Effect of laser radiation power on laser trapping of light-absorbing microparticles in air

A. P. Porfirev, S. A. Fomchenkov

We investigate the effects of changing the power of a Gaussian laser beam on the motion of light-absorbing microparticles trapped in the beam region. Laser trapping of such particles was due to the action of so-called photophoretic forces. In addition, we demonstrate the possibility of controlled movement of trapped carbon nanoparticle agglomerations, both in the direction of propagation of the laser beam and in the opposite direction.

DOI

A Laser Trapping-Spectroscopy Study on Mass Transfer Processes Across a Single Micro-Droplet/Air Interface

M.A. Jiang, Shoji Ishizaka, Terufumi Fujiwara, Yuan Gao

Clouds regulate the earth's energy balance by reflecting and scattering solar radiation and by absorbing the earth's infrared radiation. The fundamental knowledge about mass transfer processes across a micro-droplet/air interface is very important to give mathematical equations that describe the growth process of clouds for climate models. So far, experimental studies on the condensation growth of water droplets have been conducted by using either an aerosol flow tube or a vibrating orifice aerosol generator. However, the mass accommodation coefficients evaluated by such techniques are very scattered and, therefore, the detailed mechanisms of condensation growth of micrometer-sized water droplets are still controversial. The primary reason for this is difficulties in observing the growth processes of single water droplets in air. In this study, we demonstrate a novel approach for in situ observation of the evaporation and condensation processes of single water droplets levitated in air by means of a laser trapping technique.

DOI

Optical Binding Force between Two Chiral Spheres by an Incident On-axis Gaussian Beam

Yuanyuan Zhu, Zhensen Wu, , Zhengjun Li, Qingchao Shang

According to the electromagnetic scattering of two spheres, the incident on-axis Gaussian beam is expanded in terms of spherical vector wave functions (SVWFs), and the beam shape coefficients are obtained by applying the localized approximation method. Using the addition theorem, the interaction scattering fields of two chiral spheres and the internal fields are also expanded in terms of SVWFs. Based on the continuous tangential boundary conditions, the scattered field coefficients are derived analytically. Utilizing the Maxwell's stress tensor integration technique, the optical binding force between two chiral spheres is formulated explicitly. Numerical simulations of the binding force are carried out. The effects of the beam width and the radius of the sphere on the force are analyzed. The numerical results are compared with the results from references.

DOI