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

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 beyond, challenging obtain in solid-state systems, ultimately limited by loss. We present novel approach achieve tunable at room temperature without resonators. exploit gain-enhanced polarization pulling twisted birefringent medium where eigenmodes frequency-dependent. Using Brillouin gain commercial spun fibre, we experimentally 0.72 MHz dip, the narrowest backward scattering feature ever reported. Further optimization can potentially reduce linewidth <0.1 MHz. Our is simple broadly applicable, offering on-demand tunability high sensitivity, with wide microwave photonic filters, slow fast light, Tunable demonstrated The approach, which exploits medium, may be useful