Cryogenic enhancement of phononic four-wave mixing in AlScN/SiC

TL;DR

The paper investigates gigahertz guided SAW four-wave mixing in an Al$_{0.58}$Sc$_{0.42}$N/4H-SiC heterostructure at 295 K and 4 K to quantify intrinsic phononic $\chi^{(3)}_{\text{acoustic}}$ nonlinearity for Rayleigh and Sezawa modes. Using a Kerr-like undepleted model, they extract modal nonlinear coefficients $\gamma_m$ and four-wave mixing coefficients $\Gamma$, finding the Rayleigh mode to be intrinsically far more nonlinear than Sezawa, with substantial enhancement at cryogenic temperatures for both modes. The results reveal power-dependent nonlinearities and deviations from the simple Kerr description, suggesting contributions from higher-order processes and possible cavity-enhancement of pump power, which are not fully captured by the basic model. These findings establish AlScN/SiC as a promising platform for engineering temperature-tunable nonlinear phononics on-chip, with implications for classical RF signal processing and future quantum acoustic technologies.

Abstract

Surface acoustic wave platforms based on piezoelectric thin-film heterostructures provide sub-wavelength acoustic confinement, making them attractive for compact nonlinear phononic systems with applications including frequency conversion, parametric interactions, and nonlinear signal processing. Here, we investigate guided surface acoustic wave phononic four-wave mixing at gigahertz frequencies in an aluminum scandium nitride/4H-silicon carbide heterostructure operated at both room temperature (295 K) and cryogenic temperature (4 K). The 500 nm thick aluminum scandium nitride film supports guided Rayleigh and Sezawa modes with distinct displacement and strain energy density distributions, allowing a direct comparison of mode-dependent nonlinear behavior within the same device. Continuous-wave four-wave mixing measurements reveal an enhancement in the extracted modal nonlinear coefficient at 4 K relative to 295 K for both modes. In addition, the Rayleigh mode exhibits a modal nonlinearity approximately two orders of magnitude larger than that of the Sezawa mode across both temperature regimes. These results demonstrate that phononic four-wave mixing is strongly influenced by temperature, mode confinement, and strain localization while establishing aluminum scandium nitride on silicon carbide heterostructures as a promising platform for engineering enhanced nonlinear phononic interactions for future classical and quantum acoustic on-chip signal processing systems.

Figures (3)

• Figure 1: (a) The modeled normalized mechanical displacement field for the Rayleigh mode (left) and Sezawa mode (right) in the 500 nm thick Al$_{0.58}$Sc$_{0.42}$N/SiC material platform. The acoustic wavelength, $\lambda$, was set to 1.5 µ m. (b) Modeled acoustic frequency with electrically open and shorted boundary conditions on the Al$_{0.58}$Sc$_{0.42}$N surface as a function of the acoustic propagation constant. The modeled $k^2$ values for Rayleigh and Sezawa modes were 1.1% and 4.8%, respectively. (c) An image of the fabricated Al$_{0.58}$Sc$_{0.42}$N/SiC chip wirebonded onto a QBoard used for characterization. (d) A microscope image of a guided SAW delay line, which consists of an input IDT, a propagation region, and an output IDT. The inset shows a schematic of the split finger IDT design. (e) A scanning electron microscope image of the fabricated IDT electrodes.
• Figure 2: (a) S$_{21}$ as a function of frequency in the guided SAW delay line measured at 295 K and at 4 K. The local maxima across the transmission shows the Rayleigh and Sezawa modes at 3.5 GHz and at 4.7 GHz, respectively. (b,c) The measured transmission and the respective time gated transmission extracted for the delay line at 295 K for Rayleigh and Sezawa modes. (d,e) The measured transmission and the respective time gated transmission extracted for the delay line at 4 K for Rayleigh and Sezawa modes.
• Figure 3: (a) Energy-level diagrams illustrating degenerate four-wave mixing, where two phonons at one frequency and one phonon at a second frequency participate in a third-order acoustic nonlinear interaction via a $\chi^3_{\text{acoustic}}$ to generate new sideband phonons. (b) A schematic of the the four-wave mixing experimental setup. Two RF sources were used to supply two pump tones to the input IDT driven at $f_1$ and $f_2$ frequencies. The output signal was collected by an IDT and characterized using a spectrum analyzer. (c,d) Comb lines observed due to four-wave mixing at 4 K for Rayleigh and Sezawa modes, respectively with the RF signal generator power set to -10 dBm. (e) Extracted power conversion efficiency, $\eta_0$, as a function of input acoustic pump power, $P_0'^2$, for the Rayleigh and Sezawa modes at 295 K and 4 K. (f) Extracted modal nonlinear coefficient, $\gamma_m$, as a function of input acoustic power, $P_0'^2$, at 295 K and 4 K.