Fluorescence of Acridine Orange in inorganic glass matrices

Steady-state, time-resolved and polarization-resolved fluorescence studies of glasses doped with acridine orange with concentrations near 10^–3 M/L are reported. It was found that at this concentration a noticeable part of dye molecules exists in the form of dimers. By means of a polarization-resolved spectroscopic technique the structure of dimers was found to be consistent with the simple exciton theory.     

Über die spontane und stimulierte emission von ZnO nach Zweiquantenanregung

Spontaneous and stimulated emission of ZnO excited by two-quantum-absorption is investigated in the temperature range from 4.2 to 300K. The luminescence lines are attributed to free and bound excitons and their phonon replicas. Stimulated emission arises from both, bound excitons and phonon replicas of the A-exciton. Our results are compared with theory and other measurements.     

Nonequilibrium properties of electron-hole plasma in direct-gap semiconductors

The gain spectra of the electron-hole plasma recombination in CdS are investigated as a function of the excitation conditions and of the lattice temperature. From a lineshape analysis which includes such many-body effects as collision broadening, single-particle energy renormalization and excitonic enhancement, average plasma parameters are obtained. In contrast to the predictions of quasi-equilibrium theory, one finds that the electron-hole plasma does not reach a full thermal quasi-equilibrium in direct-gap materials because of the short lifetimes of the carriers. The nonequilibrium effects are shown to lead to the formation of electron-hole plasma density fluctuations. No well-defined coexistence region exists. The experimental results in the phase transition region can consistently be explained by theoretical treatments of this nonequilibrium phase transition.     

Self oscillations of a bistable element in two coupled hybrid ring resonators

A CdS crystal showing thermally induced optical bistability is incorporated into two coupled hybrid ring resonators with different delay times. Both delay times are much longer than the relaxation time of the nonlinearity. The resulting self-oscillations are investigated both experimentally and theoretically. We find two different types of oscillation modes. If the crystal is on the lower branch of the bistability during the longer delay time, step like oscillations similar to the case of a single resonator occur. If the crystal is in the lower state onlh for the shorter delay time (also the shorter of the two delay times is much longer than the relaxation time) we find more complicated modes with plateaus and stairs because the long resonator acts as a memory for the system state before the switching process. We find complex mode locking structures exhibiting Farey-tree like transitions between different oscillation modes as well as mode coexistence. Based on an adiabatic theory we compute the regions of extstence of the different oscillation modes and compare them with experimental results.     

Nonlinear optics and plasma-induced bistability in CdS

The investigation of highly excited semiconductors and of nonlinear optical properties recently tends to merge into one field of semiconductor physics. This is demonstrated here for the case of CdS. Two experimental methods are presented, which allow us both to detect optical nonlinearities and to understand their physical origins. These are the creation of laser-induced gratings and the excite- and- probe technique. The reasons for the nonlinearities are a broadening of the exciton resonances, transitions to the biexciton and the formation of an electron-hole plasma. The latter point is discussed in some detail and it is demonstrated, how the nonlinearities connected with this phase transition may be exploited to produce a dispersive and an intrinsic, absorptive bistability.     

Photoluminescence and gain spectroscopy of highly excited epitaxial GaN-layers

The spectra of photoluminescence and optical gain of GaN have been measured at low temperatures under N₂-laser excitation.At low excitation levels (I _exc10⁴W cm^–2) we observe free and bound exciton recombination. At intermediate excitation (I _exc10⁵W cm^–2), inelastic exciton-exciton scattering processes show up. At the highest excitation levels (I _exc10⁶ W cm^–2) we report here for the first time the emission from an electron-hole plasma.     

Quantitative investigation of the two-photon absorption of Ruby-laser-light in various semiconductors

With aQ-switched Ruby-laser (h v _L =1.785 eV), the two-photon absorption (TPA) coefficient of various semiconductors is determined. The gap energy,E _g , of these substances is lying in the range ofh v _L <E _g <2hv _L . For all measurements the same experimental setup is used, and for the evolution of the data the real, time-dependent intensity of the laser pulse is taken. Thus, the existing discrepancies between the TPA-coefficients as determined by various authors with different experimental techniques can be settled.     

Zur Diffusion lichtelektrisch angeregter Elektronen und über dendc-Effekt an ZnO-Kristallen

In the volume of ZnO single crystals electrons are excited into the conductionband by two-photon absorption of ruby laser light at room temperature. At the edge of the irradiated region there occur a gradient in the concentration and a diffusion of the electrons. As the diffusion is anisotropic in Li-doped ZnO crystals, an axial quadrupole results, which influences measurable charges on the electrodes of the crystal. From the amount of the charge, the product of drift mobility and drift lifetime μ_Dτ_D can be calculated for Li-doped ZnO. Cu-doped ZnO shows no anisotropic diffusion. By means of photoconductivity experiments, μ_Dτ_D can be obtained in this case. Finally the coefficients of thedc-effect of ZnO are measured with a Neodymium laser.     

Experimental evidence for the existence of biexcitons in ZnO

For the first time resonant two-photon Raman scattering via biexcitons in highly excited ZnO is reported. The existence of biexcitons is thus shown. Their binding energy is determined to be (20 ± 1) meV.