Generation of a stable femtosecond supercontinuum in a divergent laser beam for time-resolved broadband spectroscopy of laser-induced processes in matter

Time-resolved absorption spectroscopy with subnanosecond time resolution requires sources of white light with short or ultrashort durations, which can be synchronized with other laser systems. Using a near-infrared femtosecond laser source, a supercontinuum with a high spectral brightness ($\sim 10$ pJ/nm), a high pulse-to-pulse stability of about $2$–$5\%$, and a long-term (several hours) stability has been generated in the spectral range of $\sim 450$–$750$ nm. These supercontinuum characteristics have been achieved by operating in a divergent beam, which makes it possible to avoid multiple filamentation, to stabilize the spectrum (halving fluctuations and a wider energy stability range), and to broaden it by changing the dynamic balance between Kerr focusing, plasma defocusing, and diffraction. Time resolution has been achieved by means of a specially developed electronic delay system based on field-programmable gate arrays, which has made it possible to achieve subnanosecond time resolution in a wide time window (up to several milliseconds). This technique has been successfully tested in experiments to study the dynamics of silicon ablation under nanosecond laser impact.