Spectral range extension of laser-induced dichroism in composite glass with silver nanoparticles

We have studied the influence of irradiation parameters on the laser-induced shape transformation of Ag nanoparticles in glass by fs pulses. The amount of dichroism which can be achieved by this technique is related to the aspect ratio of the prepared spheroidal particles. Looking for maximal dichroism, it was found that 'off-resonant' irradiation with wavelengths considerably larger than the surface plasmon (SP) resonance at 413 nm can produce a larger spectral gap between orthogonally polarized SP bands of the reshaped nanoparticles than resonant excitation. It is also shown that subsequent irradiation with successively larger wavelengths may increase the spectral gap even further. A sequence of only two steps of irradiation at 535 and 670 nm already caused a shape transformation of initially spherical Ag nanoparticles to spheroids with an aspect ratio of (a/c) > 3.