Acousto-optic modulation of photonic bound state in the continuum

Tuning trapped light for integrated photonics Light waves trapped within an integrated circuit could pave the way for microwave photonics and quantum information processing applications. Zejie Yu and Xiankai Sun of The Chinese University of Hong Kong demonstrated controlled interactions between sound waves and locally trapped light waves that could be used in integrated photonic circuits to reduce the amount of light loss during signal transmission. The researchers fabricated a device made of a low-refractive index, polymer waveguide placed on top of a lithium niobate film. Microwaves excited surface sound waves that changed the film's refractive index and modulated the properties of locally trapped light moving inside the waveguide. Tuning the laser that introduces light into the device ultimately causes the lithium niobate film to switch between transparency and light absorbency. Photonic bound states in the continuum (BICs) have recently been studied in various systems and have found wide applications in sensors, lasers, and filters. Applying BICs in photonic integrated circuits enables low-loss light guidance and routing in low-refractive-index waveguides on high-refractive-index substrates, which opens a new avenue for integrated photonics with functional single-crystal materials. Here, we demonstrate high-quality integrated lithium niobate microcavities inside which the photonic BIC modes circulate and further modulate these BIC modes acousto-optically by using piezoelectrically actuated surface acoustic waves at microwave frequencies. With a high acousto-optic modulation frequency, the acousto-optic coupling is well situated in the resolved-sideband regime. This leads to coherent coupling between microwave and optical photons, which is exhibited by the observed electro-acousto-optically induced transparency and absorption. Therefore, our devices serve as a paradigm for manipulating and controlling photonic BICs on a chip, which will enable many other applications of photonic BICs in the areas of microwave photonics and quantum information processing.