Quantifying Microscale Gain in Photorefractive Semiconductors by Transient White-Light Interferometry.

Accurate optical gain measurement at the microscale has long been plagued by photorefractive effects that distort optical spectroscopy signals under high excitation conditions. Here, we develop a phase-sensitive microscopic transient white-light interferometry that disentangles gain dynamics from transient refractive index changes, enabling direct access to intrinsic gain in photorefractive materials. Applying this method to CsPbBr3 single-crystal microplatelets, we reveal that conventional measurements underestimate gain by up to 20%. The corrected gain exhibits a duration of 500 ps and a bandwidth of ∼30 nm. We further uncover that the optical confinement factor governs a nonmonotonic thickness dependence of gain, peaking at ∼140 nm. Exploiting this optimal cavity length in a DBR structure yields a lasing threshold of 0.07 μJ·cm-2. This work presents a general method for measuring gain in photorefractive micronano materials and offers design guidelines for high-performance microlasers and integrated photonic circuits.