Recent advances in nonlinear optics have revolutionized integrated photonics, providing on-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

Boes, Andreas

Bowers, John E.

Chang, Lin

Jin, Warren

Liu, Songtao

Moille, Gregory

Papp, Scott B.

Peters, Jon D.

Shen, Boqiang

Shu, Haowen

Srinivasan, Kartik

Vahala, Kerry

Wang, Xingjun

Xiang, Chao

Xie, Weiqiang

Yang, Qi-Fan

Yu, Su-Peng

English

arXiv

We demonstrated ultra-efficient frequency comb generation in AlGaAs-on-insulator ring resonators that have a quality factor beyond 1.5*10⁶. The threshold power is as low as 36 µW

Caltech Authors - Main

STu3H.8.pdf CLEO 2020 © OSA 2020
Ultra-efficient frequency comb generation in AlGaAs-on-insulator 
microresonators 
 
Lin Chang,1,† Weiqiang Xie,1,†,* Haowen Shu,1,2,† Qi-Fan Yang,3 Boqiang Shen,3 Andreas Boes,1,4 Jon D. Peters,1 Warren Jin,1 Chao Xiang,1 
Songtao Liu,1 Gregory Moille,5 Su-Peng Yu,6 Xingjun Wang,2 Kartik Srinivasan,5 Scott B. Papp,6 Kerry Vahala,3 and John E. Bowers1 
 
1Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA 
2State Key Laboratory of Advanced Optical Communications System and Networks, Peking University, Beijing, 100871, China 
3T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA 
4School of Engineering, RMIT University, Melbourne, VIC 3000, Australia 
5Microsystems and Nanotechnology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 
6Time and Frequency Division, National Institute of Standards and Technology, Boulder, CO 80305 USA 
weiqiangxie@ucsb.edu 
†All three authors contributed equally to this work  
 
Abstract: We demonstrated ultra-efficient frequency comb generation in AlGaAs-on-insulator ring 
resonators that have a quality factor beyond 1.5×106. The threshold power is as low as 36 µW.  
OCIS codes: (190.4390) Nonlinear optics, integrated optics; (190.4380) Nonlinear optics, four-wave mixing; (160.6000) 
Semiconductor materials. 
 
       The extensive research on integrated nonlinear photonics in the last few years has opened up many new 
opportunities for on-chip integrated photonics, ranging from spectroscopy to atomic clock applications [1], [2]. The 
demand to construct efficient nonlinear devices has motivated the development of different material platforms in 
nonlinear photonics. So far, the dominating nonlinear materials used in integrated photonics are usually based on 
dielectrics, such as silica, Si3N4 and so on.  Recently, the significant reduction of loss in those platforms has enabled 
comb generation threshold power in the sub-milliwatt region. However, it is very challenging to integrate those devices 
into advanced photonic integrated circuits (PICs) along with other photonics components, such as lasers, amplifiers, 
modulators and detectors, as the dielectric material platform is not compatible with active devices. Another drawback 
for those dielectric microresonators is that they require extremely high quality (Q) factors (> 10 million) for efficient 
nonlinear operation, which poses significant challenges in the fabrication process. 
The newly developed nonlinear platform, (Al)GaAs on insulator, provides an attractive solution to overcome 
these problems [3]. (Al)GaAs has one of the highest third order nonlinear coefficient (n2 = 2.6×10-13 cm2W-1), orders 
of magnitude higher compared to those of commonly used nonlinear materials, which greatly relieves the requirements 
for high Q factors in microresonator fabrication [4]. Furthermore, (Al)GaAs and its alloys have been widely used as 
an active medium in the PIC industry. Hence, such a platform is compatible for the integration of active and nonlinear 
optical elements, which is essential for future nonlinear photonic applications.  
Here, we present ultra-high efficiency frequency comb generation using AlGaAs on insulator microresonators 
with unprecedented high Q factors (beyond 1.5 million). The waveguides are fully etched with sub-micron dimensions 
and exhibit anomalous dispersion at C band. For a 1 THz comb, the threshold power is only ~36 µW, 100 times lower 
relative to previous semiconductor resonators and 10 times lower compared to the state-of-the-art results from 
integrated dielectric micro-resonators. With 300 µW pump power we were able to generate a frequency comb that 
covers a spectral range of ~250 nm.  
 
Fig.1. (a) Schematic drawing of the AlGaAsOI waveguide cross section; (b) simulated GVD of 400 nm thick 
AlGaAsOI waveguides with different widths; (c) SEM image of the sidewall of the waveguide. 
 
        One advantage of AlxGa1-xAs for nonlinear optical applications is that a higher Al compound increases the 
material’s bandgap, which is essential to avoid two photon absorption, especially at C-band telecom wavelengths. In 
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on September 21,2020 at 23:25:51 UTC from IEEE Xplore.  Restrictions apply. 
STu3H.8.pdf CLEO 2020 © OSA 2020
this work we used an Al concentration of x = 0.2, which corresponds to a bandgap of 743 nm. Another requirement 
for comb generation in microresonator is to use a waveguide with anomalous dispersion at the pump wavelength. In 
our case we chose the thickness of the AlGaAs layer to be 400 nm. The simulated dispersions of waveguides with 
different width are plotted in Fig. 1(b). The mode size is 4 times smaller compared to that of common Si3N4 
waveguides for comb generation, thanks to the stronger refractive index contrast. The smaller mode size greatly 
enhances the photon density and thus improves the efficiency of the nonlinear optical process. 
The fabrication for the Al0.2Ga0.8As devices is similar to the process described for the fabrication of the GaAs on 
insulator platform [5]. Here, we further optimized the lithography and etching process [6], which leads to smoother 
waveguide sidewalls, as shown in Fig. 1(c). 
Fig. 2(a) shows the transmitted spectrum of a resonance of a ring resonator with 1 THz FSR.  The intrinsic quality 
factor of this resonator is extracted to be 1.5 × 106, which corresponds to a propagation loss around 0.4 dB/cm. For 
the first time, a III-V semiconductor nanowire waveguide suitable for nonlinear optics exhibits a waveguide loss that 
is as low as those of silicon and even comparable with many other dielectric waveguides. 
 
 
Fig.2. (a) Measured transmission spectrum of resonance. The intrinsic quality factor is 1.5×106, calculated by fitting 
a Lorentzian Function; Frequency comb spectrum from a 1 THz resonator under pump power of (b) 36 µW and (c) 
300 µW 
Such high Q enabled a record low threshold power of ~ 36 µW for comb generation, two order of magnitude 
lower compared to the previous results of semiconductor resonators as shown in Fig. 2(b). When increasing the pump 
power to 300 µW, the span of the comb extended over 250 nm range (Fig. 2(c)). Such pump power is still below the 
threshold power of the state of the art integrated dielectric microresonators. 
In conclusion, we demonstrated a low loss AlGaAsOI platform and applied it to create microcavities with a Q 
factor beyond 1.5 × 106.  These high-Q devices in combination with the high nonlinearity of the AlGaAsOI waveguides 
enabled ultra-efficient frequency combs.  A record low threshold power around 36 µW for 1 THz comb was achieved. 
Moreover, pumping the resonator with 300 µW power leads to an efficient comb broadening to a span over 250 nm. 
The observation of a soliton step in AlGaAsOI platform has also been reported. This demonstration opens up many 
opportunities for ultra-efficient nonlinear photonics in this platform. Furthermore, it paves the way for fully integrated 
nonlinear PICs in the future. 
This work is supported by a DARPA MTO DODOS contract (HR0011-15-C-055). We thank Gordon Keeler for 
fruitful discussions. 
References 
[1] D. T. Spencer et al., “An optical-frequency synthesizer using integrated photonics,” Nature, vol. 557, no. 7703, pp. 81–
85, 2018. 
[2] T. J. Kippenberg, A. L. Gaeta, M. Lipson, and M. L. Gorodetsky, “Dissipative Kerr solitons in optical microresonators.,” 
Science, vol. 361, no. 6402, p. eaan8083, Aug. 2018. 
[3] L. Chang et al., “Heterogeneously integrated GaAs waveguides on insulator for efficient frequency conversion,” Laser 
Photon. Rev., 2018. 
[4] M. Pu, L. Ottaviano, E. Semenova, and K. Yvind, “Efficient frequency comb generation in AlGaAs-on-insulator,” 
Optica, vol. 3, no. 8, pp. 823–826, 2016. 
[5] L. Chang et al., “Strong frequency conversion in heterogeneously integrated GaAs resonators,” APL Photonics, vol. 4, 
no. 3, p. 036103, Mar. 2019. 
[6] L. Chang et al., “Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators,” Sep. 2019. 
 
Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on September 21,2020 at 23:25:51 UTC from IEEE Xplore.  Restrictions apply. 


Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators

Recent advances in nonlinear optics have revolutionized integrated photonics, providing on-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&nbsp;much higher nonlinear coefficients and convenience in active integration, they have&nbsp;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&nbsp;a resonator with a 1 THz free spectral range, ∼100 times lower compared to that in previous semiconductor platforms. Moreover, combs&nbsp;with&nbsp;broad spans (&gt;250 nm) have been generated with a pump power of ∼300 µW, which is&nbsp;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&nbsp;in an AlGaAs resonator

Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

eScholarship - University of California

Crossref

Caltech Authors - Main