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The rich dynamics of photon-phonon interactions in subwavelength dimension structures have generated immense scientific interest lately. Brillouin scattering self-cancellation [1] and surface Brillouin scattering [2] are among the newly discovered optoacoustic interactions in sub-wavelength silica microwires. Surface Brillouin scattering can be described as an inelastic light scattering generated by a surface acoustic wave (SAW) induced via electrostriction. Traveling at the boundary interface of dissimilar elastic stiffness regions, SAWs demonstrate clear advantages over bulk waves for their low speed and accessibility, permitting a wide range of applications, including surface studies, signal processing, time delay, etc.[3]. Unlike the simplified plane wave description of stimulated Brillouin scattering (SBS) in conventional optical fibers, the low dimension and hard mechanical boundary conditions of silica microwires confine elastic waves into eigenmodes. This feature gives rise to a variety of acoustic vibrations featuring different spatial distributions and speeds, among them hybrid shear-longitudinal acoustic waves (HAWs) and SAWs [2]. Being compatible with standard optical fiber, silica microwires simplify the explorations and manipulations of acoustic waves in an isolated mesoscopic phononics structure [4].The existence of multiple auxiliary elastic resonances in silica microwires has been experimentally verified by detecting the spontaneous Brillouin backscattering generated by thermally excited acoustic waves [2]. Although such measurements merely indicate the existence of multiple elastic resonances in the entire sample, applied techniques so far cannot resolve the exact longitudinal location of each resonance mode inside a microwire. Indeed, the existence of each acoustic mode highly depends on the local diameter of the microwire, and therefore different resonance frequencies are expected to be found at different positions of the tapered fiber section (e.g., in the transition region and waist). Here, we propose a method to stimulate and probe different classes of acoustic waves at specific locations inside an optical microwire, thus pinpointing precisely their respective locations and amplitude. We demonstrate that, by using a correlation-based distributed Brillouin technique [5][6][7], each Brillouin resonance frequency (mode) can be locally generated at any position over the tapered optical fiber with a spatial resolution of a few centimeters (or even millimeter scale). The technique employed here is based on the use of pump and probe light beams being phase-modulated with a given (same) phase pattern, so that an identical optical phase crossing will occur only at a specific microwire location (defining a so-called correlation peak). At the correlation peak, the locally generated acoustic waves (independently for each acoustic mode) have enough time to be efficiently activated, thus scattering pump light at the corresponding Brillouin resonant frequency [6,7]. This way, the phase-modulated Brilloui...

Optical nanofibers have recently emerged as attractive nanophotonic platforms for many applications ranging from quantum technologies to nonlinear optics, due to both their tight optical confinement and their wide evanescent field. Herein we examine both theoretically and experimentally the optical Kerr effect induced by the evanescent field of a silica nanofiber surrounded by different nonlinear liquids such as water, ethanol and acetone and we further compare them with air cladding. Our results show that the evanescent Kerr effect significantly dominates the usual Kerr effect into the silica core for sub-wavelength diameters below 560 nm, using acetone. We further report the observation of the evanescent Kerr effect through surrogate measurements of stimulated Raman-Kerr scattering (SRKS) in an acetone-immersed silica nanofiber. Our findings open the way towards potential applications of optical nanofibers to ultra-sensitive liquid sensing or to enhancing the nonlinear effects through the evanescent field.

Optical nanofibers (ONFs) are excellent nanophotonic platforms for various applications such as optical sensing, quantum and nonlinear optics, due to both the tight optical confinement and their wide evanescent field in the sub-wavelength limit. Other remarkable features of these ultrathin fibers are their surface acoustic properties and their high tensile strength. Here we investigate Brillouin light scattering in silica-glass tapered optical fibers under high tensile strain and show that the fundamental properties of elastic waves dramatically change due to elastic anisotropy and nonlinear elasticity for strain larger than 2%. This yields to unexpected and remarkable Brillouin strain coefficients for all Brillouin resonances including surface and hybrid waves, followed by a nonlinear evolution at high tensile strength. We further provide a complete theoretical analysis based on third-order nonlinear elasticity of silica that remarkably agrees with our experimental data. These new regimes open the way to the development of compact tensile strain optical sensors based on nanofibers.