Transfer Of Oscillator Strength Research Articles

Giant LO oscillation in the Zn 1− xBe x(Se,Te) multi-phonons percolative alloys

We enrich a percolation-based picture for the basic understanding of the atypical two-modes behavior observed by Raman scattering in the Be-VI optical range of Zn 1− x Be x (Se,Te) alloys, with contrast in the bond stiffness. The attention is focused on the longitudinal optical (LO) Be-VI spectral region within the percolation regime (0.19< x<0.81). Three apparent anomalies are discussed. First the low-frequency component is systematically overdamped. Also, the high-frequency component exhibits a marked red-asymmetry which goes with an apparent blue-shift with respect to the theoretical predictions derived on a conventional one-bond↔one-mode basis. All three apparent anomalies are accounted for by considering a discrete multi-mode description for each of the low- and high-frequency Be-VI components. They basically arise from inter- and intra-component transfer of oscillator strength resulting in the building up of a quasi-unique giant LO oscillation. The transfer comes from coupling via the common LO macroscopic polarization field.

Mechanism of Cph1 phytochrome assembly from stopped-flow kinetics and circular dichroism.

The kinetics and mechanism of the autocatalytic assembly of holo-Cph1 phytochrome (from Synechocystis) from the apoprotein and the bilin chromophores phycocyanobilin (PCB) and phycoerythrobilin (PEB) were investigated by stopped flow and circular dichroism. At 1:1 stoichiometry, pH 7.9, and 10 degrees C, SVD analysis of the kinetic data for PCB revealed three spectral components involving three transitions with time constants tau(1) approximately 150 ms, tau(2) approximately 2.5 s, and tau(3) approximately 50 s. Tau(1) was associated with a major red shift and transfer of oscillator strength from the Soret region to the 680 nm region. When the sulfhydryl group of cysteine 259 was blocked with iodoacetamide, preventing the formation of a covalent adduct, a noncovalent red-shifted complex (680 nm) was formed with a time constant of 200 ms. Tau(1) could thus be assigned to the formation of a noncovalent complex. The absorption changes during tau(1) are due to the formation of the extended conformation of the linear tetrapyrrole and to its protonation in the binding pocket. From the concentration and pH dependence of the kinetics we obtained a value of 1.5 microM for the K(D) of this noncovalent complex and a value of 8.4 for the pK(a) of the proton donor. The tau(2) component was associated with a blue shift of about 25 nm and was attributed to the formation of the covalent bond (P(r)), accompanied with the loss of the 3-3' double bond to ring A. Tau(3) was due to photoconversion to P(fr). For PEB, which is not photochromic, the formation of the noncovalent complex is faster (tau(1) = 70 ms), but the covalent bond formation is about 80 times slower (tau(2) = 200 s) than with the natural chromophore PCB. The CD spectra of the PCB adduct in the 250-800 nm range show that the chromophore geometries in P(r) and P(fr) are similar to those in plant phytochrome. The opposite rotational strengths of P(r) and P(fr) in the longest wavelength band suggest that the photoisomerization induces a reversal of the chirality. The Cph1 complex with noncovalently bound PCB was still photochromic when cysteine 259 was blocked with IAA or with the bulkier IAF. The covalent linkage to cysteine 259 is thus not required for photoconversion. The CD spectra of the noncovalently bound PCB in P(r)- and P(fr)-like states are qualitatively similar to those of the covalent adducts, suggesting analogous structures in the binding pocket. The noncovalent interactions with the binding pocket are apparently sufficient to hold the chromophore in the appropriate geometry for photoisomerization.

3d photoionization along the xenon isoelectronic sequence

The relativistic time-dependent density functional theory has been applied to3d photoionization along the Xe isoelectronic sequence, from I− to La3+.It is seen that interchannel coupling between the two spin–orbit photoionizationchannels greatly alters their partial photoionization cross sections. Changes similar tothose already observed for the Xe system are seen to affect the I−3d5/2partial cross section. For the elements following the noble gas in the isoelectronicsequence, Rydberg series of autoionization resonances associated withexcitations into the closed channels 3d3/2 → np1/2,3/2;n′f5/2are seen to strongly modulate the total and partial 3d5/2cross sections between the two spin–orbit split thresholds. Moreover, the analysisof the transfer of oscillator strength from the continuum to the discrete part of thespectrum along the series has permitted us to follow the occurrence of the collapseof the 4f wavefunction with a 3d hole, which is seen to occur already in Cs+.A global picture emerges from the comparison of 3d and 4d ionization in neutralsystems and Xe isoelectronic series.

Coulomb correlations in a two-dimensional electron gas in large magnetic fields

We present an investigation of the dynamics of the inter-Landau-level (LL) excitations of a two-dimensional electron gas (2DEG) in large magnetic fields using coherent time-resolved nonlinear spectroscopy. The results are compared directly with measurements on undoped quantum wells. We observe time-dependent Coulomb coupling between the LL's induced by the 2DEG that induces a large transfer of oscillator strength to the lowest LL. The time dependence of the nonlinear response reveals non-Markovian and memory effects of the photoexcited system that cannot be understood in terms of the random phase approximation. We introduce a theoretical approach that treats the interactions of the magnetoexcitons with the 2DEG excitations and qualitatively accounts for the most salient experimental results in terms of shake-up of the 2DEG.

Dynamic response of artificial bipolar molecules

We calculate the equilibrium properties and the dynamic response of two vertically coupled circular quantum dots populated by particles of different electrical charge sign, i.e. electrons and holes. The equilibrium density profiles are obtained and used to compute the frequencies and oscillator strengths of magnetoplasma excitations. We find a strong coupling between the modes derived from the center-of-mass modes of the individual dots which leads to an anticrossing with a pronounced oscillator strength transfer from the ``acoustic'' to the ``optical'' branch. Also, due to breaking of the generalized Kohn theorem a number of other than center-of-mass modes are excited whose oscillator strengths, however, are rather weak.

Dynamics of inter-landau-level excitations of a two-dimensional electron gas in the quantum Hall regime.

The femtosecond inter-Landau-level dynamics of a two-dimensional electron gas in a large magnetic field is investigated by degenerate four-wave mixing on modulation doped quantum wells. We observe a large transfer of oscillator strength to the lowest Landau level, and unusual dynamics due to Coulomb correlation. We interpret the effects using a model based on shakeup of the electron gas.

Optical Properties of InAs/InP Planar Quantum Dot Microcavities

ABSTRACTPlanar InAs/InP quantum dot microcavities using multi-layer SiO2/Ta2O5 Bragg reflectors have been studied in emission. The spectra exhibit collection optics-limited cavity linewidths of ∼1meV with the occasional ∼200μeV single-dot emission. Measurements as a function of incident power show quantum dot saturation behavior, with transfer of oscillator strength to the wetting layer outside the cavity stop band. Saturation behavior at fixed pump power is also observed as a function of decreasing temperature. Dispersion measurements as a function of emission angle show polarization splitting in qualitative agreement with theory.

Negatively charged polaritons in a semiconductor microcavity

We study by reflection spectroscopy the cavity polaritons in a structure consisting of a single GaAs/AlAs quantum well that contains a low density ${(n}_{e})$ two dimensional electron gas, and is embedded in a \ensuremath{\lambda}-wide ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ microcavity (MC). For ${n}_{e}&lt;5\ifmmode\times\else\texttimes\fi{}{10}^{10} {\mathrm{cm}}^{\ensuremath{-}2},$ negatively charged MC polaritons are photoexcited, as a result of the strong coupling of the MC photon and the negatively charged $[(e1:\mathrm{hh}1)1S+e]$ exciton ${(X}^{\ensuremath{-}}).$ The charged polaritons have several properties that are distinct from those of neutral polaritons [which are formed from the neutral $(e1:\mathrm{hh}1)1S$ (X) and $(e1:\mathrm{lh}1)1S$ excitons] (a) The MC-photon--${X}^{\ensuremath{-}}$ coupling strength increases as $\sqrt{{n}_{e}}.$ This is analogous to the dependence of the confined-photon--atom coupling on the density of free atoms in a metallic cavity. (b) The charged polaritons have a nonvanishing electric charge that is due to the bare ${X}^{\ensuremath{-}}$ charge. (c) Since the energy difference between the bare X and ${X}^{\ensuremath{-}}$ excitons is smaller than the coupling strength of each one with the MC photon, these two bare exciton states are admixed in the charged polariton states. The experimental reflection spectra were analyzed using a model of coupled quantum oscillators representing the excitons and the confined photon mode. From the fitted spectra it is deduced that the ${X}^{\ensuremath{-}}$ coupling strength increases with increasing ${n}_{e},$ and there is an oscillator strength transfer from X to ${X}^{\ensuremath{-}}.$ Using the cavity polariton wave functions (which are obtained from the model fitting) we calculate the effective charge and mass of all the cavity polaritons as a function of the MC-photon energy. The calculated ${(e/m)}_{\mathrm{eff}}$ ratio reaches a value $\ensuremath{\sim}200$ times larger than that of the free electron in a bare GaAs quantum well. Using the calculated dependence of the effective polariton charge and mass on the in-plane wave vector and the detuning energy, the maximum distance that the charged polariton can drift under an applied electric field is calculated. The charged polariton is expected to drift a distance at least 10 times larger than the bare ${X}^{\ensuremath{-}}.$

Crossover from Exciton to Biexciton Cavity Polaritons

We report novel oscillator strength transfer phenomena induced by the exciton–exciton correlation. A high quality semiconductor microcavity is resonantly excited by intense circularly polarised σ+ pump pulses (100 fs long) and probed at different delays by weak counter-polarised σ— test pulses. We show that the presence of σ+ excitons can completely inhibit the creation of σ— excitons. With increasing σ+ exciton density the oscillator strength of the σ— transition is transferred to the biexciton transition, permitting the first observation of the strong coupling regime between the biexciton transition and the cavity photon mode, i.e., the biexciton polariton states.

Visible-near ultraviolet ellipsometric study of Zn1−xMgxSe and Zn1−xBexSe alloys

We report pseudodielectric function data 〈ε〉=〈ε1〉+i〈ε2〉 of Zn1−xMgxSe and Zn1−xBexSe samples grown on GaAs substrates. The data were obtained from 1.5 to 6.0 eV using spectroscopic ellipsometry. Critical point parameters were obtained by fitting model line shapes to numerically calculated second energy derivatives of 〈ε〉, from which the bowing parameters and spin-orbit-splitting Δ1 of the E1 and E1+Δ1 gaps were obtained. A transfer of oscillator strength from E1+Δ1 to E1 with increasing Mg and Be composition and a positive bowing of these threshold energies are attributed to the k-linear interaction, which is large in small-band gap semiconductors.

The evolution of 4d photoabsorption in Sb with increasing ionization

The photoabsorption spectra of Sb+, Sb2+, Sb3+ and Sb4+ have been obtained in the XUV spectral region with the dual laser-produced plasma (DLP) technique. Photoexcitation from the 4d subshell is the dominant process in the 30-100 eV energy region. In Sb+, for photon energies between 30 and 50 eV, discrete structure corresponding to 4d np, n1f (n5,n14) transitions is obtained, while above 50 eV the spectrum is dominated by an intense 4df resonance. Apart from the 4d5p transitions the features are very weak. The 4df resonance is still observed in Sb2+ as the dominant feature in the spectrum, but in Sb3+ and Sb4+ there is significant transfer of oscillator strength from the continuum to the discrete spectra, in particular to the 4d nf transitions. This behaviour mirrors the effects of contraction of the 4f wavefunction, which is seen to be more gradual here than in elements with Z>54. Multiconfiguration Hartree-Fock and time-dependent local density approximation calculations successfully reproduce this behaviour.

Oscillator strength transfer fromXtoX+in a CdTe quantum-well microcavity

We report on a study of oscillator strength transfer from the neutral exciton X to the positively charged exciton ${X}^{+},$ in a modulation doped CdTe quantum well embedded in a microcavity. The strong-coupling regime between X or ${X}^{+}$ and the cavity photon mode has been analyzed for various hole densities p and the oscillator strengths ${f}_{X}$ and ${f}_{{X}^{+}}$ are derived accurately from polariton line energies. For increasing p, ${f}_{{X}^{+}}$ increases and ${f}_{X}$ is shown to decrease linearly with ${f}_{{X}^{+}}.$ We propose a model that accounts for those oscillator strength variations. This model relies on an effective cross section ${S}_{{X}^{+}}=6.6\ifmmode\times\else\texttimes\fi{}{10}^{4} {\mathrm{\AA{}}\mathrm{}}^{2}$ for the creation of ${X}^{+}$ in the vicinity of a hole, and a saturation density equal to ${10}^{11} {\mathrm{cm}}^{\ensuremath{-}2}$ for phase-space filling effect.

Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

We have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent \ensuremath{\pi}-conjugated polymer films in the spectral range of 1.5--4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenylene ethylene), and polythiophene; the nonluminescent polymers include poly(diethynyl silane) and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton ${(1B}_{u})$ and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states ${(A}_{g}),$ the strongest of which ${(mA}_{g})$ is located at an energy about 1.3 times that of the ${1B}_{u}$ exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent ${A}_{g}$ state ${(kA}_{g})$ at an energy of about 1.6 times that of the ${1B}_{u}.$ We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, $\mathrm{Im}[{\ensuremath{\chi}}^{3}(\ensuremath{-}\ensuremath{\omega};\ensuremath{\omega},0,0)],$ using a summation over states model dominated by the ground state, the ${1B}_{u}$ exciton, two strongly coupled ${A}_{g}$ states (${\mathrm{mA}}_{g}$ and ${\mathrm{kA}}_{g}$), and their most strongly coupled vibrations, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results in a distribution of excited state energies, was also incorporated into the model. The decomposition of the EA spectra due to the conjugation length distribution was then used to calculate the ${1B}_{u}$ exciton polarizability $(\ensuremath{\Delta}p)$ using first derivative analysis. For the longest conjugation lengths in our films, we found $\ensuremath{\Delta}p$ to be of order ${10}^{4}(\mathrm{\AA{}}{)}^{3}$ in luminescent polymers and ${10}^{3}{\mathrm{\AA{}}}^{3}$ in nonluminescent polymers, respectively, in good agreement with recent subnanosecond transient photoconductivity measurements. We also found that the Huang-Rhys parameter of the ${1B}_{u}$ exciton varies between 0.25 and 0.9, being in general smaller for the luminescent polymers. The consequent exciton relaxation energies were calculated to be of order 100 meV.

Quantum Wires and Dots Fabricated by Cleaved Edge Overgrowth

Recent progress in the fabrication of atomically precise quantum wires and quantum dots by cleaved edge overgrowth is reviewed. This technique is based on molecular beam epitaxy along two or three directions in space and is capable of producing low-dimensional structures in which all relevant sizes are controlled on an atomic scale. Different sample structures consisting of isolated quantum dots as well as of linear arrays of coupled quantum dots are studied by micro photoluminescence spectroscopy. Optical emission from zero-dimensional states in these dots is characterized by extremely narrow linewidths of less than 70 μeV. Resonant excitation of one of the dot levels in coupled systems leads to emission from all the other levels. As a result of the quantum mechanical coupling of the almost identical dots anti-Stokes luminescence, i.e. thermally activated transfer of oscillator strength from one dot to another, whose emission is slightly higher in energy, is observed.

The dielectric function of some low-dimensional Si and Ge structures

The optical properties of Si structures, such as hydrogen-terminated Si quantum wires, Si quantum wires embedded in Ge, and Ge quantum wires embedded in Si, are studied in a unified and consistent way, using an accurate and efficient microscopic electronic structure method. We investigate the dielectric function in a broad energy range. For the hydrogen-terminated wires we observe a splitting of the main peak and near the fundamental bandgap some narrow and strong peaks. For the embedded wires the changes are less dramatic, the main characteristic being a considerable transfer of oscillator strength to lower energy.

3d absorption spectra of Sr I through Sr IV.

The extreme ultraviolet photoabsorption spectra of neutral to three-times-ionized strontium have been recorded in a comprehensive series of experiments with the dual laser-produced plasma technique. Striking differences were found in the spectra, which can be attributed to the transfer of oscillator strength from 3d\ensuremath{\rightarrow}np to 3d\ensuremath{\rightarrow}nf transitions at ${\mathrm{Sr}}^{2+}$ due to nf wave-function contraction. In Sr and ${\mathrm{Sr}}^{+}$, 3d\ensuremath{\rightarrow}5p transitions dominate; in ${\mathrm{Sr}}^{2+}$, 3d\ensuremath{\rightarrow}nf transitions are most intense, while in ${\mathrm{Sr}}^{3+}$ the 4p subshell opens and 3d\ensuremath{\rightarrow}4p transitions are the strongest features. Partial cross sections for 3d\ensuremath{\rightarrow}\ensuremath{\varepsilon}f and 3d\ensuremath{\rightarrow}\ensuremath{\varepsilon}p photoionization were calculated and compared with experiment.

Influence of excess electrons and magnetic fields on Mott-Wannier excitons in GaAs quantum wells

Abstract We describe an experimental study of the interband optical properties of GaAs quantum wells as a function of their excess electron density and an applied magnetic field. With a negligible density of excess electrons in the well, the spectra show sharp resonances due to neutral Mott-Wannier-type excitons X. Upon increasing the density slightly the excitons capture an excess electron to form negatively charged excitons X-. The transition energy separation of X and X- agrees with the expected binding energy of the second electron. At a density roughly consistent with that determined for a homogeneous well, we observe a sharp transfer of oscillator strength from X to X-. With further density increase, X is completely quenched from the spectra, while X- evolves adiabatically into the Fermi-edge singularity observed for dense electron gases. Application of a magnetic field perpendicular to the well causes a large enhancement of the second electron binding energy of X-, due to the enhanced Coulomb inter...

Luminescence of deep phosphorous and arsenic impurities in ZnSe at high pressure.

High-pressure photoluminescence (PL) is reported for bulk and thin-film ZnSe containing ${10}^{18}$--${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ P and As acceptors using a cryogenic (4--300 K) diamond-anvil cell to 55 kbar. We focus on the deep PL bands in the red and green regions of the visible spectrum; the red emission bands (1-atm peaks at \ensuremath{\sim}6000 A in ZnSe:P and \ensuremath{\sim}7000 A in ZnSe:As) are thought to stem from ${\mathit{C}}_{3\mathit{v}}$-distorted acceptor sites. With increasing pressure in the range studied, the peak energies of all the deep bands approach but do not cross the ZnSe band gap. The average deformation potential of the valence edge relative to the P or As deep levels attributed to ${\mathit{C}}_{3\mathit{v}}$ sites is -1.2\ifmmode\pm\else\textpm\fi{}0.3 eV. The negative sign is confirmed by thermal quenching data at different pressures in ZnSe:P. These results imply that with pressure the valence edge shifts to higher energy, causing deep acceptor states to become less stable. We also observe under pressure strong transfer of oscillator strength from red to green emission, and several deep bands. However, since the shallow acceptor-bound exciton PL is not enhanced, we conclude that a deep-to-shallow transition has not occurred within the pressure range studied.

Collective electronic excitations in metal-coated C60.

A two-shell jellium-on-jellium model has been used to study the collective electronic excitations of ${\mathrm{C}}_{60}$ coated by an increasing number N of Na atoms. We predict a transition from the \ensuremath{\pi}-electron polarization of the fullerene to the metallic sodium polarization as a function of increasing N. For low coverages, the Na layer produces only a broadening and fragmentation of the \ensuremath{\pi} plasmon of ${\mathrm{C}}_{60}$. However, if the coverage is large enough, a Na surface plasmon appears. This occurs only after the electron density of the cluster clearly shows two distinct regions, the outer one having an ``average'' density similar to that in pure Na metal or Na clusters. The induced density at the collective-mode frequency shows structure corresponding to two interfaces, ${\mathrm{C}}_{60\mathrm{\ensuremath{-}}}$Na and Na vacuum, the first peak becoming less pronounced as the coverage increases due to a transfer of oscillator strength to the metallic counterpart. The static polarizability per electron also shows this trend and tends slowly to the Na value from below after an initial sharp rise in the region N\ensuremath{\sim}13--21.

Anomalous temperature- and doping-induced changes in the c-axis apical-oxygen phonon mode of Pb2Sr2RCu3O8.

We observe via polarized far-infrared-reflectance measurements of single-crystal samples of ${\mathrm{Pb}}_{2}$${\mathrm{Sr}}_{2}$R${\mathrm{Cu}}_{3}$${\mathrm{O}}_{8}$, where R is a rare-earth element, significant doping and temperature-induced changes in two c-axis phonons assigned to motion within the structural block containing the double ${\mathrm{CuO}}_{2}$ planes. In the normal state both phonons shift to lower frequency as the level of doping increases, with a monotonic transfer of oscillator strength from the lower [bending motion along c of the planar oxygen, O(3)] to the higher [vibrations of the apical oxygen, O(1)]. This doping-induced transfer of oscillator strength to the apical oxygen, accompanied by a sudden increase in asymmetry in the normal-state line shape of metallic samples which exhibit superconductivity at low temperatures, suggests that this process may play a role in the mechanism leading to the superconducting pairing. The linewidth of these two phonons shows a maximum as a function of doping near which, upon entry into the superconducting state, there is further anomalous broadening. A third phonon, attributed to vibrations of oxygen within the O(2)-Cu(1)-O(2) ``sticks'' and thus spatially removed from the ${\mathrm{CuO}}_{2}$ planes, shows no unusual behavior. These observations are consistent with a strong interaction between the phonons and the carriers of the structural block containing the ${\mathrm{CuO}}_{2}$ planes.