Dual-beam Fiber-optic Trap Research Articles

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

For an ellipsoid suspended in a dual-fiber optic trap with transverse offset, the optical force, torque and posture are theoretically analyzed. When importing a transverse offset between two fibers, in the water environment, the azimuthal angle between the long axis and the optical axis increases with the offset distance. In the vacuum environment, the ellipsoid will spin in the trap center. When the offset is great than the threshold, the ellipsoid will move around the trap center along an elliptic trajectory, accompanied with the spin motion. The results will contribute to better understanding of the spin motion in the transversely misaligned dual-beam fiber-optic trap.

Microscopic viscosity measurement using orbital rotation in dual-beam fiber-optic trap with transverse offset

We presented an optical system that could measure the viscosity coefficient of liquid in a micro-area. The orbital rotation of a polystyrene microsphere was realized by a dual-beam fiber-optic trap with a transverse offset. The rotation rate increased with the viscosity coefficient of the environmental medium. On this basis, the viscosity coefficients of ethanol solutions with different concentrations were measured successfully. The volume of solution samples was less than 1 μL. This provides a basis for the viscosity measurement of rare liquid or enchylema, which is of great significance for biological applications such as cell characteristics and reaction dynamics.

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 368nm. 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.

Launch and capture of a single particle in a pulse-laser-assisted dual-beam fiber-optic trap

The rapid loading and manipulation of microspheres in optical trap is important for its applications in optomechanics and precision force sensing. We investigate the microsphere behavior under coaction of a dual-beam fiber-optic trap and a pulse laser beam, which reveals a launched microsphere can be effectively captured in a spatial region. A suitable order of pulse duration for launch is derived according to the calculated detachment energy threshold of pulse laser. Furthermore, we illustrate the effect of structural parameters on the launching process, including the spot size of pulse laser, the vertical displacement of beam waist and the initial position of microsphere. Our result will be instructive in the optimal design of the pulse-laser-assisted optical tweezers for controllable loading mechanism of optical trap.

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.

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.

Theory of dielectric micro-sphere dynamics in a dual-beam optical trap

We investigate the dynamics of an array of polystyrene micron-sized spheres in a dual-beam fiber-optic trap. Experimental results show non-uniform equilibrium particle spacing and spontaneous self-sustained oscillation for large particle numbers. Results are analyzed with a Maxwell- Stress Tensor method using the Generalized Multipole Technique, where hydrodynamic interactions between particles are included. The theoretical analysis matches well with the experimentally observed equilibrium particle spacing. The theory shows that an offset in the trapping beams is the underlying mechanism for the oscillations and influences both the oscillation frequency and the damping rate for oscillations. The theory presented is of general interest to other systems involving multi-particle optical interactions.

Observation of Bistability and Hysteresis in Optical Binding of Two Dielectric Spheres

Using a dual-beam fiber optic trap, we have experimentally observed bistability and hysteresis in the equilibrium separations of a pair of optically bound dielectric spheres in one dimension. These observations are in agreement with our coupled system model in which the dielectric spheres modify the field propagation, and the field self-consistently determines the optical forces on the spheres. Our results reveal hitherto unsuspected complexity in the coupled light-sphere system.

Optical levitation particle delivery system for a dual beam fiber optic trap

We combine a radiation-pressure-based levitation system with a dual fiber, laser trapping system to demonstrate the potential of delivering single particles into the fiber trap. The forces versus position and the trajectory of the particle subjected to the laser beams are examined with an enhanced ray optics model. A sequence of video images taken from the experimental apparatus demonstrates the principle of particle delivery, trapping, and further manipulation.