Reducing extrinsic damping of surface acoustic waves at gigahertz frequencies

Abstract

High-frequency surface acoustic waves (SAWs) in the GHz range can be generated using absorption from an ultrafast laser in a patterned metallic grating on a substrate. Reducing the attenuation at these frequencies can yield better sensors as well as enable them to better probe phonon and electron-phonon interactions near surfaces. It is not clear from existing experiments which mechanisms dominate damping at high frequencies. We calculate damping times of SAWs due to various mechanism in the 1-100 GHz range to find that mechanical loading of the grating on the substrate dominates dissipation by radiating energy from the surface into the bulk. To overcome this and enable future measurements to probe intrinsic damping, we propose incorporating distributed acoustic Bragg reflectors (DABRs) in the experimental structure. Layers of alternating materials with contrasting acoustic impedances embedded a wavelength away from the surface serve to reflect energy back to the surface. Using numerical simulations, we show that a single Bragg reflector is sufficient to increase the energy density at the surface by more than five times. We quantify the resulting damping time to find that it is longer than the intrinsic damping time. The proposed structure can enable future measurements of intrinsic damping in SAWs at ∼100 GHz.

Effect of interfacial damping on high-frequency surface wave resonance on a nanostrip-bonded substrate

“…could also contribute to the SAW attenuation to some extent. Based on the previous research, 9 the attenuation caused by these mechanisms will contribute at least 10% to the whole energy dissipation near 10 GHz. However, in the high-frequency region beyond 10 GHz, the attenuation in the experiment is significantly lower than the calculation, which cannot be explained by the existing theory.…”

“…6,7 Although Maznev and Wright mentioned that SAW can propagate without radiating energy into the substrate in the specific zero-groupvelocity modes, 8 in the general situation, as shown in the research by Gelda et al, the mass loading of the grating on the substrate dominates dissipation by radiating energy from the surface into the bulk in the range of 1-100 GHz. 9 Although the propagation characteristics of SAWs have been widely studied, the interface problem between strips and substrate has not been investigated deeply, because their values are highly ambiguous and very difficult to obtain both theoretically and experimentally. However, interface problems such as interface delamination and degradation of interfacial bond are very common issues in the fabrication of SAW devices, and it is very important to study their influences on SAW propagation.…”