Soft dynamics of the excited state: Lambda-shaped optical spectra

Anomalous optical spectra of centres with strongly reduced local elastic constants in the excited electronic state are studied. In this case, transitions take place to the vicinity of a flat minimum of the potential energy in the configurational coordinate space. The existence of a flat minimum results in the appearance of the resonant (pseudo-local) mode of a low frequency. This leads to a strong enhancement of the low-frequency part of the phonon wing, causing a formation of the possibly modulated lambda-shaped optical spectra; the modulation comes from the Airy oscillations of the quantum accelerated motion.     

Isotopic disorder effect in the infrared reflection spectra of LixLi1−xY F4 single crystals

Polarized infrared reflection spectra of LiY F₄ single crystals with different compositions of ⁷Li and ⁶Li isotopes were measured and analysed using the model of independent oscillators. Lattice dynamics calculations were performed. It was found that, for some modes, the contribution of the isotopic disorder to the damping constant is comparable to the anharmonic width. Results of simulations agree qualitatively with the experimental data. Possible reasons for the remaining discrepancies are discussed.     

Influence of oxidation on optical diffuse reflectance spectra in nanoscale silicon powder

Received: 5 June 1997/Accepted: 30 July 1997     

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

Optics of circularly polarized light waves in one-dimensional magneto-photonic crystals: Theory

A theory is presented for propagation of electromagnetic waves through one-dimensional magnetic Bragg structures (magneto-photonic crystals). Within the self-consistent Green-function technique the transfer matrices and magneto-optical characteristics are derived in terms of circularly polarized waves propagating in periodical arrays of alternating magnetic and dielectric layers. For finite-thickness magneto-photonic crystals, the Faraday rotation and other magneto-optical responses are demonstrated to change considerably in the spectral range of stop-bands, the magnetic modulation of the in-plane reflection intensity being essentially enhanced.     

An optical spectroscopy study of defects in lithium tantalate single crystals

The congruent, stoichiometric, and Mg doped stoichiometric LiTaO₃ single crystals have been successfully grown by the Czochralski technique. The evolution of defect structures caused by varying composition and post-growth processing has been evaluated from the optical absorption and photoluminescence measurements. Optical absorption studies showed that the UV absorption edge is very sensitive to the composition of LiTaO₃ crystals. Photoluminescence of various LiTaO₃ single crystals at room temperature was observed. The emission bands centered at 360, 430, and 530 nm were assigned to different defects, which can well show the defect information in LiTaO₃ crystals.     

Theory of vibronic optical spectra of impurity centers with violation of the Condon approximation

Expressions for non-Condon generating functions of absorption and luminescence of optical centers in crystals at zero temperature have been obtained in the adiabatic approximation. A solution to the Schrödinger equation for an electronic subsystem has been considered to the first order of the perturbation theory for a vibronic interaction linear in normal coordinates of the vibrational subsystem. The non-Condon form function of the optical transition has been obtained in the form of a convolution operator acting on the normalized Condon form function. It has been proved that, if the optical transition is forbidden in the Condon approximation due to symmetry, the non-Condon form function does not contain a zero-phonon line and the non-Condon form functions of absorption and luminescence are symmetric.     

The study of optical properties of amorphous thin films of mixed oxides In<Subscript>2</Subscript>O<Subscript>3</Subscript>—SnO<Subscript>2</Subscript> system,deposited by co-evaporation

A discussion of optical properties of mixed oxides In₂O₃—SnO₂ system is presented. Film thickness, substrate temperature, composition (in molar %) and annealing have a profound effect on the structure and optical properties of these films. Initially the increase in band gap with the increase of SnO₂ content in In₂O₃ is due to the increase in carrier density as a result of donor electrons from tin. The decrease in band gap above the critical Sn content is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in band gap with film thickness is caused by the increase in free carrier density which is generated by (i) Sn atom substitution of In atom, giving out one extra electron and (ii) oxygen vacancy acting as two electrons donor. The decrease in band gap with substrate temperature and annealing is due either to the severe deficiency of oxygen, which deteriorate the film properties and reduce the mobility of the carriers, or to the formation of indium species of lower oxidation state (In²⁺).     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

Engineering absolute band gap in anisotropic hexagonal photonic crystals

We analyze the band gap spectra of two-dimensional anisotropic photonic crystals created by a hexagonal lattice of rods covered by an interfacial layer (e.g. tellurium tubes). Using the plane-wave numerical expansion method, we study the modification of the band gap spectrum when the rods are infiltrated with other material, and discuss the optimization strategies leading to the maximum value of the absolute band gap.     

Band-gap properties of two-dimensional low-index photonic crystals

We study the band-gap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure. Using numerical plane-wave method, we calculate a minimum value of the refractive-index contrast for opening both partial and full two-dimensional spectral gaps for both TM- and TE-polarized waves. We also analyze the effect of ellipticity of rods and holes and their orientation on the threshold value and the relative size of the band gaps.     

Growth and characterization of a pure and doped nonlinear optical l-histidine acetate single crystals

Single crystals of pure, Cu²⁺and Mg²⁺ doped l-histidine acetate (LHA) were grown successfully by slow evaporation technique. The X-ray diffraction (XRD) studies were carried out for the pure and doped grown crystals. Absorption of these grown crystals was analyzed using UV-vis-NIR studies, and it was found that these crystals possess minimum absorption from 200 nm to 1500 nm. The pure and doped crystals are characterized by Fourier transform Raman (FT-Raman), thermal and photoconductivity studies. Vickers microhardness tests were carried out for the pure and doped crystals and the mechanical strengths were found. The dielectric constant and the dielectric loss with frequency were also studied.     

Transmission of XeCl excimer laser pulses through optical fibers: Dependence on fiber and laser parameters

Measurements of the transmission of XeCl excimer laser pulses (308 nm) through optical all-silica fibers are reported. The observed photodegradation effects during long time irradiation are dependent on the laser energy density and the laser repetition rate. Also the influence of the fiber drawing parameters and fiber geometry have been investigated. Absorption spectra of the fibers in the wavelength range from 200nm to 400nm before and after the irradiation with the excimer laser give some insight in the physical reasons for the changes in transmission properties. A spectral analysis of the fiber fluorescence has been performed.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi₂O₅ glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190-800nm. The fundamental absorption edge located around 340nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.     

Determination of size and concentration of copper nanoparticles dispersed in glasses using spectroscopic ellipsometry

For the generation of particular optical properties the melt of a commercially manufactured glass is doped with copper compounds. The glass obtained is opaque black at the usual thickness and looks dark red after making it into bulbs of incandescent lamps. It is generally assumed that copper particles cause this colouring. A proof in a spectrophotometric way fails due to the very high absorbance even for a sample thickness below 20 μm. It will be shown that in these cases spectroscopic ellipsometry is a suitable method of investigation. The pseudo-optical constants of this material were determined as a function of wavelength in the range from 350 nm to 700 nm by ellipsometric measurements. They can be reproduced very well by those of a model that consists of a roughness layer situated on a substrate of glass containing spherical copper particles with a Gaussian size distribution with =6.5 nm and σ=0.24 and a volume concentration of 2.4×10^-3. For this modelling the dielectric function of the roughness layer was approximated by Bruggeman effective-medium theory and that of the copper-containing glass substrate was calculated on the basis of the theory of Gans and Happel. The results were verified by transmission electron microscope investigations. Received: 1 July 2001 / Published online: 10 October 2001     

Absolute band gap properties in two-dimensional photonic crystals composed of air rings in anisotropic tellurium background

We analyze the band gap spectra of two-dimensional photonic crystals created by square, triangular and honeycomb lattices of air rings with different geometrical shapes and orientations in anisotropic tellurium background. Specifically, five different shapes of rings (circular, hexagonal, elliptical, rectangular and square) are considered. Using the numerical plane wave method, we discuss the maximization of the absolute photonic band gap width as a function of air ring parameters with different shapes and orientations in three types of lattices.     

Theory of photoinduced charge separation in solids

We discuss photoinduced ionization and charge separation kinetics in solutions as a way to conserve and utilize light energy. Our interest is in the secret of the high efficiency of reaction centers and the fast and irreversible separation of ions involved in reduction and oxidation reactions in the macromolecule. We present a new theory of reversible charge transfer, which shows high efficiency of charge separation: the charge's recombination has a weak influence on the donor cation-state probability's maximum value but a large effect on its long-time behavior. We give the comparison of new results with recently published ones on charge separation kinetics in condensed media. We use the methods of independent reaction “vessels” and “conditional concentrations”. The influence of back charge transfer is essential for long-time tail of the cation-state probability; its action on the cation-state probability's maximum value grows weak with an increase of forward rate constants and acceptor's concentration.     

Investigation on optical transmission spectra of (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 single crystals

The optical transmission spectra from 0.3 to 11 μm of relaxor ferroelectric single crystals (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) were systematically studied at room temperature in this paper. The crystal is transparent between 0.45 and 5.5 μm and becomes completely absorbing around 0.4 μm in near UV region and 10 μm in infrared region. But the wavelength cutoff in near UV is much sharper than the long wavelength cutoff. As compared with other configurations, tetragonal single crystals possess the optimal transmission properties. The optical transmittance in the wavelength region from 0.45 to 5.5 μm is about 70%. The results show that tetragonal PMN-xPT single crystals are promising for a wide range of optical applications. Some discussions about the oxygen-octahedra structure that determines the basic energy level of the crystals are also presented on the optical properties of PMN-xPT single crystals.     

Zero-phonon lines: Novel manifestations of vibronic interactions in impurity centres of solids

At low temperatures the zero-phonon lines (ZPL) in optical spectra of solids have a very small homogenous width and huge peak intensity. The lines are highly sensitive to the interactions in solids, allowing for a number of applications of the ZPL-based solid state spectroscopy. The standard theory predicts the Lorentzian shape of ZPL with T ⁷ dependence of its homogeneous width on T at low temperature. However, in the last years different systems have been found, in which remarkable deviations from these laws have been observed. Some generalizations of the standard theory, allowing one to explain the observed anomalous lines, are presented.     

The effect of the crystal field strength on the optical spectra of Cr3+ in gallium garnet laser crystals

We have investigated the basic spectroscopic properties of Cr³⁺-doped gallium garnets. Weak crystal fields (Dq/B=2.39–2.55) lead to broad⁴ T ₂⁴ A ₂ fluorescence in the 700–950 nm range depending on the chemical composition of the crystals. At room temperature the⁴ T ₂⁴ A ₂ transition is homogeneously broadened, whereas the 4 K spectra of the² E⁴ A ₂ transition show inequivalent Cr³⁺ sites and inhomogeneous linewidths of 8 cm^–1 in these laser crystals. From 300 to 4 K, the oscillator strength of the⁴ T ₂⁴ A ₂ transition is reduced by a factor of about 0.6 due to the temperature dependent electron phonon coupling. The transition probability of the² E⁴ A ₂ transition correlates with the energetic position of the⁴ T ₂ level, which is admixed into the² E level via spin-orbit coupling. The comparison of the² E⁴ A ₂ phonon sideband structure in garnets of different chemical compositions allows the assignment of some peaks to GaO₄-tetrahedron modes of the garnet structure.