Germanium as a material for stimulated Brillouin scattering in the mid-infrared

Abstract

We propose buried waveguides made of germanium or alloys of germanium and other group-IV elements as a fully CMOS-compatible platform for robust, high-gain stimulated Brillouin scattering (SBS) applications in the mid-infrared regime. To this end, we present numerical calculations for backward-SBS at 4 µm in germanium waveguides that are buried in silicon nitride. Due to the strong photoelastic anisotropy of germanium, we investigate two different orientations of the germanium crystal with respect to the waveguide's propagation direction and find considerable differences. The acoustic wave equation is solved including crystal anisotropy; acoustic losses are computed from the acoustic mode patterns and previously published material parameters.

Suspended mid-infrared waveguides for Stimulated Brillouin Scattering

“…Finally, 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.…”

“…">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.…”

Predictions of Free-Carrier Electroabsorption and Electrorefraction in Germanium

“…The material also exhibits other properties that make it an attractive target for photonics, namely a high refractive index (n ¼ 4:0-4.1 for wavelengths of 2-20 m) [3], a large 3 nonlinearity [4], [5], high carrier mobility [6], and it is compatible with silicon processing. The first germanium on silicon (Ge-on-Si) waveguides for the mid-infrared were demonstrated in 2012 [7], and since then Ge-on-Si waveguides with losses G 1 dB/cm have been realized [8], as well as Ge-on-Si spectrometers [9] and multimode interferometers [8].…”

Stimulated Brillouin scattering enhancement in silicon inverse opal waveguides

“…We evaluate the acousto-optic overlap as in [21], with Q A = Ω wg B /Γ wg B , where the line width is given by the integral of the dynamic viscosity tensor [12]. Figure 2a shows the gain coefficient g wg P versus filling fraction f and silicon shell thickness d. A maximum of g wg P = 193 W −1 m −1 occurs at (f = 80.7%, d = 55 nm) which is comparable with values for chalcogenide (≈ 300 W −1 m −1 [3,9]) and germanium (≈ 488 W −1 m −1 [3]) waveguides. The gain is low for small f and large shell thickness due to poor acousto-optic overlap.…”