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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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).
The codes used for the simulations are available from the corresponding authors on reasonable request.
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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.
The authors declare no competing interests.
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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