Suspended mid-infrared waveguides for Stimulated Brillouin Scattering

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 −1 W −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. IntroductionStimulated 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]. The ability to bring in the advantages of Brillouin interaction into integrated systems, and generate a useful level of SBS gain in a short, on-chip waveguide is especially important in the mid-IR, where there is particular demand for broadband, tuneable filters for spectroscopy or IR sensors [9,10]. Furthermore, by migrating nonlinear photonics towards mid-IR range, the unwanted two-photon absorption (TPA) in the two key CMOS compatible materials: silicon and germanium, can be eliminated [10,11].A central challenge in harnessing on-chip 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) -this approach requires materials with both a high refractive index and low stiffness, and while realizations of this scheme have been reported, they are limited to a small range of materials [12] that require specialized fabrication techniques. TIR can also 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, spatially-separated supports [14][15][16], or using phoxonic crystals [17,18] which guide both photons and phonons along line defects. Each of these strategies has advantages and drawbacks. Two-dimensional phoxonic crystals offer a unique control over the propagation and co-localization of photons and phonons along line and point towards near-IR and visible spectral ranges, since shorter effective optical wavelengths would require smaller pitch. This in turn would result in more perturbation, and consequently, larger clampin...

ARRAW: anti-resonant reflecting acoustic waveguides

“…It is instructive to compare the nonlinear performance of the investigated ARRAW to other BSBS waveguiding systems. The mechanical quality factor Q m of our structure can easily reach 10 3 , which is a result comparable to that found in suspended waveguides, in which the radiative dissipation of acoustic waves is suppressed, and Q m becomes limited by the intrinsic viscous losses in silicon [8,28]. Furthermore, the maximum Brillouin gain found in our system (nearly 1000 (Wm) −1 ) is of a similar order to that reported in GHz BSBS systems, including those based on sub-micron silicon slot waveguides where the optoacoustic interaction is dominated by radiation pressure [29], or relying on materials with different optoacoustic properties, such as chalcogenides [12].…”

“…A number of designs have been put forward to address this challenge [1][2][3][4]. In some, the waveguides are suspended in air by either sparsely positioned [5][6][7] or specifically engineered supporting structures [8]. In others, both light and sound are guided along line defects of phoxonic crystals [9][10][11].…”

ARRAW: Anti-Resonant Reflecting Acoustic Waveguide for efficient Brillouin scattering

“…While larger Brillouin gain can be achieved through increasing the acoustic frequency of the modes, or shrinking the geometric dimensions of the waveguide, the advantage of our approach lies in the relative simplicity of the design, and the choice of materialssilicon and silica. Achieving similar Brillouin gain typically requires extensive engineering of the acoustic dissipation through suspending the waveguide [1,4] or relying on high refractive index, soft materials, such as chalcogenides [3].…”

Tailorable Stimulated Brillouin Scattering Laser Based on Silicon Ring Waveguides

“…This will greatly inhibit the interaction between photons and phonons, which leads to a decrease of SBS effect in SOI. In order to excite the strong photon-phonon interaction in SOI, different structures of silicon-based optical waveguides were further proposed, including silicon ridge [13], suspended silicon waveguide membrane [14,15], silicon disk [16], silicon ring [17], and silicon bullseye [18]. Independent control of acoustic and optical characteristics is allowed, since optical and acoustic modes are limited by different physical mechanisms.…”