Intensity of optical field inside a weakly absorbing spherical particle irradiated by a femtosecond laser pulse

Using the Fourier technique in combination with the Mie theory, we study numerically the spatiotemporal evolution of the intensity of the internal optical field inside micron-sized weakly absorbing spherical particles upon diffraction by these particles of a femtosecond laser field. A number of specific features of the dynamics of the spatial intensity distribution of the femtosecond pulses inside the particles are found to depend on the pulse width, the shape of the laser beam, the size of the particles, and the geometry of their irradiation. It is shown that, under conditions of nonstationary diffraction, the internal optical field is usually excited in a resonance way, with the eigenfrequencies of one or several high-Q resonance modes of the particle falling into the central part of the original pulse spectrum. This causes a time delay of the light in the particle and a reduction of the absolute maximum in the time dependence of the internal field intensity as compared with a stationary regime. The greatest reduction of the peak occurs at exact resonance. In this case, the decrease in the peak intensity may reach several orders of magnitude. Irradiation of a particle by a narrow Gaussian beam of femtosecond duration directed toward the particle center enhances the internal field intensity as compared with the case of near-edge incidence. (C) 2004 MAIK Nauka/Interperiodica.