Ultrafast lasing due to electron-hole plasma in ZnO nano-multipods

An Instantaneous Recombination Rate Method for the Analysis of Interband Recombination Processes in ZnO Crystals

Time-resolved photoluminescence (TRPL) analysis is often performed to assess the qualitative features of semiconductor crystals using predetermined functions (e.g., double- or multi-exponentials) to fit the decays of PL intensity. However, in many cases—including the notable case of interband PL in direct gap semiconductors—this approach just provides phenomenological parameters and not fundamental physical quantities. In the present work, we highlight that within a properly chosen range of laser excitation, the TRPL of zinc oxide (ZnO) bulk crystals can be described with excellent precision with second-order kinetics for the total recombination rate. We show that this allows us to define an original method for data analysis, based on evaluating the “instantaneous” recombination rate that drives the initial slope of the decay curves, acquired as a function of the excitation laser fluence. The method is used to fit experimental data, determining useful information on fundamental quantities that appear in the second-order recombination rate, namely the PL (unimolecular) lifetime, the bimolecular recombination coefficient, the non-radiative lifetime and the equilibrium free-carrier concentration. Results reasonably close to those typically obtained in direct gap semiconductors are extracted. The method may represent a useful tool for gaining insight into the recombination processes of a charge carrier in ZnO, and for obtaining quantitative information on ZnO excitonic dynamics.

Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices: Effects on Photoluminescence and Energy Transfer

Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr₃ nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr₃ nanocrystals develop bulk-like CsPbBr₃ particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr₃ nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects.

Electron-hole plasma induced band gap renormalization in ZnO microlaser cavities

We report electron-hole plasma (EHP) lasing in hexagonal ZnO microrods and thin nanobelts. Under the excitation of 325 nm line femtosecond pulsed laser, ultraviolet whispering-gallery mode (WGM) lasing was observed from hexagonal ZnO microrods. When EHP was formed at high excitation energy density, the center wavelength of the WGM lasing band presented a redshift from 387.5 nm to 397.5 nm, and the full width of half maximum (FWHM) of the WGM lasing band increased from 2.5 nm to 7 nm. Each lasing mode showed obvious blueshift and broadening. Such lasing characteristics were attributed to the band gap renormalization (BGR) due to the high carrier concentration at the EHP condition. In addition, EHP Fabry-Perot (F-P) mode lasing from thin ZnO nanobelt was also observed and discussed. According to the phenomenological BGR calculation with including the carrier density dependent screening effect, the values of the band gap of ZnO at different excitation energy densities were obtained, which agree well with the experimental results.

Room-temperature laser emission of ZnO nanowires explained by many-body theory

Are excitons involved in lasing in ZnO nanowires or not? Our recently developed and experimentally tested quantum many-body theory sheds new light on this question. We measured the laser thresholds and Fabry-Pérot laser modes for three radically different excitation schemes. The thresholds, photon energies, and mode spacings can all be explained by our theory, without invoking enhanced light-matter interaction, as is needed in an earlier excitonic model. Our conclusion is that lasing in ZnO nanowires at room temperature is not of excitonic nature, as is often thought, but instead is electron-hole plasma lasing.