U. Neukirch

Dynamical properties of excitons in Zn1−xCdxSe/ZnSe quantum wells and Zn1−xCdxSe epilayers grown by molecular beam epitaxy

Abstract Zn1−xCdxSe/ZnSe single quantum wells (SQWs) and Zn1−xCdxSe/GaAs epilayers grown by molecular beam epitaxy are investigated by picosecond time-resolved luminescence spectroscopy as well as by time integrated luminescence, reflection, and transmission spectroscopy. The well thicknesses of the SQW samples are 3.3 and 5 nm, and the Cd contents amount to 0.09, 0.18, and 0.20, respectively. Whereas the epilayer luminescence is dominated by free-exciton emission, the SQW luminescence exhibits free and bound-exciton bands of comparable intensity. The decay times of the luminescence are shorter than 140 ps in any case. When tuning the energy of detection, Edet over the free-exciton band, the decay time decreases for increasing energy. This is also known from other localizing ternary systems like Cd1−xSxSe or CdSe/ZnSe. In contrast to those systems, the transient behavior of the luminescence in the Zn1−xCdxSe samples exhibits a simple exponential rise and decay even at high Edet. This is probably caused by the short length scale of the localizing potential. At high excitation densities emission from recombining biexcitons is observed in the SQWs.

Exciton relaxation dynamics in ultrathin CdSe/ZnSe single quantum wells

Abstract Ultrathin CdSe/ZnSe single quantum wells grown by self-limiting monolayer epitaxy are investigated by means of time integrated as well as picosecond time-resolved luminescence spectroscopy. Well thicknesses are 1, 2 and 3 monolayers of CdSe. All samples exhibit a bright excitonic luminescence which shows, for an SQW of one monolayer thickness, a blue shift of 850 meV with respect to bulk CdSe. The transient behavior of the luminescence varies depending on energy of detection. At high energies, the decay is very fast and nonexponential. At the low-energy side of the luminescence band, a slower, simply exponential decay is observed. A model of excitons relaxing in a random potential is set up to describe and discuss the experimental data. Under resonant excitation into higher well states, several phonon replica of recombining, resonantly excited excitons occur.

Stretched-exponential decay of the luminescence in ZnSeZnTe superlattices

Abstract The photoluminescence decay of fractional-layer superlattices of ZnSeZnTe has been measured to investigate the origin of “slow-bands” (K. Suzuki et al. [4]). The decay of this band is found to be well described by a stretched exponential function. Distributions of effective life-time of excitons at several wavelength positions of the bands are derived from the stretched exponential parameters with which the decay is fitted. From the distributions, the radiative life-time of localized excitons in the slow bands are estimated to be 10–20 ns.

Coherent dynamics of excitons and excitonic complexes in ZnSe epilayers

The contribution of bound excitons and biexcitons to degenerate four-wave mixing signals is investigated in high-quality biaxially strained ZnSe layers. For the former complexes signatures of polarization interference with free excitons are found in time-resolved four-wave mixing, whereas spectrally resolved data point to the occurrence of quantum beating. Thus this system behaves as a mixture of independent two-level systems on the one hand and three-level systems on the other. Biexcitons contribute to the signal mainly at negative delay between the exciting pulses and are identified by means of spectrally resolved measurements and by use of different configurations of the pulse polarization. Complete agreement is found between the selection rules for biexcitons (and excitons) and the experimental data.

Dynamical Properties of Optical Excitations in II–VI Structures

Ultrafast dynamical properties and lasing in two-dimensional II–VI heterostructures reveal a manifold of physical mechanisms which are partly unique and of great interest for the general understanding of the optical behavior of these structures and related devices. We review our investigations of polariton propagation effects strongly governing transmission of fs pulses perpendicular to II–VI waveguiding layers, similar to those in respective laser structures, for low to high excitation densities, being explained by the interference of simultaneously excited modes from four polariton branches involved. Second, exciton diffusion properties can be deduced from results of transient-grating experiments. Extremely large exciton diffusion constants could be measured in ultrapure ZnSe layers. For the first time, ultrafast time-resolved laser mode and gain dynamics is determined for II–VI laser diode structures. Coupling of longitudinal ground modes and higher-order transversal modes results in a density-dependent temporal beat pattern which is well described in a model of weakly phase-coupled modes in II–VI resonator cavities. Gain in laser diodes under optical pumping exhibits a complicated spectro-temporal shape with two subsequent stimulated-emission peaks and kinetic- as well as spectral-hole-burning features, being explained on base of carrier heating and cooling effects during lasing. Challenges for the development of a microscopic theory of lasing in II–VI diodes are discussed.

Time‐Resolved Photoluminescence Measurements on ZnSe–ZnTe Superlattices

Time-resolved photoluminescence of ZnSe-ZnTe supperlattices with different Te concentrations has been measured in the picosecond time region. The samples show typical S1 (blue band) and S2 (green band) emissions in the time-integrated spectra. In the time-resolved spectra, both samples exhibit an asymmetrically bradened shallow component at the high-energy side of the luminescence band, indicating the presence of shallow localized excitons due to long-range potential fluctuations.

Luminescence of ZnCdSe/ZnSe ridge quantum wires

Blue light-emitting quantum wire structures fabricated by molecular-beam epitaxial growth on submicrometer prepatterned GaAs substrates were investigated by spatially and time resolved luminescence experiments. The quantum wires are formed due to the different growth rates of ZnCdSe on the (111) and (100) surfaces of the grated substrate. With decreasing wire width, the exciton luminescence splits into two clearly distinguished lines. These lines can be assigned to the emission of the ridge quantum wire and the emission of ZnCdSe quantum wells at the bottom of the grooves. The two-dimensional quantum confinement in the ridge wire is confirmed by a maximum of the decay time at the energy of the ridge luminescence.

Quantum beats of excitons in Zn1−xCdxSe/ZnSe multiple quantum well structures

We investigate the coherent dynamics of excitons in Zn1−xCdxSe/ZnSe multiple quantum wells by means of spectrally integrated and spectrally resolved transient four‐wave mixing. When simultaneously exciting the heavy‐ and light‐hole exciton resonance, a beating of the spectrally integrated four‐wave mixing signal is observed. The measured beating period does not exactly correspond to the energetic split of the involved free‐exciton resonances. By including a bound‐exciton state in the calculation of the four‐wave mixing signal, quite a reasonable fit can be achieved yielding the dephasing times of the excitonic transitions. The spectrally resolved signal allows to determine the origin of the oscillation as a quantum beat between the light‐ and heavy‐hole exciton.

Intensity dependence of signals obtained in four-wave-mixing geometry: influence of higher-order contributions

The nonlinear optical response of a single ZnSe quantum well is investigated in four-wave-mixing geometry. Contributions above third order in the driving fields are quantified by intensity-dependent measurements. Comparison with a microscopic density-matrix approach yield good agreement between experiment and theory. It is found that substantial contributions above third order are generated even at lowest excitation levels necessary in current experiments.

Recombination kinetics of S1 and S2 bands in ZnSełZnTe superlattices

Abstract The origin of the S1 and S2 emission bands observed in ZnSelZnTe superlattices is discussed on the basis of time-resolved photoluminescence measurements. We show that the decay characteristics of the S1 and S2 band are described each by a single time constant, irrespective of observation energy. This indicates that the S1 and S2 bands are homogeneously broadened due to electron—phonon coupling, supporting the idea that the recombination of self-trapped excitons is a relevant mechanism for these emissions.