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.

由高度分散的光-物质相互作用产生的超窄光谱特征对于光谱学、慢光和高精度传感等广泛的应用至关重要。接近亚兆赫兹或等效的Q的特征- 高达 10 亿甚至更多的因子在固态系统中难以获得，最终受到损失的限制。我们提出了一种无需谐振器即可在室温下实现可调谐亚兆赫光谱特征的新方法。我们在扭曲的双折射介质中利用增益增强的偏振牵引，其中偏振本征模与频率有关。在商用旋转光纤中使用布里渊增益，我们通过实验实现了 0.72 MHz 的光谱下降，这是迄今为止报道的最窄的反向布里渊散射特征。进一步优化可能会将线宽降低到 <0.1 MHz。我们的方法简单且适用广泛，提供按需可调性和高灵敏度，具有广泛的应用，例如微波光子滤波器、慢光和快光以及光学传感。