Coherence Length of Excitons in a Semiconductor Quantum Well

We report on the first experimental determination of the coherence length of excitons in semiconductors using the combination of spatially resolved photoluminescence with phonon sideband spectroscopy. The coherence length of excitons in ZnSe quantum wells is determined to be 300 ~ 400 nm, about 25 ~ 30 times the exciton de Broglie wavelength. With increasing exciton kinetic energy, the coherence length decreases slowly. The discrepancy between the coherence lengths measured and calculated by only considering the acoustic phonon scattering suggests an important influence of static disorder.Comment: 4 Pages, 4 figure

Direct observation of free-exciton thermalization in quantum-well structures

We report on a direct observation of free-exciton thermalization in quantum-well structures. A narrow energy distribution of free 1s excitons is created in ZnSe-based quantum wells by emission of one LO phonon after optical excitation of the continuum states with picosecond laser pulses. The subsequent relaxation dynamics within the 1s-exciton dispersion is directly monitored by time-resolved studies of the phonon-assisted photoluminescence. It is demonstrated that the free-exciton distribution remains nonthermal for some 100 ps. The observed dynamics is in reasonable agreement with numerical results of a rate-equation model which accounts for the relevant exciton-phonon coupling mechanisms

Excitons, biexcitons, and phonons in ultrathin CdSe/ZnSe quantum structures

The optical properties of CdSe nanostructures grown by migration-enhanced epitaxy of CdSe on ZnSe are studied by time-, energy-, and temperature-dependent photoluminescence and excitation spectroscopy, as well as by polarization-dependent four-wave mixing and two-photon absorption experiments. The nanostructures consist of a coherently strained Zn1−xCdxSe/ZnSe quantum well with embedded islands of higher Cd content with sizes of a few nanometer due to strain-induced CdSe accumulation. The local increase in CdSe concentration results in a strong localization of the excitonic wave function, in an increase in radiative lifetime, and a decrease of the dephasing rate. Local LO-phonon modes caused by the strong modulation of the Cd concentration profile are found in phonon-assisted relaxation processes. Confined biexcitons with large binding energies between 20 and 24 meV are observed, indicating the important role of biexcitons even at room temperature

Excitonic Spectra of Cd1−x\text{}_{1-x}Fex\text{}_{x}Te Crystals

Strong modification of the optical spectra near the band-gap edge is observed in Cd$\text{}_{1-x}$Fe$\text{}_{x}$Te crystals as compared to the spectra of a pure compound. The evolution of the luminescence spectra at the increase in Fe concentration is represented by the change of radiative recombination channels from dominantly (A$\text{}^{0}$, x) acceptor bound exciton emission in undoped CdTe to the free exciton luminescence in Cd$\text{}_{1-x}$Fe$\text{}_{x}$Te solid solutions

Energy States of Ni in Zn1−x\text{}_{1-x}Mnx\text{}_{x}Se:Ni Solid Solutions

Absorption and reflectivity measurements were carried out on Zn$\text{}_{1-x}$Mn$\text{}_{x}$Se:Ni solid solutions. The spectra demonstrate the radical change of the structure of internal Ni$\text{}^{2+}$(3d$\text{}^{8}$) transitions at the presence of the Mn ions and strong dependence on Mn concentration. The threshold energy ħω$\text{}_{th}$ for the process of Ni photoionization to the valence band Ni$\text{}^{2+}$(3d$\text{}^{8}$) + ħω$\text{}_{th}$ → Ni$\text{}^{1+}$(3d$\text{}^{9}$) + h is shifted to the lower energy by 30 meV, but hardly depends on x despite the essential increase in the energy gap

Free Exciton Luminescence in ZnCdSe Solid Solutions, ZnSe Epitaxial Films and ZnSe1−x\text{}_{1-x}Sx\text{}_{x}/ZnSe1−y\text{}_{1-y}Sy\text{}_{y} Superlattices

We studied the luminescence of bulk Zn$\text{}_{1-x}$Cd$\text{}_{x}$Se solid solutions, epitaxial ZnSe films and ZnSe$\text{}_{1-x}$S$\text{}_{x}$/ZnSe$\text{}_{1-y}$S$\text{}_{y}$ superlattices at T=2-80 K. The analysis of the bands of exciton radiative annihilation with simultaneous emission of 1 or 2 LO phonons shows that the excitons in the studied samples can be considered as free particles with thermalized distribution of kinetic energies. In ZnSe$\text{}_{1-x}$S$\text{}_{x}$/ZnSe$\text{}_{1-y}$S$\text{}_{y}$ superlattices a three-dimensional character of exciton motion was established. The periodic potential of superlattice perturbs the exciton wave vector and induces additional elastic scattering of excitons