Phononic Bandgap Engineering of Suspended Waveguides for Stimulated Brillouin Scattering in Mid-Infrared

We propose a new design of suspended mid-infrared silicon waveguides for SBS. By engineering phononic dispersion of ribs supporting the waveguide, we suppress the dissipation of vibrations and boost the Brillouin gain of the structure.

A central challenge in harnessing Stimulated Brillouin Scattering (SBS) is to design a waveguide which confines both the optical and acoustic waves. The obvious approach to this task is to confine both waves using total internal reflection (TIR). However, this approach requires materials with both high refractive index and low stiffness, and while realizations of this scheme have been reported, they are limited to a small range of materials [1] that require specialized fabrication techniques. Other approaches rely on geometric isolation of the acoustic modes from the substrate by designing suspended waveguides with few, spatially-separated supports [2], or using phoxonic crystals [3] which guide both photons and phonons along line defects.

In this work, we merge these two concepts, and propose a new class of silicon waveguides for SBS in the mid-infrared (MIR). The waveguide consists of a rectangular core supporting a low-loss optical mode, suspended in air by a series of transverse ribs. The ribs are patterned to form a finite quasi-one-dimensional phononic crystal, with the complete stop band suppressing the radiative dissipation of acoustic waves, and confining them to the core of the waveguide. The mechanical quality factor of such structures can reach about 90% of the viscosity-limited quality factor of an unsupported waveguide, indicating an almost complete elimination of the radiative dissipation of acoustic waves.

Furthermore, we extend the classical formalism of SBS designed for translationally-invariant systems to treat opto-acoustic interaction in such periodic structures, and find forward intramodal-SBS gains up to 1750 (Wm)^-1, which compares favorably with the proposed MIR SBS designs based on buried germanium waveguides [4].

This design can be further refined to explore its applicability to the backwards SBS, or the efficiency of acoustic isolation through 1D phononic crystals with partial stopband (see e.g. [5]). Besides further enhancing the Brillouin gain, enhanced control over the channels of acoustic dissipation and propagation might also pave the way to designing novel acoustic beam splitters or couplers.

[1] R. Pant, et al., Opt. Express 19, 8285 (2011).

[2] H. Shin, et al., Nat. Comm. 4, 1944 (2013).

[3] R. Zhang and J. Sun, J. Light. Technol. 35, 2917 (2017).

[4] C. Wolff, R. Soref, C. Poulton, and B. Eggleton, Opt. Express 22, 30735 (2014).

[5] A. H. Ghadimi, et al., Science 360, 764 (2018).