Ryota Omori

Observation of a single-beam gradient-force optical trap for dielectric particles in air

A single-beam gradient-force optical trap for dielectric particles, which relies solely on the radiation pressure force of a TEM(00)-mode laser light, is demonstrated in air for what is believed to be the first time. It was observed that micrometer-sized glass spheres with a refractive index of n=1.45 remained trapped in the focus region for more than 30 min, and we could transfer them three dimensionally by moving the beam focus and the microscope stage. A laser power of ~40 mW was sufficient to trap a 5- microm -diameter glass sphere. The present method has several distinct advantages over the conventional optical levitation method.

High-Q concentrated directional emission from egg-shaped asymmetric resonant cavities

We propose the use of egg-shaped asymmetric resonant cavities (ARCs), each of which consists of a half-circular part and a half-deformed part, as promising candidates in obtaining desirable whispering-gallery-mode resonances. According to numerical analysis based on a ray-optics model, more than an order-of-magnitude higher Q and more-concentrated emission from the tip of the egg region were obtained for egg-shaped ARCs than for the previously studied quadrupolar ARCs.

FORCES OF A SINGLE-BEAM GRADIENT-FORCE OPTICAL TRAP ON DIELECTRIC SPHEROIDAL PARTICLES IN THE GEOMETRIC-OPTICS REGIME

We developed a computational code to calculate the forces of a single-beam gradient-force optical trap exerted on dielectric ellipsoidal particles in the geometric-optics regime. Using this code, the axial and the transverse trapping stability of spheroidal particles, the semi-major axis of which is perpendicular (type-A) and parallel (type-B) to an incident beam axis, was evaluated and the effects of the nonspherical geometry of the particles were analyzed. As the fractional deformation ratio increased, the axial trapping stability improved for type-B particles, whereas it degraded for type-A particles. It can, therefore, be concluded that type-B particles can be trapped more stably than type-A particles. It was also observed that the axial trapping stability can be improved by the use of a TEM01*-mode beam instead of a TEM00-mode beam.

Rotation of Optically Trapped Particles in Air.

We have demonstrated optically induced rotation of microscopic dielectric particles in air. The particles were anisotropically shaped, and were simultaneously trapped three-dimensionally and rotated about the beam axis, which depended solely on the radiation pressure of an extremely focused laser light. It was observed that the rotational speed was linearly dependent on the irradiated beam power and the slope of the fitting lines revealed up to 860 rpm/mW for 3.0 µm-diameter-particles and 540 rpm/mW for 4.0 µm-diameter-particles, which was much higher than the previously reported values measured in water. This technique will be useful for micromotors and microfans assembled in microelectromechanical systems.

Two-Dimensional Measurement of Rotational Speed of a Diffuse Object Based on Laser Doppler Anemometry.

A novel measurement method based on laser Doppler anemometry that determines the pure rotational speed of a diffuse object on the condition that its axis of rotation is in a plane perpendicular to the illuminating laser lights is described. The rotating object is illuminated by three parallel laser lights and its rotational speed is calculated from beat frequencies resulting from the mixing of their backscattered lights. To verify the principle, measurements of rotational speed of a brass cylinder were performed for rotational speeds in the range of 30–500 rpm. The measured values have a fairly good agreement with the preselected values over this range. The present method is useful for detection of rotational conditions of machinery components assembled in rotating or vibrating structures.

Formation of three-dimensional spatial patterns of glass particles using a single-beam gradient-force optical trap in air

Laser manipulation is a technique to confine and manipulate microscopic objects remotely using radiation pressure of a laser beam. Recently, a single-beam gradient-force optical trap, which relies solely on the radiation pressure of a tightly focused laser beam, was demonstrated in air. In this paper we report the formation of spatial patterns consisting of glass particles of d equals 5.0 micrometers in air using the technique. Once a particle was trapped, we transferred it by moving the focus of the objective lens and the microscopic stage and put it to other particles or the surface of the glass plate. We formed spatial patterns of particles by repeating this procedure. Because the attractive force between neighboring particles in the spatial patterns are much greater than the gravitational force acting on particles, observed 3D spatial patterns were very stable. The transfer of trapped particles was generally easier in air than in water because of low viscosity of air. Moreover, in this type of trap, we can improve the stability of the trap by increasing the laser power. It is expected that the present technique will be applied in various fields including microfabrication and micromachine.