Perovskites are currently attracting extensive research interest as a wavelength-tunable lasing material. As a first step toward electrically pumped lasers, numerous investigations have recently reported amplified spontaneous emission (ASE) of optically pumped perovskites with remarkably low thresholds. Here, we investigate the optical aspects of perovskite ASE, to establish the design principle of materials and devices. We show that compared to solution-processed CsPbBr3, vacuum deposition yields superior ASE characteristics with a threshold of 35 μJ/cm2. The optical loss (R loss) during lateral photon propagation in the waveguide mode is identified as a key parameter to determine the ASE quality. With spatially resolved photoluminescence, we determine R loss as 40 and >1000 cm–1 for vacuum and solution-processed perovskites, respectively. We present a comprehensive model that relates ASE, gain length, optical loss, temperature, and density of states. Finally, we succeed in demonstrating ASE in perovskite samples with metal electrodes, mimicking a diode architecture for electrical pumping. Optical spacer layers are shown to play a crucial role in preventing metal absorption loss in waveguide modes.

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We report the first observation of Fano resonances in biased semiconductor superlattices: The excitonic Wannier-Stark ladder transitions show asymmetric absorption due to the coupling to the continua of lower transitions. In contrast to other known examples of Fano resonances, the Fano coupling can be continuously tuned in this system by changing the static field across the superlattice. The line shapes and their coupling dependence are in excellent agreement with theory. We also investigate the dephasing dynamics of the resonances and observe an increase in dephasing with increasing Fano coupling. [S0031-9007(98)06702-7]

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Time-resolved nondegenerate four-wave mixing experiments were performed on 2.6-μm-thick GaN epilayers grown by metalorganic chemical-vapor deposition using picosecond pulses at 355 and 1064 nm for grating recording and probing, respectively. Measurements of free carrier grating kinetics at various grating periods Λ in range from 3 to 10 μm allowed determination of nonequilibrium carrier bipolar diffusion coefficient D=1.7 cm2/s at 300 K. Substitution of the D value into two-dimensional carrier transport model allowed fitting of the whole set of grating kinetics at various Λ varying surface recombination velocity S and linear recombination time τR. This procedure provided us a value of S=5×104 cm/s as well as carrier lifetime of ∼1 ns.