AlGaAs-On-Insulator Nonlinear Photonics

The combination of nonlinear and integrated photonics has recently seen a surge with Kerr frequency comb generation in micro-resonators as the most significant achievement. Efficient nonlinear photonic chips have myriad applications including high speed optical signal processing, on-chip multi-wavelength lasers, metrology, molecular spectroscopy, and quantum information science. Aluminium gallium arsenide (AlGaAs) exhibits very high material nonlinearity and low nonlinear loss when operated below half its bandgap energy. However, difficulties in device processing and low device effective nonlinearity made Kerr frequency comb generation elusive. Here, we demonstrate AlGaAs-on-insulator as a nonlinear platform at telecom wavelengths. Using newly developed fabrication processes, we show high-quality-factor (Q>100,000) micro-resonators with integrated bus waveguides in a planar circuit where optical parametric oscillation is achieved with a record low threshold power of 3 mW and a frequency comb spanning 350 nm is obtained. Our demonstration shows the huge potential of the AlGaAs-on-insulator platform in integrated nonlinear photonics.

efficient nonlinear parametric processes such as low-threshold Kerr frequency comb generation once a high quality factor (Q) micro-resonator is realized. Fig. 2a shows a SEM picture of an 810-μm long race-track-shaped AlGaAsOI micro-resonator. Fig. 2b shows a coupling gap of 170 nm for the resonator where the light propagating in the 450 nm-wide bus waveguide can be evanescently coupled to the 630 nm-wide curved (17.5 μm radius) waveguide of the resonator. The resonator works in the under-coupled regime and its transmission is shown in Fig. 2c for the transverse electric (TE) mode. Only one mode family with a free spectral range (FSR) of ~0.82 nm (98 GHz) is observed in the spectrum, which implies that the resonator waveguide with anomalous dispersion can be operated in a single-mode state. Therefore, the dispersion distortion induced by inter-mode interaction between different mode families can be completely avoided, which is preferable for Kerr frequency comb generation (especially temporal soliton formation) but not attainable in all nonlinear material platforms 21 . Fig. 2d shows the measured transmission spectrum for the resonance at 1589.64 nm and the measured linewidth is around 9.6 pm which corresponds to a Q of ~165,600. The measured Q for all the devices ranges from 1.5×10 5 to 2.0×10 5 , which is more than an order of magnitude higher than previously demonstrated Q for AlGaAs micro-ring resonators 22 .
Kerr frequency comb generation is based on optical parametric oscillation (OPO), which relies on a combination of parametric amplification and oscillation as a result of the nonlinear FWM processes within the micro-resonator. As a continuous wave pump light is tuned into a cavity resonance to achieve thermal soft-locking 23 , the built-up intra-cavity power triggers OPO at a critical power threshold when the round-trip parametric gain exceeds the round-trip loss of the resonator. To satisfy the momentum conservation in the FWM process, the pump energy can only be transferred to equispaced frequencies within the supported resonances of the micro-resonator 1 and thus form a frequency comb at output as illustrated in Fig. 3a. The measured spectrum of an AlGaAsOI microresonator is shown in Fig. 3b when a pump power of 72 mW was coupled into the bus waveguide. A frequency comb with the native line spacing (single FSR) spanning over about 350 nm was observed.
The threshold power in the bus waveguide can be estimated by the expression 24 where L and λp, are the cavity length and pump wavelength, and QC and QL are the coupling and loaded quality factors of the resonator, respectively. As QL and L are correlated, we measured the threshold power for microresonators with different cavity lengths (FSR ranging from 98 GHz to 995 GHz) to find their influences on threshold power. The obtained minimum threshold power was 3 mW for a ring-shaped micro-resonator (see inset of Fig. 4a) operated in the under-coupling condition with a QL of about 10 5 . The output power of the primary OPO sideband increases significantly at threshold, as shown in Fig. 4a. Fig. 4b shows the measured output spectrum for this resonator with a pump power (4.5 mW) slightly above the threshold. It shows a typical OPO initial state in which widely spaced (multi-FSR spacing) primary sidebands are generated 25 . The measured threshold power for different devices as a function of QL is shown in Fig. 4c, where most of micro-resonators are seen to have milliwatt-level thresholds, although micro-resonators with smaller FSR exhibit slightly higher threshold. The measured data follows the theoretically predicted threshold trend as shown by the coloured bands, and a sub-mW threshold power can be expected with further improvement of resonator design 26 and fabrication processes.
As the dynamics of Kerr frequency comb generation have been extensively studied 23-25 and mode-locked combs have been demonstrated 21,27 , Kerr frequency comb technologies are approaching practical applications.
Planar integration platforms are critical for practical systems because of their robustness and potential for a fully integrated comb system with on-chip light sources. Low threshold is crucial for the realization of such a system. Table 1 summarizes several planar integrated nonlinear material platforms where a frequency comb has been demonstrated at telecom wavelengths, including Hydex 4 , Si3N4 5 , AlN 11 , Diamond 12 and AlGaAs. The linear refractive index of AlGaAs is the highest among these platforms, which makes it the most suitable platform for compact circuits. In addition, the material nonlinear index of AlGaAs is orders of magnitude larger than those of the other platforms. These intrinsic material properties make AlGaAsOI an ultra-efficient platform for nonlinear parametric processes. Therefore, even though the QL of our device is relatively low, the OPO threshold power for the AlGaAsOI platform is the lowest compared with the other platforms. In line with the fast-growing hybrid integration trend to combine different materials in multiple levels on a single CMOS compatible chip, the AlGaAsOI platform is very promising for realizing a fully-integrated comb system.
Because of the wide transparency window of AlGaAs materials, the frequency comb can potentially be extended into the mid-infrared with proper dispersion engineering. Besides comb generation, we have recently demonstrated wavelength conversion of a serial data signal at a record high speed (beyond terabaud) in this platform 28 . An ultra-broad bandwidth and ultra-high efficiency make AlGaAsOI an excellent platform for optical signal processing in telecommunications. Moreover, AlGaAs exhibits strong χ (2) effects 29 due to its non-centrosymmetric crystal structure. Therefore, the AlGaAsOI platform is also suitable for combining both χ (2) and χ (3) effects to obtain, for example, multi-octave spanning combs 30 .

Methods
Device fabrication. The preparation of AlGaAsOI samples includes wafer growth, wafer bonding, and substrate removal. A 320-nm thick layer of Al0.17Ga0.83As was grown in a low-pressure metalorganic vapour phase epitaxy (MOVPE) reactor on a 50-mm GaAs (100) substrate with sacrificial layers. After depositing a 3-μm thick silica layer on the AlGaAs layer using plasma-enhanced chemical vapour deposition (PECVD), a 90-nm thick Benzocyclobutene (BCB) layer was used as a bonding polymer between the grown wafer and another 50-mm semiconductor substrate covered with a 10-nm thick silica layer. The bonding process was performed under partial vacuum (~3×10 -2 Pa) at 250 ⁰C for one hour in a wafer bonding system, while a force of 750 N was applied to the wafers. Subsequently, the GaAs substrate and the sacrificial layers were removed by wet etching. It is crucial to use proper etch stop layer and etchant as the etching by-products may result in roughness and absorption sites on the AlGaAs film surface ( Supplementary Fig. 1). Electron beam lithography (EBL, JEOL JBX-9500FS) was used to define the device pattern in the electron beam resist hydrogen silsesquioxane (HSQ, Dow Corning FOX-15). To get a smooth pattern definition for bent waveguides in micro-resonators, a multi-pass exposure was applied to mitigate the stitching between pattern segments fractured during the EBL ( Supplementary Fig. 2). The device pattern was then transferred into the AlGaAs layer using a boron trichloride (BCl3)-based dry etching process in an inductive coupled plasma reactive ion etching (ICP-RIE) machine. As the refractive index of HSQ is relatively low (similar to SiO2), it was kept on top of the AlGaAs device pattern. Finally, clad in a 3-μm thick silica layer using PECVD, the chip was cleaved to form the input and output facets where nano-tapers enabled efficient chip-to-fibre coupling for characterization.
Modelling. Waveguide dispersion was calculated using a finite-element method mode solver (COMSOL) for the TE mode in the wavelength range between 1300 nm and 1800 nm. The Sellmeier equation was used to incorporate the dispersion of SiO2 and the modified single effective oscillator model was used for AlGaAs. The dispersion of HSQ, a leftover electron beam resist on top of AlGaAs, was assumed to be the same as that of SiO2.

Measurement of nonlinearity.
A FWM measurement was performed using two continuous wave signals (one for the pump and one for the signal) in a 3-mm long AlGaAsOI nano-waveguide with a cross-section of 320 nm×630 nm. The pump-to-signal detuning was less than 1 nm to ensure phase matching 14 . We measured the dependence of conversion efficiency on pump power (Supplementary Fig. 4) and extracted the nonlinear coefficient γ by fitting the expression 18     was not observed in the micro-resonator as shown in c. However, the nonlinear loss can be significantly reduced to a negligible level as the bandgap is increased 8 . Therefore, a slightly larger bandgap e.g. 1.64 eV (Al 0.17 Ga 0.83 As) results in a significant reduction of TPA and thus enables OPO in the micro-resonator with much lower pump power as shown in d.