Jizong Xu

Langmuir-Blodgett films of a series of new fluorescent ternary europium(III) complexes

Three europium complexes 1-methyl-4-octadecylpyridinium tetrakis(α-thenoyl trifluoroacetonato) europium(III), tris(α-thenoyl trifluoroacetonato) mono-(9-(4-hexadecylanilino)-4,5-diazafluorene) europium(III), and tris(α-thenoyltrifluoro acetonato) mono(2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline) europium(III) have been newly synthesized. They were capable of forming stable Langmuir films on pure water subphase(pH 5.6, 18 °C) with collapse pressures of 39.0, 50.0 and 20.5 mN/m, respectively. Langmuir-Blodgett films of these complexes have been prepared and studied by surface pressure-area isotherms, ultraviolet, fluorescent spectroscopy, fluorescent lifetime measurements and second-harmonic generation.

Subband separation energy dependence of intersubband relaxation time in wide quantum wells

Subband separation energy dependence of intersubband relaxation time in a wide quantum well (250 A) was studied by steady-state and time-resolved photoluminescence. By applying a perpendicular electrical field, the subband separation energy in the quantum well is continuously tuned from 21 to 40 meV. As a result, it is found that the intersubband relaxation time undergoes a drastic change from several hundred picoseconds to subpicoseconds. It is also found that the intersubband relaxation has already become very fast before the energy separation really reaches one optical phonon energy.

A new azo rare earth coordination compound for second-order non-linear optical Langmuir-Blodgett films

Abstract An amphiphilic lanthanide complex, ( E )-1-methyl-4-(2-(4-(dihexadecylamino)phenyl)azo)pyridinium tetrakis(1-phenyl-3-methyl-4-benzoyl-5-pyrazolonato)lanthanum(III), has been synthesized. It forms a stable Langmuir monolayer film on a pure water subphase (pH 5.6, 18°C) when spread from a chloroform solution. Langmuir-Blodgett (LB) films were prepared by transferring the monolayers onto hydrophilic substrates of quartz and calcium fluoride with a transfer ratio close to unity. From a second-harmonic generation measurement, the second-order molecular hyperpolarizability β of the complex was estimated to be about 1.4 × 10 −47 C m 3 V −2 , which is one of the largest values for azo-based, second-order non-linear optical materials reported so far. The infrared and variable-temperature ultraviolet spectra of the film are also reported.

Photoluminescence studies of GaAs/AlGaAs single quantum well intermixed by Ga+ ion implantation

Ga(+)ion implantation followed by rapid thermal annealing (RTA) was used to enhance the interdiffusion in GaAs/AlGaAs single Quantum Wells(SQWs). The extent of intermixing was found to be dependent on the well depth, number of implanted ions and annealing time. A very fast interdiffusion process occurs at the initial annealing stage. After that, the enhanced diffusion coefficient goes back to the umimplanted value. We propose a two-step model to explain the diffusion process as a function of the annealing time : a fast diffusion process and a saturated diffusion process. The interdiffusion coefficient of the fast diffusion was found to be of well depth dependence and estimated to be in the range of 5.4x10(-16) similar to 1.5x10(-15)cm(2)s(-1). Copyright (C) 1996 Published by Elsevier Science Ltd

Thermal population from hole mixing tunneling in asymmetric coupled double wells

The high energy tails in continuous wave photoluminescence spectra of asymmetric coupled double wells are studied under various excitation intensities and at different lattice temperatures. Hole mixing tunneling induced thermal population of hot carriers is firstly demonstrated and distinguished from photon heating. The corresponding thermalization process is shown to be dominated by acoustic phonon scattering.

Exciton dynamics in GaAs/GaAlAs, InGaAs/GaAs, and InGaAs/AlGaAs quantum wells

Photoluminescence (PL) and time-resolved PL measurements were used to study the exciton recombination processes in GaAs/AlGaAs, InGaAs/GaAs, and InGaAs/AlGaAs quantum wells (QWs). An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/AlGaAs samples, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. In InGaAs/GaAs strained QWs the measured exciton lifetimes were found to be of In composition dependence: the more the indium composition, the shorter the lifetime. The mechanisms, including the random alloy disordering and the degeneration of quasi two-dimensional properties of excitons were inferred to explain the experimental results. The nonradiative recombination was stressed in our InGaAs/AlGaAs QWs. A combined analysis of cw PL and time-resolved PL measurements allows us to separate the radiative and nonradiative decay times in our sample. The observed dominant nonradiative recombination has been tentatively ascribed to the poor quality of AlGaAs.

The thermalization of photoexcited hot carriers in InxGa1−xAs/GaAs strained single quantum well structures

Abstract Both hot carrier photoluminescence and picosecond time-resolved luminescence demonstrate that photoinduced carrier relaxation processes in In x Ga 1− x As GaAs strained single quantum well structures are significantly slower than in the bulk and depend on the alloy composition of the InxGa1−xAs. These results are interpreted in terms of strain induced confinement of the LO-phonon as a possible source of the reduced electron-phonon interaction.

Hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures

By using of the nonlinear luminescence correlation technique, a new time resolved optical spectroscopy technique has been developed and applied to investigating of hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures. It has been found that the well width has a significant effect on the relaxation processes. For a sample with L z =40?, the time constant of the LO-phonon relaxation was found to be as long as 40ps. The physical me-chanism of this weakened electron-phonon interaction is also discussed.Abstract Based on the nonlinear luminescence correlation technique, a new time resolved spectroscopy technique has been developed and applied to the study of hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures. We have found that the well width has a significant effect on the relaxation processes. For a sample with L Z =40 A, the time constant of the LO-phonon relaxation is found to be as long as 42 ps.