Observation of Stimulated Brillouin Scattering in Silicon Nitride Integrated Waveguides

Silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore, understanding all nonlinear optical properties of ${\mathrm{Si}}_3{\mathrm{N}}_4$ is important. So far, only stimulated Brillouin scattering (SBS) has not yet been reported. Here we observe, for the first time, backward SBS in fully cladded ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguides. The Brillouin gain spectrum exhibits an unusual multipeak structure resulting from hybridization with high-overtone bulk acoustic resonances of the silica cladding. The reported intrinsic ${\mathrm{Si}}_3{\mathrm{N}}_4$ Brillouin gain at 25 GHz is estimated as $4\times {10}^{-13}\mathrm{m}/\mathrm{W}$. Moreover, the magnitude of the ${\mathrm{Si}}_3{\mathrm{N}}_4$ photoelastic constant is estimated as $|p_{12}|=0.047\pm 0.004$, which is nearly 6 times smaller than for silica. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of ${\mathrm{Si}}_3{\mathrm{N}}_4$ to handle high optical power, central for integrated nonlinear photonics.

Figure 1

On-chip SBS in a ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide. (a) Artist’s view of SBS in a ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide. Note that the length of the inverse tapers is exaggerated for illustrative purpose. The actual taper length is $300\mu \mathrm{m}$, while the full length of the ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguide is 5 mm. White and blue shading represents the material density variation generated by SBS when the pump and probe beams overlay. Inset: false-color SEM of the waveguide cross section of $2\times 0.8\mu {\mathrm{m}}^2$. (b) Normalized electric field distribution for the optical TE-like mode. The arrows represent the direction and strength of the electric field in the cross-section plane. (c) Normalized displacement field norm (instantaneous value) for the fundamental acoustic mode. The vertical black line visible in (b) and (c) is indicative of the waveguide symmetry.

Figure 2

High sensitivity SBS gain measurement scheme. (a) Measured waveguide transmission spectrum as a function of the relative frequency starting at 193.55 THz. (b) Illustration of the gains due to Kerr effect (phase-to-intensity conversion) and SRS as a function of frequency. (c) Illustration in the complex plane of different contributions of the measured probe signal amplitude $A_{\det }$. The signal generated by pump 1 is a sum of the Kerr effect, SRS and SBS, while the signal generated by pump 2 only contains the Kerr effect and SRS. (d) Simplified TIM experimental setup. ECDL, external-cavity diode laser; DFB laser, distributed feedback laser; EDFA, erbium-doped fiber amplifier; IM, intensity modulator; PD, photodetector.

Figure 3

Measurement and simulation results of ${\mathrm{Si}}_3{\mathrm{N}}_4$ Brillouin gain spectrum. (a) Measured signal as a function of the pump-probe detuning frequency in logarithmic scale, showing the 1-m-long silica patchcord SBS to the left and ${\mathrm{Si}}_3{\mathrm{N}}_4$ SBS at 25 GHz, with more than 3 orders of magnitude difference. (b) Measured ${\mathrm{Si}}_3{\mathrm{N}}_4$ SBS gain spectrum, in agreement with the simulated eigenmodes (eigenfrequency and gain for each eigenmode). (c) SEM image showing the sample cross section. The precisely measured geometry parameters are used to build the simulation model. (d) Normalized acoustic displacement field norm (instantaneous value) of four peaks extracted from the simulations. The top ${\mathrm{SiO}}_2$ dome-shaped interface is due to fabrication (see Supplemental Material S2 [48]).