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Ultra-Efficient Frequency Comb Generation in Algaas-On-Insulator

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Ultra-Efficient Frequency Comb Generation in Algaas-On-Insulator ARTICLE https://doi.org/10.1038/s41467-020-15005-5 OPEN Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators ✉ Lin Chang1,7, Weiqiang Xie1,7 , Haowen Shu1,2,7, Qi-Fan Yang 3, Boqiang Shen 3, Andreas Boes 1,4, Jon D. Peters1, Warren Jin1, Chao Xiang1, Songtao Liu 1, Gregory Moille5, Su-Peng Yu6, Xingjun Wang2, ✉ Kartik Srinivasan 5, Scott B. Papp 6, Kerry Vahala 3 & John E. Bowers1 Recent advances in nonlinear optics have revolutionized integrated photonics, providing on- 1234567890():,; chip solutions to a wide range of new applications. Currently, state of the art integrated nonlinear photonic devices are mainly based on dielectric material platforms, such as Si3N4 and SiO2. While semiconductor materials feature much higher nonlinear coefficients and convenience in active integration, they have suffered from high waveguide losses that prevent the realization of efficient nonlinear processes on-chip. Here, we challenge this status quo and demonstrate a low loss AlGaAs-on-insulator platform with anomalous dispersion and quality (Q) factors beyond 1.5 × 106. Such a high quality factor, combined with high nonlinear coefficient and small mode volume, enabled us to demonstrate a Kerr frequency comb threshold of only ∼36 µW in a resonator with a 1 THz free spectral range, ∼100 times lower compared to that in previous semiconductor platforms. Moreover, combs with broad spans (>250 nm) have been generated with a pump power of ∼300 µW, which is lower than the threshold power of state-of the-art dielectric micro combs. A soliton-step transition has also been observed for the first time in an AlGaAs resonator. 1 Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA. 2 State Key Laboratory of Advanced Optical Communications System and Networks, Peking University, Beijing 100871, China. 3 T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA. 4 School of Engineering, RMIT University, Melbourne, VIC 3000, Australia. 5 Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA. 6 Time and Frequency Division, National Institute of Standards and ✉ Technology, Boulder, CO 80305, USA. 7These authors contributed equally: Lin Chang, Weiqiang Xie, Haowen Shu. email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2020) 11:1331 | https://doi.org/10.1038/s41467-020-15005-5 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-15005-5 here has been extensive research on integrated nonlinear nonlinear coefficients that are typically orders-of-magnitude lar- Tphotonics in the last few years, driven by breakthroughs ger than those of the dielectric materials noted above26. Also, in microcombs and other on-chip nonlinear devices. This their refractive indices are higher, which can be used to achieve a has opened up many new opportunities for on-chip integrated high index contrast and therefore a small modal cross sectional photonics, ranging from spectroscopy to atomic clock applica- area (high intensity). These two factors are very attractive as they tions1–3. The need for efficient nonlinear devices has motivated reduce the turn-on power of nonlinear optical processes in the development of different material platforms in nonlinear semiconductor-based nonlinear optical platforms. Moreover, photonics. A common goal of these efforts is the reduction of III–V materials feature both a second- and a third-order non- the waveguide propagation loss to enable high-Q cavities. These linear coefficient, both of which are required in frequency-comb are used to resonantly enhance power and therefore increase the systems27. Furthermore, the wide usage of semiconductors in efficiency of the nonlinear optical processes4. In this regard, silica integrated photonics20, as passive and active (laser and detector) on silicon resonators5–7 have long been dominant offering Q elements, provides the potential for the integration of nonlinear factors of nearly 1 billion6. These devices can access a wide range devices into PICs. of nonlinear effects including microwave rate soliton micro- However, most of the III–V semiconductor photonic plat- combs8. However, over the last 5 years, there has been remark- forms suffer from high waveguide losses, usually in the order of able progress to significantly improve the Q factors of resonators several dB cm−128–30, which correspond to quality factors of in many other nonlinear integrated optical material platforms. ∼105 or lower. The lack of sufficiently high Q in semiconductor One example is the Si3N4 platform, which delivers high perfor- platforms has limited the ability to harness their attractive 9–11 mance in Kerr comb generation on-chip .Si3N4 micro- material properties for nonlinear optics applications like resonators have enabled the generation of efficient frequency microcombs. Recent demonstrations of whispering gallery combs with repetition rates from microwave to THz fre- mode or partially etched waveguide cavities in (Al)GaAs31,32 quencies12 and feature Q factors beyond 30 million13,14. Simi- achieved quality factor in the millions. However, significant larly, LiNbO3 offers additional opportunities for integrated challenges remain to obtain such high-Q factors within a nonlinear devices, due to its strong second order nonlinearity waveguide that is both suitable for nonlinear applications such and electro-optic efficiency15. The significant reduction of as Kerr microcombs or second-harmonic generation (SHG) waveguide loss in the lithium niobate on insulator (LNOI) while also being compatible with integration. For example, fully platform16,17 enabled the demonstration of resonators with Q dry etched surfaces are required in these structures as opposed factors beyond 10 million18. And its combined third and second to partially etched waveguides. order nonlinearities have also allowed soliton microcomb for- In this work, we make a key step in this direction by mation with intrinsic second-harmonic generation19. demonstrating compact micro-ring resonators in the AlGaAs- However, despite significant progress, there remain many on-insulator (AlGaAsOI) platform with an intrinsic quality challenges in the current technologies of integrated nonlinear factor beyond 1.5 × 106. The waveguides are fully etched with photonics. One problem is efficiency as measured by threshold or sub-micron dimensions and exhibit anomalous dispersion at a turn-on power. Focusing on Kerr-effect microcombs, all of the wavelength of ∼1.55 µm. Their high quality factor, high Kerr fi dielectrics noted above feature weak nonlinear Kerr indices (n2 is nonlinear coef cients and compact mode volume, enabled us to usually in the 10−19 m2 W−1 range or less). To reduce turn-on demonstrate ultra-efficient frequency-comb generation. For a 1 power and comb operation power to levels accessible by inte- THz comb, the threshold power is only ∼36 µW, which is a grated lasers (tens of mW)20, there are primarily two approaches 100 times reduction relative to previous semiconductor reso- in use. The first is to boost optical Q factor, which provides a nators and 10 times lower compared with the state-of-the-art quadratic benefit to lowering the Kerr-effect parametric thresh- results from integrated dielectric microresonators. With only old21. However, to obtain high quality factors, the material 300 µW pump power we were able to generate a frequency absorption and waveguide scattering need to be carefully sup- comb that covers a spectral range of ∼250 nm. A soliton-step pressed during microfabrication. A second approach is to reduce transition has also be observed. Furthermore, the waveguide the mode volume of the resonator in order to decrease the platform features much simpler fabrication procedures com- required stored optical energy at threshold. Here, however, simply pared with previous high-Q nonlinear platforms, and thus is reducing the diameter of the resonator is not always an option as suitable for high volume, low cost production. This demon- certain applications require large resonator diameters so as to stration paves the way to ultra-efficient nonlinear photonics of obtain electronic-rate microcombs. Therefore, the reduction of semiconductors, and provides a valuable solution to nonlinear the modal cross sectional area is also used to improve efficiency14. PICs in the near future. And while this approach has been particularly fruitful in silicon nitride microcombs, it does increase the challenge and complexity of maintaining high optical Q factor in the highly confined Results waveguides. As a result, it is common to have a large number of Device design. The nonlinear optical platform used in this work additional fabrication steps for high quality resonators12,14. These is AlGaAsOI. One key reason for selecting AlGaAs as a nonlinear include chemical-mechanical polishing (CMP) and high tem- material is its relatively large bandgap compared with other perature anneals. These additional steps can complicate the fab- commonly used semiconductors in photonics, such as Si (1.1 eV rication process, increase the cost, and sacrifice the yield and (1127 nm)) or InP (1.34 eV (925 nm)). By changing the Al mole fi precise control of geometry, all of which add dif culties for high fraction, the bandgap of AlxGa1−xAs varies from 1.42 eV (872 volume, low cost production in industry. A final and significant nm) to 2.16 eV (574 nm)33, which can
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