Light guiding by narrow waveguides (WGs) has triggered the remarkable advances in integrated optics during recent years. One interesting question is whether GHz vibrations can also be efficiently guided along curved paths on a surface. These guided SAWs can to act as efficient acoustic actuators, filters, acousto-optic modulators, as well as conveyer belts for electrons and phonons. Due to the complex nature of the SAW fields, however, guiding requires a very high acoustic contrast between the core and cladding WG regions, and has so far only been demonstrated using metal layers on piezoelectric insulators.
We recently demonstrated the guiding of GHz SAWs along a ring racetrack consisting of an epitaxial Ge ridge WG on a Si wafer (cf. Fig. 1(a)). Acoustic guiding becomes possible due to the high acoustic contrast between Ge and Si. Rayleigh SAWs are generated on the linear sections of a WG racetrack by high-frequency interdigital transducers (IDTs) placed on a piezoelectric ZnO island. As shown by the interferometric SAW profile of Fig. 1(b), the waves are focused by an acoustic horn into a narrow (thickness equal to the SAW wavelength of 2.8 μm) WG section and guided along a circular path towards a second IDT. Here, the SAWs can be electrically detected, as illustrated by the spectrum for rf-transmission coefficient s21 for the ring delay line displayed in Fig. 2.
The SAW WGs employ crystalline materials, thus enabling low absorption at high frequencies. In addition, they are fully compatible with the Si CMOS technology and can provide a powerful tool to interconnect phononic elements on Si chips.
P. Boucher, S. Rauwerdink, A. Tahraoui, C. Wenger, Y. Yamamoto, and P. V. Santos, Ring waveguides for gigahertz acoustic waves on silicon, Appl. Phys. Lett. (in print).