Abstract

We report on a Raman microprobe technique where micron-sized solid particles or liquid droplets are trapped in stable optical potential wells using only the force of radiation pressure from a continuous gas laser. We demonstrate this technique with Raman spectra from spherical as well as nonspherical single particles of sizes ranging between ~5 and 40μm. The particles are stably supported by a vertically directed focused TEM₀₀-modecwargon-ion laser of ~500 mW. From the optically levitated single microparticles we took the light-scattering spectra, which, in the case of spherical particles, show pronounced structural resonances. The observed resonances could be assigned by using the well-known Lorenz-Mie formalism, and a good correlation is found between experimental and theoretically predicted spectra. It was further found that, for particular liquid droplets, quite high-order numbers of the natural modes of oscillation of a sphere play a dominant role. We describe the intensities of the resonances in the Raman-Mie spectra particularly through the volume averaged internal field intensity. We also show that the internal angle-averaged electric field intensity is localized near, but not confined to, the sphere surface.

© 1986 Optical Society of America