R. Hauschild

Ordered, uniform-sized ZnO nanolaser arrays

Ordered ZnO nanorod arrays with almost uniform rod size have been grown perpendicularly on GaN∕Al2O3 substrates by a controlled vapor phase transport growth method. The ZnO nanorods are [0001] oriented single crystals with diameter of 200nm and length of 4.7μm, with a rod-to-rod spacing of 500nm. Photoluminescence spectra of the rod arrays indicate that the rods are of high crystal quality: very strong, well-separated bound and free exciton emission in the ultraviolet (UV) region are resolved at low temperature. Time resolved microphotoluminescence measurements are performed on single nanorods standing on the substrate which demonstrates lasing behavior with multiple UV lasing modes. Under quasistationary excitation lasing is observed up to room temperature. The lasing emission peaks are sharp, with a linewidth about 0.1nm, and have a fast decay time of ∼8ps. These high crystal quality nanorod arrays may be promising candidates for UV nanolaser devices.

Surface-state related luminescence in ZnO nanocrystals

We investigate the optical properties of four different samples of ZnO nanocrystals, with a particle size average varying from 70 up to 380nm. The photoluminescence (PL) of all samples shows at low temperature an emission band around 3.31eV, which is several orders of magnitude stronger compared to the PL of bulk ZnO at this energy. This band shows a clear dependence on the surface to volume ratio of the nanocrystals and is therefore assigned to surface states. Temperature dependent measurements reveal that this band plays a major role up to room temperature for all examined ZnO powders. Additionally, intensity dependent measurements display that the origin of this emission band can be assigned to bound exciton complexes (BECs). Compared to the well known shallow BECs the measured lifetimes of these relatively strong bound excitons states are much longer.

Guided modes in ZnO nanorods

For the case of hexagonal ZnO waveguides the Helmholtz equation is solved numerically taking into account the anisotropy of both index of refraction and gain, as well as the material dispersion. The intensity distribution of all waveguide modes, their energy dependence, and the confinement as a function of the nanorod diameter are discussed. The authors apply these model calculations to determine the coupling of nanorod lasers in dense arrays and to optimize the geometry of nanorods for single mode lasing.

Absolute external luminescence quantum efficiency of zinc oxide

We report on the measurement of the absolute external luminescence quantum efficiency of various ZnO samples using a miniature integrating sphere fitted into a cryostat. Even the absolute luminescence quantum efficiencies per spectral interval are directly accessible. Measurements have been carried out on high quality bulk samples and different commercially available ZnO powders from 8K up to room temperature. Activated processes lead to an overall decrease in the efficiency with temperature. All efficiencies are considerably below unity, making the identification of the luminescence decay time with the radiative life time very questionable.

Influence of ZnO seed crystals and annealing on the optical quality of low-temperature grown ZnO nanorods

The influence of ZnO seed crystals and postgrowth annealing on low-temperature aqueous chemically grown ZnO nanorods is analyzed. At the seed crystal/nanorod interface a high density of structural defects leads to emission at 3.332 eV, attributed to excitons bound to structural defects. This peak is absent for seed crystals, very pronounced for rods of shorter lengths grown on seed crystals, and reduced for longer nanorods. After annealing in oxygen and nitrogen atmosphere, the near-band-edge excitonic transitions sharpen and deep-level emission is strongly reduced. Time-resolved photoluminescence measurements show a striking similarity between donor-bound excitons and excitons bound to structural defects.

Programmed Metalloporphyrins for Self-Assembly within Light-Harvesting Stacks: (5,15-Dicyano-10,20-bis(3,5-di- tert -butylphenyl)porphyrinato)zinc(II) and Its Push−Pull 15- N , N -Dialkylamino-5-cyano Congeners Obtained by a Facile Direct Amination

The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic combination of pi-stacking and a CN-Zn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of meso-bromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over 1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.

Macroscopic energy transport in ZnO monitored by spatiotemporally resolved luminescence

We report on an experimental study of macroscopic energy transfer in ZnO bulk crystals. We observe the ultraviolet photoluminescence (PL) emission from the near band edge (3.4eV at room temperature), a green emission band from a deep center transition, and an orange emission band. Unusually, the orange PL is not only visible at the excitation spot and at crystal edges, but we also observe a slow spatial expansion of the light emission in the orange band up to a distance of several millimeters. In contrast to that, the green and ultraviolet emission originate only from the excitation spot or are scattered from the crystal edges. We investigate the temporal dynamics of the orange PL as a function of the distance from the laser spot, by turning on and off the laser excitation. We describe the results with a two-dimensional diffusion model and discuss some possible mechanisms that may cause this energy transfer, such as Forster/Dexter transfer or a thermally induced hopping process.

Lasing dynamics in ZnO nanorods

The gain mechanisms and the lasing dynamics of ZnO nanorods are investigated in this work as a function of temperature and excitation power. Measurements are obtained using a streak camera which allows to resolve the onset and decay of the lasing emission on a ps-timescale for a single lasing peak. Observations show multimode lasing for some nanorods caused by higher excitation densities.

Processes of Stimulated Emission in ZnO

In this contribution we present a critical review of experimental data and theoretical considerations concerning the stimulated emission in ZnO. The main processes discussed in literature leading to stimulated emission at RT are inelastic exciton‐exciton scattering (P‐band) and recombination of an inverted band‐to‐band transition in an electron‐hole plasma (EHP). While the latter process becomes dominant at RT for densities close to 1019 cm−3 theory predicts a lower threshold for inelastic exciton‐free carrier scattering compared to the P‐band. In addition to that the exciton‐nLO phonon process might also have a lower threshold than the P‐band. A detailed analysis of the temperature dependence of the band gap and of the homogenous broadening of the exciton resonance allows us to distinguish between the various processes but also casts some doubt on the frequently given claim of excitonic RT lasing in ZnO. Therefore we argue that inelastic scattering processes with carriers, phonons or plasmons in a still ...

Temperature dependent dynamics of the excitonic photoluminescence in zinc oxide nanorods

The temporal dynamics of the exciton photoluminescence (PL) in ZnO nanorod samples was investigated experimentally as a function of temperature and excitation intensity. Excitonic photoluminescence is observed up to room temperature. The excitation dependence of the PL dynamics reveals a saturable non‐radiative recombination center. Under high excitation conditions the time‐resolved photoluminescence shows two components: the ZnO M‐band which decays with a temperature independent sub‐100 ps time constant, and the intrinsic exciton PL with a time constant of several 100 ps increasing with temperature. Exciton‐exciton scattering effects are notably absent, which is attributed to the reduced polariton phase space resulting from the small nanorod diameter of 50 nm.