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.

Waves that are perfectly confined in the continuous spectrum of radiating waves without interaction with them are known as bound states in the continuum (BICs). Despite recent discoveries of BICs in nanophotonics, full routing and control of BICs are yet to be explored. Here, we experimentally demonstrate BICs in a fundamentally new photonic architecture by patterning a low-refractive-index material on a high-refractive-index substrate, where dissipation to the substrate continuum is eliminated by engineering the geometric parameters. Pivotal BIC-based photonic components are demonstrated, including waveguides, microcavities, directional couplers, and modulators. Therefore, this work presents the critical step of photonic integrated circuits in the continuum, and enables the exploration of new single-crystal materials on an integrated photonic platform without the fabrication challenges of patterning the single-crystal materials. The demonstrated lithium niobate platform will facilitate development of functional photonic integrated circuits for optical communications, nonlinear optics at the single photon level as well as scalable photonic quantum information processors.

Polarization control of light waves is an important technique in optical communication and signal processing. On-chip polarization rotation from the fundamental transverse-electric (TE) mode to the fundamental transverse-magnetic (TM) mode is usually difficult because of their large effective refractive index difference. Here, we demonstrate an on-chip wideband polarization rotator designed with a genetic algorithm to convert the TE mode into the TM mode within a footprint of 0.96 μm ×4.2  μm. In simulation, the optimized structure achieves polarization rotation with a minimum conversion loss of 0.7 dB and the 1-dB bandwidth of 157 nm. Experimentally, our fabricated devices have demonstrated the expected polarization rotation with a conversion loss of ∼2.5  dB in the measured wavelength range of 1440-1580 nm, where the smallest value reaches ∼2  dB. The devices can serve as a generic approach and standard module for controlling light polarization in integrated photonic circuitry.

dielectric materials on a 2D material followed by postfabrication of PICs with lithography methods. [19][20][21][22] The layer transfer process of 2D materials on prefabricated PICs has the potential disadvantage of introducing strain and possible distortion of the 2D lattice if the strong van der Waals attraction between the 2D material and the dielectric surface produces conformal coverage over the sharp waveguide ridge corners, leading to reduced electron mobility, more phonon scattering centers, and possible degradation of device performance. While it is possible to planarize optical waveguides by additional processing involving deposition of low-refractive-index dielectrics followed by chemical-mechanical polishing, [6,8,13,23,24] such processes will reduce the optical overlap with the 2D material. For growing and patterning thin-film dielectric materials on 2D materials, the properties of both the thin-film dielectrics and 2D materials are usually affected adversely. [25][26][27][28] For example, the excellent properties of 2D materials can be destroyed by high-energy ions during the material growth process. Although some fabrication techniques for integrating 2D materials with dielectric materials like polymers [29] and chalcogenide glass [22] produce negligible effects on the material properties, they cannot universally be applied to many other types of dielectrics on 2D materials. In addition, few 2D materials can be grown on single-crystal materials with excellent optical properties. Therefore, a generic approach for integrating any types of 2D materials with any types of singlecrystal dielectrics is highly desired.The concept of "bound states in the continuum (BICs)" was first proposed by von Neumann and Wigner in 1929 with the mathematical construction of a 3D potential which can support perfectly confined states in a continuous band. [30] The radiation loss of these confined states can be eliminated by engineering their destructive interference with the continuous modes. [31][32][33][34][35][36][37][38][39] Harnessing BICs in PICs allows for low-loss light guidance and routing with a low-refractive-index waveguide on a high-refractive-index substrate. The light guided by the low-refractive-index waveguide can be confined to a region of the high-refractive-index substrate below the low-refractive-index waveguide. [40] Because the substrate is naturally flat, transferring a 2D material onto the high-refractive-index Integration of 2D materials on dielectric planar optical waveguides can make available new functionalities from the 2D materials' enhanced optoelectronic properties, such as nonlinearity, light emission, modulation, photodetection, and saturable absorption. However, the conventional integration schemes involving either the transfer of 2D materials onto prepatterned nonplanarized topology of photonic integrated circuits (PICs) or the growth and patterning of dielectric materials on 2D materials can degrade the properties of either the dielectric or the 2D material. Here, a fundamentally new ...

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