Fig. 1: Slow light induced linewidth narrowing.

a Schematic of an Er:TFLN ring resonator cavity coupled to through and drop ports with electro-optic tuning. Inset: schematic cross-sectional view of the waveguide. b Optical image of the fabricated microring resonator, which was loaded onto a stage at a temperature of  ~2 K and with an estimated magnetic field B = 0.1–0.2 T aligned along the c-axis of the TFLN. The gold colored regions show the electrodes utilized in the EO tuning. Light was coupled in and out using grating couplers and an additional drop port at the top is used for the slow light measurements. c Schematics of a bare cavity mode with no absorption (left) and ultra high-Q resonator mode (right) formed in a narrow transparency window in a region of high absorption that creates a highly dispersive medium with ultra-low group velocity. d Measured transmission through a resonance tuned away from the Er absorption (left) and a narrowed resonance (right) showing narrowing by nearly three orders of magnitude due to preparing a spectral hole in a region of high absorption.