Evanescent Kerr effect in liquid-immersed optical nanofibers

Optical nanofibers have recently emerged as attractive nanophotonic platforms for several applications ranging from quantum technologies to nonlinear optics due to both the tight optical confinement and their wide evanescent field. In this work, we report on a theoretical and experimental investigation of the optical Kerr effect in the evanescent field of silica nanofibers immersed in several highly nonlinear liquids such as ethanol, acetone and water and we further compare them with air cladding. We provide formula of the effective nonlinear coefficients including the contribution of the nanofiber silica core and of the evanescent field for varying nanofiber diameter and for different surrounding media. Our results show that the contribution of the silica core inversely scales with the effective mode area, while the cladding contribution via the evanescent field depends both on the taper diameter and the nonlinear properties of the liquid. More specifically, it is shown in a silica nanofiber immersed in acetone that the evanescent field contribution to the total Kerr effect is greater than that of the silica core for a taper diameter smaller than 560 nm. We further report the observation of a strong evanescent Kerr effect through measurements of the stimulated Raman-Kerr scattering in a silica nanofiber immersed in acetone. The evanescent Kerr effect is shown to give rise to a strong asymmetric spectral broadening of the first Raman order generated in the nonlinear liquid. Finally, the evanescent Kerr and Raman effects demonstrated in this work may find potential applications to ultra-sensitive liquid sensing and Raman spectroscopy, as the optical mode propagating in the nanofiber essentially interacts with the outer environment without any major contribution from the nanofiber itself.