Photolithography allows high-Q AlN microresonators for near octave-spanning frequency comb and harmonic generation

: Single-crystal aluminum nitride (AlN) possessing both strong Pockels and Kerr nonlinear optical eﬀects as well as a very large band gap is a fascinating optical platform for integrated nonlinear optics. In this work, fully etched AlN-on-sapphire microresonators with a high-Q of 2.1 × 10 6 for the TE 00 mode are ﬁrstly demonstrated with the standard photolithography technique. A near octave-spanning Kerr frequency comb ranging from 1100 to 2150 nm is generated at an on-chip power of 406 mW for the TM 00 mode. Due to the high conﬁnement, the TE 10 mode also excites a Kerr comb from 1270 to 1850nm at 316 mW. In addition, frequency conversion to visible light is observed during the frequency comb generation. Our work will lead to a large-scale, low-cost, integrated nonlinear platform based on AlN.


Introduction
Integrated and miniaturized optical microresonators have a wide range of applications such as nonlinear optics, quantum electrodynamics, biochemical sensing and all-optical signal processing due to their small mode volume, high intra-cavity optical power density and strong interaction between the light field and the material [1][2][3][4]. Since the first demonstration in a silica microtoroid [5], the field of optical frequency comb (OFC) generated by cascaded four-wave mixing (FWM) in high-Q microcavities has made great progress and significantly impacted applications ranging from telecommunications and spectroscopy calibration to optical ranging and astronomy [6][7][8][9]. Various material platforms for OFC generation have been demonstrated, such as MgF 2 crystal cavities [10], high index silica glass (Hydex) [11], diamond [12] and silicon [13]. Silicon based materials such as silicon nitride [14][15][16] and silicon dioxide (SiO 2 ) [17,18] have attracted more attention due to the low propagation loss and mature manufacturing processes, in conjunction with the merits of CMOS compatibility and chip-scale integration. AlN featuring both strong Pockels ( χ 2 ) and Kerr ( χ 3 ) nonlinear effects [19] has been successfully used for second-harmonic generation (SHG) [20,21], third-harmonic generation (THG) [22] and electro-optic devices [23]. Especially the second-order nonlinearity can help broaden the spectrum of OFC into the visible regime, which is essential to achieve the key function of f -2f (or 2f -3f ) self-referencing [24,25]. With a wide band gap (∼6.0 eV), it shows optical transparency over a broad spectral range covering from the ultraviolet (UV) to the visible and infrared (IR) [26]. AlN is therefore a very versatile and useful platform for integrated nonlinear interactions, frequency conversion and self-referencing frequency combs.
Compared with AlN sputtered on silicon, wurtzite AlN epitaxially grown on sapphire exhibits superior crystalline quality and usually has higher optical Q [27,28]. For example, ultrahigh-Q UV microring resonators with potential applications in on-chip UV spectroscopy and nonlinear optics have been attained on a single-crystal AlN platform [29]. In the telecom C band, the first high-Q single-crystal AlN microresonator with an intrinsic Q (Q int ) of ∼2.5 million was demonstrated by X. Liu, et al. [28], and the highest Q int of ∼2.8 million for microring resonators based on the 1-µm-thick single-crystal AlN film has been reported recently [30]. Considering its low loss and high nonlinearity ( χ 2 and χ 3 ), a near octave-spanning Kerr comb (from 1075 to 2075nm) with 1 W on-chip power has been realized with a partially etched structure [31]. Mode-locked Kerr optical soliton generation was demonstrated [32]. A 17000%/W second-harmonic conversion efficiency in single-crystal AlN microresonators was obtained [21].
To date, lithography and etching challenges have made it very difficult to achieve fully etched devices with thick (>1.2 µm) AlN-on-sapphire films due to the strength of the Al-N bonds. Deep ultraviolet (DUV) lithography has been successfully implemented to fabricate high-Q microresonators and realized the ultra-low threshold OFC generation based on some material platforms, such as S 3 iN 4 , AlGaAs, etc. [33][34][35]. AlN microresonator fabrication, however, rarely utilizes simple photolithography instead using electron beam lithography (EBL), although the former method will be essential for manufacturing large-scale integrated devices. In this work, for the first time, standard photolithography and inductively coupled plasma (ICP) etching are used to fully etch 1.2-µm-thick AlN-on-sapphire films, which allows easy integration of microcavities with other optical elements. By optimizing the fabrication process, a high Q int of 2.1 × 10 6 is obtained for the transverse-electric fundamental (TE 00 ) mode. The corresponding propagation loss is estimated to be 0.17 dB/cm, which is comparable to the results of devices fabricated with EBL. Due to the enhanced confinement for modes, a broadband Kerr frequency comb ranging from 1100 to 2150 nm is generated by pumping the transverse-magnetic fundamental (TM 00 ) mode around 1560 nm at 406 mW power. Red and green emissions are also observed with a visible CCD camera due to the harmonic generation and sum frequency generation.

Fabrication and measurement
The single-crystal AlN film is grown on a sapphire substrate (0001) by metal organic chemical vapor deposition (MOCVD) using the process reported in Ref. [36]. A 1.2-µm-thick AlN film is selected to reduce the scattering loss caused by lattice mismatch at the interface between AlN and sapphire, while ensuring the control of geometric dispersion. A schematic illustration of the fabrication process is shown in Fig. 1. After depositing an 800-nm-thick SiO 2 hard mask by plasma enhanced chemical vapor deposition (PECVD), a 70-nm-thick chromium (Cr) is further deposited, prior to the photoresist spinning, as a metal mask to etch SiO 2 due to the low etching selectivity between SiO 2 and photoresist. A 900-nm-thick SPR955-0.9 photoresist is spin coated onto the film without bottom anti-reflective coatings. The ring resonators and bus waveguides are defined using a purchased stepper reticle and the Nikon NSR-2005i9C Stepper System based on i-line UV illumination (365 nm Hg spectral peak). Then, a post-exposure bake is used to cure the surface roughness of the photoresist pattern. Next, the pattern is transferred to the Cr mask and SiO 2 hard mask sequentially by ICP-RIE using Cl 2 /O 2 and SF 6 /Ar, respectively. After removing the Cr mask, the SiO 2 mask is used to etch the AlN layer completely with an optimized Cl 2 /BCl 3 /Ar-based ICP-RIE dry etching process. Finally, devices are encapsulated in 1.5-µm-thick PECVD-SiO 2 and cleaved prior to the measurement. No additional annealing is performed throughout the fabrication process. This process provides a new route for AlN-on-sapphire fabrication, requiring photolithography instead of EBL as in previous reports. The etching rate of AlN is about 205 nm/min, which is slightly lower than the reported previously [28]. In addition, a 1.4-µm-thick SiO 2 mask can be etched completely due to the thicker SPR photoresist and the high etching selectivity between SiO 2 and Cr. With a selectivity of 2.4:1 between AlN and SiO 2 , a 3.1-µm-thick AlN is allowed to be fully etched, which is thicker than 1.4 µm estimated previously [21]. A chip with a set of ∼360 GHz free-spectral-range (FSR) microresonators (ring radius of 60 µm) is fabricated. For OFC generation, the crucial question is the maximum achievable bandwidth which is strongly limited by the mismatch between equidistant comb modes and the non-equidistant resonator modes which caused by the dispersion in the cold cavity [37]. When high light intensities are circulating within cavity, the cross-phase and self-phase modulation arising from the Kerr effect will potentially make the resonator modes equidistant. It always leads to a resonance redshift at higher power and consequently it is only possible to compensate an appropriately anomalous group velocity dispersion (GVD). Typically, the evolution of Kerr frequency combs is governed by the mean-field Lugiato-Lefever equation and indicates that a small GVD and a large nonlinearity are indispensable to access broadband Kerr comb [38]. Due to the normal dispersion found in bulk AlN material, we need to adjust the geometric size of the ring resonator to acquire an overall anomalous dispersion.
The GVD profiles of the microring resonators are calculated theoretically using a finite-element method (FEM) solver. Here, we have considered the experimental structure with the radius of 60 µm and 75°sidewall slope angle induced by dry etching of AlN. Figure 2 compares the simulated dispersion of different modes in the resonators with widths of 1.8, 2.6 and 3.6 µm. We can learn that the anomalous dispersion increases with the decrease of the ring width for both TM and TE fundamental modes. The large and near-zero anomalous-GVD bandwidth spanning an octave is achievable for the TM 00 mode when the width is 3.6 µm. For the TE 00 mode, a 2.6-µm width is more suitable as 3.6 µm presents normal dispersion when the operating wavelength lower than 1700nm and 1.8 µm has a rather large anomalous dispersion. For the higher-order modes, due to the strong interaction with the geometric boundary caused by the larger mode spot, they are often accompanied by a higher anomalous dispersion than the fundamental mode. We can find that when the width is 3.6 µm, the TE 10 mode shows anomalous dispersion above 1300 nm and is considered to have the potential to directly generate Kerr frequency comb by cascaded FWM as found in subsequent studies.
The microscope and scanning electron microscopy (SEM) images of the fabricated AlN microresonators are shown in Fig. 3. All microring resonators are side coupled to the straight waveguides of different widths with varying gaps. The waveguides from the coupling region to both ends are tapered increasing from ∼0.91 to 4 µm to improve the fiber coupling efficiency and are offset 10°from horizontal to prevent reflection, as shown in Fig. 3(a). Figure 3(b) shows the coupling area between the microring and the bus waveguide after AlN etching. Clearly, the 0.5 µm gap was defined very well by the photolithography and ICP dry etching. The fully etched cross section of rings with 75°sidewall slope angle induced by dry etching of AlN is illustrated in Fig. 3(c). Figure 3(d) shows a smooth sidewall of the microresonator after etching. In the following, we will present and discuss the measurement results for the microresonator with 1.2 × 3.6 µm 2 cross section, 60 µm radius and the bus waveguide with 0.91 µm width, while the gap between them is 0.5 µm. The performance of the microresonators are measured using a Santec TSL-710 tunable laser (linewidth < 100 kHz) with -10 dBm output power. By recording the output power with a Santec power monitor (MPM210) synched with the tunable laser, we can obtain the transmission spectrum with 0.1 pm resolution. Figure 4(a) shows the transmission spectra for the TM and TE modes. The microresonator exhibits various resonance modes when excited with different polarized light which is controlled by a fiber polarization controller (FPC). As shown in Fig. 4(a), the total chip insertion loss is about 8 dB (4 dB/facet) for both polarization states. Among them, the FSRs of TM 00 , TE 00 and TE 10 Figures 4(b) and 4(c) show the Lorentz fitting curves for the TM 00 and TE 10 mode at 1559.47 and 1559.75 nm, respectively. According to the fitted full width half maximum (FWHM), the loaded Q (Q load ) of the two modes are calculated to be 6.1 × 10 5 and 6.2 × 10 5 . The Q int can be estimated as 1.5 × 10 6 and 1.2 × 10 6 , corresponding to the loss of 0.26 and 0.3 dB/cm, respectively. Figure 4(d) shows a resonance splitting for TE 00 mode due to the coherent backscattering of light from fabrication imperfections or surface roughness [30]. The estimated Q int based on the measured resonance spectra is 2.1 × 10 6 , corresponding to the loss of 0.17 dB/cm. Higher-order modes also can be strongly confined with low transmission loss, which is attractive for achieving phase matching in frequency conversion and enhancing the utilization of microresonators. Extinction ratios for the modes can reach as high as 17 dB. Therefore, the simple photolithography and dry etching process enable a high-Q microresonator with a gap of only 500 nm which is close to critical coupling for all TE and TM modes. Critical coupling improves the intracavity power buildup and reduces the pump power required for nonlinear optical processes.

Frequency comb generation
The experimental set-up for Kerr comb generation is illustrated in Fig. 5(a). CW light is amplified by an erbium-doped fiber amplifier (EDFA) and launched into the microresonator through the bus waveguide. The OFC can be initiated by tuning the pump wavelength into resonance. For TM 00 mode at 1559.47 nm, by setting the pump (on-chip) power at 270 mW, we tune the pump wavelength gradually from blue shifted to red shifted and plot the comb spectrum. The threshold power is measured to be around 50 mW. In addition, based on the nonlinear index n 2 = 3.5 × 10 −19 m 2 ·W −1 reported in [31], the threshold power can also be calculated by the formula: where n (∼2.1 around 1560 nm) is the refractive index of AlN and V eff is the effective mode volume calculated with FEM at the pump wavelength. Taking into account the coupling Q (Q c = 1.1 × 10 6 ) from the measured result, the threshold of the TM 00 mode is determined to be 44 mW. Figure 5(b) indicates that the evolution (from state i-vi) accords with the scenario of multiple-mode-spaced (MMS) combs [39]. For instance, the first parametric sidebands of TM 00 mode are generated at a spacing of multiple FSRs and referred to as primary comb lines. The primary comb lines are preserved as pump lines to generate subcombs via degenerate or non-degenerate FWM processes at later stages of the comb evolution. When the power coupled to the cavity is increased further, the subcombs merge and lead to spectral lines with a shape similar to parametric gain lobes. The new lines may also appear between two previously existing strong lines by non-degenerate processes. Eventually, broadband Kerr combs are observed yielding single-FSR-spaced lines. The RF beat note accompanying the formation of the OFC is recorded in Fig. 5(c) using an electrical spectrum analyzer (ESA). The broad RF beat note observed in this system indicates the formation of a high-noise OFC and verifies the generation of MMS combs. Although this comb state therefore exhibits low coherence and is unsuitable for metrology, the transition to low noise has been proven to exist and implemented in many material platforms [39,40]. For the TE 00 mode, the formation of Kerr OFC was not observed. The broadband Kerr OFC is obtained through TE 10 mode and the evolution process is similar to the TM 00 mode. This is consistent with the GVD simulation results above. The resonance frequencies of a microresonator can be Taylor-expanded around a central pump frequency ω 0 , using the relative mode numbers µ r = µ -µ 0 : where D 1 /2π is the FSR of the resonator and D 2 /2π is the difference of the two FSRs adjacent to the pump frequency ω 0 and linked to the GVD. The integrated dispersion D int is introduced to describe the deviation of a given resonance from an equidistant frequency grid defined by the FSR around the central pump frequency: For a microresonator with anomalous GVD (D 2 > 0), the third-order nonlinearity leads to a nonlinear phase shift that can compensate for the resonant frequency deviation to reach a certain spectral bandwidth. Beyond this, the power of the microcomb lines in the spectral endpoints can be enhanced where the linear phase-matching condition D int (µ r ) = 0 is satisfied, and such a dispersive wave-like (DW-like) bump offers an effective way to extend the comb spectra into the normal GVD regime [41]. Figure 6 shows the simulated D int for the TM 00 and TE 10 modes with the red curves, where the D 2 /2π is 7.3 and 14 MHz at the pump wavelength, respectively. For the TM 00 mode at 1559.47 nm, when the on-chip power is increased to 406 mW, a broadband comb spectrum ranging from 1100 to 2150 nm is realized, recorded by two optical spectrum analyzers (OSAs) covering different wavelength ranges. A DW-like bump is observed around 1180 nm for the comb spectrum, which agrees well with the simulation result. For the TE 10 mode at 1559.75 nm, the excited comb ranges from 1270 to 1850nm when the power is 316 mW, where the DW-like bump is observed around 1300 nm. Therefore, the fully etched AlN microring resonator not only achieves nearly octave-spanning OFC at low power, but also, for the high-order TE 10 mode, directly realizes OFC produced by FWM. Considering that Si 3 N 4 is heavily researched and has a linear and nonlinear refractive index similar to AlN, we compare the performance of AlN and Si 3 N 4 integrated microresonators reported previously and summarize in Table 1. Si 3 N 4 microresonators have reached Q factor more than 10 million and realized the octave-spanning soliton [42,43]. Here, an optical comb spectral ranging close to 1050 nm is achieved, which is the widest Kerr comb for reported AlN microresonators while the power is much lower. Even though our results are outstanding in current AlN microresonators, the Q factors still need to be improved further to reach the value for the Si 3 N 4 resonators. More importantly, the fabrication using standard photolithography is proven to be feasible and will create more attractive applications in the large-scale manufacturing of low-loss and high-power nonlinear devices.

Harmonic generation
The Kerr comb from FWM is not the only nonlinear phenomenon observed from the AlN material system. Usually, it is difficult to produce OFCs at short wavelength due to normal dispersion of the material. Fortunately, AlN features strong χ 2 and χ 3 nonlinearities, so other nonlinear effects such as SHG and THG can be generated to get the frequency lines from green light to the IR region [20,22,47]. Deeply etched waveguides certainly reduce the volume of modes and enhance nonlinear optical interaction. In addition, as mentioned above, the low transmission loss of high-order modes makes it possible to confine the phase-matched higher-order visible modes within the ring resonators. Therefore, resonant enhanced wave-mixing in microcavities are leveraged to generate new frequency lines in the red and green regions. For efficient wavelength conversion, the phase matching condition should be satisfied. For fully etched microring resonators, confinement of both the fundamental and higher-order modes mean that there are more opportunities for achieving phase matching during frequency conversion. The wavelength conversion for TM 00 mode from IR to visible wavelength is monitored by a visible CCD camera. Figures 7(a) and 7(b) show the red and green emission from the ring resonator under different states of OFC generation, which may mean that SHG, THG or sum frequency generation are all occurring. The visible emission in the form of combs in the red and green region needs further exploration.

Summary
For the first time, simple photolithography and ICP dry etching process are applied to fully etched microresonators based on a 1.2-µm-thick AlN film. A 0.5 µm gap can be achieved to realize nearly critical coupling for TM and TE modes. By optimizing the dry etching condition, a high Q int of 1.5 × 10 6 for TM 00 mode is obtained and a near octave-spanning Kerr comb is generated from 1100 to 2150 nm at an on-chip power of 406 mW. A high Q int of 2.1 × 10 6 is also obtained for the TE 00 mode. Thanks to the full etching, the TE 10 mode also has a high Q int of 1.2 × 10 6 and is used to generate OFC spanning from 1270 to 1850 nm at 316 mW. These prove that the performance of the device manufactured by this process is comparable to the results of EBL. With a strong χ 2 and χ 3 optical nonlinearity, in conjunction with more confinement of high-order modes, frequency conversion to visible light is observed. Based on the superior performance, octave-spanning Kerr soliton combs should be developed for this platform in the near future. Photolithography will make a strong impact on the development and application of comb-based microresonators due to its simplicity guaranteeing a high yield of devices.

Disclosures
The authors declare no conflicts of interest.