Investigations of multiple component systems by means of optical trapping and Raman spectroscopy

Abstract

The Brownian motion of microdroplets in emulsions makes it very difficult to observe a single microparticle over a long time range without additional arrangements. The application of an optical trap to stabilize the droplet horizontally and vertically can overcome this problem. The trapping of microparticles has been first reported by Ashkin [l]. Ashkin et al. [2] have shown that optical trapping of dielectric particles can be achieved by a single beam gradient force trap. This kind of laser trapping of small particles has been currently studied in several research groups [3,4]. An electromagnetic wave carries a certain momentum which causes a pressure that is equal to the energy density of the wave. Because a momentum is conserved, light that is scattered, exert a force on a scattered object. The direction of the force depends strongly on the refractive index, the particle size and the incident laser power. A microparticle with a diameter up to about 30 urn can be optically trapped, when a laser beam is focused on the particle in the medium with refractive index n(medium) Qarticle) the situation is reversed and single laser beam trapping is impossible. The particle is repelled by the laser beam. However, using a rotating focused laser beam one can create a potential well around the particle which then traps the particle. This technique enabled us to investigate not only emulsion droplets or particles but also single gas bubbles [5]. Most of the studies dealing with optical trapping were focused on the physical understanding of laser trapping and the chemical or analytical aspect of this technique has been rarely explored. If single particles of micron size can be manipulated and characterized, the chemistry and physics of small particles can be studied. In this paper, we report about an efficient trap for single microparticles in water using the gradient field of a highly focused laser beam and the spectroscopical characterization of these particles.