Non-Hermitian non-equipartition theory for trapped particles.

The equipartition theorem is an elegant cornerstone theory of thermal and statistical physics. However, it fails to address some contemporary problems, such as those associated with optical and acoustic trapping, due to the non-Hermitian nature of the external wave-induced force. We use stochastic calculus to solve the Langevin equation and thereby analytically generalize the equipartition theorem to a theory that we denote the non-Hermitian non-equipartition theory. We use the non-Hermitian non-equipartition theory to calculate the relevant statistics, which reveal that the averaged kinetic and potential energies are no longer equal to kBT/2 and are not equipartitioned. As examples, we apply non-Hermitian non-equipartition theory to derive the connection between the non-Hermitian trapping force and particle statistics, whereby measurement of the latter can determine the former. Furthermore, we apply a non-Hermitian force to convert a saddle potential into a stable potential, leading to a different type of stable state.

Momentum-Topology-Induced Optical Pulling Force.

We report an ingenious mechanism to obtain robust optical pulling force by a single plane wave via engineering the topology of light momentum in the background. The underlying physics is found to be the topological transition of the light momentum from a usual convex shape to a starlike concave shape in the carefully designed background, such as a photonic crystal structure. The principle and results reported here shed insightful concepts concerning optical pulling, and pave the way for a new class of advanced optical manipulation technique, with potential applications of drug delivery and cell sorting.

Subwavelength optical trapping and transporting using a Bloch mode.

Multi-functional optical manipulations, including optical trapping and transporting of subwavelength particles, are proposed using the Bloch modes in a dielectric photonic structure. We show that the Bloch modes in a periodic structure can generate a series of subwavelength trapping wells that are addressable by tuning the incident wavelength. This feature enables efficient optical trapping and transportation in a peristaltic way. Since we are using the guiding Bloch mode in a dielectric structure, rather than using plasmonic or dielectric resonant cavities, these operations are wide band and free from joule loss. The Bloch mode in a simple periodic dielectric structure provides a new platform for multi-functional optical operations and may find potential applications in nanophotonics and biomedicine.

Self-Induced Backaction Optical Pulling Force.

We achieve long-range and continuous optical pulling in a periodic photonic crystal background, which supports a unique Bloch mode with the self-collimation effect. Most interestingly, the pulling force reported here is mainly contributed by the intensity gradient force originating from the self-induced backaction of the object to the self-collimation mode. This force is sharply distinguished from the widely held conception of optical tractor beams based on the scattering force. Also, this pulling force is insensitive to the angle of incidence and can pull multiple objects simultaneously.

Giant and tunable optical torque for micro-motors by increased force arm and resonantly enhanced force.

Micro-motors driven by light field have attracted much attentions for their potential applications. In order to drive the rotation of a micro-motor, structured optical beams with orbital angular momentum, spin angular momentum, anisotropic medium, and/or inhomogeneous intensity distribution should be used. Even though, it is still challenge to increase the optical torques (OT) in a flexible and controllable way in case of moderate incident power. In this paper, a new scheme achieving giant optical torque is proposed by increasing both the force arm and the force amplitude with the assistance of a ring resonator. In this case, the optical torque doesn't act on the target directly by the incident beam, but is transmitted to it by rotating the ring resonator connected with it. Using the finite-difference in time-domain method, we calculate the optical torque and find that both the direction and the amplitude of the torque can be tuned flexibly by modifying the frequency, or the relative phases of the sources. More importantly, the optical torque obtained here by linearly polarized beams can be 3 orders larger than those obtained using the structured beams. This opt-mechanical-resonator based optical torque engineering system may find potential applications in optical driven micro-machines.

Optical trapping of nanoparticles with tunable inter-distance using a multimode slot cavity.

Optical trapping of nano-objects (i.e., the nano-tweezers) has been investigated intensively. Most of those nano-tweezers, however, were focused on the trapping of a single nanoparticle, while the interactions between them were seldom considered. In this work, we propose a nano-tweezers in a slot photonic crystal cavity supporting multiple modes, where the relative positions of two trapped nanoparticles can be tuned by selective excitation of different resonant mode. Results show that both the nanoparticles are trapped at the center of the cavity when the first order mode is excited. When the incident source is tuned to the second order mode, however, these two nanoparticles push each other and are trapped stably at two separated positions. Also, the inter-distance between them can be tuned precisely by changing the relative power of the two modes. This provides a potential method to control the interactions between two nano-objects via optically tuning the separation between them, and may have applications in various related disciplinary.

Optical pulling using evanescent mode in sub-wavelength channels.

Optical evanescent wave in total internal reflection has been widely used in efficient optical manipulation, where the object is trapped by the intrinsic intensity gradient of the evanescent wave while transported by the scattering force along the orthogonal direction. Here, we propose a distinct optical manipulation scheme using the attenuated modes in subwavelength optical channels, where both the trapping and transportation forces are along the channel direction. We create such a mode in a sub-wavelength photonic crystal waveguide and quantitatively obtain the net pushing and pulling forces, which can overcome the Brownian motion within a critical length. Due to the presence of the physical channel, subwavelength trapping on the transverse direction is natural, and manipulation along bend trajectories is also possible without the assistance of the self-acceleration beams provided a channel is adopted. This optical manipulation method can be extended to any other channels that support attenuation mode, and may provide an alternate way for flexible optical manipulation.

Flexible optical manipulation of ring resonator by frequency detuning and double-port excitation.

Optical force exerted on a ring resonator, which can move freely in plane, is investigated using the finite-difference in time-domain method. In order to manipulate the ring resonator more flexibly, two assistant waveguides are introduced to form a microring resonator based add-drop device. Results show that a blue tuned source is more suitable for the manipulation of the ring, rather than the central resonant frequency as expected. A red-tuned frequency, however, is difficult to trap the ring stably. When the frequency detuning is combined with selected double-port excitation, the ring can be trapped stably at some discrete positions, some determined regions, or be transported continuously along the waveguide. This optically reconfigurable opto-mechanical resonant system may find potential applications in tunable photonic devices and precise sensing.

Spin-controlled orbital motion in tightly focused high-order Laguerre-Gaussian beams.

Spin angular momentum can contribute to both optical force and torque exerted on spheres. Orbit rate of spheres located in tightly focused LG beams with the same azimuthal mode index l is spin-controlled due to spin-orbit coupling. Laguerre-Gaussian beams with high-order azimuthal mode are used here to study the orbit rate of dielectric spheres. Orbit rates of spheres with varying sizes and refravtive indices are investigated as well as optical forces acting on spheres in LG beams with different azimuthal modes. These results would be much helpful to investigation on optical rotation and transfer of spin and orbital angular momentum.

Wide-angle and polarization independent perfect absorber based on one-dimensional fabrication-tolerant stacked array.

We propose a wide-angle, polarization independent and fabrication-tolerant perfect absorber, which is based on a one-dimensional stacked array consisted of vertically cascaded two pairs of metal-dielectric bilayers. The results show that the absorption peaks are over 99% at the wavelength of 5.25 µm for different polarization angles, and remain very high within wide ranges of incident and azimuthal angles. We attribute those excellent performances to the excitation of the magnetic resonance (MR) and the guided mode resonance (GMR) for the TM and TE polarization, respectively, and are further expounded by the inductor-capacitor (LC) circuit model and the eigen equation of the GMR, respectively. More importantly, this one-dimensional absorber is very robust to the spacing distance between the neighboring stacks and the metallic strip thickness, which releases degrees of freedom in design and makes the absorber extremely flexible and simple in fabrication, thus it can be a good candidate for many fascinating applications.

Equilibrium orientations of oblate spheroidal particles in single tightly focused Gaussian beams.

Based on a hybrid discrete dipole approximation (DDA) and T-matrix method, a powerful dynamic simulation model is used to find plausible equilibrium orientation landscapes of micro- and nano-spheroids of varying size and aspect ratio. Orientation landscapes of spheroids are described in both linearly and circularly polarized Gaussian beams. It's demonstrated that the equilibrium orientations of the prolate and oblate spheroids have different performances. Effect of beam polarization on orientation landscapes is revealed as well as new orientation of oblate spheroids. The torque efficiencies of spheroids at equilibrium are also studied as functions of tilt angle, from which the orientations of the spheroids can be affirmed. This investigation elucidates a solid background in both the function and properties of micro-and nano-spheroidal particles trapped in optical tweezers.

Equilibrium orientations and positions of non-spherical particles in optical traps.

Dynamic simulation is a powerful tool to observe the behavior of arbitrary shaped particles trapped in a focused laser beam. Here we develop a method to find equilibrium positions and orientations using dynamic simulation. This general method is applied to micro- and nano-cylinders as a demonstration of its predictive power. Orientation landscapes for particles trapped with beams of differing polarisation are presented. The torque efficiency of micro-cylinders at equilibrium in a plane is also calculated as a function of tilt angle. This systematic investigation elucidates in both the function and properties of micro- and nano-cylinders trapped in optical tweezers.