Chip-scale implementations of second-order nonlinear optics benefit from increased optical confinement that can lead to nonlinear interaction strengths that are orders of magnitude higher than bulk free-space configurations. Here, we present thin-film-based ultraefficient periodically-poled lithium niobate nonlinear waveguides, leveraging actively-monitored ferroelectric domain reversal engineering and nanophotonic confinement. The devices exhibit up to 4600 %W⁻¹cm⁻² conversion efficiency for second-harmonic generation, pumped around 1540 nm. In addition, we measure broadband sum-frequency generation across multiple telecom bands, from 1460 to 1620 nm. As an immediate application of the devices, we use pulses of picojoule-level energy to demonstrate second-harmonic generation with over 10% conversion in a 0.6-mm-long waveguide. Our ultracompact and highly efficient devices address growing demands in integrated-photonic frequency conversion, frequency metrology, atomic physics, and quantum optics, while offering a coherent link between the telecom and visible bands.

Integrated optical signal processors, in combination with conventional electrical signal processors, are envisioned to open a path to a new generation of signal processing hardware platform that allows for significant improvement in processing bandwidth, latency, and power efficiency. With its well-known features and potential, silicon photonics is considered as a favorable candidate for the device implementation, particularly with high circuit complexity, and hence has been the focus of the study. As an outlook from the previous discussions on such processors, we are considering new building blocks in silicon photonics platform for further extending the processor capabilities and adding practical features, particularly the miniaturized devices that enable ultra-dense integration of complex circuits into such processor chips. As enlightening examples, we review here our recent contribution together with representative works from other groups of compact designs of silicon photonics devices that enrich functionalities of processor building blocks such as multiplexing, polarization handling, and optical I/Os. The results shown in this review reflect the significance and maturity of the state-of-the-art photonic fabrication technology and contribute to the implementation of high-capacity, general-purpose optical signal processing functionalities on the chip scale.

Achieving ultrahigh-speed electro-optic modulators (subterahertz modulation bandwidths) is shown to be feasible in the thin-film lithium niobate integrated photonic platform. Design guidelines for optimization of the main radio-frequency and optical parameters are presented, and 3-dB modulation bandwidth up to 400 GHz is proved attainable in 3-mm-long devices. Such unprecedented bandwidths pave the path towards utilizing the devices in advanced optical communication systems.

Dispersion engineering for efficient supercontinuum generation (SCG) is investigated in a hybrid nonlinear photonic platform that allows cascaded third- and second-order optical nonlinearities in transverse-electric (TE) guided modes. The highly nonlinear chalcogenide waveguides enable SCG spanning over 1.25 octaves (from about 1160 nm to more than 2800 nm at $20\mathrm{dB}$ below maximum power), while the TE polarization attained is compatible with efficient second-harmonic generation in a subsequent thin-film lithium niobate waveguide integrated monolithically on the same chip. A low-energy pump pulsed laser source of only 25 pJ with 250 fs duration, centered at a wavelength of 1550 nm, can achieve such wideband SCG. The design presented is suitable for the $f$-to-$2f$ carrier-envelope offset detection technique of stabilized optical frequency comb sources.

We present a new class of photon pair sources on a silicon chip. The source is based on nanophotonic periodically poled lithium niobate waveguides and presents MHz rate pair generation.