Abstract
We theoretically investigate a new class of silicon waveguides for achieving Stimulated Brillouin Scattering (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 stopband suppressing the transverse leakage of acoustic waves, confining them to the core of the waveguide. We derive a theoretical formalism that can be used to compute the opto-acoustic interaction in such periodic structures, and find forward intramodal-SBS gains up to 1750 m-1W-1, which compares favorably with the proposed MIR SBS designs based on buried germanium waveguides. This large gain is achieved thanks to the nearly complete suppression of acoustic radiative losses.
Highlights
Stimulated Brillouin Scattering (SBS), which describes the coherent nonlinear interaction between optical and acoustic fields[1,2], is a key effect for a wide range of photonics capabilities, including wideband tunable, ultra-narrow RF filters[3,4], acousto-optical storage[5,6], non-reciprocal photonic elements[7] and new laser sources[8]
We present the formalism for Brillouin gain computations in a periodic system, and use this to estimate SBS gain for a realistic silicon platform. We find that these structures exhibit gains that are comparable with the predicted gains for mid-IR structures in germanium[9], and so represent a viable alternative for harnessing SBS in this spectral range
As we show in the inset, the peculiar splitting of modes in the latter case is due to the anti-crossing between two modes of structure with unpatterned ribs of length around 5Λ = 11.25 μm
Summary
Stimulated Brillouin Scattering (SBS), which describes the coherent nonlinear interaction between optical and acoustic fields[1,2], is a key effect for a wide range of photonics capabilities, including wideband tunable, ultra-narrow RF filters[3,4], acousto-optical storage[5,6], non-reciprocal photonic elements[7] and new laser sources[8]. TIR can be achieved by geometric softening of the guided acoustic modes[13] to reduce their phase velocities below that of the substrate and surface waves, thereby prohibiting acoustic loss Another class of strategies relies on geometric isolation of the acoustic modes from the substrate, for example, by designing suspended waveguides with few, spatiallyseparated supports[14,15,16], or using phoxonic crystals[17,18] which guide both photons and phonons along line defects. We discuss the creation of acoustic stopband at the frequency of the mechanical vibrations of the waveguide by patterning of the ribs, compute the resulting acoustic confinement and investigate the geometrical dependence of the acoustic loss This results in a set of design guidelines for creating these types of suspended, softlyclamped waveguides for SBS applications over a broad range of wavelengths. We find that these structures exhibit gains that are comparable with the predicted gains for mid-IR structures in germanium[9], and so represent a viable alternative for harnessing SBS in this spectral range
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