Kai Dou

Luminescence of Mn2+ doped ZnS nanocrystallites

Abstract Optical properties of Mn2+-doped ZnS colloids are reported. The band to band excitation energy transfer to Mn2+ is more efficient compared to Mn2+ direct excitation, which is different from the case for bulk ZnS: Mn. Aging effects and the radiation-induced luminescence enhancement (RILE) effect are reported and an explanation for this behavior is presented.

Temperature dependence of photoluminescence in CdS nanocrystals prepared by the sol-gel method

The temperature-dependent photoluminescence spectra of CdS semiconductor nanocrystals-doped glasses prepared by a sol-gel method were measured at different excitation intensities. Based on the temperature dependence of the peak energy, linewidth and intensity of the luminescence, the characteristics and origins of the band-edge luminescence were discussed.

Photodarkening effect and optical nonlinearity in CdSSe-doped glasses

We systematically investigated the photodarkening effect in CdSSe-doped glasses by studying the dependences of phase-conjugation efficiency, luminescence efficiency and transmittance on the exposure time. A new result is that the fading of the trap-to-valence-band luminescence is much faster than that of band-to-band luminescence. This fact leads us to propose a four-level model to explain the photodarkening effect.

Nonexponential hole burning in Sm2+ doped mixed crystals

Abstract Kinetics of nonexponential hole burning in Sm2+ doped BaFCl0.5Br0.5 is investigated at 77 K. Recombination in two-photon hole burning and tunnelling in single-photon hole burning are considered to result in the decrease of the hole-burning rate with the increase of hole depth.

Mn-related absorptive optical nonlinearity and bistability in Zn1−xMnxSe

Abstract ZnMnSe exhibits a strong nonlinear absorption related to Manganese ions. This absorption is easily bleached when the another narrow-band laser with high intensity illuminates the crystals. The first observation of the absorptive optical bistability is observed in a wider range of excitation power density and excitation wavelength. It is thought that the bistable effect is attributed to the bleaching of absorption concerning Mn energy bands combined with the positive-feedback provided by the light reflection at the perfect crystal surfaces.

EPR study of ZnS : Mn2+ nanocrystals and pyrex glasses

Pyrex glasses with different ZnS: Mn2+ contents were prepared by melting method. It has been found that Mn ion may occupy two sites: (Mn2+)sub, and (Mn2+)int from photoluminescene (PL) and photoluminescence excitation (PLE) spectra. The results were confirmed by the further electron panmagnetic resonance (EPR) experiments and the three types of states (Mn2+)sub, (Mn2+)int,and Mn clusters were identified. It was observed that theg-factor and the hyperfine structure (HFS) constant increase with the decreasing size of nanocrystallite. This may result from hybridization of sp3 electron states of ZnS and 3d5 electron states of Mn by the effects of quantum confinement and the surface states.

Studies on deep levels in GaAs epilayers grown on Si by metal-organic chemical vapour deposition

Much attention has been paid to the growth of heteroepilayers on Si. Recently the electrical and optical properties of GaAs epilayers grown on Si substrates (GaAs/Si) have been studied ex.tensively because of their potential applications for optoelectronics [1-4]. In this work, we studied the temperature and excitation intensity dependent .near-infrared photoluminescence (NIPL) related to the deep levels present in GaAs/Si by metal-organic chemical vapour deposition (MOCVD). Taking into account the Frank-Condon (FC) and bandgap shifts, we reformulated the energy relationship for the recombination luminescence of donor-acceptor pair (DAP) and proposed that the 0.96 eV emission could be interpreted asthe recombination luminescence of the DAP, composed of an arsenic vacancy donor VAs and a gallium vacancy acceptor VGa. The 1.2 gin thick GaAs epilayers used for the experiments were grown by MOCVD on the n-type (1 00) Si substrates misorientated 4 towards one of the (1 1 0) directions using the two-step method [1]. The Si substrates were chemically treated in NH4OH, H202, H20 and HC1, H202, H20, and etched in HF for 1 min. In HJAsH3 ambient, they were heated initially at 1000 °C for 10 min, and the temperature was lowered to 400-450 °C for GaAs buffer growth with thickness of 25 nm. Trimethylgallium (TMG) and arsine in hydrogen were used as source chemicals. Then, the temperature was raised to 700 °C, and the top GaAs epilayers were grown with the ratio of [As]/[Ga] 20 and with a growth rate of 100nmmin -1. The samples were not intentionally doped. The carrier concentration at room temperature was 2 x 1017cm -3. The temperature and excitation intensity dependent NIPL spectra of GaAs/Si epilayers were obtained with an ordinary gating monochromator and were detected by a liquid nitrogen-cooled Ge

Thermally activated barrier crossing and hole-filling in donor-acceptor electron transfer systems

An electron donor-acceptor-doped polymer for photon-gated spectral hole burning (PHB) was described as a two-level system (TLS). Thermal hole-filling was studied by the temperature cycling experiment. A thermally activated barrier crossing model in a TLS is used to explain the hole-filling process was verified by numerically fitting between the theoretical and the experimental results. The maximum barrier heights in several PHB materials were obtained.

Study of excited state processes by using dynamic gratings in the frequency domain

Abstract Ultrafast relaxation of excited states in a system with three energy levels has been investigated theoretically and experimentally by using laser frequency-domain techniques. The picosecond process of electron transfer in binary molecules of porphyrin-porphyrin or porphyrin-phthalocyanine was studied and is discussed here.

Dephasing processes of coherent exciton migration in mixed aggregates with barriers of PIC-I and azaPIC-I

Abstract Coherence decay processes of Frenkel excitons in the mixed aggregates with barriers at low temperature were investigated using an accumulated photon echo technique. The lengthening of the dephasing times T 2 with increase of the molar fraction of azaPIC-I was observed, which is contrary to the shortening of T 2 in the mixed aggregate with traps. Exciton coherence lengths were studied theoretically and experimentally. It is demonstrated that the effective distance, passed through by excitons, is less than the exciton coherence length.