Observation of Elastic Anisotropy in Strained Optical Nanofibers using Brillouin Spectroscopy

Abstract

Optical nanofibers (ONFs) are excellent nanophotonic platforms for many applications such as optical sensing, quantum photonics, and nonlinear optics, due to both tight optical confinement and their evanescent field. From an acoustic viewpoint, it has recently been reported the observation of a new class of Brillouin acoustic resonances in optical nanofibers, including hybrid shear/longitudinal acoustic waves (HAWs) and surface acoustic waves (SAWs) [1–2]. It has been later shown that, under axial tensile strain, the Brillouin frequency shifts (BFS) of these elastic resonances are fundamentally different from that of standard optical fibers [3]. This is principally due to the hybrid nature of acoustic waves and thus classical theory used for standard fibers can no longer be used [4]. Here, we develop a theoretical model based on third-order elasticity of silica to predict the strain dependence of acoustic waves in ONFs. We show in particular that the fundamental elastic properties of silica dramatically change due to elastic anisotropy and transverse hardening. The agreement with experimental results is excellent. Figure 1(a) shows a scheme of the tapered optical fiber under test including the uniform nanofiber section with a diameter of 660 nm and a length of 80 mm. The backward Brillouin spectrum was measured by using heterodyne detection and the tensile strains were applied by stretching the ONF using translation stages, as shown in the bottom of Fig. 1(a). The Brillouin spectrum of the unstrained fiber taper is shown in red in Fig. 1 (b). We can note several peaks in the GHz range due to HAWs, SAWs, and the untapered fiber (SMF), as indicated by black arrows [1]. Tensile strain up to 5% in elongation was then applied on the ONF and the results are plotted in Fig. 1(c) as a colormap. We can clearly see a continuous frequency shift of all the Brillouin resonances (red lines) with different slopes from 98 MHz/% for SAWs up to 350 MHz/% for HAWs. We also note the crossing of the two SAWs around 5%.