Interferometric Measurements of High Quality Acoustic Microcavities (MCs) on Anisotropic Surfaces

Applications that couple high frequency surface acoustic waves (SAW) to quasi 0-D systems require high-Q acoustic MCs. We use optical interferometry to measure the surface displacement patterns between pairs of interdigital transducers (IDTs) configured to form resonant MCs with Q ~ 1800. Our focusing Al/Ti IDT with 0.6 rad (34○) aperture and wavelength Λ=5.6 μm emits a SAW beam along the [110] direction on GaAs with focusing comparable to the predicted 1.2Λ beam waist and 8Λ Rayleigh length. Reflections from the anisotropy corrected outer edges of the IDT fingers play an important role in performance. Previous assumptions of parabolic dispersion overcorrect for anisotropy and move the device focus closer to the IDT. Our resonant MC is created by placing additional floating fingers to form a 2.5Λ cavity around the focus. Since the inner cavity fingers are in the near field of the beam focus, their positions must also be adjusted to account for the Gouy phase shift. The resulting IDT + MC system shows highly localized elastic strain at frequencies within the electrical resonance band of the driving IDT. These 514 MHz, 5.6 µm devices should scale well to the submicron wavelength and GHz frequency domain appropriate to quantum dot applications.