Spectral Broadening and Pulse Compression in Molecular Gas-Filled Hollow-Core Fibers

Gas-filled hollow-core fibers have over the last three decades emerged as a key technology for ultrafast nonlinear optics and strong-field physics. Today, noble gas-filled capillary and microstructured fibers are used to generate broadband, coherent supercontinuum spectra through self-phase modulation, which can then be compressed to yield few-cycle driving pulses spanning the ultraviolet to mid-infrared. More recently, the use of molecular gases for spectral broadening has attracted significant interest, due to the interplay of the rotational and vibrational degrees of freedom with the electronic nonlinearity. Depending on the pulse duration of the driving laser, the complex interplay between instantaneous Kerr effect and the 'delayed' rotational and vibrational Raman nonlinearities can induce novel behavior, such as four-wave mixing, stimulated Raman scattering, and soliton self-frequency shifting, which can combine with self-phase modulation to realize unique few-cycle sources in spectral regions inaccessible the laser gain medium. In this Invited Paper, we discuss new routes to spectral broadening in molecular gas-filled hollow-core capillary fibers, with a particular focus on the generation of few-cycle pulses from ytterbium-doped laser amplifiers. We review the physics underlying the rotational enhancement of optical nonlinearity in linear molecules and explore nonlinear propagation in molecular gas-filled fibers through both simulations and experiments. We demonstrate pulse compression, using dispersive mirrors and bulk media to produce few-cycle output pulses in different spectral regions, and discuss the challenges and opportunities for average power scaling. Finally, we describe the prospects for generating few-cycle sources in the mid-infrared through red-shifted broadening initiated by long-wavelength driving lasers.