Gas-filled hollow-core photonic crystal fibers (HC-PCF) have attracted immense attention in gas-based nonlinear optics because they offer long fundamental-mode interaction lengths, high intensities per unit output, a high optical damage threshold and pressure-tunable dispersion. For large core diameters, phase matching condition typically requires low gas pressures, which reduces the gas nonlinearity. Moreover, it is very difficult to simultaneously remove both wavevector mismatch and group velocity walk-off.
We have proved that acoustically-induced perturbation for the gas pressure can compensate for a nonzero wavevector mismatch for nonlinear frequency conversions in gas-filled HC-PCFs, while group velocity walk-off is suppressed by manipulating the pressure-tunable dispersion. The nonlinear generation can be switchable by turning on/off the acoustically-induced perturbation.
We consider a physical model: acoustically-induced quasi-phase-matching (QPM) for third-harmonic generation (THG) in argon-filled HC-PCFs. An acoustic wave produces a density modulation of gas, which in turn modulates the material dispersion and nonlinearity in fibers. 266-nm impulses are generated from 800-nm ultrafast Ti:sapphire laser impulse via THG in argon-filled HC-PCFs.
In this proof-of-principle study we have found that an acoustic perturbation in gas-filled structures compensates for a wavevector mismatch, offering a different type of QPM for gas-based nonlinear optical processes. The conception of acoustically-induced QPM is applicable to other kinds of nonlinear optical processes in gas-filled structures such as four-wave mixing and sum frequency generation.
Our research results have been published in the journal of "Optik-International Journal for Light and Electron Optics" under the title of "Acoustically-switchable nonlinear frequency conversion in gas-filled hollow-core photonic crystal fibers"(https://doi.org/10.1016/j.ijleo.2023.171414).
