Intramodal and Intermodal Modulations in Exposed Silicon Waveguides with Surface Acoustic Waves Excited by Aluminum Scandium Nitride Films

Coupling surface acoustic waves with optical waves via piezoelectricity provides a pathway to connect the optical and microwave domains. Here, intramodal and intermodal acousto-optic modulations in the silicon-on-insulator platform are demonstrated with surface acoustic waves excited by the interdigital transducers (IDTs) on the piezoelectric AlScN films, where optical waveguides are formed in the exposed silicon layer without any cladding. The fabricated devices achieved a maximum intramodal acousto-optic modulation efficiency of −23.92 dB with 50 IDT finger pairs and a microwave power of 20 dBm in the straight waveguide over a broad range of optical wavelengths, corresponding to a V_πL of 2.34 V cm. In addition, the modulation efficiencies with 100 IDT finger pairs significantly improved from −31.06 dB in the straight waveguide to −14.34 dB in the modified spiral waveguide with five interaction segments at 2.325 GHz, which can obtain a further enhancement of 3.8 dB if 50 IDT finger pairs were used instead. Besides, the demonstrated intermodal acousto-optic modulations enable the conversion between optical modes TE₀₀ and TE₁₀ with single-sideband contrast of >30 dB over an optical 3 dB bandwidth of 68 nm. These acousto-optic devices hold promise as complementary metal–oxide–semiconductor (CMOS)-compatible on-chip acousto-optic modulators, frequency shifters, and microwave photonic filters, which also pave the way for magnetic-free nonreciprocal devices.