EFFECTS OF THE LASER-RADIATION PRESSURE IN PHOTON-CORRELATION SPECTROSCOPY

Effects of the radiation pressure of illuminating laser light, even with a low power less than a few tens of mW, on a photon correlation function and a particle size obtained by using photon correlation spectroscopy with the homodyne detection are investigated experimentally for suspensions of polystyrene microspheres. The diffusion coefficient of particle suspension, which is determined from the measured correlation time, decreases with an increase in the power of illuminating laser light. This result stems from suppression of the brownian motion of particles within the probe volume due to a radial component of the radiation pressure of illuminating laser light. It turns out that the particle size evaluated from the correlation time is misled to be larger than the true value under illumination of the laser light. Complemental measurements by photon correlation velocimetry are performed in order to demonstrate the effects of the laser radiation pressure on the particle sizing by means of photon correlation spectroscopy and their dependence on the size of microspheres.