Room‐Temperature Exceptional‐Point‐Driven Polariton Lasing from Perovskite Metasurface

Abstract

Excitons in lead bromide perovskites exhibit high binding energy and high oscillator strength, allowing for a strong light‐matter coupling regime in the perovskite‐based cavities localizing photons at the nanoscale. This opens up the way for the realization of exciton‐polariton Bose–Einstein condensation and polariton lasing at room temperature – the inversion‐free low‐threshold stimulated emission. However, polariton lasing in perovskite planar photon cavities without Bragg mirrors has not yet been observed and proved experimentally. In this study, perovskite metasurface is employed, fabricated with nanoimprint lithography, supporting so‐called exceptional points to demonstrate the room‐temperature polariton lasing. The exceptional points in exciton‐polariton dispersion of the metasurface appear upon optically pumping in the nonlinear regime in the spectral vicinity of a symmetry‐protected bound state in the continuum providing high mode confinement with the enhanced local density of states beneficial for polariton condensation. The observed lasing emission possesses high directivity with a divergence angle of 1° over one axis. The employed nanoimprinting approach for solution‐processable large‐scale polariton lasers is compatible with various planar photonic platforms suitable for on‐chip integration.

Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton–Polariton Perovskite Metasurfaces

“…This modulation depth is a record-high one among the known modulators in the visible range. The underlying mechanism behind it is the ultrafast polariton condensation, , which is consistent with the previous work for this type of halide perovskites and photon cavity design …”

“…As the fluence surpasses the threshold, the laser emission further intensifies and broadens across the spectrum, as indicated in Figure c. The emission is associated with the formation of an exciton–polariton condensate driven by exceptional points, a phenomenon comprehensively investigated previously …”

“…The increase of the maximum optical modulation can be attributed to polariton accumulation in the condensed state. However, at the highest pump fluences, the appearance of abnormally high optical gain disrupts the conditions necessary for exceptional points, leading to the disappearance of polariton condensation and transmission modulation . In turn, the reduction of τ mod is attributed to the fact that stimulated polariton relaxation is a nonlinear process, with its rate scaling proportionally to the square of the polariton concentration. , This nonlinear scaling results in an accelerated stimulated polariton relaxation rate to the condensed state and, consequently, a reduction in the modulation time with an increasing pump fluence, as shown in Figure d.…”

“…The presence of room-temperature excitons in MAPbBr 3 along with the strong optical field confinement (Figure c) enables the realization of a strong light-matter coupling regime, which is described in detail in the SI. Previous research also has demonstrated the existence of exciton–polaritons in similar structures. ,, Namely, in such systems, the behavior of the interaction of the modes can be described via Hamiltonian: Ĥ false( θ false) = true( .25ex2ex .25ex2ex E + ( θ ) U U E − ( θ ) true) − i true( .25ex2ex .25ex2ex γ n r ( θ ) + γ r γ r γ r γ n r ( θ ) + γ r true) where E +,– (θ) are the real parts of the mode eigenvalues with positive and negative group velocity, respectively, which can be found as the spectral mode position. The parameters γ nr , r delineate the optical losses stemming from nonradiative and radiative processes.…”

“…In a previous study, we demonstrated that applying a nonresonant optical pump to such structures induces optical gain, reducing nonradiative losses. This effect can lead to the degeneracy of the real and imaginary parts of the eigenmodes near θ = 0, which is referred to as the exceptional points (Figure d) …”