Gas-based nonlinear optics
Although solid-core fibers have found widespread use in different fields, they have certain drawbacks, such as the fact that they are restricted to specific wavelengths due to transmission bands of materials, they cannot handle high-peak-power operations due to high nonlinear refractive indices, and any undesired physical response cannot be easily removed. Nonetheless, the advent of hollow-core fibers has solved these issues.
Hollow-core fibers have expanded the study of nonlinear optics from solids to gases and plasmas. Unlike densely packed solids, gases show simpler atom interactions, resulting in clearer physical effects. For instance, molecular gases have a more narrowband Raman response than solid materials, and it decays more gradually, causing an abundance of physical phenomena depending on the pulse duration and Raman dephasing time. Moreover, by using inert gases in hollow-core fibers, scientists can exclusively study the electronic Kerr response and create few-cycle pulses with impressive peak power.
Currently, we focus on studying the nonlinear dynamcis of Raman scattering in molecular gases, aiming to generate wavelengths that cannot be achieved directly through solid-state gain media. Besides wavelengths, we intend to generate a pulse with high pulse energy which is only enabled by low nonlinearity of gases but hindered by strong nonlinearity in conventional solid-core fibers. With hollow-core fibers, we hope to improve practicality and versatility of fiber lasers.
Spectrum and pulse temporal profile of the soliton after soliton self-frequency shift in a hydrogen-filled hollow-core fiber 
 Y.-H. Chen, P. Sidorenko, E. Antonio-Lopez, R. Amezcua-Correa, and F. Wise, “Efficient soliton self-frequency shift in hydrogen-filled hollow-core fiber,” Opt. Lett. 47, 285–288 (2022)