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Optimized pulse shaping experiments were carried out on the control of two-photon excitation efficiency of an a-perylene crystal in the temperature region from 30 to 290 K. It was Ibund that a pulse train with a pulse interval of 90 fs and an alternately reversing phase relation increased the excitation efficiency by a factor of 2 for the whole temperature region. The pulse shape characteristic for effective efficiency increase was educed by double pulse experiments in which the dependence of the emission intensity on the pulse interval and relative phase between pulses were measured. The mechanism of the efficiency increase is briefly discussed using a sliding-window Fourier transform of the pulse shape. [ABSTRACT FROM AUTHOR]

Studies the effect of thermal annealing on the indium aluminum arsenide (InAlAs)/indium aluminum arsenide (InGaAs) modulation-doped structure, passivated with a silicon nitrogen (SiN) film. Degradation in the electron gas degradation; Secondary ion mass spectroscopy measurements ; Hall effect measurements.

Provides information on a study which examined the molecular-beam epitaxial growth conditions for the superlattices of indium arsenic/gallium arsenic (GaAs) on GaAs substrates. Reflection high-energy electron diffraction intensity oscillations; Observation on the growth of short period superlattices.

By means of tunable infrared pump–probe experiment, vibrational relaxation rates (v=1→0) of OH stretching mode of Bro\nsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The high repulsive states of HgAr and HgNe van der Waals complexes, correlating with Hg 6s6d atomic states have been investigated by double resonance spectroscopy, through the first excited state A 30+ and B 31 of the complexes. The repulsive potentials have been fitted through numerical Franck–Condon simulations. They have been characterized by perturbative calculation as quasi-pure 6dΣ potentials in Hund’s case a. The strong Hg–rare gas electrostatic interaction potential overruns the spin–orbit interaction at distances shorter than 7 Å. These observed repulsive states are mostly of Ω=1 character correlating with 3D3 at infinite distances. The contribution from the potential of Ω=0- symmetry correlating with 1D2 is of minor importance. Therefore, the absorption in the repulsive states of the complex arises mostly from proximity induced absorption in an optically forbidden transition 3P1→3D3. A perturbative model accounts well for the bound free absorption intensities experimentally observed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An optical pulse shaping system using a spatial light modulator (SLM) controlled by genetic algorithm (GA) optimization was improved and we demonstrated that the time for finding the optimal condition could be shortened by a proposed “thinning out” method. In this method, GA controls only every N (N=1, 2, 4, 8, 16) pixel in the SLM. The conditions of the remaining pixels are derived by interpolation. The effectiveness of this method was verified by the pulse compression of chirped femtosecond pulses and the increase in the two-photon excitation efficiency of α-perylene crystals. The influence of the thinning out on the pulse shape is discussed, and an advanced method is proposed. It is shown that the disadvantages of the thinning out method can be compensated for by changing the value of N during optimization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quantitative relation between fluorine (F) accumulation and Si donor concentration in n-InAlAs layers on InP substrate was investigated for several kinds of step-doped InAlAs samples using secondary ion mass spectroscopy. From the depth profile of F and Si donors in a periodic i-/n-InAlAs sample, we found that F accumulates only in n-InAlAs layers, passing through i-InAlAs layers. We also found that the amount of F accumulation in an n-InAlAs layer depends on the Si doping concentration. The experimental results can be explained by considering two states of F. In one state, F is bound to a Si donor and immobile, and in the other it is free and can diffuse. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Thermally activated triplet-to-singlet upconversion is attractive from both fundamental science and exciton engineering, but controlling the process from molecular configuration is still unrevealed. In particular, the flexibility of the freedom of molecular geometry is of major importance to understand the kinetics of the phonon-induced upconversion. Here, we focus on two linearly connected donor-acceptor molecules, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) and hexamethylazatriangulene-2,4,6-triphenyl-1,3,5-triazine (HMAT-TRZ), as the model system. While DMAC-TRZ possesses a rotational degree of freedom in the dihedral angle between the donor and acceptor moieties, i.e., C-N bond in tertiary amine, the rotation is structurally restricted in HMAT-TRZ. The rotationally flexible DMAC-TRZ showed significant triplet-to-singlet upconversion caused by thermal activation. On the other hand, the rotation-restricted HMAT-TRZ showed negligible thermal upconversion efficiency. We elaborate on the origin of the photophysical properties from the viewpoint of the geometries in the excited states using time-resolved infrared spectroscopy and quantum chemical calculations. We uncovered that the structural restriction of the intramolecular flexibility significantly affects the optimized geometry and phonon modes coupled to the spin conversion. As a result of the rotation restriction, the spin flipping in HMAT-TRZ was coupled to bending motion instead of the rotation. In contrast, the free rotation fluctuation in the DMAC-TRZ mixes local-excitation and charge-transfer characters, leading to successful activation of the delayed fluorescence as well as the reverse intersystem crossing. Our discovery sheds light on the mechanism of the triplet-to-singlet upconversion, providing a microscopic strategy to control the optoelectronic properties from a molecular viewpoint.

Lead halide perovskites are promising materials for optoelectronic applications because of their exceptional performances in carrier lifetime and diffusion length; however, the microscopic origins of their unique characteristics remain elusive. The organic-inorganic hybrid perovskites show unique dielectric functions, i.e., ferroelectric-like phonon responses in the 0.1-10 THz region and liquid-like rotational relaxation in the 1-100 GHz range. To reveal the role of the dielectric responses is of primal importance because the dielectric screening is a key to understanding the optoelectronic properties governed by polarons in the perovskites. Here, we conducted comparative studies of broadband dielectric spectroscopy on both all-inorganic CsPbBr 3 and organic-inorganic hybrid (CH 3 NH 3 )PbBr 3 single crystals to uncover the origin of the liquid-like dielectric relaxation in the 1-100 GHz range. We confirmed the absence of the dielectric response in the range of 10 6 -10 10 Hz in CsPbBr 3 , which was clearly present in the hybrid (CH 3 NH 3 )PbBr 3 . This suggests that the response is almost purely due to the rotational motions of the organic dipoles in the hybrid perovskites. We evaluated the lifetimes of the polarons using surface-free transient photoluminescence. The lifetime in CsPbBr 3 was up to 1.6 µs, while the lifetime in (CH 3 NH 3 )PbBr 3 was 18 µs. The lifetime in the hybrid (CH 3 NH 3 )PbBr 3 was significantly longer than in CsPbBr 3 , also confirmed by transient infrared spectroscopy. We concluded that the liquid-like dielectric response inhibits polaron recombination due to the efficient separation of opposite charges by the additional dynamic disorder.

The atomic and electronic dynamics in the topological insulator (TI) Bi2Te3 under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi2Te3 trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results.

We constructed an instrument for time-resolved photoemission electron microscopy (TR-PEEM) utilizing femtosecond (fs) laser pulses to visualize the dynamics of photogenerated electrons in semiconductors on ultrasmall and ultrafast scales. The spatial distribution of the excited electrons and their relaxation and/or recombination processes were imaged by the proposed TR-PEEM method with a spatial resolution about 100 nm and an ultrafast temporal resolution defined by the cross-correlation of the fs laser pulses (240 fs). A direct observation of the dynamical behavior of electrons on higher resistivity samples, such as semiconductors, by TR-PEEM has still been facing difficulties because of space and/or sample charging effects originating from the high photon flux of the ultrashort pulsed laser utilized for the photoemission process. Here, a regenerative amplified fs laser with a widely tunable repetition rate has been utilized, and with careful optimization of laser parameters, such as fluence and repetition rate, and consideration for carrier lifetimes, the electron dynamics in semiconductors were visualized. For demonstrating our newly developed TR-PEEM method, the photogenerated carrier lifetimes around a nanoscale defect on a GaAs surface were observed. The obtained lifetimes were on a sub-picosecond time scale, which is much shorter than the lifetimes of carriers observed in the non-defective surrounding regions. Our findings are consistent with the fact that structural defects induce mid-gap states in the forbidden band, and that the electrons captured in these states promptly relax into the ground state.

Optimized pulse shaping experiments were carried out on the control of two-photon excitation efficiency of an alpha-perylene crystal in the temperature region from 30 to 290 K. It was found that a pulse train with a pulse interval of 90 fs and an alternately reversing phase relation increased the excitation efficiency by a factor of 2 for the whole temperature region. The pulse shape characteristic for effective efficiency increase was reduced by double pulse experiments in which the dependence of the emission intensity on the pulse interval and relative phase between pulses were measured. The mechanism of the efficiency increase is briefly discussed using a sliding-window Fourier transform of the pulse shape., ((c) 2004 American Institute of Physics)