Proposal for Forward Brillouin Inter-Modal Scattering in Non-suspended Lithium Niobate Waveguides at Visible Wavelengths

TL;DR

The paper addresses the challenge of realizing efficient forward Brillouin scattering at visible wavelengths in non-suspended lithium niobate waveguides by breaking the confinement-phase-matching trade-off with a quasi-phase-matching scheme founded on periodic width modulation. By injecting a controllable momentum $K=2\pi/\Lambda$, the authors enable inter-modal coupling between TE modes with a 3 GHz acoustic mode, achieving near-complete conversion over a short interaction length. Optimized parameters (LN thickness $400$ nm, etch depth $300$ nm, average width $0.7\,\mu$m) yield a coupling strength $g_{\text{eff}}/\sqrt{\hbar\Omega} \approx 5.05\times10^{4}\ \mathrm{m}^{-1}\mathrm{W}^{-1/2}$ and a predicted conversion length $L\approx1.1$ mm at $P_{\text{aco}}(z=0)=1$ mW, with optical and acoustic 3-dB bandwidths of about 2.0 nm and 2.1 MHz respectively. The work also analyzes how acoustic loss limits performance (e.g., 8 dB/mm extends the required length to ~2.5 mm unless acoustic power is increased to ~2.2 mW) and highlights the robustness and practicality of non-suspended LN waveguides for visible AO devices, enabling compact, efficient, and broadband visible-wavelength acousto-optic functionality on integrated platforms. These results advance visible-wavelength quantum and photonic technologies by leveraging strong LN piezoelectricity and scalable fabrication.

Abstract

Thin-film lithium niobate on sapphire provides an excellent platform for simultaneously confining acoustic and optical modes without suspended structures, enabling efficient acousto-optic modulation through strong piezoelectric coupling. Here, we identify the challenges in realizing the forward Brillouin interaction at visible wavelengths, and overcome the limitation by introducing a quasi-phase-matching scheme through periodic waveguide width modulation. We predict a complete inter-modal optical conversion over 1.1 mm using only 1 mW acoustic power. Our study paves the way for high-performance visible-wavelength acousto-optic devices on integrated platforms.

Figures (4)

• Figure 1: (a) Waveguide structure schematic and (b) phase-matching condition illustration of forward inter-modal Brillouin scattering. (c) Waveguide structure schematic and (d) phase-matching condition illustration of proposed QPM forward Brillouin scattering scheme.
• Figure 2: (a) Dispersion curve and (b) confinement factor of acoustic modes. (c) and (d) are profiles of two acoustic modes annoted with circle and square in (a) and (b). (e) Dispersion curves of optical modes: TE mode (red) and TM mode (blue). (f) and (g) are TE$_{00}$ and TE$_{01}$ mode optic profiles. (h) Dependence of acoustic mode wavenumbers and optical modes propagation constant differences on the waveguide width.
• Figure 3: (a) Coupling coefficient under different propagation direction. $\theta$ is the angle between propagation direction and LN $+y$ axis and $\theta=$ 90° corresponds to propagating along LN $+z$ axis. (b) Coupling coefficient under different LN film thickness and etch depth. (c) Coupling coefficient under different average waveguide width.
• Figure 4: (a) Pump optic mode and signal optic mode transmission about interaction length. (b) Complete inter-modal conversion length under different acoustic mode power and acoustic propagation loss. The white area represent parameter space where complete conversion can not be realized regardless of the interaction length. The star represent the conversion length under parameter combination in (a). Optic mode conversion efficiency as a function of optic wavelength (c) and acoustic frequency (d).