Solution-processed metal–halide perovskites are promising gain media for compact, wavelength-tunable, and low-cost coherent light sources due to their high absorption coefficients, strong photoluminescence, defect tolerance, and large optical gain. However, translating their intrinsic gain into stable, spectrally controlled, low-threshold lasing requires simultaneous optimization of the perovskite material quality and the optical cavity architecture. Here, we present a material–cavity co-design strategy for realizing low-threshold amplified spontaneous emission and cavity-mediated lasing from MDACl₂-passivated mixed-cation perovskite thin films integrated with lithographically defined glass nanopillar photonic crystal substrates.

The perovskite gain medium, based on Cs/FA lead iodide–bromide compositions with controlled MDACl₂ incorporation, was first optimized through steady-state absorption, photoluminescence, time-resolved photoluminescence, and femtosecond transient absorption spectroscopy. Moderate MDACl₂ passivation improves the optical quality of the films by reducing non-radiative recombination pathways and extending the carrier lifetime. The optimized 3% MDACl₂-doped film exhibits the most favorable gain behavior, with an amplified spontaneous emission threshold of approximately 2.4 µJ cm⁻², significantly lower than that of the undoped film. Femtosecond transient absorption measurements reveal the formation of broadband optical gain in the near-infrared spectral region, with the gain window emerging near the band edge and broadening with increasing excitation fluence. These ultrafast measurements directly link defect passivation, carrier relaxation, and gain formation, confirming the suitability of the optimized perovskite film as an efficient light-emitting gain medium.

To further reduce the lasing threshold and achieve spectral control, the optimized perovskite film was deposited onto periodically patterned SiO₂ nanopillar photonic crystal cavities fabricated by electron-beam lithography and dry etching. The nanopillar geometry was designed to provide strong spatial overlap between the optical cavity modes and the perovskite gain region. By tuning the lattice periodicity of the nanopillar arrays, the cavity resonance was systematically aligned with the perovskite gain spectrum. Room-temperature micro-photoluminescence measurements demonstrate narrowband, cavity-mediated stimulated emission with wavelength tunability across the near-infrared region. The nanopillar cavities reduce the emission threshold to approximately 0.7 µJ cm⁻², more than three times lower than that of the unpatterned optimized film, while narrowing the emission linewidth to approximately 3.3 nm. The emission wavelength shifts systematically with the nanopillar lattice period, confirming deterministic cavity control over the lasing mode.

These results demonstrate that defect-passivated perovskite thin films integrated with glass nanopillar photonic crystal cavities provide an effective platform for low-threshold, spectrally tunable, and solution-processable coherent light emission. The combination of ultrafast gain spectroscopy, compositional passivation, and scalable nanophotonic cavity engineering offers a promising pathway toward integrated perovskite lasers for on-chip photonics, optical communication, sensing, and emerging light-emitting technologies.