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Gate-tunable frequency combs in graphene–nitride microresonators

Abstract

Optical frequency combs, which emit pulses of light at discrete, equally spaced frequencies, are cornerstones of modern-day frequency metrology, precision spectroscopy, astronomical observations, ultrafast optics and quantum information1,2,3,4,5,6,7. Chip-scale frequency combs, based on the Kerr and Raman nonlinearities in monolithic microresonators with ultrahigh quality factors8,9,10, have recently led to progress in optical clockwork and observations of temporal cavity solitons11,12,13,14. But the chromatic dispersion within a laser cavity, which determines the comb formation15,16, is usually difficult to tune with an electric field, whether in microcavities or fibre cavities. Such electrically dynamic control could bridge optical frequency combs and optoelectronics, enabling diverse comb outputs in one resonator with fast and convenient tunability. Arising from its exceptional Fermi–Dirac tunability and ultrafast carrier mobility17,18,19, graphene has a complex optical dispersion determined by its optical conductivity, which can be tuned through a gate voltage20,21. This has brought about optoelectronic advances such as modulators22,23, photodetectors24 and controllable plasmonics25,26. Here we demonstrate the gated intracavity tunability of graphene-based optical frequency combs, by coupling the gate-tunable optical conductivity to a silicon nitride photonic microresonator, thus modulating its second- and higher-order chromatic dispersions by altering the Fermi level. Preserving cavity quality factors up to 106 in the graphene-based comb, we implement a dual-layer ion-gel-gated transistor to tune the Fermi level of graphene across the range 0.45–0.65 electronvolts, under single-volt-level control. We use this to produce charge-tunable primary comb lines from 2.3 terahertz to 7.2 terahertz, coherent Kerr frequency combs, controllable Cherenkov radiation and controllable soliton states, all in a single microcavity. We further demonstrate voltage-tunable transitions from periodic soliton crystals to crystals with defects, mapped by our ultrafast second-harmonic optical autocorrelation. This heterogeneous graphene microcavity, which combines single-atomic-layer nanoscience and ultrafast optoelectronics, will help to improve our understanding of dynamical frequency combs and ultrafast optics.

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Fig. 1: Conceptual design and implementation of the gate-tunable graphene–nitride heterogeneous microcavity.
Fig. 2: Gate-tuning the graphene microring resonator.
Fig. 3: Observations of the gate-tunable graphene Kerr frequency combs.
Fig. 4: Soliton crystals of the gated graphene–nitride microresonator.

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Acknowledgements

We thank J. Yang, B. Li, T. Itoh, H. Liu and X. Xie for discussions. Graphene fabrication was supported by the Nanoelectronics Research Facilities (NRF) of UCLA. The authors acknowledge support from the National Science Foundation (NSF; DMR-1611598, CBET-1520949 and EFRI-1741707), the University of California National Laboratory research program (LFRP-17-477237), the Office of Naval Research (N00014-16-1-2094) and the Air Force Office of Scientific Research (FA9550-15-1-0081). X.F.D. acknowledges support from the Office of Naval Research (N00014-15-1-2368) and Y.H. acknowledges support from the NSF (EFRI-1433541). This work is also supported by the National Science Foundation of China (61705032) and the 111 project of China (B14039).

Reviewer information

Nature thanks T. Tanabe and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations

Authors

Contributions

B.Y. and S.-W.H. designed and led the work on the graphene–nitride frequency combs including the first and detailed measurements, the gate-tuned ultrafast optics measurements and the numerical designs. Y.L. and Z.Y.F. performed the graphene–nitride integration, conducted relevant electrical measurements and device optimizations. B.Y., C.C., S.-W.H., M.H., M.Y. and D.-L.K. performed silicon nitride chip and device processing. Y.H. and X.F.D. supervised graphene material preparation, device fabrication and electronic measurements. B.Y., A.K.V., S.-W.H. and C.W.W. performed the measured data analysis, on the frequency comb, radiofrequency and ultrafast correlation measurements. S.-W.H., B.Y. and Y.N.L. provided the theory and numerical calculations. All authors discussed the results. B.Y., S.-W.H., Y.L., Y.J.R. and C.W.W. prepared the manuscript. C.W.W. led and supported this research.

Corresponding authors

Correspondence to Baicheng Yao, Shu-Wei Huang, Xiangfeng Duan or Chee Wei Wong.

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Extended data figures and tables

Extended Data Fig. 1 Measured gate-tunable coupling and dispersions in a GMR.

a, Dips at approximately 1,600 nm, with different VG. b, Correlation of the round-trip transmissions and the bus transmissions for the resonator, obeying T = (α−|t|)2/(αα|t|)2. Here, 1−α is the cavity loss per round trip, and 1−t is the bus-to-cavity coupling rate. In our experiment, the graphene ring resonator is under-coupled originally, as the blue dot shows. c, Group velocity dispersion in range of 1,500 nm to 1,700 nm. Here, the curves show the calculated results, while dots show measured data. d, Calculated third-order dispersion in range of 1,500 nm to 1,700 nm.

Extended Data Fig. 2 Comparative optical transmissions of the heterogeneous graphene–nitride ring.

a, Spectral transmission of the silicon nitride ring resonator under the silica overcladding. b, Spectral transmission of the silicon nitride ring resonator after buffer-oxide etching to remove the silica overcladding. c, Spectral transmission of the graphene/ion-gel-based nitride ring resonator, heavily p-doped (VG = −2 V). d, Loaded Q factor around 1,600 nm. e, FSR, which is sensitive to the geometry modification. f, Mode non-equidistances, D2. d and e are measured at λ = 1,600 nm. In this figure, the error bars denote the typical system error.

Extended Data Fig. 3 An implementation of the graphene primary frequency comb gate-modulation.

a, Method for measuring the modulated comb. Keeping bias VG = −1.2 V, we control the laser-cavity detuning to generate a primary comb such as the grey spectrum shown here. To filter off the 1,600-nm continuous-wave pump, we apply a C-band filter, selecting the comb lines in the C-band only. A signal generator (maximum amplitude of 2 V, HP3312) is applied to modulate the gate voltage between −1.2 V and −1.8 V. In this process, primary comb lines in the filter window are modulated by the gate signal; the modulation is monitored by using an oscilloscope (500 MHz, Rigol DS1054) and an electrical spectrum analyser (ESA, 3 GHz, Agilent CXA9000A). b, Examples of radiofrequency spectra of the modulated combs, filtered by an optical filter (1,530 nm to 1,570 nm).

Extended Data Fig. 4 Example measurements of the graphene soliton comb formation.

a, Under VG = −1.2 V (Fermi level 0.59 eV), when the wavelength of the pump (λp) is tuned from 1,600.00 nm to 1,600.23 nm, the Kerr frequency comb is generated gradually. When λp is tuned between 1,600.15 nm and 1,600.19 nm, two multi-soliton states with low phase noise are achieved (states iv and v). b, Corresponding radiofrequency (RF) amplitude noise of the six states. In a and b, the pump power is kept at 34.5 dBm. Cherenkov radiation of the multi-soliton comb is narrow and sharp. c, Zoom-in of the eight-soliton crystal spectrum. The FSR changes from 89 GHz to 718 GHz, owing to the soliton-crystal-based longitude mode interaction. d, Beat note for the comb lines of the eight-soliton state (red; ninth comb line offset from the pump) and the four-soliton state (green; 56th comb line offset from the pump).

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Yao, B., Huang, SW., Liu, Y. et al. Gate-tunable frequency combs in graphene–nitride microresonators. Nature 558, 410–414 (2018). https://doi.org/10.1038/s41586-018-0216-x

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