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Sub-megahertz spectral dip in a resonator-free twisted gain medium

Abstract

Ultra-narrow optical spectral features resulting from highly dispersive light–matter interactions are essential for a broad range of applications such as spectroscopy, slow-light and high-precision sensing. Features approaching sub-megahertz or, equivalently, Q-factors up to one billion and beyond, are challenging to obtain in solid-state systems, ultimately limited by loss. We present a novel approach to achieve tunable sub-megahertz spectral features at room temperature without resonators. We exploit gain-enhanced polarization pulling in a twisted birefringent medium where polarization eigenmodes are frequency-dependent. Using Brillouin gain in a commercial spun fibre, we experimentally achieve a 0.72 MHz spectral dip, the narrowest backward Brillouin scattering feature ever reported. Further optimization can potentially reduce the linewidth to <0.1 MHz. Our approach is simple and broadly applicable, offering on-demand tunability and high sensitivity, with a wide range of applications such as microwave photonic filters, slow and fast light, and optical sensing.

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Fig. 1: Graphic illustrating the fixed and rotating frames of reference of an SBF.
Fig. 2: Simulation results showing the formation of a sub-megahertz spectral dip due to Brillouin gain-enhanced polarization pulling effect in an SBF.
Fig. 3: Simulation results showing the depth and linewidth of the spectral dip as a function of SBF parameters.
Fig. 4: Simulation results showing the real-time tunability of the dip frequency, linewidth and depth.
Fig. 5: Experimental demonstration of a sub-megahertz spectral dip in the Brillouin gain spectrum of SBF.
Fig. 6: Experimental demonstration of the real-time tunability of the dip frequency, linewidth and depth.

Data availability

The data that have been used to produce the results reported in this manuscript and supplementary file are available in an open-access data repository (ref. 55).

Code availability

The codes used for the simulations are available from the corresponding authors on reasonable request.

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Acknowledgements

The authors gratefully acknowledge Natural Science and Engineering Research Council (NSERC) (RGPIN-2019-07019, RGPAS-2019-00113, and CREATE 484907-16 to L.Q.), Canada Foundation for Innovation (CFI) (Innovation Fund 33415, Leaders Opportunity Fund 203429 and New Opportunities Fund 9650 to L.Q.) for funding this research. Y.L. acknowledges financial support from International Postdoctoral Exchange Fellowship sponsored by the China Postdoctoral Council and Wuhan University of Technology.

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Contributions

L.Q. supervised the project. N.C. formulated and coded the theoretical framework, performed the calculations and data analysis, and together with L.Q. determined the underlying mechanism and significance of the dip. L.Q. conceived and designed the experiment. R.G. made the first experimental observation of the spectral dip. Y.L. improved the experimental data acquisition and along with R.G. showed the tunability of the dip, experimentally. N.C. and L.Q. experimentally determined that homogeneous gain broadening is the dominant broadening mechanism. N.C. and L.Q. wrote the paper and revised it.

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Correspondence to Neel Choksi or Li Qian.

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Nature Photonics thanks Gustavo Wiederhecker, Thibaut Sylvestre and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs 1–4, Discussion and Tables 1–4.

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Choksi, N., Liu, Y., Ghasemi, R. et al. Sub-megahertz spectral dip in a resonator-free twisted gain medium. Nat. Photon. 16, 498–504 (2022). https://doi.org/10.1038/s41566-022-01015-w

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