Thermoelastic surface acoustic waves in low-loss silicon nitride integrated circuits

Acousto-optic modulation in photonic integrated circuits harness the applications that include signal processing, quantum photonics and microwave photonics. However, silicon nitride ($\rm{Si_3N_4}$), as a main-stream low-loss scalable photonic platform, suffers from the lack of piezoelectric effect and therefore the hybrid co-integration with other materials is always required for acousto-optic modulation. Here, we employed thermoelastic surface acoustic waves (SAW) in a 8 dB/m propagation loss $\rm{Si_3N_4}$ integrated circuits without adding extra materials. A phase modulation efficiency enhancement of 13.6 dB is realized with a multi-pass configuration. Furthermore, a single-sideband intermodal scattering with a suppression ratio of 8 dB is measured and an intensity modulation is observed by incorporating the phase modulation into a ring resonator spectral. This thermoelastic SAW technique, as an initial step of acousto-optic modulation in low-loss $\rm{Si_3N_4}$ platform, is promising for integrated microwave photonics and programmable photonics applications.

💡 Research Summary

This paper presents a pioneering demonstration of acousto-optic modulation in silicon nitride (Si3N4) photonic integrated circuits using thermoelastic surface acoustic waves (SAWs), eliminating the need for hybrid integration with piezoelectric materials. Silicon nitride is a mainstream photonic platform prized for its ultra-low propagation loss, broad transparency window, and high-power handling capability. However, its inherent lack of a piezoelectric effect and low photoelastic coefficient have historically impeded the development of efficient, high-speed acousto-optic modulators on this platform, often requiring loss-compromising heterogeneous integration.

The core innovation lies in harnessing the thermoelastic effect. The researchers fabricate a thin gold metallic grating on top of the Si3N4 waveguide circuit. When an intensity-modulated pump laser illuminates this grating, periodic heating induces mechanical stress, launching SAWs directly into the underlying silica cladding and Si3N4 layer. These propagating SAWs create a dynamic refractive index grating via the photoelastic effect, which can modulate co-propagating probe light.

The work showcases three key functionalities, all achieved while maintaining a low waveguide propagation loss of 8 dB/m:

1. Enhanced Phase Modulation: A multi-pass waveguide geometry is designed where a single SAW intersects the optical waveguide multiple times. Leveraging the low optical loss of Si3N4, this extends the effective acousto-optic interaction length. Using a 4-μm period grating at 0.81 GHz, they achieved a 13.6 dB improvement in phase modulation efficiency with up to 9 passes, demonstrating the direct benefit of the platform’s low-loss property.

2. Single-Sideband Intermodal Scattering: By tilting a 6-μm period metallic grating at a specific angle relative to a multimode waveguide, the SAW wavevector is designed to match the wavevector difference between the fundamental TE0 and first-order TE1 optical modes. This phase-matching condition enables selective scattering of light from the TE0 to the TE1 mode, frequency-shifting it by +f_SAW, while suppressing the corresponding lower sideband (-f_SAW) by 8 dB at 0.76 GHz.

3. Phase-to-Intensity Modulation Conversion: The phase-modulated sidebands generated by the SAW are positioned on the slope of a ring resonator’s transmission spectrum. This converts the phase modulation into detectable intensity modulation, a crucial function for microwave photonic signal processing. Signals were observed at 0.95 GHz (a fundamental SAW mode) and 1.75 GHz (a second-harmonic mode).

The study thoroughly details the fabrication process using low-pressure chemical vapor deposition (LPCVD) for the Si3N4 and cladding layers, and electron-beam lithography for patterning. Heterodyne and vector network analyzer measurement setups are described for characterizing the modulation effects.

In discussion, the authors acknowledge that the current modulation efficiency of thermoelastic SAWs is lower than that of electrically-driven devices. However, the technique offers significant advantages: it requires no additional active material layers, preserves the low-loss nature of the Si3N4 platform, offers design flexibility, and avoids impedance-matching circuits. Future work to optimize cladding thickness, grating design, and integrate on-chip pump delivery could further enhance performance.

In conclusion, this research represents a critical first step in enabling self-contained acousto-optic functionality on the ultra-low-loss Si3N4 platform. By successfully demonstrating phase modulation, sideband control, and intensity modulation via the thermoelastic mechanism, it opens promising avenues for developing advanced integrated microwave photonics and programmable photonic circuits without sacrificing the platform’s foundational advantages.