Optical absorption spectra of Na-5, Na-6 and Na-7 clusters: a theoretical study

In the frame of time-dependent density functional theory, the: dynamical polarizabilities of Na-5, Na-6 and Na-7 clusters are calculated using a time-dependent local density approximation. By using Fourier transformation, the optical absorption spectra of Na-5, Na-6 and Na-7 clusters are obtained from their dynamical polarizabilities. It is shown that experimentally measured optical absorption spectra of Na-5, Na-6 and Na-7 clusters are reproduced in our calculations. Furthermore, the calculations of Na-6 and Na-7 clusters are in good agreement with the results of configuration interaction method. Compared with the three-dimensional structure of Na-6, the calculated optical absorption spectra of Na-6 with the two-dimensional structure are more close to the experimental data.

The effects of oxygen partial pressure on the optical absorption edge and the UV emission energy of ZnO films

The optical absorption edge and ultraviolet (UV) emission energy of ZnO films deposited by direct current (DC) reactive magnetron sputtering at room temperature have been investigated. With the oxygen ratio increasing, the structure of films changes from zinc and zinc oxide coexisting phase to single-phase ZnO and finally to the highly (002) orientation. Both the grain size and the stress of ZnO film vary with the oxygen partial pressure. Upon increasing the oxygen partial pressure in the growing ambient, the visible emission in the room-temperature photoluminescence spectra was suppressed without sacrificing the band-edge emission intensity in the ultraviolet region. The peaks of photoluminescence spectra were located at 3.06---3.15 eV. From optical transmittance spectra of ZnO films, the optical band gap edge was observed to shift towards shorter wavelength with the increase of oxygen partial pressure.

Intersubband optical absorption in quantum dots-in-a-well heterostructures

The theoretical analysis of intersubband optical transitions for InAs/ InGaAs quantum dots-in-a-well ( DWELL ) detectors are performed in the framework of effective-mass envelope- function theory. In contrast to InAs/ GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR ) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window ( 8 - 12 mu m ) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data. (c) 2005 American Institute of Physics.

Intersubband optical absorption in a step asymmetric semiconductor quantum well driven by a terahertz field

The nonlinear optical absorption in a three-subband step asymmetric semiconductor quantum well driven by a strong terahertz (THz) field is investigated theoretically by employing the intersubband semiconductor-Bloch equations. We show that the optical absorption spectrum strongly depends on the intensity, frequency, and phase of the pump THz wave. The strong THz field induces THz sidebands and Autler-Townes splitting in the probe absorption spectrum. Varying the pump frequency can bring not only the new absorption peaks but also the changing of the energy separation of the two higher-energy levels. The dependence of the absorption spectrum on the phase of the pump THz wave is also very remarkable.

Preparation and optical properties of Ag/PPy composite colloids

Ag/PPy (polypyrrole) composite colloids were prepared through the reaction of silver nitrate with pyrrole solution in DNIF either in the dark, or under the irradiation of femtosecond laser (fs) pulse or UV lamp. The UV-vis spectra of the nanocomposite colloid display an intense absorption band around 620 nm, accompanied by a weak one around 470 nm. The colors and optical absorption spectra of as-synthesized colloids can be reversibly tuned between blue and red, corresponding to absorption band of 620 urn and 526 urn, within few seconds by adding base and acid solutions or gases in turn into the composite colloid suspension. In addition, excess of H+ solution enhanced the absorption band around 470 nm and, at the same time, depressed that around 620 nm. The possible mechanism for the formation and optical absorption properties of the Ag/PPy composite colloid was proposed. (C) 2006 Elsevier B.V. All rights reserved.

Effects of annealing on the color, absorption spectra, and light yield of Ce : YAlO3 single crystal grown by the temperature gradient technique

YAlO3 single crystal doped with Ce3+ at concentration 1% was grown by the temperature gradient technique. The as-grown crystal was pink. After H-2 annealing or air annealing at 1400degreesC for 20 h, the crystal was turned into colorless. We concluded there were two kinds of color centers in the as-grown crystal. One is F+ center attributed to absorption band peaking at about 530 nm, the other is O- center attributed to absorption band peaking at about 390 nm. This color centers model can be applied in explaining the experiment phenomena including the color changes, the absorption spectra changes, and the light yield changes of Ce:YAP crystals before and after annealing. (C) 2004 American Institute of Physics.

First-principles study of the electronic structures and absorption spectra for the PbMoO4 crystal with lead vacancy

The electronic structures and absorption spectra for the perfect PbMoO4 crystal and the crystal containing lead vacancy V-Pb(2-) with lattice structure optimized are calculated using density functional theory code CASTEP. The calculated absorption spectra of the PbMoO4 crystal containing V-Pb(2-) exhibit three absorption bands peaking at 2.0 eV (620 nm), 3.0 eV (413 run) and 3.3 eV (375 nm), which are in good agreement with experimental values. The theory predicts that the 390 nm, 430 nm and 580 run absorption bands are related to the existence of V-Pb(2-) in the PbMoO4 crystal.

First-principles calculations of electronic structures and absorption spectra of YAlO3 crystals with F center

National Nature Science Foundation of China (Grant No. 60607015)

The plasmon resonance absorption of Ag/SiO2 nanocomposite films

The plasmon resonance absorption of the Ag/SiO2 nanocomposite film is investigated. The measured absorption spectra are compared with those calculated by the Mie theory. The results indicate that the Mie theory on the basis of classical electrodynamics can only partially explain the optical absorption spectra of the Ag/SiO2 nanocomposite film. We believe that the plasmon resonance absorption is mainly an intrinsic quality of the metal particle, and can be explained only with the electronic structure of the metal particle. In the latter, surface resonance state is introduced to systematically discuss the optical absorption spectra of the Ag/SiO2 nanocomposite film. (C) 2003 Elsevier Science B.V. All rights reserved.

Properties and turning of intraband optical absorption in InxGa1-xAs/GaAs self-assembled quantum dot superlattice

We investigated properties of intraband absorption in In-x Ga1-xAs quantum dots (QDs) superlattice. Energy levels in conduction band in QDs were calculated for a cone-shaped quantum dot associated with coupling between QDs in the framework of the effective-mass envelope-function theory. Theoretical results demonstrated that energy levels in conduction band were greatly affected by the vertical coupling between quantum dots, which can be used to modify transition wavelength by adjusting the space layer thickness. Intraband transition is really sensitive to normal incidence and the absorption peak intensity is dependent on the polarization. A satisfying agreement is found between theoretical and experimental values. This result opens up prospects for the fabrication of QDs infrared detectors, which work at atmospheric windows.

Fine structure of the plasmon resonance absorption peak of Ag nanoparticles embedded in partially oxidized Si matrix

Ag/Si nanocomposite films were prepared by the radio-frequency magnetron cosputtering method. The fine structure of the plasmon resonance absorption peak was found in film samples. X-ray photoelectron spectroscopy analysis indicated that the samples were composed of a two-layer structure, which accounted for the structure of the optical absorption spectra. The peak located near 445 nm is the plasmon resonance absorption peak of Ag nanoparticles embedded in a partially oxidized Si matrix. Its intensity decreases with decreasing film thickness and disappears in a very thin sample. The peak located near 380 nm originates from the plasmon resonance absorption of the thoroughly oxidized surface layer of the sample. Its intensity does not change with increasing thickness, but it cannot be observed in the very thick sample. (C) 2001 American Institute of Physics.

Absorption spectra of nanocrystalline silicon embedded in SiO2 matrix

Nanocrystalline silicon (nc-Si) embedded SiO2 matrix has been formed by annealing the SiOx films fabricated by plasma-enhanced chemical vapor deposition (PECVD) technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there is an "aUrbach-like" b exponential absorption in the spectral range of 2.0-3.0 eV. The relationship of (alpha hv)(1/2) proportional to(hv - E-g) demonstrates that the luminescent nc-Si have an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between electron states and hole states but also between shallow trap states of electrons and holes. (C) 2000 Elsevier Science B.V. All rights reserved.