular research: More than just a measuring tool.

[This corrects the article DOI: 10.3389/fbioe.2022.947918.].

Analysis and Suppression of Laser Intensity Fluctuation in a Dual-Beam Optical Levitation System.

Levitated micro-resonators in vacuums have attracted widespread attention due to their application potential in precision force sensing, acceleration sensing, mass measurement and gravitational wave sensing. The optically levitated microsphere in a counter-propagating dual-beam optical trap has been of particular interest because of its large measurement range and flexible manipulation. In this system, laser intensity fluctuation directly influences the trap stability and measurement sensitivity, which makes it a crucial factor in improving trapping performance. In this paper, a time-varying optical force (TVOF) model is established to characterize the influence of laser intensity fluctuation in a dual-beam optical trap. The model describes the relationship between the laser intensity fluctuation, optical force and the dynamic motion of the micro-sized sphere. In addition, an external laser intensity control method is proposed, which achieved a 16.9 dB laser power stability control at the relaxation oscillation frequency. The long-term laser intensity fluctuation was suppressed from 3% to 0.4% in a one-hour period. Experiments showed that the particle's position detection sensitivity and the stability of the relaxation oscillation could be improved by laser intensity fluctuation suppression.

Controllable Formation and Real-Time Characterization of Single Microdroplets Using Optical Tweezers.

Existing preparation methods for microdroplets usually require offline measurements to characterize single microdroplets. Here, we report an optical method used to facilitate the controllable formation and real-time characterization of single microdroplets. The optical tweezer technique was used to capture and form a microdroplet at the center of the trap. The controllable growth and real-time characterization of the microdroplet was realized, respectively, by adjusting experimental parameters and by resolving the Raman spectra by fitting Mie scattering to the spike positions of the spectra during the controllable growth of microdroplets. The proposed method can be potentially applied in optical microlenses and virus detection.

Simultaneous and independent capture of multiple Rayleigh dielectric nanospheres with sine-modulated Gaussian beams.

This study investigates the propagation properties and radiation forces on Rayleigh dielectric particles produced by novel sine-modulated Gaussian beams (SMGBs) because of the unique focusing properties of four independent light intensity distribution centers and possessing many deep potential wells in the output plane of the target laser. The described beams can concurrently capture and manipulate multiple Rayleigh dielectric spheres with high refractive indices without disturbing each other at the focus plane. Spheres with a low refractive index can be guided or confined in the focus but cannot be stably trapped in this single beam trap. Simulation results demonstrate that the focused SMGBs can be used to trap particle in different planes by increasing the sine-modulate coefficient g. The conditions for effective and stable capture of high-index particles and the threshold of detectable radius are determined at the end of this study.

Simultaneous Trapping of Two Types of Particles with Focused Elegant Third-Order Hermite-Gaussian Beams.

The focusing properties of elegant third-order Hermite-Gaussian beams (TH3GBs) and the radiation forces exerted on dielectric spherical particles produced by such beams in the Rayleigh scattering regime have been theoretically studied. Numerical results indicate that the elegant TH3GBs can be used to simultaneously trap and manipulate nanosized dielectric spheres with refractive indexes lower than the surrounding medium at the focus and those with refractive indexes larger than the surrounding medium in the focal vicinity. Furthermore, by changing the radius of the beam waist, the transverse trapping range and stiffness at the focal plane can be changed.