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.
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.
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.
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.
Magnet outside is one of the main reasons that induce the bias shift of the Ring Laser Gyroscope (RLG), and the
most effective way to handle it so far, is to employ a magnetic shielding device. In this paper, we design a new magnetic
shielding box for RLG in Fe-Ni soft magnetic alloy. By 3-D modeling and simulation calculation in finite element
method, we plot the distribution of magnetic flux density of the box in uniform magnet; Besides, distribution of magnetic
shielding effectiveness (SE) on the gaining plane of RLG where is most sensitive to the change of magnet outside, is
given out, too. We also discuss the factors that influence the distribution of SE such as permeability of shielding material,
the thickness of the box and punching on the surfaces of the box. And by the SE test of the box samples, the result of the
simulation analysis was finally proved true. The result shows that, the structure of the box decides the distribution of SE
in it and the permeability of the material plays the predominant role in evaluating the average value of SE. Punching on
the surfaces of the box makes the distribution of SE severely inhomogeneous, and with increase of the permeability, the
influence will be more significant. At last, we put some practical steps forward to enlarge the average value of SE, and
these measures are also applicable in other similar magnetic shielding devices.