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Although the first lasers invented operated in the visible, the first on-chip devices were optimized for near-infrared (IR) performance driven by demand in telecommunications. However, as the applications of integrated photonics has broadened, the wavelength demand has as well, and we are now returning to the visible (Vis) and pushing into the ultraviolet (UV). This shift has required innovations in device design and in materials as well as leveraging nonlinear behavior to reach these wavelengths. This review discusses the key nonlinear phenomena that can be used as well as presents several emerging material systems and devices that have reached the UV–Vis wavelength range.

Photoswitchable organic molecules can undergo reversible structural changes with an external light stimulus. These optically controlled molecules have been used in the development of "smart" polymers, optical writing of grating films, and even controllable in vivo drug release. Being the simplest class of photoswitches in terms of structure, azobenzenes have become the most ubiquitous, well-characterized, and implemented organic molecular switches. Given their predictable response, they are ideally suited to create an all-optically controlled switch. However, fabricating a monolithic optical device comprised solely of azobenzene while maintaining the photoswitching functionality is challenging. In this work, we combine integrated photonics with optically switchable organic molecules to create an optically controlled integrated device. A silica toroidal resonant cavity is functionalized with a monolayer of an azobenzene derivative. After functionalization, the loaded cavity Q is above 105. When 450 nm light is coupled into cavity resonance, the azobenzene isomerizes from trans isomer to cis isomer, inducing a refractive index change. Because the resonant wavelength of the cavity is governed by the index, the resonant wavelength changes in parallel. At a probe wavelength of 1300 nm, the wavelength shift is determined by the duration and intensity of the 450 nm light and the density of azobenzene functional groups on the device surface, providing multiple control mechanisms. Using this photoswitchable device, resonance frequency tuning as far as 60% of the cavity's free spectral range in the near-IR is demonstrated. The kinetics of the tuning are in agreement with spectroscopic and ellipsometry measurements coupled with finite element method calculations. [ABSTRACT FROM AUTHOR]

On-chip optical resonators have proven to be a promising platform for generating Kerr frequency combs. Whispering gallery mode resonators are particularly attractive because of their small footprint as well as low threshold and power consumption. This performance can be attributed to two characteristics: the cavity quality factor (Q) and the cavity dispersion. The input optical field into the cavity is amplified by the cavity Q, enabling nonlinear processes to occur with low input powers. In addition, the total span of the optical comb is governed by the dispersion. In an optical cavity-based comb, the dispersion is governed by the geometric dispersion of the cavity and the material dispersion. While many different materials have been explored, the realization of ultrahigh Q (UHQ) on-chip frequency comb sources is still challenging for most materials. One exception is the family of integrated silica devices. However, because the silica attracts water molecules from the ambient environment, the lifetime of the device performance is fundamentally limited, unless packaged in a controlled atmosphere. Here, we demonstrate the generation of environmentally stable frequency combs fabricated from SiO1.7N0.13 microtoroidal resonators. Due to the UHQ factors of the cavities, parametric oscillations with submicrowatt thresholds are demonstrated. Based on their geometry and material properties, the cavities have normal dispersion. However, due to avoided mode crossing, frequency combs are generated. Finally, unlike SiO2, SiO1.7N0.13 inhibits the formation of a water monolayer, allowing submicrowatt performance to be maintained for over a week in devices stored with minimal environmental controls. [ABSTRACT FROM AUTHOR]