Xiang Han

Evanescent wave optical binding forces on spherical microparticles.

In this Letter, we demonstrate stable optical binding of spherical microparticles in counter-propagating evanescent optical fields formed by total reflection at a dielectric interface. The microspheres are observed to form one-dimensional chains oriented parallel to the direction of propagation of the beams. We characterize the strength of the optical binding interaction by measuring the extent of Brownian position fluctuations of the optically bound microspheres and relating this to a binding spring constant acting between adjacent particles. A stronger binding interaction is observed for particles near the middle of the chain, and the dependence of the binding strength on incident laser power and number of particles in the chain is determined.

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.

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.

Controllable rotation of microsphere chain in dual-beam fiber-optic trap with transverse offset

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.

Numerical simulations of dual-waveguide trap with rough and tilted endfaces

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.

Distinguishing the number of captured microspheres in dual-beam optical trap by measuring the back light signal

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.

Brownian dynamics of the optically trapped spinning microparticles in low pressures

Optical trap has become a powerful tool of biology and physics, since it has some useful functions such as optical rotator, optical spanner and optical binding. We present the translational motions in the transverse plane of a 4.4μm-diameter vaterite particle which is optically trapped in low pressures utilizing the Monte-Carlo method. We find that the air pressure around the microparticle plays an important part in the determination of dynamics of the trapped particle. According to the energy equipartition theorem, the position fluctuations of the optically trapped particle satisfy Maxwell-Bolzmann distributions. We present the features of particles’ displacements and velocities changing with air pressures in detail, and find that the modulation of the trap stiffness makes a higher position variance. The mechanical quality factor Q larger than 10 induces a high peak of power spectral density. Our research presents a powerful tool towards further discovery of dynamical characteristics of optically trapped Brownian particles in low air pressures.

Correlated fluctuations of optically trapped particles

We present a study of correlated Brownian fluctuations between optically confined particles in a number of different configurations. First we study colloidal particles held in separate optical tweezers. In this configuration the particles are known to interact through their hydrodynamic coupling, leading to a pronounced anti-correlation in their position fluctuations at short times. We study this system and the behavior of the correlated motion when the trapped particles are subject to an external force such as viscous drag. The second system considered is a chain of optically bound particles in an evanescent wave surface trap. In this configuration the particles interact both through hydrodynamic and optical coupling. Using digital video microscopy and subsequent particle tracking analysis we study the thermal motion of the chain and map the covariance of position fluctuations between pairs of particles in the chain. The experiments are complemented by Brownian motion simulations.