Role of external focusing geometry in supercontinuum generation in bulk solid-state media

We present a detailed experimental and numerical study of supercontinuum generation in sapphire crystal at the vicinity of its zero group velocity dispersion point (1.3 μm) when pumped by relatively long (210 fs) femtosecond pulses. We uncover very different evolutions of the spectral broadening versus the input pulse energy when the incident beam is focused either onto the input face or inside the 4-mm-thick sapphire sample. In particular, when the input beam was focused inside the crystal, we captured a surprising variation of the spectral width as a function of the input pulse energy, demonstrating supercontinuum generation, its suppression, and eventually, its recovery. The experimental findings were nicely reproduced by the numerical simulations, revealing a specific supercontinuum generation scenario, which relies on strong reshaping of the wave packet due to defocusing and absorption of free electron plasma, subsequent replenishment of the pulse on the propagation axis, and its splitting. We also demonstrate that this particular supercontinuum generation scenario takes place regardless of the external focusing geometry.