Zhou Jianying

Phase Controlled Laser Interference for Tunable Phase Gratings in Dye-Doped Nematic Liquid Crystals

A phase controlled multi-beam interference is applied to excite the doped liquid crystals. Control of the phase difference between the exciting beams allows the external control of the interference pattern inside the liquid crystal. The dynamic variation of the grating is made possible with a time-dependent phase change to derive the nonlinear refractive index as well as the responding speed of the material. The induced grating structure is numerically modelled with reorientational phase gratings in the liquid crystal, and the diffraction dynamics is found to be in good agreement between theoretical and experimental results.

Time-Resolved Fluorescence Study of C60 Solution

Using a picosecond excitation source and a time-resolved streak camera detection system, the time-resolved fluorescence spectra of purified C60 have been recorded from toluene solution at room temperature. A fluorescence peak at about 730 nm was detected and its lifetime was directly determined to be 1.1±0.1 ns. Two fluorescence peaks obtained from the mixture of C60 and C70 at 676 and 696 nm were mainly related to C70 content.

EXCITON RECOMBINATION DYNAMICS IN CdTe/CdZnTe QUANTUM WELLS

Quantum wells of CdTe/CdZnTe were grown by molecular beam epitaxy. The highest order of satellite peak of the sample is 5 from XRD spectra and excition emission linewidth is about 4.8 nm at 77 K. It was shown that our samples are very good. The recombination dynamics of exciton in high-quality CdZnTe/CdTe multiquantum wells were investigated by means of time-resolved photoluminescence(PL) spectra with different excitation power at 77 K and photoluminescence spectra with different temperature. When weaker excitation was used, radiative recombination decay time of the exciton was reduced as the excitation intensity was decreased; the results indicate that the dominant mechanism may be the quenching of exciton emission by impurities and defects. The linewidth of the exciton emission becomes broader with increasing temperature, the linewidth at low temperature is only due to the well thickness, and the broadening linewidth at high temperature is contributed by the interactions among the exciton and LO and TO phonons and ionized donor impurities. The PL intensities are reduced with increasing temperature, which is mainly due to the thermal dissociation of excitons, i.e., the electrons or holes jump into the barriers from the wells by thermal excitations. The exciton emission from n=2 heavy hole exciton at 77 K has been observed, and the n=2 heavy hole exciton luminescence decay time is shorter than that from n=1. The investigation indicates that there is the exciton energy relaxation from the n=2 to n=1 heavy hole exciton state by phonons.

Photonic Band Gap Structures with Periodically Arranged Atoms in a Two-Dimensional Photonic Crystal

Linear transmission, reflection and absorption spectra for a new two-dimensional photonic crystal with periodically arranged resonant atoms are examined. Numerical results show that a twin-gap structure with forbidden bands displaced from a non-doped bandgap structure can be produced as a result of atomic polarization. The absorption spectrum is also significantly altered compared to the single atom entity.

Generation and Evaluation of Sub-15 fs Laser Pulses

Full width at half maximum (FWHM) and rms formalisms are employed to characterize temporal and spectral profiles of sub-15 fs laser pulses. Furthermore, intracavity spectral and spatial control is applied to improving the laser pulse quality. As a result, high quality sub-15 fs laser pulses have been generated, which possess a satellite-free temporal profile, a smooth spectrum and a nearly single transverse mode.

Pattern Formation and Quasicrystal Structure in Azobenzene Polymer Film

Pattern formation in azobenzene polymer film by degenerate four-wave mixing is reported. Island arrays with specific patterns are analyzed with scanning electron microscopy and polarizing optical microscopy. It is demonstrated that the control of photo-induced nanostructure sized micropattern in the nonlinear organic film is possible by using properly polarized writing beams with the total incident power exceeding a certain threshold.

Photoluminescence of fullerene[60] in ultrathin ordered multilayers: The dissociation of molecular aggregation

The aggregation behavior of a substituted fullerene compound, 1-benzyl-2-hydro[60]fullerene (C60-Be), in Z-type ultrathin ordered (e.g. LB) multilayers has been studied by UV-vis absorption, steady state and time-resolved fluorescence spectroscopies. The LB multilayers of pure C60-Be showed J-aggregate formation, characterized by a large red-shift of two absorption peaks (ΔE∼2 098 cm1 from 260→275 nm and ΔE∼1 076 cm-1 from 328→340 nm, respectively). The main fluorescence emission bands were found at 710, 646, 603, 600 and 595 nm for pure (C60-Be, mixed C60-Be (molar percentage 50%, 25% and 20%. respectively) multilayers and monomeric C60-Be in solution, respectively. The aggregates were dissociated by mixing with some inert materials. The fluorescence decay profiles of C60-Be, mixed C60-Be (molar percentage 50%, 25% and 20%, respectively) in LB films could be best-fitted by using a bi-exponential decay model.

Time decay behavior of fullerene-C60 studied by time-resolved photoluminescence

The time-resoved photoluminescence spectra of C60 have been studied from toluence solution at room temperature and from thin film at both room temperature and 77 K. A photoluminescence peak at about 730 nm was detected from solution at room temperature and film at 77 K, in which lifetimes was determined to be 1.1 and 0.9 ns, respectively. At room temperature, the photoluminescence peak of thin film shifted to 740 nm with fast decay behavior which was fitted well to a double-exponential lifetimes with τ1 = 0.087 ns, τ2 = 0.68 ns. Two relaxation mechanisms are given tentatively in explaining this phenomenon.