H. Vogelsang

Optical properties of γ-AgI nanocrystals synthesized in reverse micelles

Abstract AgI nanocrystals were synthesized in reverse micelles. From X-ray diffraction patterns the nanocrystals were found to exhibit predominantly a zincblende structure. The exciton absorption was blueshifted and inhomogeneously broadened due to spatial confinement and the particle size distribution. “Free” exciton luminescence was weak. Instead, in addition to donor–acceptor recombination, a strong LO multiphonon Raman scattering was observed under resonant excitation in the exciton absorption. An LO phonon energy of 15.4 meV was derived and the incoming and outgoing resonances with the exciton were investigated. All results were consistent with the case of weak exciton confinement.

Resonant Brillouin scattering in biaxially strained ZnSe

Abstract Resonant Brillouin scattering experiments have been performed in the 1S exciton region of ZnSe layers grown on GaAs substrate. Information about the biaxial strain in the ZnSe epilayer, the exchange splitting δ, and the longitudinal-transverse splitting Δ LT can be obtained by comparing the experimental Stokes shifts with those calculated from the dispersion of the 1S exciton-polariton in the strained crystal. For these calculations we used a Hamiltonian which contains the kinetic energy of the exciton center-of-mass motion, the electron-hole exchange interaction and the strain-induced effects. The polariton dispersion is calculated using a multi-component oscillator model. We find a quantitative agreement between the calculated and experimental Stokes shifts for a biaxial strain of +0.03% and exchange parameters δ=0.4 meV and Δ LT =1.2 meV.

Exciton-phonon coupling in indirect gap AgBr nanocrystals

Abstract The resonance Raman scattering observed in the exciton region of indirect AgBr nanocrystals with mean particle radius 〈R〉 ≈ 5 nm is analyzed. Considering the processes as absorption followed by emission and calculating the size dependent exciton energies by the donor-like exciton model of Ekimov et al. [1], the 1TO(L) and 2TO(L)&LO(Γ) resonance behavior can be well-reproduced. The analysis of the Raman spectral lineshape based on linear exciton-phonon coupling shows a substantial increase of coupling strength with excitation photon energy. It is attributed to enhanced acoustic and surface phonon interaction of higher exciton states.

Resonant brillouin scattering from polaritons in ZnSe/GaAs epitaxial layers

Abstract The first observation of resonant Brillouin scattering in an epitaxially grown semiconductor layer is reported. The material is ZnSe grown by metalorganic vapor phase epitaxy on a GaAs substrate. We have studied the spectra and kinetics of the emission occurring under resonant optical excitation in the exciton-polariton region by a tunable picosecond laser. The Brillouin scattering data are used to analyze in detail the effect of strain in the sample and determine the exchange splitting of the exciton states, which is found in quantitative agreement with the reflectivity.

Photoinduced instability of MnO4- molecular defects in potassium iodide

Abstract An instability of the MnO4- molecular ion in KI is reported that occurs under intense resonant excitation in the 1A1 to 1T2 optical transition of the impurity. The intensities of the resonantly excited vibrational Raman lines and the absorption strength are used to monitor the time dependence of this effect which is investigated for different temperatures and excitation powers. A physical model is discussed that involves ionization of the molecular ion and qualitatively accounts for the experimental observations.

Confined excitons in AgI nanocrystals in crystalline KI matrix

Abstract Low-temperature exciton spectra are investigated in AgI nanocrystals in a monocrystalline KI matrix. The occurrence of the Z1,2 together with the A and B exciton absorption bands demonstrate that both zincblende (γ) and wurtzite (β) structure nanocrystals have formed. The exciton blue shift implies a case of weak confinement and an average particle radius of about 2.5nm. Size-selective excitation results in line narrowing and reveals detailed structure in emission due to the LO phonon coupling and the fine structure of the A exciton state. From the analysis the short-range electron–hole exchange energy is found to vary between 2 and 7meV with decreasing particle radius. The origin of another structure with size-dependent energy shift of 7–11meV is discussed. A size-dependent scattering of the B exciton state into the A exciton state is observed.

Exciton–phonon interaction and energy relaxation in ZnSe-based low-dimensional heterostructures

Using a tunable laser excitation we have studied with a high resolution the PL and PLE spectra of localized excitons in ZnCdSe/ZnSe MQWs and ZnSe superlattices (SML) with submonolayer insertions of CdSe. Detailed study of fine structure of PLE spectra and the PL spectra at resonant monochromatic excitation indicates a strong coupling of localized excitons to the phonons all over the Brillouin band, suggesting a tight localization of exciton states. We believe that such a localization is produced by composition fluctuations within QWs or SML, the role of which is strongly enhanced by the low-dimensional character of exciton states.

A 95 GHz ODMR study of AgCl nanocrystals embedded in crystalline KCl matrix

Abstract The W-band (95 GHz) ODMR spectra of AgCl nanocrystals (NCs) embedded in a KCl single crystal lattice have been measured for various orientations of the magnetic field in a {100} crystal plane. Only self-trapped exciton (STE) signals are observed while no ODMR is found from distant electron-hole pairs. This confirms the conclusion of a previous ODMR study in the Q-band (at 35 GHz) concerning the spatial confinement effect and impurity exclusion on exciton recombination in NCs of AgCl. Through increased resolution, more accurate g- and zero-field splitting values of the triplet state were obtained in good agreement with those of the STE in bulk AgCl.

Resonant Raman scattering and photochromic effects in KI:MnO 4 −

A study is reported of photoinduced bleaching in the resonant Raman (RR) scattering spectra of KI:MnO4− crystals under laser excitation of the MnO4− centers in resonance with the 1A1→1T2 optical transition. The bleaching effect is found to exist for all RR lines and is studied in detail as a function of time, temperature, and laser excitation power for the ν1 line. A reversible and an irreversible process are observed, whose contributions to the total effect depend on temperature. It is established that the decrease in RR intensity is accompanied by a decrease in integrated impurity absorption. The observed photochromic effects are attributed to photoinduced instability of the MnO4− ions. The results find qualitative interpretation within a three-center model including the following photoelectronic processes: photothermal ionization of the permanganate, electron ejection from the excited state of the impurity center into the conduction band, and electron capture by traps of two types, with subsequent trap reactivation. The intensity distribution in a multiphonon RR scattering spectrum obtained at 5 K is used to find the main electron-phonon interaction constants. The introduction of the permanganate ion is found to excite a local lattice vibration, which is observed both near the exciting line and in combination with the fully symmetric intramolecular vibration of the impurity ion itself.

Confined excitons in silver halide nanocrystals: Impurity trapping and self-localization

Abstract The self-trapped and I− bound excitons are investigated in nanocrystals of AgCl and AgBr:I−, respectively. Based upon the intermediate confinement model that was previously successfully employed to describe free excitons, the luminescence and excitation spectra including the time and temperature dependences are analyzed and discussed with regard to spatial confinement effects. Even though in both cases the reduced size influences the diffuse electron in a similar way, it is found that the individuality of the two localized states is preserved through the different character of the hole. For AgCl at low temperature, the data suggest tunneling of the self-trapped hole into the nanocrystal center.