ABSTRACT
The formation of both local second- and third-harmonic generations (SHG and THG) induced by a train of femtosecond laser pulses in silver-doped phosphate glasses is addressed. Based on modeling calculations, including various diffusion and kinetic processes, THG is shown to result from the formation of silver clusters. The latter organize into a ring-shape structure, leading to the emergence of a static electric field. By breaking the glass centro-symmetry, this field gives rise to a local effective second-order susceptibility, inducing SHG. Both theoretically predicted SHG and THG evolutions with respect to the number of pulses in the train are in good agreement with experimental observations. In particular, the observed reaching of a maximum in the nonlinear optical responses after a few thousands of pulses is explained by the competition of various physical processes. A cooling of the glass is shown to improve the process efficiency of the laser writing of second-order nonlinearity.
ABSTRACT
We report on two-photon excited fluorescence in the oriented Eu(3+)doped LYB monoclinic crystal under femtosecond laser tight focusing. Due to spatial walk-off, the two polarization modes of the incident femtosecond beam simultaneously provide the independent excitation of two distinct focuses, leading to a single-beam dual-voxel nonlinear excitation of fluorescence below material modification threshold. These observations emphasize on the anisotropy of both two-photon absorption as well as fluorescence emission. They demonstrate the localized control of the nonlinear energy deposit, thanks to the adjustment of both the input power and polarization, by properly balancing the injected energy in each voxel. Such approach should be considered for future direct laser writing of waveguides in propagation directions out of the dielectric axes, so as to optimally cope with the highly probable anisotropy of laser-induced material modification thresholds in these crystals. These results open new ways for further potential developments in direct laser writing as the simultaneous inscription of double-line structures for original waveguides processes.
