High-sensitivity force sensing using a phonon laser in an active levitated optomechanical system.

Force detection with high sensitivity is of paramount importance in many fields of study, from gravitational wave detection to investigations of surface forces. Here, we propose and demonstrate a force-sensing method based on gain-enhanced nonlinearity in a nonlinear phonon laser. Experimental and simulation results show that the input force leads to the frequency shift of phonon laser, due to nonlinearity. In addition, we further investigate the influences of the pumping power, numerical aperture, and microsphere's refractive index on the performance of this force-sensing system, regarding the sensitivity and the linear response range. Our work paves a new way towards the realization of precise metrology based on the nonlinearity of phonon laser.

Structured-light displacement detection method using split-waveplate for dual-beam optical tweezers.

Structured-light displacement detection method is an innovative approach with extremely high sensitivity for measuring the displacement of a levitated particle. This scheme includes two key components, a split-waveplate (SWP) and a single-mode fiber. In this work, we further investigated the influence of SWP installation on this method regarding the sensitivity of displacement detection. The results indicate that the sensitivity increases with the expanding of SWP offset in the effective range. In addition, we found this method has a significant tolerance rate, with an extensive SWP offset effective range of 5%-25%. However, an excessive offset can render this method ineffective. More interestingly, we demonstrated the feasibility of rotating the SWP to detect displacement in different directions. Our research contributes to guiding the structured-light detection methods in practical applications and expanding their applications in fundamental physics.

Characteristics of the phonon laser in the active levitated optomechanical system.

Phonon lasers, coherent oscillations of phonons, have gradually become one of the emerging frontiers in the last decades, and have promising applications in quantum sensing, information processing, and precise measurement. Recently, phonon lasers based on dissipative coupling have been realized in an active levitated optomechanical (LOM) system for the first time. Here, we further investigated the characteristics of the phonon laser in the system above regarding the oscillator amplitude and the phonon laser linewidth. We established both the experimental system and a physical model of the phonon laser. On the basis of simulations and experiments, the influences of pumping power, numerical aperture, the microsphere's diameter and refractive index on the performance of the phonon lasers are sufficiently discussed. Our work is of great significance for the high-quality phonon lasers generated by the appropriate parameters, which is the basis for the in-depth research and practical application.

Enhanced optical trapping of ZrO2@TiO2 photonic force probe with broadened solvent compatibility.

Optical trapping and manipulating nanoparticles are essential tools for interrogating biomedicine at the limits of space and time. Typically, silica or polystyrene microspheres are used as photonic force probes. However, adapting those probes to organic solvents is an ongoing challenge due to the limited solvent compatibility and low refractive index mismatch. Here we report on the optical force enhancement and solvent compatibility that utilizes ZrO2@TiO2 core-shell nanoparticles. We experimentally demonstrate that the 450-nm-diameter ZrO2@TiO2 core-shell nanoparticles achieve the lateral and axial trap stiffness up to 0.45 pN µm-1 mW-1 and 0.43 pN µm-1 mW-1 in water, showing more than fivefold and ninefold improvement on the ordinary SiO2 particle of the same size. In addition, ZrO2@TiO2 core-shell nanoparticles can realize stable three-dimensional trapping in both polyethylene glycol and glucose solutions. This optical trapping enhancement property, coupled with solvent compatibility, expands the range of feasible optical trapping experiments and will pave the way toward more advanced biological applications.

Differential displacement measurement of the levitated particle using D-shaped mirrors in the optical tweezers.

Displacement measurement using a D-shaped mirror is a key technology in optical tweezers, which have emerged as an important tool for precision measurement. In this paper, we first study the influences of installation errors for the D-shaped mirror on the displacement measurement. The calibration factor and sensitivity of the different installation parameters are quantified. The results show that the variation of the calibration factor obeys the cosine curve with the angle error, and the sensitivity increases exponentially with the translation error. Besides, we find that the translation error will also lead to crosstalk between transverse and axial displacement. Our work will contribute to improving the performance of optical tweezers for the application in precision measurement and basic physics.

Customizable trajectory of trapped particle in quadruple-beam optical trap.

We have presented and demonstrated a customizable trajectory of a trapped particle in the Quadruple-beam optical trap. The orbital motion of the trapped microsphere was realized by modulating the trapping power. The motion trajectories could be designed by adjusting the modulation frequency, amplitude, and phase. By using this method, we have realized the triangle, bowknot, ellipse, straight line, and hooklike trajectories. The motion frequencies and circumferences were also modulated. The customizable trajectory in the optical trap may result in more possibilities for directional movement, microfluidic mixing, driven machines, and even painting freely.

Optical Pulling Using Chiral Metalens as a Photonic Probe.

Optical pulling forces, which can pull objects in the source direction, have emerged as an intensively explored field in recent years. Conventionally, optical pulling forces exerted on objects can be achieved by tailoring the properties of an electromagnetic field, the surrounding environment, or the particles themselves. Recently, the idea of applying conventional lenses or prisms as photonic probes has been proposed to realize an optical pulling force. However, their sizes are far beyond the scope of optical manipulation. Here, we design a chiral metalens as the photonic probe to generate a robust optical pulling force. The induced pulling force exerted on the metalens, characterized by a broadband spectrum over 0.6 µm (from 1.517 to 2.117 µm) bandwidth, reached a maximum value of -83.76 pN/W. Moreover, under the illumination of incident light with different circular polarization states, the longitudinal optical force acting on the metalens showed a circular dichroism response. This means that the longitudinal optical force can be flexibly tuned from a pulling force to a pushing force by controlling the polarization of the incident light. This work could pave the way for a new advanced optical manipulation technique, with potential applications ranging from contactless wafer-scale fabrication to cell assembly and even course control for spacecraft.

Self-feedback induced bistability in dual-beam intracavity optical tweezers.

The intracavity optical tweezers is a new, to the best of our knowledge, cavity optomechanics system, implementing a self-feedback control of the particle's position by trapping the particle inside an active ring cavity. This self-feedback mechanism efficiently constructs a novel potential in the cavity. Here we predict and give experimental evidence for the self-feedback induced optical bistability in dual-beam intracavity optical tweezers. Then the characteristics of these bistable potential wells are investigated. The results show that we can prevent the bistable behaviors from destabilizing the trapping stability through tuning the foci offset of two propagating beams in the cavity. This contributes to the use of intracavity optical tweezers as a powerful tool for optical manipulation. Importantly, the thermally activated transition of the trapped particle in the bistable potential is observed for particular experimental parameters. Further investigation of this phenomenon could underlie the mechanism of many metastable-related processes in physics, chemistry, and biology.

Dual-beam intracavity optical tweezers with all-optical independent axial and radial self-feedback control schemes.

The feedback control to optical tweezers is an obvious approach to improve the optical confinement. However, the electronic-based feedback controlling system in optical tweezers usually consists of complex software and hardware, and its performance is limited by the inevitable noise and time-delay from detecting and controlling devices. Here, we present and demonstrate the dual-beam intracavity optical tweezers enabling all-optical independent radial and axial self-feedback control of the trapped particle's radial and axial motions. We have achieved the highest optical confinement per unit intensity to date, to the best of our knowledge. Moreover, both the axial and radial confinements are adjustable in real-time, through tuning the foci offset of the clockwise and counter-clockwise beams. As a result, we realized three-dimensional self-feedback control of the trapped particle's motions with an equivalent level in the experiment. The dual-beam intracavity optical tweezers will significantly expand the range of optical manipulation in further studies of biology, physics and precise measurement, especially for the sample that is extremely sensitive to heat.

Optical confinement efficiency in the single beam intracavity optical tweezers.

Single beam intracavity optical tweezers characterizes a novel optical trapping scheme where the laser operation is nonlinearly coupled to the motion of the trapped particle. Here, we first present and establish a physical model from a completely new perspective to describe this coupling mechanism, using transfer matrices to calculate the loss of the free-space optical path and then extracting the scattering loss that caused by the 3D motions of the particle. Based on this model, we discuss the equilibrium position in the single beam intracavity optical tweezers. The influences of the numerical aperture, pumping power, particle radius and refractive index on the optical confinement efficiency are fully investigated, compared with standard optical tweezers. Our work is highly relevant for guiding the experiments on the single beam intracavity optical tweezers to achieve higher optical confinement efficiency.

Coupling between axial and radial motions of microscopic particle trapped in the intracavity optical tweezers.

Intracavity optical tweezers have been proposed and demonstrated recently, which allows orders-of-magnitude higher optical confinement with lower-numerical-aperture lens and lower laser power in contrast to the standard optical tweezers. We further investigate its characteristics about the position stability of trapped particles. The dependence of the radial and axial position stability on the laser intensity acting on the particle of 10-µm diameter in intracavity optical tweezers and standard optical tweezers are compared experimentally. Result shows that higher laser intensity can make stronger optical confinement in intracavity optical tweezers under the condition of good trap operation, compared with standard optical tweezers. We demonstrate and analyze the coupling between the particle's radial and axial motion, and then provide two approaches to reduce it. Our work will benefit the further enhancement of position stability for the trapped particle in intracavity optical tweezers.

All-fiber interferometer for displacement and velocity measurement of a levitated particle in fiber-optic traps.

We propose an all-fiber interferometer based on laser Doppler velocimetry in a dual-beam fiber-optic trap to measure the displacement and velocity of a trapped particle. ABCD matrices are used to compute the contrast ratio of the interference. The influence of the reflectivity of the fiber end face is discussed. We have designed an optimized reflectivity based on the parameters of our setup. The antireflective coatings on the fiber end face are employed to achieve the given reflectivity. The displacement and velocity of the trapped microparticle are successfully measured by the period and frequency of the interference signal, respectively. The sensitivity of the displacement detection is 368 nm. By describing the miniaturization of the detection system, this paper provides a simple and practical scheme to achieve the integration of the entire optical trapping setup.

Back-focal-plane displacement detection using side-scattered light in dual-beam fiber-optic traps.

In optical traps the position of a trapped bead is usually determined by measuring the intensity distribution of the forward-scattered light and the back-scattered light. In this paper we demonstrate that this position can be determined using the side-scattered light. A quadrant photodiode is used to monitor the position of an optically trapped object in a dual-beam fiber-optic trap by measurement of intensity shifts in the back focal plane of the objective that is perpendicular to the propagating beam. An approximated model based on ray optics is presented with numerical results that describe the use of the side-scattered light for position detection. The influences of system parameters, including fiber separations, the numerical apertures (NA), and the radii of microspheres, are discussed in details. We find out that the displacement sensitivity of the detector is null for some critical radii and numerical apertures. In addition, the noises in laser powers are analyzed, and one power difference regime is proposed to weaken the influences.

Optically bound colloidal lattices in evanescent optical fields.

In this Letter, we demonstrate the formation of a stable two-dimensional lattice of colloidal particles in the interference pattern formed by four evanescent optical fields at a dielectric interface. The microspheres are observed to form a two-dimensional square lattice with lattice vectors inclined relative to the beam propagation directions. We use digital video microscopy and particle tracking to measure the Brownian motion of particles bound in the lattice, and use this to characterize fluctuations in the local ordering of particles using the bond orientational order parameter, the probability distribution of which is shown to be a chi-squared distribution. An explanation for the form of this distribution is presented in terms of fluctuations of the modes of a ring of particles connected by springs.

Characteristics of the orbital rotation in dual-beam fiber-optic trap with transverse offset.

The orbital rotation is an important type of motion of trapped particles apart from translation and spin rotation. It could be realized by introducing a transverse offset to the dual-beam fiber-optic trap. The characteristics (e.g. rotation perimeter and frequency) of the orbital rotation have been analyzed in this article. We demonstrate the influences of offset distance, beam waist separation distance, light power, and radius of the microsphere by both experimental and numerical work. The experiment results, i.e. orbital rotation perimeter and frequency as functions of these parameters, are consistent with the theoretical model in the present work. The orbital rotation amplitude and frequency could be exactly controlled by varying these parameters. This controllable orbital rotation can be easily applied to the area where microfluidic mixing is required.

Dynamics analysis of microsphere in a dual-beam fiber-optic trap with transverse offset.

A comprehensive dynamics analysis of microsphere has been presented in a dual-beam fiber-optic trap with transverse offset. As the offset distance between two counterpropagating beams increases, the motion type of the microsphere starts with capture, then spiral motion, then orbital rotation, and ends with escape. We analyze the transformation process and mechanism of the four motion types based on ray optics approximation. Dynamic simulations show that the existence of critical offset distances at which different motion types transform. The result is an important step toward explaining physical phenomena in a dual-beam fiber-optic trap with transverse offset, and is generally applicable to achieving controllable motions of microspheres in integrated systems, such as microfluidic systems and lab-on-a-chip systems.

Theoretical analysis on the rotation-induced frequency difference in ring lasers with coupled cavities.

We analyzed the effective scale factor of ring laser gyros with coupled cavities in a general way. The coupled cavities can be made of both an odd and even number of mirrors, or even fiber coil. Compared with the "zero-vector-area" design in previous publications, we use the propagation loss rather than transmittance and reflectivity of mirrors to characterize the coupled cavities, which are more universal and controllable. In addition, we found the area of the coupled cavities could further enhance the effective scale factor by 1+l/L, where l and L are the round-trip length of the ring lasers and the coupled cavity, respectively. Therefore, the scheme using coupled cavities to enhance the sensitivity is more practical. These findings are important to realize highly sensitive ring laser gyros.

Temperature compensation method using readout signals of ring laser gyroscope.

Traditional compensation methods using temperature-related parameters have little effect when the ring laser gyroscope (RLG) bias changes rapidly. To solve this problem, a novel RLG bias temperature compensation method using readout signals is proposed in this paper. Combined with the least squares support vector machine (LS-SVM) algorithm, the novel method can improve the precision of the RLG bias. Experiments show that by utilizing the readout signals in the LS-SVM model, the RLG bias stability can be significantly raised compared to the original data. The novel method proposed in this paper is shown to be feasible, even when the RLG bias changes rapidly.