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

TL;DR

This work tackles the lack of piezoelectricity in the low-loss Si$_3$N$_4$ photonic platform by implementing thermoelastic surface acoustic waves (SAWs) to enable acousto-optic modulation without adding active materials. Using an intensity-modulated pump driving a gold grating, a multi-pass interaction in an $8~\mathrm{dB/m}$ Si$_3$N$_4$ waveguide yields a phase-modulation efficiency improvement of $13.6~\mathrm{dB}$ at $f_{\rm mod}=0.81~\mathrm{GHz}$ with a $4~\mu\mathrm{m}$ grating period. The device demonstrates single-sideband intermodal scattering with an extinction of $8~\mathrm{dB}$ at $f_{\rm mod}=0.76~\mathrm{GHz}$ using a tilted $6~\mu\mathrm{m}$ grating and achieves phase-to-intensity modulation by spectral placement on a ring resonator, showing modulation at $0.95~\mathrm{GHz}$ and $1.75~\mathrm{GHz}$. These results validate thermoelastic SAW as a viable, material-efficient route to programmable microwave photonics on Si$_3$N$_4$, with future work aimed at optimizing cladding, grating design, and chip-integrated pump delivery for higher efficiency and broader functionality.

Abstract

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.

Figures (3)

• Figure 1: (a) Conceptual figure. A gold metallic grating is illuminated by an intensity-modulated pump light. The thermoelastic SAW interacts with optical waves in the low-loss $\rm{Si_3N_4}$ waveguides. (b) Microscope photo of the phase modulation design. The black lines are multiple waveguide-passes and the yellow blocks are metallic gratings. (c) Chip cross-section view. The dimension is not to scale.
• Figure 2: Phase modulation and intermodal scattering configurations. (a) A 4-$\mu$m period metallic grating is placed parallel to a multiple waveguide-passes. Each path constructively contributes to the modulation efficiency. (b) A 6-$\mu$m period metallic grating is titled at an angle $\theta$ with respect to the waveguides to match the wavevetor difference between $\rm{TE_0}$ and $\rm{TE_1}$ optical modes. However, only the forward path (green arrow) matches, the backward path (red arrow) does not. (c) Phase-matching plots of phase modulation and intermodal scattering processes. (d) The heterodyne measurement setup. The intensity-modulated pump light is amplified and then delivered to the metallic grating with a 40° facet-polished fiber. The probe light is partially guided to the device, and the rest to an frequency shifter. After passing the device, they combine and beat on the PD. The beating signals are observed on ESA. (e) The measured sidebands power on ESA vs. the total interaction length. (f) A modulation efficiency enhancement of 13.6 dB with a 9-path configuration. (g) The intermodal scattering performs a 8-dB suppression ratio between two sidebands. IM: intensity modulator, SG: signal generator, EDFA: erbium-doped fiber amplifier, DUT: device under test, PD: photo detector, and ESA: electrical spectrum analyzer.
• Figure 3: Intensity modulation on a ring resonator. (a) The experimental setup. (b) Transmission spectrum of the ring resonator when turning on and off the EDFA. (c) The vector network analyzer (VNA) S21 measurement result. Two signals at 0.95 GHz and 1.75 GHz are generated from one of the fundamental modes of SAW and a second-harmonic generation of another fundamental mode, respectively. FSR: free spectral range.