ARRAW: Anti-Resonant Reflecting Acoustic Waveguides
2School of Mathematical and Physical Sciences, University of Technology Sydney
In this work, we consider an acoustic analogue of optical ARROW waveguides, and demonstrate realistic designs of planar and cylindrical waveguides capable of simultaneous guidance of optical, and GHz acoustic waves.
Conventional optical step-index waveguides are unsuitable for guiding acoustic fields - acoustic waves which would propagate in the high refractive index core (e.g. Si) of the waveguide are not reflected on the interface with the low-refractive index cladding (e.g. SiO2). Instead, they propagate into the cladding and dissipate. This is due to the fact that high refractive index materials support propagation of sound at velocities typically higher than the low index materials, reversing the order of velocities in a step-index waveguide. As a result, optical and acoustic fields become spatially separated, drastically limiting the efficiency of their interaction [1].
We suggest a novel approach to address this problem, based on the concept of optical ARROW waveguides, in which light is confined to a low-refractive-index (or fast) core by engineering anti-resonance in the high-refractive-index (slow) cladding [2-3]. We propose to extend this idea to the acoustic domain, and introduce ARRAW (Anti-Resonant Reflecting Acoustic Waveguide); capable of guiding acoustic waves through the fast core due to the anti-resonance in the slow cladding. This concept can be realized in typical slab or cylindrical waveguides with Si core and SiO2 cladding.
Furthermore, we note that, unlike optical waves, sound can propagate in a solid medium in the form of both transverse and longitudinal waves, suggesting that ARRAW waveguides should have a richer structure of modes than their optical counterparts. We explore the similarities between the two, by considering the special optics-like cases of planar and cylindrical waveguides in which shear waves become decoupled from the longitudinal components, and the system becomes reminiscent of the ARROW.
We finally demonstrate how such ARRAW waveguides can be used to enhance the stimulated backward Brillouin interaction between acoustic and optical modes co-localized in the core of the waveguide.
[1] C. Wolff, M.J. Steel, B.J. Eggleton, and C.G. Poulton, Phys. Rev. A 92, 013836 (2015).
[2] M.A. Duguay, et al., Appl. Phys. Lett. 49, 13–15 (1986).
[3] N.M. Litchinitser, et al., Opt. Express 27, 1592 (2002).
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