Integrating nanoscale electromechanical transducers and nanophotonic devices potentially can enable new acousto-optic devices to reach unprecedented high frequencies and modulation efficiency. We demonstrate acousto-optic modulation of a photonic crystal nanocavity using acoustic waves with frequency up to 19 GHz, reaching the microwave K band. Both the acoustic and photonic devices are fabricated in piezoelectric aluminum nitride thin films. Excitation of acoustic waves is achieved with interdigital transducers with periods as small as 300 nm. Confining both acoustic wave and optical wave within the thickness of the membrane leads to improved acousto-optic modulation efficiency in the new devices than that obtained in previous surface acoustic wave devices. Our system demonstrates a novel scalable optomechanical platform where strong acousto-optic coupling between cavity-confined photons and high frequency traveling phonons can be explored.

Exploitation of light-sound interactions in various types of media has led to a plethora of important optical technologies ranging from acousto-optic devices for optical communication to photo-acoustic imaging in biomedicine. With advances of integrated photonics and nanofabrication technology, it is now more feasible to miniaturize and integrate acousto-optic devices to augment their speed and performance so they can assume indispensable roles in integrated photonic systems for chip-scale optical communication. Moreover, in addition to electromechanical excitation, acoustic waves or localized mechanical vibrations can also be optically stimulated through optomechanical forces including radiation pressure, gradient force and electrostriction. Such optomechanical effects recently has been extensively investigated in various optomechanical systems with dimensional scales ranging from meters to nanometers. Therefore, with these recent developments, acousto-optics is entering a new era with plenty of research opportunities.

To combine acoustic and photonic devices, previously we and other groups have used piezoelectric aluminum nitride (AlN) film deposited on silicon wafers with a layer of silicon dioxide (SiO₂). Since AlN has a relatively high refractive index of about 2.1, photonic waveguides and cavities can be fabricated in AlN with the SiO₂ layer as the cladding. At the same time, acoustic waves can be exited in the AlN layer. On this platform, we have demonstrated SAW wave with frequency up to 12 GHz and its acousto-optic modulation of optical ring resonators and photonic crystal nanocavities. However, an important drawback of above devices based on SAW is that the SiO₂ layer has a lower sound velocity than both the top AlN layer and the bottom silicon substrate. As a result, the excited acoustic waves in the AlN layer tend to leak into and be guided in the SiO₂ layer whereas the optical modes are highly confined in the top AlN layer because of its high refractive index. Consequently, the modal overlap between the optical and acoustic modes is low, leading to relatively weak acousto-optic coupling efficiency.

To circumvent this problem, we further implemented integrated acousto-optic devices on a suspended AlN membrane. When the membrane thickness is less than or comparable to the acoustic wave, the generated acoustic wave will propagate in Lamb wave mode with very high acoustic velocity. The removal of the substrate enforces the optical mode and the acoustic wave to maximally overlap within the thickness of the membrane. With this approach, we demonstrate acousto-optic modulation of a photonic crystal nanocavity at frequency up to 19 GHz with significantly improved modulation efficiency. The system overcomes the limitation of the unsuspended AlN platform where the acoustic wave leaks into the substrate layer without contributing to the acousto-optic interaction. The platform is promising for studying interaction between cavity confined photons and propagating phonons of microwave frequency. In additional, the strong and high frequency acoustic waves realized in this platform can provide spatially-coherent time-domain modulation to induce non-reciprocity and break time-reversal symmetry in photonic systems. At this high acoustic frequency, this system can also be applied for microwave photonics technology where optical and microwave channels of communication can be linked and interchanged.