摘要:

Resonant optical transitions in asymmetric quantum wells yield huge second order optical nonlinearities but also strong absorption. It is shown that, under these specific conditions where the conversion length may be far smaller than the coherence length, the second harmonic generation equations can be put into a simple dimensionless form. The influence of the quantum well parameters on the optical nonlinearities may thus be easily studied. It is shown that, by optimizing these parameters, second harmonic generation efficiency as large as 20% may be obtained for moderate pump power (≊15 MW/cm2, i.e., before saturation) without any phase match procedure.

ISSN:0021-8979    

DOI:10.1063/1.353179

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Photoluminescence decays fromn‐GaAs/Na2S contacts following picosecond pulse excitation are presented. Decays measured at several different potentials from depletion to accumulation regimes all exhibit a strong dependence on excitation power when the photon flux is greater than 1010photons/cm2/pulse. Using a flux of 4×1012photons/cm2we model the potential dependence of the decays by adjusting only the value of the surface minority trapping velocity. These results are used to evaluate the applicability of the modified dead layer model in the analysis of photoluminescence intensity versus potential measurements as a means of measuring surface minority trapping velocities in photoelectrochemical cells.

ISSN:0021-8979    

DOI:10.1063/1.353180

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electronic band structure and absorption coefficient for intersubband transitions in a Si/Si1−xGexsuperlattice under external electric fields are reported using a full Brillouin‐zone energy‐band description. Full spectra of the absorption coefficient are calculated from zero to high electric fields. The effect of Wannier–Stark localization on intersubband transitions is theoretically studied for the superlattice and the charge densities are given for various electric fields. A finite‐length superlattice instead of an infinite superlattice is considered to show the effects of electric fields on the optical absorption. The Stark shift is observed toward the shorter photon wavelength for an intersubband transition in a quantum well.  

ISSN:0021-8979    

DOI:10.1063/1.353181

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Oligosilanes bridging microgaps between silicon microcrystallites were investigated as a possible model for the origin of the intense visible photoluminescence of porous silicon by means of the semiempirical molecular‐orbital method. The incomplete structural relaxation of the oligosilane bridge after the photoexcitation was found to be a key factor to give the visible photoluminescence. The calculated structure‐insensitive photoexcitation energy around 3.3 eV and the structure‐sensitive light emission energy around 1.2–2.1 eV are consistent with the experimental evidence. The sufficient transition probabilities between the concerning electronic states support the high efficiency of photoluminescence. Durabiliy of the structure under the photoexcitation was also suggested. The model is valid even if the silicon microcrystallites are partly or thoroughly replaced by silicon oxides particles as is more realistic for the porous silicon exposed to the air.

ISSN:0021-8979    

DOI:10.1063/1.353182

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Absorption spectra of Tm3+ions in yttrium scandium aluminum garnet are reported between 1.9 and 0.25 &mgr;m at 4 K. Laser‐excited fluorescence was observed at 4 K from Tm3+(4f12) multiplet manifolds1D2,1G4,3F2, and3H4to the ground‐state manifold3H6. Emission from1D2and1G4also includes transitions to Stark levels in manifolds3F4and3H5. Upconversion excited fluorescence was observed between1D2and3F4at 10 K. Analysis of the fluorescence emission confirms assignments given to individual Stark levels based on an analysis of the absorption spectra. A crystal‐field splitting calculation was carried out in which a parameterized Hamiltonian (including Coulombic, spin‐orbit, and crystal‐field terms inD2symmetry) was diagonalized for all multiplets of the Tm3+(4f12) configuration. The rms deviation between 42 experimental and calculated Stark levels was 8 cm−1.

ISSN:0021-8979    

DOI:10.1063/1.354080

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An apparatus and an experimental procedure, mainly designed to investigate the thermodynamics and kinetics of hydrogen evolution in materials, are described. Employing a 30 ns ruby laser pulse as a heat source, this method probes H behavior at higher temperatures and on a much shorter time scale than classic thermal desorption. Precise calibration techniques for the laser fluence and the desorption yield have been developed. The absolute calibration agrees well with a measurement of the H content by elastic recoil detection. Particular attention has been paid to lateral uniformization of the laser intensity, to allow valid one‐dimensional modeling of laser heating and H evolution, and extraction of the activation energies and kinetic factors. As an example, a cursory study of intrinsic H in Be is presented. The method is also applicable to other volatile dopants.

ISSN:0021-8979    

DOI:10.1063/1.353183

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The behavior of H‐ion‐implantedc‐Si was investigated at temperatures up to the melting point, on the 10−8s time scale, using pulsed‐laser annealing in conjunction with quantitative analysis of the released gas. Laser reflectivity, scanning electron microscopy, and surface profilometry were also used to characterize implantation and annealing effects. Computational kinetic modeling of H release as a function of laser energy is applied to the interpretation of the data. The desorption of H implanted at 1 or 2 keV takes place at ≥1000 K, without extensive surface deformation, and can be fitted by detrapping with an activation energy (≊2 eV) that slowly decreases with the H/Si ratio in the range 4%–27%; contrary to expectation, no limitation by diffusion is observed. Implanted at 5 or 10 keV and H/Si≥20%, H is released at ≥550 K by blister rupture. In spite of the differences between the results for low and high implantation energies, a unified picture emerges, involving a layer with a high‐temperature H mobility greater than that of ordinary atomic diffusion.

ISSN:0021-8979    

DOI:10.1063/1.353184

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Photoluminescence (PL) spectroscopy, cathodoluminescence (CL) spectroscopy and imaging, and preferential defect etching and optical microscopy have been used to characterize compositionally graded Si1−xGexalloy layers grown by molecular beam epitaxy. Si1−xGexcapping layers grown on the compositionally graded layers have low threading dislocation densities, and both PL and low‐beam energy CL spectra show bound exciton luminescence features identical with those observed in bulk Si1−xGexalloys and relatively weak dislocation relatedD‐band features. Increasing the beam energy increases the relative strength of theDbands in the CL spectra, indicating that they are associated with the misfit dislocations in the compositionally graded layer. This has been confirmed by combined chemical etching and PL spectroscopy measurements. The misfit dislocations can be observed by monochromatic CL imaging at a high‐beam energy using a narrow band pass filter centerd on theD4 band.

ISSN:0021-8979    

DOI:10.1063/1.353185

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The first simultaneous measurements of the ground‐state translational, rotational, and vibrational energy distributions of a laser ablated diatomic molecule (YO) liberated during a 351 nm excimer laser ablation of YBa2Cu3O7−&dgr;, are reported. In addition, the translational energy distributions of Y, Y+, Ba, Ba+, and Cu are reported. Measurements made at low and high fluences on YO indicate that there areverylarge differences in the ‘‘temperatures’’ inferred between the translational and the vibrational/rotational energy channels, implying that energy is partitioned in a very nonequilibrium fashion in the expanding plume. A measurement of the rotational and vibrational temperatures shows that the internal degrees of freedom are equilibrated in the expansion plume (Boltzmann distributions) with a temperature of approximately 1000 K being observed at normal processing fluences. The translational energy distributions of YO are observed to fit a shifted Boltzmann distribution at high fluences. The atomic species monitored all display non‐Boltzmann velocity distributions at high fluences. The peak of the observed velocity distributions corresponds to energies as high as 10 eV for the YO neutral diatomic and up to several hundred eV for Ba+at ablation fluences of 5 J/cm2. Bimodal velocity distributions are observed for the Cu, Ba, and Y neutral atoms at high incident fluences, implying ion‐electron recombination as a partial source of the high neutral velocities.

ISSN:0021-8979    

DOI:10.1063/1.353186

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Melts of the eutectic alloy Ag‐39.9 at. %Cu were cooled below the liquidus temperature by the glass flux method. The solidification behavior was followed by measuring the temperature change (recalescence), and the resulting microstructures were investigated by optical and electron microscopy. Up to a certain critical undercooling &Dgr;T*≊76 K, the recalescence rate is less than 100 K s−1, and a feathery, lamellar microstructure is formed, with typically one or two growth centers. When the undercooling exceeds &Dgr;T*, a sudden increase in the recalescence rate (up to 104K s−1) is observed, and the microstructure consists of a dendritic, irregular two‐phase component surrounded by a lamellar eutectic. The critical undercooling temperature coincides with the temperature (T0) at which the Gibbs free energies of the liquid and the supersaturated Ag‐Cu solid solution are equal. The critical behavior is therefore thought to be due to the formation of this metastable phase. A model is proposed to explain the observed behavior and microstructures.  

ISSN:0021-8979    

DOI:10.1063/1.353187

出版商:AIP    

年代:1993

数据来源: AIP