Photonic waveguides are the most fundamental element for photonic integrated circuits (PICs). Waveguide properties, such as propagation loss, modal areas, nonlinear coefficients, etc., directly determine the functionalities and performance of PICs. Recently, the emerging waveguides with bound states in the continuum (BICs) have opened new opportunities for PICs because of their special properties in resonance and radiation. Here, we review the recent progress of PICs composed of waveguides with BICs. First, fundamentals including background physics and design rules of a BIC-based waveguide will be introduced. Next, two types of BIC-based waveguide structures, including shallowly etched dielectric and hybrid waveguides, will be presented. Lastly, the challenges and opportunities of PICs with BICs will be discussed.

The monolithic integration of Fabry-Perot cavities has many applications, such as label-free sensing, high-finesse filters, semiconductor lasers, and frequency comb generation. However, the excess loss of integrated reflectors makes it challenging to realize integrated Fabry-Perot cavities working in the ultrahigh-Q regime (>106). Here, we propose and experimentally demonstrate what we believe is the first silicon integrated million-Q Fabry-Perot cavity. Inspired by free-space optics, a novel monolithically integrated retroreflector is utilized to obtain near-unity reflectance and negligible reflection losses. The corner scattering in the retroreflector is prevented by the use of the TE1 mode, taking advantage of its zero central field intensity. Losses incurred by other mechanisms are also meticulously engineered. The measurement results show resonances with an ultrahigh intrinsic Q factor of ≈3.4×106 spanning an 80-nm bandwidth. The measured loaded Q factor is ≈2.1×106. Ultralow reflection losses (≈0.05dB) and propagation losses (≈0.18dB/cm) are experimentally realized.

Integrated spectrometers with both wide optical bandwidths and high spectral resolutions are required in applications such as spectral domain optical coherence tomography (SD-OCT). Here we propose a compact integrated scanning spectrometer by using a tunable micro-ring resonator (MRR) integrated with a single arrayed waveguide grating for operation in the 1265–1335-nm range. The spectral resolution of the spectrometer is determined by the quality factor of the MRR, and the optical bandwidth is defined by the free spectral range of the arrayed waveguide grating. The spectrometer is integrated with on-chip germanium photodetectors, which enable direct electrical readout. A 70-nm optical bandwidth and a 0.2-nm channel spacing enabled by scanning the MRR across one free spectral range are demonstrated, which offer a total of 350 wavelength channels with 31-kHz wavelength scanning speed. The integrated spectrometer is applied to measure different spectra and the interference signals from an SD-OCT system, which shows its great potential for future applications in sensing and imaging systems.

Mid-infrared (MIR) integrated photonics has attracted broad interest due to its promising applications in biochemical sensing, environmental monitoring, disease diagnosis, and optical communication. Among MIR integration platforms, germanium-based platforms hold many excellent properties, such as wide transparency windows, high refractive indices, and high nonlinear coefficients; however, the development of MIR germanium photonic devices is still in its infancy. Specifically, MIR high-Q germanium resonators with comparable performance to their silicon counterparts remain unprecedented. Here we experimentally demonstrate an MIR germanium nanocavity with a Q factor of ～18,000, the highest-to-date of reported nanocavities across MIR germanium-based integration platforms. This is achieved through a combination of a feasible theoretical design, Smart-Cut methods for wafer development, and optimized device fabrication processes. Our nanocavity, with its high Q factor and ultrasmall mode volume, opens new avenues for on-chip applications in the MIR spectral range.