Objective All-inorganic CsPbX₃ quantum dots (QDs) are promising gain media for next-generation lasers due to their excellent optoelectronic properties and superior stability. However, achieving ultralow-threshold amplified spontaneous emission (ASE) remains a challenge. While hybrid perovskites integrated with photonic crystals (PCs) suffer from environmental instability, the integration of stable CsPbX₃ QDs with PC microcavities is still in its infancy. Here, we engineer a PC microcavity that synergistically couples with CsPbBr₃ QDs to enhance the spontaneous emission rate via the Purcell effect, thereby dramatically reducing the ASE threshold under single-photon pumping and elucidating the underlying gain dynamics.

Methods  A strategy that integrates CsPbBr₃ QDs with SiO₂-based PCs to construct a composite microcavity (CsPbBr₃-QD-PCs) was proposed. The PC microcavity was first optimized using the finite-difference time-domain (FDTD) method. The physical model consisted of close-packed SiO₂ microspheres (refractive index n = 1.46) on a quartz substrate (n = 1.54), coated with CsPbBr₃ QDs (n = 2.3~2.5). The resonance wavelength was governed by a modified Bragg diffraction equation, and the Purcell factor F_p was computed to quantify spontaneous emission enhancement. The simulation parameter space included SiO₂ microsphere diameters d = 50~1000 nm, diffraction orders m = 1 and 2, and QD overlayer thickness increments ΔH. Experimentally, CsPbBr₃ QDs were synthesized via the ligand-assisted reprecipitation (LARP) method at room temperature, using CsBr and PbBr₂ as precursors with oleic acid and oleylamine as ligands dissolved in DMF/DMSO, followed by a two-step centrifugation purification. The PC template was fabricated via an SDS-assisted lifting method using SiO₂ microspheres of approximately 253 nm diameter, forming a hexagonal close-packed periodic structure. CsPbBr₃ QD solution was infiltrated into the PC template under low vacuum (0.3~0.6 Pa). A dense CsPbBr₃ QD film on quartz served as the control. Characterizations included XRD, SEM, steady-state absorption and PL spectroscopy, time-resolved photoluminescence (TRPL), and femtosecond transient absorption (TA) spectroscopy. The pump source was a Ti:sapphire femtosecond laser amplifier (Spectra-Physics Solstice Ace) frequency-doubled via a BBO crystal to produce 400 nm pulses (~35 fs, 1 kHz).

Results and Discussions FDTD simulations revealed three key features of F_p within the CsPbBr₃ QD emission window (λ = 500-550 nm): 1) high F_p at diffraction orders m = 1 and m = 2; 2) the Fp peak red-shifts with increasing SiO₂ diameter d owing to variations in the lattice constant and filling fraction; 3) the maximum F_p exhibits a near-periodic distribution with m. Fine spectral scans identified d = 170 nm (m = 1) and d = 250 nm (m = 2) as optimal diameters, both with F_p peaking near 540 nm. The d = 250 nm design was selected for its superior robustness: the ΔH tolerance range maintaining F_p > 2.5 was approximately 300 nm, substantially broader than the ~200 nm range for d = 170 nm, attributable to stronger electric field localization.

XRD confirmed the orthorhombic phase of CsPbBr₃ QDs. SEM revealed the periodic hexagonal close-packed structure and uniform QD coverage, with a continuous QD layer (D₁ ≈ 436 nm) fully encapsulating the SiO₂ microspheres (D₂ ≈ 253 nm), yielding ΔH ≈ 183 nm consistent with FDTD optimization. Despite lower absorption due to the SiO₂ volume, the PL intensity of CsPbBr₃-QD-PCs was enhanced 3.8-fold compared with bare QD films, accompanied by a PL redshift of approximately 15 nm. This redshift is primarily attributed to the spectrally selective Purcell enhancement (F_p peaking at λ ≈ 540 nm), with additional contributions from the modified local dielectric environment and enhanced reabsorption of shorter-wavelength photons. TRPL measurements showed that the PL lifetime was shortened from 1.96 ns (bare QDs) to 1.24 ns (QD-PCs), directly evidencing the Purcell-enhanced spontaneous emission rate.

Under 400 nm femtosecond laser pumping, CsPbBr₃-QD-PCs exhibited clear ASE: a narrow emission peak emerged at ~540 nm with superlinear intensity growth. The ASE threshold was determined to be 8.7 μJ/cm², with the FWHM narrowing to approximately 6.7 nm. Bare CsPbBr₃ QD films showed no ASE under identical conditions, confirming the essential role of the PC microcavity.

TA spectroscopy further elucidated the gain dynamics. Both samples shared four spectral features: excited-state absorption (470-500 nm), ground-state bleaching (GSB, 500-530 nm), transient photoinduced absorption (530-550 nm), and its subsequent inversion into stimulated emission. The PC microcavity shifted the GSB center wavelength consistently with the PL redshift and accelerated the GSB rise time from 1501 ps to 960 ps, indicating Purcell-enhanced carrier relaxation to the band edge. Critically, the optical gain lifetime at ~540 nm was drastically shortened from 1924 ps (bare QDs) to 618 ps (QD-PCs), a ~3.1-fold acceleration. This demonstrates that the PC structure activates a highly efficient stimulated emission channel, enabling the excited state to be rapidly depleted via stimulated emission and outcompeting nonradiative recombination. A systematic comparison with reported CsPbBr₃-based gain media—including thermally evaporated films (~35 μJ/cm²), spin-coated films (26-100 μJ/cm²), and glass-embedded nanocrystals (~54.5 μJ/cm²)—confirms the competitiveness of the 8.7 μJ/cm² threshold achieved here. The solution-based self-assembly approach offers the advantage of large-area, low-cost fabrication without complex lithography.

Conclusions In summary, we have demonstrated ultralow-threshold ASE by integrating CsPbBr₃ QDs with a PC microcavity. FDTD optimization yielded a CsPbBr₃-QD-PCs composite with F_p > 2.5 and strong electric field confinement, fabricated via vacuum-assisted self-assembly. The PC microcavity enhanced the PL intensity 3.8-fold, shortened the PL lifetime from 1.96 ns to 1.24 ns, and accelerated the optical gain lifetime from 1924 ps to 618 ps, enabling an ASE threshold of 8.7 μJ/cm² under single-photon pumping. This work elucidates the Purcell effect-driven mechanism by which PC microcavities enhance perovskite optical gain, and provides a platform for developing low-power, high-performance all-inorganic perovskite laser devices.