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Abstract
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 × 106 for the TE00 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 TM00 mode. Due to the high confinement, the TE10 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.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. 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 [1–4]. 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 [6–9]. Various material platforms for OFC generation have been demonstrated, such as MgF2 crystal cavities [10], high index silica glass (Hydex) [11], diamond [12] and silicon [13]. Silicon based materials such as silicon nitride [14–16] and silicon dioxide (SiO2) [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 (χ2) and Kerr (χ3) 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 (Qint) of ∼2.5 million was demonstrated by X. Liu, et al. [28], and the highest Qint 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 (χ2 and χ3), 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 S3iN4, AlGaAs, etc. [33–35]. 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 Qint of 2.1 × 106 is obtained for the transverse-electric fundamental (TE00) 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 (TM00) 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.
2. 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 SiO2 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 SiO2 due to the low etching selectivity between SiO2 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 SiO2 hard mask sequentially by ICP-RIE using Cl2/O2 and SF6/Ar, respectively. After removing the Cr mask, the SiO2 mask is used to etch the AlN layer completely with an optimized Cl2/BCl3/Ar-based ICP-RIE dry etching process. Finally, devices are encapsulated in 1.5-µm-thick PECVD-SiO2 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 SiO2 mask can be etched completely due to the thicker SPR photoresist and the high etching selectivity between SiO2 and Cr. With a selectivity of 2.4:1 between AlN and SiO2, 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 TM00 mode when the width is 3.6 µm. For the TE00 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 TE10 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.
Fig. 2. Simulation provides GVD of (a) TM modes and (b) TE modes for 1.8, 2.6 and 3.6-µm-wide microring resonators.
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 µm2 cross section, 60 µm radius and the bus waveguide with 0.91 µm width, while the gap between them is 0.5 µm.
Fig. 3. (a) Microscope image of the fabricated devices. SEM images for (b) top-view of enlarged coupling regime, (c) side-view of the cleaved ring resonator and (d) top-view of enlarged resonator, showing a smooth sidewall.
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 TM00, TE00 and TE10 modes around 1560 nm are about 363.4, 375.2 and 376.2 GHz, which are consistent with the simulation results 364.3, 375.3 and 376.5 GHz. Figures 4(b) and 4(c) show the Lorentz fitting curves for the TM00 and TE10 mode at 1559.47 and 1559.75 nm, respectively. According to the fitted full width half maximum (FWHM), the loaded Q (Qload) of the two modes are calculated to be 6.1 × 105 and 6.2 × 105. The Qint can be estimated as 1.5 × 106 and 1.2 × 106, corresponding to the loss of 0.26 and 0.3 dB/cm, respectively. Figure 4(d) shows a resonance splitting for TE00 mode due to the coherent backscattering of light from fabrication imperfections or surface roughness [30]. The estimated Qint based on the measured resonance spectra is 2.1 × 106, 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.
Fig. 4. (a) Spectra of the resonance near 1560 nm, where three sets of modes fundamental, first-order and second-order for different polarizations are marked with circle, triangle, and square symbols, respectively. Zoom views of (b) TM00 and (c) TE10 mode resonance spectrum and the Lorentzian fitting. (d) Zoom view of TE00 mode resonance spectra, wherein the resonance shows doublet splitting due to coherent backscattering. The insets show simulated electrical field distributions in the cross section of the ring.
3. 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 TM00 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 n2 = 3.5 × 10−19 m2·W−1 reported in [31], the threshold power can also be calculated by the formula:
Fig. 5. (a) Schematic of the setup used to generate Kerr combs using the Santec TSL-710 tunable laser (FPC: fiber polarization controller, EDFA: erbium-doped fiber amplifier, PD: photodetector, OSA: optical spectrum analyzer, ESA: electrical spectrum analyzer). (b) Evolution of the optical frequency comb spectrum for the TM00 mode with pump power of 270 mW. (c) RF beat note generated in the resonator while tuning the pump wavelength into resonance.
The resonance frequencies of a microresonator can be Taylor-expanded around a central pump frequency ω0, using the relative mode numbers µr = µ - µ0:
Fig. 6. Comb spectrum generated from 1100 to 2150 nm at the on-chip power of 406 mW for the TM00 mode (upper) and from 1270 to 1850nm with 316 mW for the TE10 mode (lower). The red curve is the simulation of the Dint.
Considering that Si3N4 is heavily researched and has a linear and nonlinear refractive index similar to AlN, we compare the performance of AlN and Si3N4 integrated microresonators reported previously and summarize in Table 1. Si3N4 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 Si3N4 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.
Table 1. Performance comparison of Kerr combs based on AlN and Si3N4 integrated microresonators
4. 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 χ2 and χ3 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 TM00 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.
Fig. 7. (a) Red and (b) green light images collected by the CCD camera under conditions of strong comb formation.
5. 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 Qint of 1.5 × 106 for TM00 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 Qint of 2.1 × 106 is also obtained for the TE00 mode. Thanks to the full etching, the TE10 mode also has a high Qint of 1.2 × 106 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 χ2 and χ3 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.
Funding
National Natural Science Foundation of China (61861136001); Science Foundation Ireland (17/NSFC/4918).
Disclosures
The authors declare no conflicts of interest.
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