Lin Wei-Zhu

Ultrafast Relaxation of Coherent Control Photocurrent in AlGaAs/GaAs Multi Quantum Wells

We theoretically and experimentally study the ultrafast dynamics of coherent control photocurrent in AsGaAs/GaAs multi quantum wells at room temperature. A ballistic photocurrent relaxation time of ~200 fs is deduced based on the theory of density matrix and is evidenced with the three-colour femtosecond pump–probe measurements.

Transient photoluminescence spectra of GaAs/AlGaAs quantum wells, quantum well wires, and quantum well boxes

The interaction of the electrons with the phonons in GaAs/AlGaAs quantum wells, quantum well wires, and quantum well boxes has been studied by meansuring their time resolved photoluminescence (PL) spectra. In the quantum well (QW) system, relaxation time of the electron energy of the narrower well is shorter than that of the wider well. It is attributed to the stronger interaction of the electrons with the interface mode phonons. In the QW wires and QW boxes, the shortening of the relaxation time is obvious. It is due to the stronger electron-phonon interaction caused by the reduction of screening effect. Our experiments also demonstrated the incident laser intensity dependence of the relaxation time.

Ultrafast Energy Transfer and Enhanced Two-Photon Absorption in a Novel Porphyrin Side-Chain Polymer

Ultrafast relaxation processes and transient two-photon absorption are studied in a novel porphyrin side-chain polymer, 5-hydroxy-10,15,20-triphenyl-porphyrin-poly(glycidyl methacrylate) (HTPP-PGMA), by using picosecond luminescence spectroscopy and femtosecond pump–probe techniques. HTPP-PGMA exhibits the ultrafast initial luminescence decay (~300 ps), which is absent in the conventional porphyrin monomer such as TPP. Enhanced two-photon absorption was observed in HTPP-PGMA; the corresponding Im χ(3) is about 2.8×10−11 esu, which is almost one order of magnitude larger than that of the conventional porphyrin monomer (TPP) (~1.3×10−12 esu). The ultrafast energy transfer plays an important role in the excited-state relaxation dynamics observed in HTPP-PGMA. The potential application of HTPP-PGMA in optical switching is discussed.

Dynamics of carrier capture in AlGaAs / GaAs multiple quantum wells

Dynamics of carrier relaxation and capture in AlGaAs/GaAs multiple quantum wells (MQW) at 80 K is studied using picosecond luminescence and femtosecond absorption saturation measurements. Carriers generated in the wells and in the barriers scatter initially out of the excited states to a quasi-equilibrium state in 35 and 400 femtoseconds, respectively, before they are captured into the bound states of the quantum wells. Carrier capture occurs during carrier cooling and recombination. A carrier capture time of 25 ps has been deduced from time-resolved luminescence.

Existence of the transverse relaxation time in optically excited bulk semiconductors

Two basic types of depolarization mechanisms, carrier-carrier (CC) and carrier-phonon (CP) scattering, are investigated in optically excited bulk semiconductors (3D), in which the existence of the transverse relaxation time is proven based on the vector property of the interband transition matrix elements. The dephasing rates for both CC and CP scattering are determined to be equal to one half of the total scattering-rate-integrals weighted by the factors (1 - cos chi), where chi are the scattering angles. Analytical expressions of the polarization dephasing due to CC scattering are established by using an uncertainty broadening approach, and analytical ones due to both the polar optical-phonon and non-polar deformation potential scattering (including inter-valley scattering) are also presented by using the sharp spectral functions in the dephasing rate calculations. These formulas, which reveal the trivial role of the Coulomb screening effect in the depolarization processes, are used to explain the experimental results at hand and provide a clear physical picture that is difficult to extract from numerical treatments.

The effect of intermolecular interactions on photoluminescence of a porphyrin side-chain polymer

Photoluminescence properties and exciton decay dynamics in a porphyrin side-chain polymer, poly[porphyrin acrylate- acrylonitrile (abbreviated p[(por)A-AN]), have been investigated by femtosecond time-resolved photoluminescence spectroscopy. All the luminescences of p[(por)A-AN] films are due to the emissive decay of the photoexcited singlet excitons in the porphyrins. The luminescence efficiencies and lifetimes are increased for samples from pure films to dilute blend films. However, they are increased as the intrachain concentration of the porphyrin sidechain groups is decreased. The intrachain rotation motions of porphyrin sidechain groups result in the initial ultrafast luminescence decays, which are much faster than those due to the interchain interactions. All the samples show no significant red-shift and broadening of the transient luminescence spectra. The interchain and intrachain nonradiative exciton relaxation processes may play an important role in the luminescence dynamics in the p[(por)A-AN] films. The possible origin of different intrachain and interchain dynamic behaviours in p[(por)A-AN] films is discussed.

Ultrafast Exciton Trapping Dynamics in ZnSe/(CdSe) m (ZnSe)) n Short Period Superlattice Quantum Wells

The ultrafast exciton trapping dynamics in ZnSe/ZnCdSe short period superlattice quantum wells has been studied by using transient photoluminescence spectroscopy. A rapid exciton trapping time of less than 7 ps from the barriers to the wells is measured. In Te doped samples self-trapped excitons give rise to Stokes shifted broad band luminescence and exhibit much longer lifetime.

Coherent optical phonon excitation by femtosecond laser pulse in YBa2Cu3O7-δ thin films

With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa2Cu3O7-δ thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO2 plane.

Effects of bandgap shrinkage in femtosecond absorption saturation measurements of GaAs film

Using CPM dye laser and self-mode-locked Ti sapphire laser as pump-probe optical sources, the effects of bandfilling and bandgap shrinkage on the femtosecond absorption saturation spectra of GaAs film have been studied. For exciting photon energy of 1.97 eV and carrier density of 1 × 1018 cm-3, an optical-induced absorption increase is observed and is attributed to bandgap shrinkage. The dependence of the absorption coefficient change on the carrier temperature and the carrier densities is discussed.

Azobenzene polymer films for optical storage and high-order diffraction (I)

The properties of phase conjugated (PC) beam and stable optical storage in an azobenzene polymer film were studied by degenerate four-wave mixing. With low-intensity incident waves (peak value of the pulses of the order of W/cm2), the PC beam as well as diffraction beams were observed and then disappeared after turning off one of the three incident beams. When the peak intensities of the incident beams reached the order of 10 W/cm2, the PC beam did not disappear but decreased to some extent after turning off the writing beams. As the peak intensities of incident beams increased to the order of 103 W/cm2, diffraction up to 4th-order was observed. When all of the three incident waves were turned on, three groups of high-order forward and backward diffraction beams were observed. Turning off two of the three incident beams, the diffraction beams of the other incident beams did not decrease obviously, i.e., stable optical storage was realized. After the experiments, brilliant hologram spot was observed and could be maintained over 120 days without obvious change.