Measuring the force parameters of a gradient-force optical trap for dielectric microobjects

Light-induced gradient force acting on extended dielectric microscopic objects in air is analytically calculated and measured in a direct manner. The experiments with filiform objects (cotton filament) make it possible to relate the measured gradient force to the elastic parameters of the studied material and to determine the optical parameters of the studied objects and the physical parameters of the trap. The potential application of the method for measuring optical parameters of small weight amounts of high-molecular substances and nanoobjects is analyzed.     

Paramagnetic particles in an optical trap

A setup based on an optical trap combined with homogeneous magnetic fields is presented. The system allows one to accurately control the alignment of multiple particles within the trap by controlling the external magnetic field. I study how two and three paramagnetic particles interact in the trap, and show that the experimental results can be explained in terms of dipolar interactions. It is also demonstrated that the system can be used to measure the magnetic moment of paramagnetic particles with a resolution of 10⁻¹⁵ Am².     

An integrated fiber-based optical trap for single airborne particles

We present a compact fiber-based optical airborne particle trap that is integrated on a microscope slide. Light guided in a short glass capillary is used to inject micron-sized droplets into an optical trap formed by the same light and a counterpropagating beam emanating from a single-mode optical fiber. The glass capillary ensures separation of trapped particles from remaining aerosol in the injection chamber. Permanent alignment of the fibers and the capillary allows for adjustment-free operation. Mie resonances arising in the radiation pressure exerted on trapped droplets are detected via a position stabilization feedback loop and are used to precisely determine the size evolution of trapped glycerol–water droplets.     

Trapping and manipulation of low-index particles in a two-dimensional interferometric optical trap

We demonstrate optical trapping and manipulation of low-index spheres in two dimensions, using the pattern produced by two interfering plane waves. This technique shows, for what is believed to be the first time, alignment of an array of hollow spheres and simultaneous manipulation of high- and low-index particles in the horizontal plane. Furthermore, rodlike particles (up to 30microm in length) are manipulated simultaneously with the low-index particles. This technique offers a practical method for manipulating bubbles, low-index droplets, or rodlike biological samples.     

Observation of nonlinear optical recoil-induced resonances in a dark magneto-optical trap

Nonlinear optical recoil-induced resonances have been observed in a gas of cold rubidium atoms trapped in a dark magneto-optical trap. These resonances are used to measure the temperature and velocity distribution of the cold atoms. The velocity distribution of the atoms in the dark magneto-optical trap has an excessive amount of fast atoms as compared to the Maxwell distribution and to the distribution in a standard magneto-optical trap.     

Calculation of axial optical forces exerted on medium-sized particles by optical trap

Optical trapping has become an efficient technique of trapping and manipulating micrometer and sub-micrometer dimension particles. Particles in the range between the applicable regime of ray optics theory (ROT) and the Rayleigh regime (so-called medium-sized particles) are focused on. By using ROT and the generalized Lorenz–Mie theory (GLMT), axial optical forces and their dependences on particle sizes and beam waists are presented. Furthermore, by comparing the numerical results of these two theories, the applicability of the GLMT to particles of arbitrary size and the limit of ROT in the region of small particles are analyzed. A new criterion of the applicable region of ROT is obtained, i.e. the relative particle size β=2πa/λ20, where a is the particle radius and λ is the wavelength of light. The theoretical results will be of great help to the design and optimization of the most efficient optical trap.     

Realization of a single-beam mini magneto-optical trap: A candidate for compact CPT cold atom-clocks

We have demonstrated the experimental realization of a single-beam mini magneto-optical trap of ⁸⁷Rb atoms, originally designed for a cold atom-clock with coherent population trapping (CPT). Only one beam is used as cooling, trapping and repumping beams rather than the three pairs of orthogonal beams of the standard magneto-optical trap. The core optics, which consists of a modified pyramidal funnel type mirror, a quarter-wave plate and a retroreflect mirror, is installed inside a mini titanium cubic chamber. The vacuum system, rubidium source, magnetic field coils and beam expander are designed in a compact geometry. As many as 1.1 × 10⁷ rubidium atoms are cooled and trapped, and thus the mini trap is ready for the implementation of a novel compact coherent population trapping cold atom-clock.     

光阱中布朗粒子动力学分析

通过对光阱中的布朗粒子进行动力学方程求解,由解的结果分析得到观测系统的性能要求．结果表明：光阱中布朗颗粒服从Boltzmann分布;对其运动的观测需要纳米量级的空间分辨率和毫秒量级的时间分辨率．     

Dynamics of charged particles in a Penning trap

An automodel solution of self-consistent equations for an ellipsoidal cluster of Coulomb particles in a Penning trap is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 18–21, July, 1994.     

Dust particles in a thermophoretic trap in a plasma

The feasibility of confining dust particles in a plasma by thermophoretic forces was demonstrated. An extended dust structure in a positive glow discharge column was experimentally obtained at liquid nitrogen temperature. The dust structure was confined in an electrostatic-thermal trap, in which vertical stability was provided by the summed action of longitudinal electrostatic field and thermophoretic forces. Traps of this kind can be analyzed in terms of the general principles developed for confining particles in traps with the use of electric and magnetic multipole fields. We were able to change the shape and volume of the structure and even separate it into parts by varying temperature fields.     

Simulation of stabilizing process of dielectric nanoparticle in optical trap using counter-propagating pulsed laser beams

<正>The stabilizing process of glass particle in water by optical trap using the pulsed counter-propagating Gaussian beams is investigated.The influence of the optical power and the particle dimension on the rate and time of the stabilizing process is simulated and discussed.     

Emission of particles from a magnetic trap

Three types of magnetic traps that provide three-dimensional potential wells for neutral atoms and ferromagnetic particles are considered. The possibility of forming directed particle beams is discussed.     

Giant enhancement of optical nonlinearity in mixtures of graded particles with dielectric anisotropy

To investigate the effective linear dielectric constant and third-order nonlinear susceptibility of composite media, in which graded inclusions with radial dielectric anisotropy are randomly embedded in a linear isotropic matrix, we develop an nonlinear anisotropic differential effective dipole approximation (NADEDA). Alternatively, based on a first-principles approach, the exact expressions for and are also derived for the linear dielectric profiles with small slopes. Then, excellent agreement between the two methods is numerically demonstrated. As an application, we further apply the NADEDA to a nonlinear metal-dielectric composite, in which the metal particles possess spatially varying radial dielectric anisotropy, in an attempt to study the nonlinearity enhancement and the figure of merit of the composite. To this end, it is shown that the presence of gradation in the radial dielectric constant plays a crucial role in enhancing the optical nonlinearity as well as the figure of merit.Received: 16 August 2003, Published online: 30 January 2004PACS: 77.22.Ej Polarization and depolarization - 42.65.-k Nonlinear optics - 42.79.Ry Gradient-index (GRIN) devices - 77.84.Lf Composite materials     

Spectroscopy of francium in a magneto optical trap

The francium atom offers an excellent laboratory to study electron-nucleus interactions. As the heaviest alkali, its atomic properties can be calculated with high precision, and laser trapping methods now allow precision optical spectroscopy of many isotopes. Recent measurements of the 7P_1/2 hyperfine structure, when coupled with previous measurements of the ground state hyperfine structure reveal a hyperfine anomaly. The change in anomaly between an even-N isotope and an odd-N isotope is sensitive to the radial distribution of the neutron magnetization. This revised version was published online in August 2006 with corrections to the Cover Date.     

Suppression of noise in a noisy optical trap

We have experimentally explored and clarified the concept of noise suppression in intrinsically noisy systems by adding noise at the input using a microscopic bead held in a moving and intensity-modulated optical trap. By coupling the stiffness of the optical trap to its position, we have explicitly constructed an experimental model system in which added fluctuations in the trap position result in reduced variance of the observed bead position as compared to a stationary trap. This reduction in variance and the spectral properties of the observed output noise agree with theoretical predictions. Our experiment demonstrates that the essential aspect of noise reduction in such a system is that the added fluctuations drive the system into states with a reduced intensity of intrinsic noise sufficiently often.     

Improved S-approximation for dielectric particles

The range of the applicability of the soft particles approximation (S-approximation) has been extended by means of the procedure which may be considered as a specific form of the analytic continuation. The initial element is represented by the standard version of the approximation while the branch of the continuation should be chosen in correspondence with the short wavelength asymptotic of the optical characteristic under consideration. As for the extinction efficiency of dielectric spherical particles, the improved version of the S-approximation fairly well describes the behaviour of Mie curves for particles with the refractive indices up to 2.0 or more and may be useful in order to get in the analytical form the “smoothed” Mie curves when the ripple-type fluctuations were averaged. The leading role of the van de Hulst approximation both for the electric and magnetic components of the extinction efficiency has been shown.     

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.     

Selective optical trap for low-temperature plasma

The possibility is demonstrated of utilizing resonant light pressure to provide prolonged selective optical confinement of a low-temperature plasma with resonant ions. For this purpose it is proposed to employ rectified gradient forces in strong interfering bichromatic fields and laser cooling methods. State University, Krasnoyarsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 96–103, January, 1997.