Understanding the nonlinear-optical response of a liquid-core photonic-crystal fiber

Liquid-core waveguide structures have long been known and intensely used in nonlinear optics. Photonic-crystal fiber (PCF) technologies enhance performance and offer new functionalities of liquid filled waveguides as tools for nonlinear optics. We demonstrate a hollow core PCF that supports single-mode guiding at wavelengths longer than 600 nm in a 4-$\mu $m-diameter liquid-filled core, thus offering an attractive platform for nonlinear-optical experiments in the liquid phase. This PCF is employed to demonstrate that liquid-phase materials can radically modify the nonlinear-optical response of a waveguide structure relative to a typical nonlinear response of a silica waveguide. We show that the strong inertia of optical nonlinearity, characteristic of highly nonlinear liquid-phase materials, gives rise to a pulse-width dependent spectral red shift of the spectrally broadened fiber output. This wavelength shift remains strong even for pulse widths as large as several hundreds of femtoseconds.