Showing papers on "Stark effect published in 1988"

Nonequilibrium theory of the optical Stark effect and spectral hole burning in semiconductors.

Abstract: We consider the influence of intense coherent laser fields on the electronic and optical properties of semiconductors. Using nonequilibrium Green's-function techniques and exploiting the analogies to superconducting and Bose-condensed systems, we discuss the nature of the renormalizations and the collective excitations in the collisionless regime. Experimentally, this situation can be realized (i) under nonresonant excitation of virtual electron-hole pairs and (ii) under resonant excitation with ultrashort pulses. We explain the recently observed optical Stark effect as well as spectral hole burning and derive from first principles the longitudinal and transverse dielectric functions including exciton correlations.

Vibrational Stark effect of CO on Ni(100), and CO in the aqueous double layer: Experiment, theory, and models

Abstract: The vibrational Stark effect (VSE), the effect of applied electrostatic field on a molecule’s vibrational frequency, is observed for adsorbates in ultrahigh vacuum (UHV) and in the electrochemical double layer. The VSE has been measured and compared with theory. Models of the double layer have also been compared with experiment. The vibrational Stark tuning rate of c(2×2) CO on Ni(100) at 300 K was measured using IR spectroscopy. The observed Stark tuning rate in terms of the applied E field δνE was (5.27±0.27)×10−7 cm−1/(V/cm). Quantum mechanics has been used to express δνE in terms of a molecule’s potential energy function and dipole moment function. The quantum theory confirms the accuracy of δνE predicted by a classical model. The theory, with potential energy and dipole moment functions observed for the experimental system, gives δνE =(4.7±1.1)×10−7 cm−1/(V/cm), in agreement with the present measurement. The fully resolved CO vibrational line shape was asymmetric with a 16 cm−1 full width at half‐max...

Quantum electrodynamics of an atom making two-photon transitions in an ideal cavity

Abstract: It is shown that a single atom making two-photon transitions in an ideal cavity (Q = ∞) in the presence of an initial coherent state field will, as expected, continue to show the collapses and revivals of atomic inversion and the intensity–intensity correlation of the field; however, these collapses and revivals are both compact and regular, in contrast to the one-photon case. This is because, although the several Rabi frequencies involved are incommensurate, for the two-photon case, unlike for the one-photon problem, they become commensurate in the limit of intense field. We derive analytic expressions for the atomic inversion and the intensity–intensity correlation in the case of direct, as well as homodyned, detection of the intracavity field. The effects of the Stark shift are discussed.

Electroabsorption of highly confined systems: Theory of the quantum‐confined Franz–Keldysh effect in semiconductor quantum wires and dots

Abstract: Semiconductor quantum wells are known to show large electroabsorption (e.g., the quantum‐confined Stark effect) that results in low‐energy optical modulating and switching devices. We show theoretically that the electroabsorption and associated electrorefraction in lower dimensional structures could be much larger, suggesting very low energy devices. We illustrate the theory with specific calculations for hypothetical GaAs‐like quantum wires and dots.

Indirect-direct anticrossing in GaAs-AlAs superlattices induced by an electric field: Evidence of Gamma -X mixing.

Abstract: We demonstrate that a GaAs-AlAs superlattice can be switched from indirect to direct, in both real and reciprocal spaces, by application of a modest axial electric field The crossover region is found to be an anticrossing, manifesting the presence of GAMMA-X mixing by a potential measured to be of the order of 1 meV This value is corroborated by time-decay measurements performed on different superlattices

Time-dependent theory of multiphoton ionization of xenon

Abstract: Calculations of the single and multiphoton ionization of xenon valence-shell electrons are reported for several wavelengths and laser intensities. The model follows a single valence electron in the field of an effective core potential, the remaining valence electrons, frozen in their ground-state orbitals, and a linearly polarized laser field. Ionization rates and cross sections are determined using a direct numerical solution of the time-dependent Schr\"odinger equation using a finite-difference technique. Excellent agreement with experimental rates and previous theoretical cross sections has been obtained. Bound states, shifted into resonance by the ac Stark effect, are found to affect the ionization dynamics. Departures from perturbation theory at high intensities are demonstrated and discussed.

Computer modeling of the electric field dependent absorption spectrum of multiple quantum well material

Abstract: The authors present a simple computer model for the electric field dependence of the absorption of semiconductor multiple-quantum-well (MQW) structure that will be used to optimize the performance of MQW modulators. This model has been compared to absorption spectra derived from photocurrent measurements on a GaAs/(GaAl)As MQW p-i-n diode and it has been found that the well-established Stark shifts of the exciton and subband continua energies are significantly overestimated. This might be linked to uncertainty in knowing the electric fields over the wells; and if a drop of 1.4 V somewhere in the device is assumed, a much better match can be achieved between the theoretical and experimental shifts. Given this improved match in the shifts the reduction in the oscillator strengths and the broadening are modeled very well. It is concluded that the model is likely to prove a useful tool for optimizing electroabsorption modulator design. >

Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers.

Abstract: The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, [unk]Δμ[unk], and the angle [unk] between Δμ and the transition dipole moment for the electronic transition. [unk]Δμ[unk] and [unk] for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. [unk]Δμ[unk] for this transition is substantially larger than for the Qy transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of charge-transfer states by Stark effect spectroscopy.

Infrared spectroscopy of the hydrogen cyanide dimer

Abstract: Sub‐Doppler resolution infrared spectra have been obtained and assigned for four vibrational bands of the hydrogen cyanide dimer, namely the ν1 and ν2 C–H stretching fundamentals and two hot bands. Accurate molecular constants are reported for all of these. In the case of the free C–H stretch (ν1 ), Stark spectra have also been recorded in order to determine the dipole moments of this complex in both the ground and vibrationally excited states. The linewidths obtained for the individual rovibrational transitions indicate that the vibrational predissociation lifetime of this complex is highly mode specific.

Transient and steady-state optical nonlinearities in semiconductors

Abstract: Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed. A simple description of the physical processes underlying the nonlinearities is given. The discussion is focused on the spectral region around the fundamental absorption edge, and it covers coherent oscillations, the optical Stark effect as well as the bleaching of the exciton resonance with increasing excitation intensity, plasma screening and band-filling phenomena.

Microwave spectroscopy of molecular ions in the laboratory and in space

Abstract: The SO+ molecular ion has been detected radioastronomically via the J = §—*§ rotational transitions, whose rest frequencies were determined by earlier laboratory spectroscopic studies. This ion was detected in seven interstellar sources, including both giant molecular clouds and a cold dark cloud, and thus appears to be very widely distributed in the Galaxy and to play an important role in interstellar chemistry. We have obtained rotational spectra of HCO + in a wide variety of vibrational states in three isotopic forms, leading to the equilibrium structural parameters that are consistent to a high degree in redundant determinations. These results will be compared to similar structures we have obtained for HCN and HNC. The high bending vibrational states observed for HCO + and HCN exhibit the effects of Stark broadening due to the electric fields present in the discharge plasmas. The effects observed for ions against neutrals will be compared, as will those for normal glow discharges against those for magnetically confined abnormal discharges.

Stark-effect studies in xenon autoionizing Rydberg states using a tunable extreme-ultraviolet laser source.

Abstract: The Stark spectra of autoionizing Rydberg states of Xe converging to the second ionization limit 5p5 P1/22 have been investigated as a function of applied electric field over the range 262362 V/cm. Single-photon excitation from the Xe ground state was achieved using coherent radiation near 92.5 nm, generated by frequency tripling the pulsed output of a frequency-doubled dye laser in a free jet of rare-gas atoms. For the lowest electric field only the ns and nd series were observed. As the electric field was increased the np series appeared because of the mixing of wave functions with different l values. Further increase of the electric field caused the appearance of hydrogenlike Stark manifolds. The Stark spectrum was simulated in a two-step procedure with no adjustable parameters. Using a jl-coupling basis set the Hamiltonian matrix was diagonalized to determine the energy levels and corresponding eigenfunctions for a given electric field strength. The intensities and line shapes were then calculated using the formalism developed by Fano for a many-discrete- levelsingle-continuum configuration interaction. This treatment was sufficient to reproduce the positions, widths, and shapes of most observed features. At field strengths between 1000 and 2000 V/cm, however, an additional modulation structure with a spacing of about 2.7 cm-1 appeared on top of the M=1 autoionizing resonances, which could not be explained by our simple theoretical model. © 1988 The American Physical Society.

Optical Stark effect in organic dyes probed with optical pulses of 6-fs duration.

Abstract: We report the first observation of the dynamic optical Stark shift on a femtosecond time scale. Pump pulses of 50-fs duration with peak intensities of ${10}^{11}$-${10}^{12}$ W/${\mathrm{cm}}^{2}$ were used to cause a Stark shift of the ${S}_{0}$ to ${S}_{1}$ transition in the organic dye molecule rhodamine-B. The perturbed spectra were observed with the use of probe pulses of 6-fs duration. The inferred effective transition dipole moment of 7 D is in good agreement with the value derived from the absorption line strength. At the higher pump powers, line-shape distortions and a reduction in the apparent line strength are also noted and discussed.

On the Dynamical Stark Effect of Excitons. The Low-Field Limit

Abstract: The dynamical Stark effect of excitons in semiconductors is treated in three and two dimensions (bulk material and MQW structures, respectively). Within the low-field limit, theoretical results are presented for the gap function, the excitonic enhancement of the Stark shift, and the induced density. Higher bound states and the excitonic continuum are fully taken into account using the Coulomb Green's function. Der dynamische Stark-Effekt von Exzitonen in Halbletern wird in drei (Volumenmaterial) und zwei Dimensionen (MQW-Strukturen) behandelt. Fur den Grenzfall kleiner Feldstarken werden theoretische Ergebnisse fur die Gapfunktion, die exzitonische Verstarkung der Stark-Verschiebung und die induzierte Dichte vorgelegt. Mit Hilfe der Coulomb-Green-Funktion konnen hohere Bindungszustande und das exzitonische Kontinuum voll berucksichtigt werden.

Electrodispersive multiple quantum well modulator

Abstract: Quantum‐confined Stark effect is combined with a Fabry–Perot resonance to build a multiple quantum well electro‐optic modulator. The structure consists of GaAs/AlGaAs quantum wells between two epitaxial AlAs/AlGaAs dielectric multilayer mirrors, all grown by molecular beam epitaxy. The modulator uses refractive index changes induced by applied electric fields. In reflection mode of operation, the modulator demonstrates >5:1 contrast ratio and >50% absolute maximum reflectivity with 17 V applied.

Polarized electroabsorption spectra of amorphous semiconductors

Abstract: Transverse electroabsorption spectra of amorphous Se and hydrogenated amorphous Si (a-Si: H) are presented. These spectra are anisotropic, with fields F parallel to the polarization E of light, causing much larger signals than F perpendicular to E. At temperatures below 350 K the signal depends little on temperature. At higher temperatures the field-induced change δα in the absorption increases in a-Si: H. In Se the anisotropy of δαdepends on photon energy. At low energies the spectrum becomes isotropic and may arise from a quadratic Stark effect of tail states with rather large polarizabilities. The electroabsorption spectra cannot be explained by the Franz-Keldysh effect or by an exciton model. It is proposed that they arise from field mixing of tail states which increases their dynamic polarizability. The mixing is most effective for states near the mobility edges.

Calculation of saturation line shapes and intensities in coherent anti-Stokes Raman scattering spectra of nitrogen

Abstract: Line shapes and intensities for coherent anti-Stokes Raman scattering (CARS) at saturation laser intensities are determined for nitrogen vibrational Q-branch lines by solving the time-dependent density-matrix equations numerically. We have previously performed measurements of saturated CARS line shapes in pure nitrogen by using nearly Fourier-transform-limited pump and Stokes lasers. The experimental laser pulse shapes, Stark effects, Doppler broadening, and the nonresonant background are incorporated in the numerical calculations. The numerical results show good agreement with the high-resolution measurements of saturated CARS line shapes. The lines show prominent saturation dips, and agreement between theory and experiment is excellent in terms of the depth and the width of the dips. The numerical results indicate that the Doppler effect tends to broaden and to deepen the dip in highly saturated lines, an effect that cannot be explained by a steady-state theory.

Photoluminescence of two-dimensional excitons in an electric field: Lifetime enhancement and field ionization in GaAs quantum wells.

Abstract: Two-dimensional excitons in GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As single quantum wells exposed to electric fields perpendicular to the layers are studied by means of time-resolved photoluminescence. The temporal decay of the excitonic emission distinguishes two characteristic field regimes: In the small-field regime, luminescence lifetime increases with increasing field, and a pronounced Stark shift is additionally observed. In the high-field domain \ensuremath{\ge}50 kV/cm, the lifetime decreases with increasing field because of excitonic field ionization, leading to carrier tunneling through the barriers. We discuss these features within the framework of a simple semiclassical model. Quantitative agreement is obtained for quantum wells of different well widths and barrier thicknesses with respect to lifetime, luminescence intensity, and tunneling current. Thus a consistent description of the dynamics of two-dimensional excitons exposed to an electric field is obtained.

Theory of the Optical Stark Effect in Semiconductors

Abstract: The light-induced changes of the properties of a semiconductor caused by an ultra-short strong laser pulse with a central frequency well below the fundamental absorption edge are investigated. Renormalization and time evolution of one- and two-particle properties are discussed using a nonequilibrium Green's function formalism. Numerical results for the absorption spectra are presented. Es werden die lichtinduzierten Anderungen der Eigenschaften eines Halbleiters untersucht, die durch einen starken ultrakurzen Laserimpuls mit einer Mittenfrequenz unterhalb der Grundgitter-absorptionskante verursacht werden. Mit einem Nichtgleichgewichts-Greenfunktionen-Formalismus werden Renormierung und Zeitentwicklung von Ein- und Zweiteilcheneigenschaften diskutiert. Numerische Ergebnisse fur die Absorptionsspektren werden angegeben.

Type I-type II anticrossing and enhanced Stark effect in asymmetric coupled quantum wells

Abstract: We have observed the transition from type I (spatially direct) to type II (spatially indirect) in GaAs/AlGaAs asymmetric coupled quantum well structures at 10 K. The transition is manifested as a strong electric field induced quenching of the photoluminescence which correlates well with the results of a single particle calculation of the electron‐hole overlap. By properly designing the coupled well structure, photoluminescence quenching (90%–10%) is observed for a change in bias field of only 5 kV/cm. Observations of the absorption spectrum clearly demonstrate that the type I–type II transition occurs by an electronic level anticrossing. Owing to the large level repulsion, a Stark shift of 5 meV is observed when the bias field is switched by only 18 kV/cm.