Sub-MHz spectral dip in a resonator-free twistedgain medium
Abstract Ultra-narrow optical spectral features have broad applications in spectroscopy, slow light, and sensing. Features approaching sub-MHz, or equivalently, Q-factors approaching 1 billion and beyond, are challenging to obtain in solid-state systems, ultimately limited by loss. We present a new paradigm to achieve tunable sub-MHz 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 fiber, 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, opening a new paradigm for microwave photonic filters, slow light, and optical sensing.
