Electronic Optical Transitions Research Articles

On self-focusing and defocusing of few-cycle pulses in hydrogen-containing ferroelectrics

The role of diffraction in the propagation of few-cycle pulses is studied in hydrogen-containing ferroelectrics in the case of the active role of optical electronic and tunnelling transitions. It is shown that, unlike optical electronic transitions, tunnelling proton transitions can produce the defocusing effect if they are overlapped by the spectral components of the pulse. The parameters of a medium and the pulse are estimated at which the defocusing effect dominates over self-focusing. It is shown that nonresonance quasi-monochromatic pulses in the optical range are subjected to self-focusing in such media.

Interband optical absorption in a small-radius quantized spherical film

The electronic conditions and direct optical transitions in a semiconductor quantized spherical film under ‘strong’ quantization regime are considered when the size of the system is smaller than that of a bulk exciton. A physically adequate model is suggested that allows to find analytically the energy spectrum and wave functions of the charge carriers in the layer. In an approximation of simple non-degenerate bands, the absorption coefficient for interband and ‘intersubband’–inband transitions is found. Along with the frequency dependence, the explicit dependence of the absorption coefficient on the geometrical size of the model is revealed.

Combined exciton–electron optical transitions in modulation doped QWs

Combined exciton–electron processes were found and studied in reflectivity and photoluminescence spectra taken from modulation-doped quantum well structures with different 2DEG densities. At low magnetic fields a Stokes shift between reflectivity and PL exciton and trion peaks due to the renormalization of the exciton and trion self-energy have been found. This shift disappears at high magnetic fields.

Drag effect of one-dimensional electrons upon photoionization of D(−) centers in a longitudinal magnetic field

A theory is elaborated for the impurity photon drag effect in a semiconductor quantum wire exposed to a longitudinal magnetic field B directed along the axis of the quantum wire. The phonon drag effect is associated with the transfer of the longitudinal photon momentum to localized electrons in optical transitions from D(−) states to hybrid-quantized states of the quantum wire, which is described by a confinement parabolic potential. An analytical expression for the drag current density is derived within the model of a zero-range potential in the effective mass approximation, and the spectral dependence of the drag current density is examined at different magnitudes of B and parameters of the quantum wire upon electron scattering by a system of impurities with short-range potentials. It is established that the spectral dependence of the drag current density exhibits a Zeeman doublet with a clear beak-shaped peak due to optical transitions of electrons from D(−) states to states with the magnetic quantum number m=1. The possibility of using the photon drag effect in a longitudinal magnetic field for the development of laser radiation detectors is analyzed.

Magnetooptical oscillations in bismuth at T≥77 K

The oscillations caused by optical interband electron transitions at the point L of the Brillouin zone of bismuth are studied in the temperature range of 77–280 K and magnetic fields up to B=22 T. It is shown that the method of simulation of the experimental line shape allows one not only to refine the band parameters of the material, but also to obtain detailed information about physical reasons for the specific features observed in the experimental magneto-optical spectra.

Electronic states and optical properties of silicon nanocrystals

Photoluminescence properties of nanometer Si-based materials have motivated a great deal of experimental and theoretical research effort because they exhibit favourable applications in opto-electronic devices. The quantum confinement effect of photoexcited carriers within nanocrystallites was mainly proposed to be responsible for the visible luminescence from these materials. In this work, the electronic states and optical transition properties of Si nanocrystals are studied by means of an sp3s* semiempirical tight-binding approximation and supercell model, in which the silicon nanocrystals are columns of square cross-section with width from a to 7a, where a is the lattice constant. The calculations have been carried out for light polarized in the [100] direction, i.e., perpendicular to the wire alignment. We present the dependence of the imaginary part of the dielectric function on the quantum confinement within two different schemes, which are applied and compared.

Proceedings of the Tenth International Conference on High Pressure Semiconductor Physics (HPSP‐X)

AbstractThe Tenth International Conference on High Pressure Semiconductor Physics (HPSP‐X) was held as a satellite meeting of the International Conference on the Physics of Semiconductors in Guildford from 5 to 8 August 2002. It brought together scientists and engineers who use high pressure to study the science and technology of semiconductors. The HPSP‐X conference reviewed the latest issues and developments in the physics of both bulk and low‐dimensional semiconductor materials and devices under pressure. It covered all aspects of fundamental and applied high‐pressure semiconductor research, including experimental and theoretical investigations under isotropic or anisotropic stress conditions. Topics such as electronic structures, vibrational and optical properties, transport phenomena, defect states, phase transitions, and novel materials or structures are included in this issue.Following the meetings HPSP‐VII in Schwäbisch Gmünd, phys. stat. sol. (b) 198, No. 1 (1996); HPSP‐VIII in Thessaloniki, phys. stat. sol. (b) 211, No. 1 (1998), and HPSP‐IX in Sapporo, phys. stat. sol. (b) 223, No. 1/2 (2001), the proceedings of this successful conference series are published in physica status solidi for the fourth time.

Free Energy Relationships for the Outer-Sphere Anion–Anion and Anion–Cation Optical Electron Transitions Involving Common Cyanometalate (II) and (IV) Donors: Evidence for the Frequency-Distorted Reorganization Dynamics

This work is an extension of the research project on the outer-sphere optical charge transfer (CT) initiated by Prof. R.R. Dogonadze at the Laboratory founded by him in Tbilisi, Georgia. We report the band deconvolution procedures and accomplished free energy relationships for recently observed novel outer-sphere anion–anion and anion–cation optical electron transitions involving common cyanometalate II and IV ions, viz. [M(CN)x]4–, M = Fe, Ru, Os (x = 6) and M = Mo, W (n = 8) as electron donors, and [FeIII(CN)6]3– and [RuIII(NH3)6]3+ as electron acceptors, respectively. The CT band maximum vs. the redox asymmetry free energy relationships, constructed through the consideration of spin-orbit splitting effects, exhibit linear character with slopes close to unity as predicted theoretically, and fully confirm the assignment of these transitions to outer-sphere optical electron transitions as outlined in the preceding works. The preliminary comparative analysis of presented relationships based on the contemporary charge transfer theory and recent differential DO-D stretching overtone data strongly suggests the essential redox asymmetry for the reorganization free energy increments attributed to first solvating shells of cyanometalate ions involved.

Effect of laser-induced antidiffusion on excited conduction electron dynamics in bulk semiconductors

By including the effect of local fluctuations in the electron kinetic energies, the kinetic Fokker-Planck-type equation for excited conduction electrons in bulk semiconductors (such as GaAs, Si, etc.) is systematically derived in the presence of a pulsed laser beyond the classical limit. A new contribution from an antidiffusion process is found as a correction to the spontaneous-phonon emission from drifting electrons in addition to contributions from joule heating and a field-dependent diffusion of electrons. The stimulated interband optical transitions of electrons from single-photon absorption are included as one of the source terms of the equation. Some possible types of damage in semiconductors including optical, electrical, and structural damage are explored. The calculated results demonstrate the existence of a kinklike feature in the electron distribution function around the edge of the conduction band due to antidiffusion. The energy spectra of the electron distribution function are studied at different times and used to analyze the transient behavior of both the conduction electron density and the hot-electron temperature.

Nonlinear effects of energy band structures on optical transitions in quantum dots

The quantum theory of nonlinear effects for optical transitions of electrons in quasi-zero dimensional (Q0D) quantum dots fabricated from n-type III–V compound semiconductor materials such as n-type GaAs has been studied due to the nonparabolicity of energy band structures. We use the effective mass approximation for carriers in the quantum dots. Most realistic quantum dot systems contain the box with a thickness c and the lateral width ( a, b). Using the time-independent perturbation theory, the first order correction of the eigenfunctions and eigenvalues for the system has been calculated. And the free-carrier absorption coefficient may be calculated for Q0D quantum dots from n-type GaAs where the polar optical phonon scattering is dominant. Our results show that the free-carrier absorption coefficient in Q0D quantum dots increases quite rapidly with increasing temperature in the region of low temperatures. When temperatures are larger than 100 K, the free-carrier absorption coefficient increases slowly with temperature. This shows that the nonlinear property of energy band structures due to the nonparabolicity plays an important role in low temperatures. The discussion about the dot size effect of the quantum confinement region in n-type GaAs quantum dots has also been given.

Optical Properties of Solid Pentacene

We have prepared pentacene films on Si, Al2O3, and glass substrates by thermal evaporation and have investigated their optical properties at room temperature over a wide range of frequencies from infrared to ultraviolet (5 meV to 6.5 eV). A series of optical phonon modes and electronic transitions have been observed. The internal vibrational modes in the infrared region match well their molecular counterparts but the electronic transitions show substantial changes from those of a free pentacene molecule. The HOMO–LUMO gap energy of the pentacene films deposited on Al2O3 and glass substrates is 1.85 eV.

Electronic states and optical transitions in small Si quantum boxes

Electronic and optical properties of small Si quantum boxes (QBs) with hydrogen saturators (referred to as a N x × N y × N z structure, where N x , N y and N z are the number of Si monolayers along the [100], [010] and [001] directions, respectively) are studied using the extended Hückel-type nonorthogonal tight-binding method. It is found that a clear transition between the bulk-like and surface-like conduction band (CB) edge states takes place alternately by changing N z , among others, in the 5×5× N z ( N z =5,7,9,…) QBs, resulting in an oscillation behavior in band gap and oscillator strength. The occurrence of the surface-like CB edge state, which is responsible for the enhanced oscillator strength, is found to be ascribed to an inter-hydride interaction between the trihydride units at the side of the (00 1 ̄ ) surface.

Electron confinement and optical enhancement inSi/SiO2superlattices

We have performed first-principles calculations of Si/SiO$_2$ superlattices in order to examine their electronic states, confinement and optical transitions, using linearized-augmented-plane-wave techniques and density-functional theory. Two atomic models having fairly simple interface structure are considered; they differ in the way dangling bonds at interfaces are satisfied. We compare our first-principles band structures with those from tight-binding calculations. The real and imaginary parts of the dielectric function are calculated at the Fermi-Golden-rule level and used to estimate the absorption coefficients. Confinement is confirmed by the dispersionless character of the electronic band structures in the growth direction. Optical enhancement is shown to exist by comparing the direct and indirect transitions in the band structures with the related transitions in bulk-Si. The role played by the interface on the optical properties is assessed by comparing the absorption coefficients from the two models.

Water- and Oxygen-Induced Decay Kinetics of Photogenerated Electrons in TiO2 and Pt/TiO2: A Time-Resolved Infrared Absorption Study

Electrons photogenerated in TiO2 and Pt/TiO2 catalysts were observed by time-resolved IR absorption spectroscopy. Transient IR absorption of an identical spectrum appeared on TiO2 and Pt/TiO2 irradiated by a 355 nm pump pulse. The absorption was attributed to optical transition of photogenerated electrons trapped in shallow midgap states. Electron- and hole-consuming reactions of adsorbates controlled the decay kinetics of the electrons competing with the electron−hole recombination. On TiO2, O2 from the gas-phase captured the electrons and accelerated the decay rate at a delay time of 10−100 μs. In water vapor, holes reacted with surface hydroxyls within 2 μs, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of the water splitting reaction affected the decay kinetics in different time domains. Photogenerated holes oxidized water within 2 μs, whereas electrons reduced water at 10−900 μs.

Terahertz injection electroluminescence in multiperiod quantum-cascade AlGaAs/GaAs structures

Terahertz electroluminescence in the range ≈1.5 THz was observed in a quantum-cascade GaAs/AlGaAs structure containing 40 periods of tunnel-coupled wells. The luminescence is caused by the spatially indirect optical electron transitions between the ground states of neighboring quantum wells.

Enhanced intraband Stark effects in stacked InAs/GaAs self-assembled quantum dots

We present a theoretical study of the electronic properties and intersubband optical transitions in vertically aligned double InAs self-assembled quantum dots (QDs) which are subject to an electric field along their growth axis. The electron properties are calculated as a function of the applied electric field by using an eight-band strain-dependent k⋅p Hamiltonian. Transitions between ground s states and excited p states are found to be almost three times stronger than in single dot, with strong field anisotropy. The system also exhibits field tunable transitions between the bonding and antibonding s states, with polarization along the growth axis. Midinfrared photodetectors consisting of vertically coupled double-quantum-dot layers are expected to exhibit enhanced sensibility and voltage tunability, compared to devices using single-quantum-dot layer.

Linear Interaction (Transformation) of de Broglie Waves of the Γ- and X-Valleys and Intraband Optical Transitions in GaAs/AlAs Quantum-Well Heterostructures in a Lateral Electric Field

We discuss the linear interaction between the de Broglie waves of different electron valleys in heterostructures where a constant electric field is applied along the heterointerfaces (lateral field). The probabilities of phononless and dipole optical transitions of electrons between the Γ- and X-subbands in GaAs/AlAs quantum-well heterosructures with a lateral electric field are calculated. It is shown that the electric field has a strong effect on the probabilities of phononless and direct optical dipole Γ-X electron transitions. Moreover, a lateral electric field changes the spectrum of the intersubband Γ-X light absorption, i.e., there is an intersubband analogue of the Franz-Keldysh effect. The considered linear interaction of de Broglie waves is compared with the well-known cases of linear wave interaction in a magnetized plasma, the Zener breakdown, and the Franz-Keldysh effect in solids.

Interaction of intense chirped pulses with molecules in condensed phase as electron transfer between “moving” potential

A nonperturbative analytic approach to the problem of the interaction of high-power chirped pulses with molecular systems: the picture of “moving potentials” (J. Chem. Phys., 109 (1998) 4523) has been generalized to different relaxation times in the ground and in the excited electronic states. The calculation results agree qualitatively with the experimental results by Shank et al. and Bardeen, Wilson et al. An electronic optical transition induced by chirped pulses, can be considered as an electron transfer reaction between a “moving photonic replication” and the corresponding term occuring at their instantaneous intersection. This clear picture explains even fine details of the behaviour of the excited state population as a function of the chirp rate.

In vivo optical/near-infrared spectroscopy and imaging of metalloproteins

A number of medical applications of near-infrared spectroscopy are growing closer to clinical acceptance, and new techniques involving both spectroscopy and imaging are evolving rapidly. In vivo spectroscopy and, more recently, imaging techniques are largely based upon optical electronic transitions involving the metal centers of hemoglobin (blood), myoglobin (muscle) and cytochrome aa 3 (mitochondria). The wide variety of near-IR based applications includes heart and stroke research, monitoring cerebral oxygenation of premature babies, and ‘functional activation’ (response of brain to mental tasks). All of these applications are founded upon changes in hemoglobin O 2 saturation; these changes are monitored by following trends in the near-infrared absorptions of deoxyhemoglobin (760 nm) and oxyhemoglobin (920 nm). The same absorptions provide a basis for imaging regional variations in blood oxygenation. This report presents and discusses examples, both from the literature and from our recent work, of near-infrared spectroscopy and imaging in medical applications.

A Theory of Electron Transfer and Steady-State Optical Spectra of Chromophores with Varying Electronic Polarizability

Electron transfer (ET) reactions and optical transitions are considered in chromophores with both the dipole moment and the electronic polarizability varying with the transition. An exact solution for reaction free energy surfaces of ET along a reaction coordinate has been obtained in the Drude model for the solute and solvent polarizabilities. The ET surfaces manifest the following effects of a nonzero polarizability variation: they (i) are asymmetric, (ii) have different curvatures at their minima, and (iii) become infinite for reaction coordinates outside a one-sided fluctuation band. The physical origin of these effects is the renormalization of the solute dipole by the solvent reaction field depending on the nonequilibrium solvent configuration. The reorganization energies of ET are substantially different for forward and backward transitions with higher reorganization energy in the state with hider solute polarizability. The dependence of the ET rate on the equilibrium energy gap is quadratic near the rate maximum and is linear at large energy gaps. The energy gap curves are also flatter from the side of exothermic reactions and broader for the state with a higher polarizability. The bandshape analysis of optical transitions is extended to the case of a nonzero polarizability variation.