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Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials.

Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals.

The PL lifetimes are very similar for solutions and for films (Supplementary Fig. PL excitation spectrum from an NC film closely resembles the absorption spectrum (Supplementary Fig. Clear signatures of the ASE emission—narrowing of the emission peaks and threshold behaviour with a steep rise in intensity above the threshold—are readily obtained from 300- to 400-nm thick films produced by drop-casting colloidal solutions onto glass substrates (Fig.

When the stripe length reaches the threshold value where propagation losses are compensated by the optical amplification, the PL spectrum starts to show an additional ASE component that grows with stripe length (see Fig. Considering these gain values, the build-up time for ASE was estimated to be 140 fs, considerably shorter than the ASE threshold lifetime of 300 ps.

Another characteristic and expected feature of ASE, seen in time-resolved experiments (Fig.

5) due to a narrow gain in bandwidth and is red-shifted by ca. When the ASE spectrum is overlaid with the Tauc plot of the direct-bandgap absorption (Supplementary Fig.

3), the ASE spectral maxima coincide with the end of the shallow absorption tail (Urbach tail).

This red-shifted ASE may have its origins in re-absorbance during single-exciton lasing for all other compositions (Supplementary Fig. Notably, the ASE linewidth increases for samples with a lower ASE threshold suggesting that a larger portion of the emission falls within the optical gain conditions, enlarging the optical gain bandwidth.

In addition to the ASE threshold, the net modal gain is an important figure-of-merit that, from a practical point of view, indicates the efficiency of light amplification in the material and the quality of the resonator needed for achieving lasing, where the excitation light is shaped into a line of variable lengths on the sample surface (see the schematics in the inset of Fig.3), is the acceleration of radiative recombination due to switch from individual to collective emission. 3e), ASE lifetimes of 60 ps were estimated, again with clean, single-exponent line shape.At the ASE threshold, an ASE lifetime of 300 ps can be roughly estimated from a bi-exponential fit assuming competing ASE and PL processes.100 me V, high PL QYs of up 90% and high photochemical stability have been achieved for Cd-chalcogenide NCs as the result of two decades of research efforts to precisely engineer core-shell morphologies with independent control of the core and shell compositions and thicknesses (for example, Cd Se. These NCs readily form uniform, compact films of sub-micron thickness on drop-casting onto a glass substrate.For the spectroscopic studies in this work, we selected monodisperse samples of cubic-shaped NCs with mean sizes of ca. NCs is controlled via compositional modulations, for example, by altering the Cl/Br ratio for the 410–530 nm range, and Br/I ratio for the 530–700 nm range (Fig. A pronounced excitonic peak is preserved in the absorption spectrum of the Cs Pb Br NC film is estimated to be 1.85–2.30 at 400–530 nm from the optical reflectance and absorption spectra (Supplementary Fig. The PL from this NC film exhibits a peak with a narrow linewidth of 25 nm (110 me V), Stokes-shifted by 13 nm (57 me V) with respect to the excitonic absorption peak (Fig. The PL QYs of the same NCs in the solution reach values of up to 70–90% (for green-to-red-emitting NCs) indicating a high degree of electronic surface passivation.In this case, with a 300-ps ASE lifetime at 5–10 μJ cm(a) Evolution of the emission spectra with the increase of pumping fluence and (b) corresponding dependence of the emission at 535 nm on pump fluence.