Lasing with guided modes in ZnO nanorods and nanowires

A vertically arranged nearly parallel array of ZnO nanorods and randomly oriented nanowires has been grown by low pressure chemical vapor deposition (CVD) on silica substrates and on stainless steel gauze woven from a wire with a diameter of 40 μm, respectively. The quality of the produced material is high enough to act as a gain medium for stimulated emission in the ultraviolet spectral region. Multiple sharp lasing peaks were realized from single hexagonal nanorods and arrays of hexagonal ZnO nanorods. The lasing peaks display successive onset and saturation with increasing excitation power density and fit well the expected resonance spectrum of guided modes in hexagonal nanorods. The behavior of lasing spectra from shot to shot of pumping in randomly oriented nanowires along with the independence of the lasing threshold on the excitation spot area suggest that the emission spectrum results from the superposition of lasing modes in individual nanowires, rather than from random lasing due to photon coherent scattering.     

Linear and nonlinear optics,dynamics, and lasing in ZnO bulk and nanostructures

In linear optics, we report on measurements of the absolute external quantum efficiency of bulk ZnO and powders using an integrating sphere. At low temperature the near band edge emission efficiency can reach 0.15 in the best samples. For deep center luminescence this value may be even higher. When going to room temperature (RT) the quantum efficiency drops by about one order of magnitude. From time resolved luminescence measurements we deduce the lifetime of the free and bound excitons to be in the sub ns regime and find for the latter a systematic increase with increasing binding energy.Concerning lasing, we discuss the role of excitonic processes and the recombination in an inverted electron–hole plasma (EHP). While excitonic processes seem well justified at lower temperatures and densities, doubts arise concerning the concept of excitonic lasing at RT in ZnO. The densities at laser threshold at RT are frequently close to the Mott density or above but below the density at which population inversion in an EHP is reached. We suggest alternative processes which can explain stimulated emission in this density regime in an EHP at RT.     

Relaxation mechanism of the 1Δg state of liquid O2

Collisional deactivation of the first excited electronic ¹Δ_g(υ = 0) state of O₂ involves intersystem crossing to higher vibrational levels (υ < 5) of the electronic ground state ³Σ^?_g. It is followed by rapid vibrational-vibrational energy exchange which populates the first excited ³Σ^?_g(υ = 1) vibrational level. The suggested relaxation mechanism is supported by experimental results on the time dependence of the populations of the ¹Δ_g(υ = 0) and ³Σ^?_g(υ = 1) states in liquid natural O₂ and ¹⁸O₂.     

Experimental investigation of the electron-hole plasma expansion in CdS

Using a two beam method for gain and reflection spectroscopy, we introduce a new technique allowing for a spatial and temporal resolution of 5 μm and 2 nsec, respectively. With this method we are able to investigate the optical gain, the reflection and the spatial extension of an electron-hole plasma under stationary excitation conditions. The experiments are performed with the II–VI compound semiconductor CdS. Our results are compared with those from other authors who deduced the electron-hole plasma properties mainly from luminescence experiments.     

The dispersion curves of the excitonic polariton and the excitonic molecule in CdS and their interaction

By means of the two-photon Raman scattering (TPRS) process we investigate the dispersion relation of the excitonic polariton in the energetic regions around the A-exciton resonance and near half the bi-exciton energy in CdS. In a high resolution experiment an anomaly is observed due to two-polariton transitions to the excitonic molecule (biexciton) state. This anomaly is explained on the basis of a previously developed theory of the intensity dependent dielectric function. Excitation spectroscopy of the TPRS-lines yields information about the damping of the excitonic molecule. Luminescence assisted two polariton spectroscopy (LATS) experiments are performed to determine the eigenenergy of the biexciton as well as its dispersion curve.     

Two-Photon Raman scattering and high excitation luminescence in ZnTe

Two-Photon-Raman Scattering (TPRS) and the luminescence of ZnTe are investigated when the samples are highly excited with a tunable narrow-band dye-laser. In luminescence, one observes emission bands due to the well-known inelastic exciton-exciton scattering at intermediate excitation intensities, and the recombination radiation of an electronhole plasma (EHP) at the highest excitation levels. For the first time, TPRS is reported in ZnTe. From the change in the TPRS lines in magnetic fields up to 10T we deduce a diamagnetic shift of 1.2°10^–2 meV/T² of the free longitudinal exciton. This value is in good agreement with results obtained by other authors from reflection spectroscopy.     

Gain spectroscopy and plasma recombination in CdS

The spectral dependence of optical amplification (gain) is measured in highly excited CdS as function of sample temperature, excitation intensity and polarization. The results indicate the existence of an electron hole plasma with a density of 2,8·10¹⁷cm^-3, a critical temperature of 75 K and a binding energy relative to the free A_Γ6 exciton of approximately 9 meV. The spectral dependence of the gain is consistent with a momentum conserving recombination of the electron hole pairs in the plasma. We compare our experimental findings with some recent results of other authors.     

Measurement of the diffusion-length of carriers and excitons in CdS using laser-induced transient gratings

Using a laser-induced grating arrangement we measure the probe beam diffraction intensity under stationary excitation conditions as function of the grating constant. We developed a mathematical model and extract by comparison with experimental data the diffusion lengths of the quasiparticles existing at various temperatures and excitation levels in the CdS sample from the experimental findings.     

Growth of CdS/ZnS strained layer superlattices on GaAs(0 0 1) by molecular-beam epitaxy with special reference to their structural properties and lattice dynamics

Strained layer CdS/ZnS superlattices have been grown on GaAs(0 0 1) by molecular-beam epitaxy using CdS and ZnS compound sources. The samples were investigated with special reference to their structural properties and lattice dynamics by means of X-ray diffraction and Raman spectroscopy. The results of four superlattices with different period length are discussed in detail. X-ray diffraction profiles show superlattice satellite peaks up to the fourth order indicating a high degree of periodicity. The lateral and in-depth homogeneity of the period length is also confirmed by Raman investigations. Folded longitudinal acoustic phonons in CdS/ZnS superlattices were observed for the first time. The experimental values agree very well with theoretical calculations based on the Rytov model and show the expected dependency on the superlattice period. The behaviour of the optical phonons is mainly determined by strain induced shifts caused by the high lattice mismatch (−7%) for this system. A good agreement between theoretical predictions and detected frequencies is obtained.     

Steady-state and time-resolved spectroscopy of porous silicon

We present experimental data on steady-state properties, time-resolved properties and on polarization characteristics of porous silicon photoluminescence and models for the decay processes of the red-orange band. The manifold manifestation of inhomogeneous broadening of this band in emission, excitation, polarization, kinetics and degradation supports the model in which porous silicon is treated as a network of crystallites connected via an oxide interface. Spectral inhomogeneties of the red-orange band can be described in terms of varying shape and size of silicon clusters. The polarization of emission is explained by coexistence of dot-like and wire-like entities, i.e. spherical and non-spherical clusters. The relative weight of these species determines the polarization degree, whereas the kinetics are controlled by the transport of excitations among the clusters. The decay is modeled by a modified stretched exponential function with the local lifetime, the migration lifetime, and a scaling factor. The latter is determined by the dimensionality of the space available for migration which was found to be close to but less than unity. On the nanosecond range two distinct bands in the blue-green region are evaluated that need further studies for interpretation. Generally, arguments are proposed in favor of a quantum confinement origin of the red-orange band and a bridge between quantum-wire and quantum-dot models is provided.