The experimental realization of a Kerr frequency comb represented the convergence of research in materials, physics, and engineering, and this symbiotic relationship continues to underpin efforts in comb innovation today. While the initial focus developing cavity-based frequency combs relied on existing microresonator architectures and classic optical materials, in recent years, this trend has been disrupted. This paper reviews the latest achievements in frequency comb generation using resonant cavities, placing them within the broader historical context of the field. After presenting well-established material systems and device designs, the emerging materials and device architectures are examined. Specifically, the unconventional material systems as well as atypical device designs that have enabled tailored dispersion profiles and improved comb performance are compared to the current state of art. The remaining challenges and future outlook for the field of cavity-based frequency combs is evaluated.
Ultra-high-quality-factor (UHQ) optical resonators have enabled numerous fundamental scientific studies and advanced integrated photonic device technology. While freestanding devices can be fabricated from many different materials, only silica (SiO 2 ) devices have been successfully integrated onto silicon wafers in large arrays. However, the UHQs (Q > 10 8 ) are transient, gradually decaying over time due to the presence of hydroxyl groups on the silica surface that attract water. Here, we overcome this challenge by using silicon oxynitride (SiO x N y ) instead of silica. Unlike SiO 2 , SiO x N y presents a mixture of −OH and −F groups to the environment, thus inhibiting the formation of a high optical loss water layer. As a result, quality factors in excess of 100 million are able to be maintained for longer than 14 days with no environmental controls on device storage. Over the same time frame, quality factors for SiO 2 devices stored in the same manner degraded by approximately an order of magnitude.
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 switch. Given their predictable response, they are ideally suited to create an all-optically controlled switch. However, fabricating a monolithic optical device comprised solely from 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 10 5 . 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 the 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 sixty percent of the cavity's free spectral range in the near-IR is demonstrated. The kinetics of the * Both authors contributed equally. a) Corresponding author: armani@usc.eduOptical resonant cavities are a fundamental element of on-chip integrated optical circuits, serving as amplifiers, filters, and buffers.[1-7] These devices have two key features that differentiate them from other components: the ability to isolate and to store pre-defined or resonant wavelengths (λo). In many cases, it is desirable to change the λo, for example, tuning an add-drop filter or encoding an optical signal. Because the λo is governed, in part, by the device refractive index, a common strategy is to leverage the electro-optic effect.[8-10] However, many optical cavities are fabricated from materials like silica with low to negligible electro-optic coefficients. Additionally, while the electro-optic effect can achieve fast tuning over small wavelength ranges, performing large shifts, comparable to the free spectral range of the cavity is challenging. Recent efforts have explored using the thermo-optic effect or the photo-acoustic effect to accomplish larger range tuning. [11][12][13][14][15] However, these control mechanisms are susceptible to cross-talk with adjacent optical components, thus, decreasing the density of the optical circuit.An alternative strategy can be found in photoswitchable or light-triggerable organic materials. This emerging class of materials has rapidly gained promin...
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
Due to their high circulating intensities, ultra-high quality factor dielectric whispering gallery mode resonators have enabled the development of low threshold Raman microlasers. Subsequently, other Raman-related phenomena, such as cascaded stimulated Raman scattering (CSRS) and stimulated anti-Stokes Raman scattering (SARS), were observed. While low threshold frequency conversion and generation have clear applications, CSRS and SARS have been limited by the low Raman gain. In this work, the surface of a silica resonator is modified with an organic monolayer, increasing the Raman gain. Up to four orders of CSRS are observed with sub-milliwatt (mW) input power, and the SARS efficiency is improved by three orders of magnitude compared to previous studies with hybrid resonators.
The next frontier in photonics will rely on the synergistic combination of disparate material systems. One unique organic molecule is azobenzene. This molecule can reversibly change conformations when optically excited in the blue (trans-to-cis) or mid-IR (cis-to-trans). Here, we demonstrate SiO2 optical resonators modified with a monolayer of azobenzenecontaining 4-(4-diethylaminophenylazo)pyridine (Aazo) with quality factors over 10 6 . Using a pair of lasers, the molecule is reversibly flipped between molecular conformations, inducing resonant wavelength shifts, and multiple switching cycles are demonstrated. The magnitude of the shift scales with the relative surface density of Aazo. The experimental data agrees with theoretical modeling.
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