Non-linear propagation effects of intense femtosecond pulses on low order harmonics in solids

TL;DR

This study investigates how intense femtosecond NIR pulses non-linearly propagate in wide bandgap solids, affecting low-order harmonic generation and spectral properties, with implications for tailored XUV sources.

Contribution

It provides new insights into non-linear propagation effects on low-order harmonics in solids, including spectral shifts, broadening, and polarization dependence, under intense ultrafast excitation.

Findings

Spectral shifts and broadening of harmonics due to non-linear effects.

Strong polarization dependence of second and third harmonic generation.

Potential for controlling harmonic spectra for optical diagnostics.

Abstract

The non-linear propagation of the intense near-infrared (NIR) driving field in wide bandgap materials pose a challenge and an opportunity to control the spectral properties of high harmonic generation (HHG) in solids. Here, we have investigated the non-linear propagation effects of the ultrafast intense near-infrared (NIR) driving field at 800 nm of 40 fs pulse duration operating at a repetition rate of 1 kHz focused on the wide bandgap dielectrics such as MgO, Chromium (Cr) doped MgO (Cr: MgO), Sapphire (Sa) crystals and fused silica (FS). Furthermore, we have generated second and third harmonic (TH) in these materials to explore the non-linear response at a strong field. To quantify the non-linear propagation effects, low-order harmonics have been generated in reflection and compared with the harmonics generated in transmission. We observe spectral shifts and broadening of the driving field spectrum which is imprinted on the harmonics. We attribute these effects to strong photoionization, generation of free-carrier density and self-phase modulation effects. We have also studied the polarization dependence of second harmonic generation (SHG) and TH in FS. The linear polarization dependence of below bandgap harmonics in FS and Sa generated in reflection demonstrated the sharp anisotropy than in transmission. This work shows the sensitivity to control the spectral profile of harmonics by manipulating the driving field, showing the possibility of new tailored solid-state XUV sources for optical diagnostics.