Study on Nano Photonics of AlN Thin Films for Generation and Self Reference of Ultra High Frequency Soliton Micro Combs

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1. Background for Studying AlN Film for High Frequency Soliton Micro Comb

The frequency comb developed from solid-state or fiber mode-locked lasers has evolved into a micro comb source based on photonic chips, with enormous potential for miniaturization and cost reduction. In order to achieve phase coherent micro combs, researchers have made significant efforts in soliton mode locking and revealed rich soliton physical mechanisms. The octave soliton comb can allow f-2f self reference and achieve phase locking of carrier envelope bias frequency, which is a prerequisite for the development of chip level optical clocks and frequency synthesizers. So far, silicon nitride nanophotonics has been developed for the use of terahertz repetition rate soliton micro combs. However, such a large repetition rate is not suitable for direct photoelectric detection and poses a challenge for accessing bias frequencies up to terahertz. Meanwhile, silicon nitride thin films lack intrinsic second-order nonlinearity and typically require external frequency multipliers and off chip optical circuits to obtain bias frequencies. These off chip optical components damage the scaling advantage of the micro comb and significantly hinder the frequency self locking micro comb for portable applications.

Therefore, a research team developed a frequency doubling micro comb generator and a second harmonic multiplier based on aluminum nitride nanophotonics, and for the first time verified the on-chip integrated f-2f self reference technology based on resonator micro combs. This method utilizes mature epitaxial growth techniques to obtain AlN thin films with uniform thickness, and achieves comb like spectral expansion through dispersion waves controlled by group velocity dispersion. This demonstrates the use of multi frequency (433, 360, 220GHz) octave micro combs, and the research results can be reproduced from batch to batch preparation. They further utilize the on-chip integrated phase matched aluminum nitride optical waveguide to achieve second harmonic from f to 2f frequency, demonstrating the detection process of bias frequency. This research result indicates that the aluminum nitride nanophotonic platform has great potential to achieve portable frequency locking micro combs in the near future.

2. Research Progress for Beyond-Octave Soliton Microcomb Based on AlN Thin Film

The research team utilize third-order and second-order nonlinearities in non centrosymmetric optical media, realizing the functions of harmonic soliton micro comb and second harmonic doubling respectively. To demonstrate f-2f self reference, they use an auxiliary laser to pump the phase matched optical waveguide, ensuring sufficient second harmonic power. In the future, improving the second harmonic efficiency through the use of microring architecture can further eliminate the limitations of auxiliary lasers. The AlN film is prepared by metal organic chemical vapor deposition (MOCVD) on a sapphire substrate, with broadband transparency from ultraviolet to mid infrared. At the same time, the selected aluminum nitride thin film has a uniform thickness, which is a key factor in achieving group velocity dispersion control to achieve octave micro comb.

Fig. 1 (a) Schematic diagram of on-chip self reference achieved by the nanophotonic platform; (b) Epitaxial AlN layer on sapphire substrate, as well as images of crystal cell units and 2-inch AlN wafers; (c) Thickness mapping of aluminum nitride thin films based on spectroscopic ellipsometry

Then, utilize a fast frequency scanning scheme to achieve soliton micro comb mode locking. Fig. 2 (top) shows the soliton spectrum of a 50um radius aluminum nitride microresonator, with a repetition frequency of 433GHz and a spectral span of 1.05-2.4um, exceeding one optical octave band. Meanwhile, soliton induced dispersion wave radiation occurs at both ends of the spectrum, consistent with the predicted integrated dispersion curve, while high-frequency dispersion waves exhibit a significant blue shift from the phase matching frequency, mainly attributed to the soliton redshift caused by Raman (relative to the pump frequency). Benefiting from the uniform thickness control of AlN single crystal films, they predict the octave soliton micro comb at various repetition frequencies. For example, the dispersion model indicates that at the optimal width of 3.3-3.5um, a 100um radius aluminum nitride microresonator can obtain a doubling reduction in repetition frequency in the octave band spectrum. Fig. 2 (bottom) shows the soliton comb spectrum recorded at a microresonator width of 3.5um, with a repetition rate of approximately 220GHz, which is expected to be directly detected using state-of-the-art single row carrier photodiodes.

Fig. 2 (a) The integrated dispersion results of a 50um radius AlN micro resonator, as well as the experimentally recorded soliton micro comb spectrum, with a repetition frequency of 433GHz; (b) The soliton micro comb spectrum of a 100um radius AlN micro resonator with reduced repetition frequency to 220GHz

Due to the second harmonic of the auxiliary laser (1940-2000nm) in the laboratory exceeding the coverage range of the soliton micro comb spectrum shown in Fig. 2, it is necessary to further adjust the size of the micro resonator to expand the micro comb spectrum to below 1um. As shown in Fig. 3 (above), by increasing the radius of the AlN microresonator to 60um while maintaining its width of approximately 2.3um, the phase matching condition for high-frequency dispersion wave emission can be met below 1um wavelength. At the same time, by controlling the width of the microresonator, the emission spectrum of dispersion waves can be tuned. Based on the dispersion modeling results, they prepared corresponding aluminum nitride micro resonators and recorded the octave soliton micro comb spectrum with a repetition frequency of about 360 GHz. The position of the dispersion wave can be changed by adjusting the width of the micro resonators, as shown in Fig. 3 (bottom), which is consistent with the prediction of the integrated dispersion curve.

Fig.3 (a) Integrated dispersion curves of 60um radius AlN micro resonators at different widths; (b) The soliton micro comb spectra with resonator widths of 2.3, 2.4, and 2.5um respectively, with a repetition frequency of 360GHz, and the dispersion wave positions represented by vertical arrows.

Then, based on the second-order nonlinearity of AlN films, researchers achieve on-chip integrated second harmonic generation to match dispersion waves below 1um. To ensure the overlap between the second harmonic and the micro comb spectrum, they adopt a simple straight waveguide structure to increase the phase matching range. However, compared with the double resonant micro cavity structure, the conversion efficiency is lower. Fig. 4 (left) shows the on-chip integrated micro comb generator and second harmonic multiplier prepared, with a basic wavelength of 1970nm. The second harmonic with an off chip power exceeding 50uW was obtained from the modal phase matching waveguide, and its spectrum is shown in Fig. 4 (right).

Fig. 4 (a) On chip integration of micro comb generator and second harmonic multiplier; (b) The second harmonic spectrum collected in the phase matched waveguide is illustrated as wavelength dependent second harmonic power, with a 3-dB phase matched bandwidth of approximately 0.8nm

Finally, by combining the selected octave soliton comb and optimized second harmonic frequency doubling spectrum, researchers can roughly estimate the f-2 f bias frequency to be about 32 GHz through a calibrated spectral analyzer, which is limited by the resolution of the spectrometer. They further adopt the scheme shown in Fig. 5 (above) to electronically record the bias frequency after down conversion in real-time. Fig. 5 (bottom) records the signals after two down converters. Based on the local oscillator frequency and the relative frequency positions of the auxiliary laser and adjacent comb teeth, the equivalent bias frequency is obtained as shown in the illustration. In down conversion scheme, the local oscillator frequency can be freely adjusted to 40 GHz and 20 GHz, thus greatly expanding the measurement range of bias frequency. Meanwhile, the local oscillator can synchronize with the external frequency reference, indicating that the down converted f-2 f signal can be used for frequency locking of the micro comb teeth in the feedback loop.

Fig. 5 (a) Schematic diagram of bias frequency detection based on nanophotonic devices; (b) The equivalent bias frequency beat frequency signal recorded in the experiment has a signal-to-noise ratio of 10 dB at a resolution bandwidth of 1 MHz

3. Conclusion for Octave Soliton Micro Comb and Self Referencing Study Based on AlN

This study is based on an asymmetric AlN nanophotonic platform, showcasing on-chip integrated octave soliton micro combs and phase matched second harmonic, thereby demonstrating f-2f self reference based on nanophotonic devices. At the same time, the octave soliton micro comb has superior sub-terahertz repetition frequency (220-433 GHz) characteristics and can be directly detected by single row carrier photodiodes.

In the future, if the micro comb generator is integrated with high-efficiency micro cavity second harmonic on a chip, it is expected to directly double the selected comb lines near low-frequency dispersion waves, thereby solving the limitation of using auxiliary lasers in current work. By changing the second harmonic phase matching condition, the octave soliton micro comb can also be extended to the near visible band, thereby obtaining a frequency locked near visible light micro comb for precision measurement.

This study represents an important milestone that is expected to fully unleash the potential of octave band micro comb technology in portable applications.

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