Unambiguous evidence of two plasmon decay during ultrafast laser writing in glass

The interaction of femtosecond laser pulses with transparent media has been a focus of research due to its unique properties. It has been established that above a certain threshold, self-assembled nanogratings in silica glass can be induced [1]. Although the mechanism that triggers the nanostructure is still unclear, a theory has been introduced involving the mechanism of nanogratings formation based on two plasmon parametric decay [2]. A signature of two-plasmon decay is the generation of the 3/2 harmonic. Previously, in transparent media, only a weak 3ω/2 emission was observed at high energy thresholds [3]. Thus it remains unclear if two plasmon decay can be associated with self-assembled nanogratings formation. Here we present a thorough survey of 2^nd, 3^rd and 3/2 harmonic generations in fused silica for varying laser fluences within multiple regimes of optical laser writing and self-assembled nanostructures. We demonstrate that 3ω/2 can be observed at the energies close to the threshold of permanent material modification.