Xinlin Chen

Orbital Rotation of Trapped Particle in a Transversely Misaligned Dual-Fiber Optical Trap

We propose and demonstrate a novel optical orbital rotation technique for trapped particle in a transversely misaligned dual-fiber optical trap. The orbital rotation frequency can be controlled by varying the power of two counterpropagating beams. We theoretically analyze an optical force field exerted on a trapped 10-μm-diameter polystyrene particle and simulate its dynamic trajectory within a geometric optics regime framework. Results show that orbital rotation is realized by an optical force field with a vortex distribution that inherently stems from a transversely misaligned dual-fiber optical trap. The orbital rotation trajectory of the particle depends on the configuration of the transversely misaligned dual-beam optical trap rather than its initial position.

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.

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.

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.

Flexible multifunction optical micro-manipulation technique based on PDMS chip

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.

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.