Polarization Dependent Excitation and High Harmonic Generation from Intense Mid-IR Laser Pulses in ZnO.

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3-4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

A Few-Minute Synthesis of CsPbBr3 Nanolasers with a High Quality Factor by Spraying at Ambient Conditions.

Inorganic cesium lead halide perovskite nanowires, generating laser emission in the broad spectral range at room temperature and low threshold, have become powerful tools for the cutting-edge applications in the optoelectronics and nanophotonics. However, to achieve high-quality nanowires with the outstanding optical properties, it was necessary to employ long-lasting and costly methods of their synthesis, as well as postsynthetic separation and transfer procedures that are not convenient for large-scale production. Here we report a novel approach to fabricate high-quality CsPbBr3 nanolasers obtained by rapid precipitation from dimethyl sulfoxide solution sprayed onto hydrophobic substrates at ambient conditions. The synthesis technique allows producing the well-separated nanowires with a broad size distribution of 2-50 µm in 5-7 min, being the fastest method to the best of our knowledge. The formation of nanowires occurs via ligand-assisted reprecipitation triggered by intermolecular proton transfer from (CH3)2CHOH to H2O in the presence of a minor amount of water. The XRD patterns confirm an orthorhombic crystal structure of the as-grown CsPbBr3 single nanowires. Scanning electron microscopy images reveal their regular shape and truncated pyramidal end facets, while high-resolution transmission electron microscopy ones demonstrate their single-crystal structure. The lifetime of excitonic emission of the nanowires is found to be 7 ns, when the samples are excited with energy below the lasing threshold, manifesting the low concentration of defect states. The measured nanolasers of different lengths exhibit pronounced stimulated emission above 13 µJ cm-2 excitation threshold with quality factor Q = 1017-6166. Their high performance is assumed to be related to their monocrystalline structure, low concentration of defect states, and improved end facet reflectivity.