Showing papers on "Stark effect published in 1991"

Quantum crystallites and nonlinear optics

Abstract: This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.

Dispersion of bound electron nonlinear refraction in solids

Abstract: A two-hand model is used to calculate the scaling and spectrum of the nondegenerate nonlinear absorption. From this, the bound electronic nonlinear refractive index n/sub 2/ is obtained using a Kramers-Kronig transformation. The authors include the effects of two-photon and Raman transitions and the AC Stark shift (virtual band blocking). The theoretical calculation for n/sub 2/ shows excellent agreement with measured values for a five-order-of-magnitude variation in the modulus of n/sub 2/ in semiconductors and wide-gap optical solids. Beam distortion methods were used to measure n/sub 2/ in semiconductors. The observations result in a comprehensive theory that allows a prediction of n/sub 2/ at wavelengths beneath the band edge, given only the bandgap energy and the linear index of refraction. Some consequences for all-optical switching are discussed, and a wavelength criterion for the observation of switching is derived. >

Spatial orientation of molecules in strong electric fields and evidence for pendular states

Abstract: IN typical collisional or spectroscopic experiments, molecules rotate freely with random spatial orientations. The resulting isotropic averaging obscures or suppresses much stereodynamical information and has remained a recalcitrant problem. The only practical means for orienting a molecule itself, rather than just its axis of rotation, has been electric field focusing1,2. But this is applicable only to certain rotational states of symmetric top molecules (or equivalent) that exhibit a first-order Stark effect. Orientation of molecules other than symmetric tops has long been considered to be quite unfeasible3. Recently, however, it has been pointed out4,5 that by exploiting the extreme rotational cooling that can occur in supersonic molecular beams, substantial orientation of diatomic, linear or asymmetric top molecules should become possible at accessible field strengths. The anisotropy of the Stark effect allows molecules in the lowest few rotational states to be trapped in 'pendular states' and thereby confined to librate (oscillate about the field axis) over a limited angular range. Here we describe an experiment which demonstrates that oriented pendular states can be obtained for a diatomic molecule with modest field strengths. With anticipated improvements, this technique should become widely applicable.

Electric-field-induced second-harmonic generation with reduced absorption in atomic hydrogen.

Abstract: We show that dc-electric-field coupling of the 2s and 2p states in atomic hydrogen leads to resonantly enhanced second-order susceptibility with reduced absorption at the second-harmonic wavelength, and exact phase matching at the center of the Stark-split components.

Analytic solution of a two-dimensional hydrogen atom. I. Nonrelativistic theory

Abstract: The two-dimensional hydrogen problem is solved analytically. In the nonrelativistic case, exact formulas for energy eigenvalues and eigenfunctions for both the discrete and continuous parts of the spectrum, dipole matrix elements, dc Stark effect, single- and two-photon transition rates, and fine and hyperfine structures are obtained. Comparison is made between the two- and the three-dimensional cases. Some interesting aspects of the solution unique to the two-dimensional case are discussed.

Model atom for multiphoton physics.

Abstract: We describe in detail some properties of a one-dimensional model atom that has been used for the study of multiphoton processes. We discuss static properties of the atom such as its energy eigenvalues, dipole moment matrix elements, and dipole sum rule, and also some aspects of its time-dependent response to a weak laser field, including second-order level shifts, and exact polarizability.

Electroabsorption (stark effect) spectroscopy of mono- and biruthenium charge-transfer complexes : measurements of changes in dipole moments and other electrooptic properties

Abstract: Electroabsorption (Stark effect) spectra are reported for the charge-transfer transitions of (NH3)SRuL2+ and [(NH3)5R~]2L4+*5+, where L is pyrazine (pz) or 4,4'-bipyridine (4,4'-bpy). The spectra permit experimental estimates of the susceptibility of the transition dipole moment to an electric field, the change in polarizability (TrAu), and the magnitude of the change in permanent electric dipole moment (IApI) associated with many of the metal-to-ligand and metal-to-metal charge-transfer (MLCT and MMCT, respectively) transitions in these complexes. The observed electroabsorption spectra of the MLCT transitions of the monoruthenium complexes are interpreted as arising predominantly from Au and Ap. When L = pz and 4,4'-bpy, the observed values of IApl are (5.3 0.8)lfand (15.8 * 0.2)/fD, respectively, compared with the values of 16.5 and 27.1 D expected for full charge transfer from the metal to the geometric center of the ligand (fis a local electric field correction). Protonation of the monoruthenium complexes has relatively small effects on the observed Au and Ap when L = 4,4'-bpy, but when L = pz, Au appears to change its sign while Ap virtually disappears. A simple electrostatic model qualitatively accounts for the results and indicates that pyrazine allows a much greater degree of delocalization from the ruthenium than 4,4'-bipyridine whose pyridyl rings are probably not coplanar. The electroabsorption spectra of the MLCT region of the biruthenium complexes are very complicated and not quantitatively interpretable on the basis of current information, though interesting and qualitatively suggestive features appear. For the MMCT transitions in the biruthenium mixed-valence complexes where L is pz and 4,4'-bpy, the observed values of lApl are (0.7 * O.l)/fand (28.5 A 1.5)/fD, respectively, compared with the values of 32.7 and 54.3 D expected fur charge transfer in fully localized complexes. These latter results demonstrate that electronic delocalization between the two metals is essentially complete when the bridging ligand is pyrazine, whereas it is significant but incomplete when the bridge is 4,4'-bipyridine. In all complexes, the angle dependence of the electroabsorption demonstrates that Ap and the other field-interactive molecular properties are parallel to the transition dipole moment, as expected if the transition moment lies along the metal-ligand axis. The electroabsorption spectra of all of the monoruthenium and [(NH3)5R~]2L4+ complexes also show evidence for transitions that are weak or obscured in conventional absorption spectra; possible assignments are discussed within the context of a molecular orbital model.

Mechanical properties and force field parameters for polyethylene crystal

Abstract: in contrast to the optical Zeeman effect, which can be observed only with II circularly polarized laser. The differences between the optical Stark and Zeeman effects are as fundamental as those between the conventional Stark and Zeeman effects, induced respectively with a static electric and magnetic field. In the optical Zeeman effect there should also be the equivalent of the Paschen-Back effect, an anomalous optical Zeeman effect as the optical conjugate product is increased. In diatomics and polyatomics in which there is net electronic angular momentum, the Hund coupling models can be used to describe the various patterns as in Townes and Schawlow, chapter 1 1 .36

On the possibility of orienting rotationally cooled polar molecules in an electric field

Abstract: It has long been thought that only symmetric top molecules (or equivalent) can be appreciably oriented in an electric field. This assessment is unduly pessimistic. In molecular beams produced by supersonic expansion, the rotational temperature can be made very low (∼ 1 K). For many diatomic, linear, or asymmetric top molecules, quite substantial orientation can thereby be attained for a large fraction of the beam at accessible field strengths (∼ 100 kV/cm or less). We present model calculations and an experimental design to evaluate the method by observing the fluorescence of photofragments from oriented molecules. Nomograms are provided from which the orientation can be estimated for linear molecules from the dipole moment, rotational constant, rotational temperature, and field strength.

The dipole moment of water. I. Dipole moments and hyperfine properties of H2O and HDO in the ground and excited vibrational states

Abstract: Molecular beam electric resonance spectroscopy has been used to study H2O in its ground state and in each of its singly excited vibrational states. HDO was studied in its ground state and first excited O–H stretching state. Precise dipole moments have been obtained for a total of 14 rotational levels of six vibrational states. Centrifugal distortion corrections to the Stark coefficients were made and rotationless moments were calculated for each vibrational state. The magnitude of the H2O moment can be written μ(v1v2v3) =1.857 04+0.005 08(v1+ (1)/(2) ) −0.031 66(v2+ (1)/(2) ) +0.022 46(v3+ (1)/(2) ). Individual components of the HDO moment were obtained for the ground and excited states. Hyperfine properties were also determined for each of the vibrational states studied. In the following paper, the dipole moments are combined with infrared transition moments to obtain an improved dipole moment function for water.

Spectroscopic measurements of electron density of capillary plasma based on Stark broadening of hydrogen lines.

Abstract: A set of measurements of the electron density of plasma jets, generated by a high-pressure discharge capillary operating at quasi-steady-state, is described. The method of measurement is based on the dependence of Stark broadening of the hydrogen ${\mathrm{H}}_{\mathrm{\ensuremath{\alpha}}}$ and ${\mathrm{H}}_{\mathrm{\ensuremath{\beta}}}$ spectral lines on the electron density. Spectra were sampled electronically, time integrated over the electrical pulse duration, for various capillary currents and at different axial locations along the emerging plasma jet. The comparison of model predictions [Loeb and Kaplan, IEEE Trans. Magn. 25, 342 (1989)] with electron densities deduced from these spectra, by applying the theory of Stark broadening [Griem, Spectral Line Broadening by Plasma (Academic, New York, 1974)], indicates a good agreement over the tested current (1--8 kA) and density (${10}^{17}$--${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) ranges. At larger densities, self-absorption might be a problem.

Dispersion of Bound Electronic Nonlinear Refraction

Abstract: A two-band model is used to calculate the scaling and spectrum of the nondegenerate nonlinear absorption Aa(w,; wJ. From this, the bound electronic nonlinear refractive index n2 is obtained using a Kramers-Kronig transformation. We in- clude the effects of two-photon and Raman transitions and the ac Stark shift (virtual band blocking). The theoretical calcu- lation for n, shows excellent agreement with measured values for a five order of magnitude variation in the modulus of n2 in semiconductors and wide-gap optical solids. We also present new measurements of n, in semiconductors using the Z-scan method. The observed change of sign of n2 midway between the two-photon absorption edge and the fundamental absorption edge is also predicted. Thus, we now have a comprehensive the- ory that allows a determination of n2 at wavelengths beneath the band edge, given only the bandgap energy and the linear index of refraction. Such information is useful for a variety of applications including optical limiting, laser-induced damage, and all-optical switching. Some consequences for all-optical switching are discussed, and a wavelength criterion for the ob- servation of switching is derived.

Excitonic effects in coupled quantum wells

Abstract: Electron-sublevel-anticrossing effects have been studied in coupled quantum wells where the exciton binding energy is comparable to the minimum sublevel splitting. The anticrossing was induced by applying an electric field to align the first and second sublevels of adjacent wells. In this situation the electron-hole Coulomb interaction has a strong effect on the splittings measured by optical techniques, because the optical spectra typically measure exciton energies rather than single-particle energies. The most striking effect is that the minimum splitting of the excitons associated with each of the split electron levels does not occur at the same field as for the minimum splitting of the bare-electron levels. One unexpected but readily observable consequence is that when the same electron-sublevel splitting is measured using two different pairs of intrawell and interwell exciton transitions, the field for minimum exciton splitting can differ by up to \ensuremath{\sim}10% from one pair of transitions to the other. We have constructed a variational model of the coupled excitons that explains these effects in terms of Coulomb mixing of the delocalized electron states. We have measured the exciton splittings directly by photocurrent spectroscopy in three GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well structures. The samples were similar in design except that the ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As barrier thickness varied from 15 to 35 \AA{}. By fitting our variational model to the experimental anticrossing data, we have been able to deduce the actual bare-electron level splittings rather than the exciton splittings. Within the experimental accuracy, we find that the minimum splitting decreased exponentially with increasing barrier thickness, as would be expected for simple quantum-mechanical tunneling.

Pure and Nd3+‐, Pr3+‐Ion Doped Trigonal Acentric LaBGeO5 Single Crystals Nonlinear Optical Properties, Raman Scattering, Spectroscopy, Crystal‐Field Analysis, and Simulated Emission of Their Activators

Abstract: Complex spectral investigations are made on new Nd3+- and Pr3+-doped and pure nonlinear-laser trigonal LaBGeO5 crystals. Raman spectra and nonlinear optical properties are studied including second harmonic generation, absorption, and luminescence spectra, which allow to determine the energy of Stark levels of the activator ions, luminescence-intensity characteristics, and pulse stimulated-emission parameters of Nd3+ ions at two intermanifold 4F3/2 4I11/2 and 4F3/2 4I13/2 channels as well as of Pr3+ ions at the visible 3P0 3H6 transition. A preliminary theoretical analysis is made of the crystal field at activator ions using a new many-electron semiempirical method. All registered induced transitions are identified. LaBGeO5:Nd3+ is shown as a promising material for self-frequency doubled laser. [Russian Text Ignored.]

Exciton Stark ladder in semiconductor superlattices

Abstract: We present a method for calculating the 1s exciton states of type-I and type-II superlattices in an applied static electric field. Our approach is based upon choosing localized exciton wave functions as the basis in which to expand the exciton eigenstates. We calculate the excitonic optical-absorption spectra of a number of very different type-I GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As superlattices, as a function of electric field, and find our results for the 65-\AA{} superlattice of Agull\'o-Rueda, Mendez, and Hong [Phys. Rev. B 40, 1357 (1989)] to be in good agreement with the experimental photocurrent spectra over the full range of field strengths. In addition, we explain many of the low-field effects generally seen, such as the ladder absorption asymmetry, oscillations in the absorption of individual levels, and the nonlinear dependence of the exciton-Stark-ladder energy levels on the field strength. Finally, we discuss the electric-field range for which a modified single-particle picture provides an adequate picture of the qualitative features of the exciton Stark ladder.

High sensitivity optical image processing device based on CdZnTe/ZnTe multiple quantum well structures

Abstract: We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

Microwave spectrum and ab initio calculations of ethylbenzene: potential energy surface of the ethyl group torsion

Abstract: The weak rotational spectrum of the ground and three vibrational states belonging to the ethyl group torsional progression of ethylbenzene has been measured using conventional microwave spectroscopy. The small dipole moment (μa = 0·49(4), μc = 0·2(1) D) has been determined from a second order Stark effect, and is compared with a variety of ab initio calculations at the RHF/6-31G, MP2/6-31G, RHF/6-31G*, MP2/6-31G* and RHF/6-31G** levels. The potential energy surface for the rotation of the ethyl group with respect to the ring has also been estimated on the basis of vibrational satellite intensities, shifts of rotational constants upon torsional excitation, and of ab initio calculations. A conformation in which the ethyl group is perpendicular to the nominal ring plane is found to be the most stable.

Energy levels and crystal quantum states of trivalent holmium in yttrium aluminum garnet

Abstract: Absorption spectra of Ho3+ ions in yttrium aluminum garnet (Ho3+:YAG) are reported between 2.16 and 0.23 μm at various temperatures between 4 K and room temperature. Laser‐excited excitation spectra and emission spectra from the 5F4 and 5S2 multiplet manifolds to the ground state manifold 5I8 were obtained at 4, 20, and 55 K. The majority of Ho3+ ions substitute for Y3+ ions in sites of D2 point‐group symmetry in the lattice. Over 1000 temperature‐dependent transitions (hot bands) establish 280 experimental Stark levels of the 4 f 10(2S+1LJ) multiplet manifolds. Symmetry labels Γ1, Γ2, Γ3, or Γ4, appropriate to D2 symmetry, have been assigned to Stark levels up to 43 000 cm−1. Experimental levels are compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin‐orbit, and interconfiguration interaction terms for the 4 f 10 atomic configuration of Ho3+ and crystal‐field terms in D2 symmetry. The Hamiltonian was diagonalized within the 50 lowest 2S+1LJ manifol...

Transient many-body effects in the semiconductor optical Stark effect: A numerical study.

Abstract: A comprehensive numerical study of the optical Stark effect and associated phenomena in semiconductors is presented. The results are obtained from numerical solutions of the full semiconductor Bloch equations for bulk and quantum-well structures. Coherent oscillations, the optical Stark effect, and adiabatic following, i.e., ultrafast bleaching and recovery of the exciton, are discussed. The relative importance of the different contributions in the semiconductor Bloch equations is identified. Optical-absorption spectra are computed for different exciton dephasing models and a large variety of pump-probe excitation parameters.

Role of resonances and quantum-mechanical interference in the generation of above-threshold-ionization spectra.

Abstract: A general technique for modeling the behavior of an atom in the presence of a laser field is described. This involves the integration of the time-dependent Schr\"odinger equation in the Kramers-Henneberger frame. Results are presented for a one-dimensional calculation for a model hydrogen atom across a range of frequencies and intensities. The resultant peaks in the photoelectron spectra produced are shown to be partly the result of Stark-shifted bound-state multiphoton resonances. The spectra also contain rainbow features due to interference between the electron amplitude generated on the rising and falling edge of the pulse. These rainbow features are a purely time-dependent effect that can only be easily reproduced in time-dependent calculations.

Electric field measurement in the cathode sheath of a hydrogen glow discharge

Abstract: Spatially resolved electric field measurements in the cathode region of a dc glow discharge in hydrogen are performed using polarization‐dependent Stark broadening of plasma‐induced emission of the Balmer lines. The large concentrations of excited atoms in the sheath provide an accurate, sensitive measure of discharge electric fields.

Low‐voltage, high‐saturation, optically bistable self‐electro‐optic effect devices using extremely shallow quantum wells

Abstract: Symmetric self‐electro‐optic effect devices (S‐SEEDs) using extremely shallow GaAs/Al0.04Ga0.96As multiple quantum wells are demonstrated. By exploiting mainly exciton ionization, rather than the usual quantum‐confined Stark shift, room‐temperature optical bistability is obtained with no applied bias. The extremely shallow symmetric‐SEED (symmetric E‐SEED) exhibits contrast ratios (CRs)≂3.5, with biasses<5 V, demonstrating system applicability and compatability with electronics. Large system tolerances Δλ≂6 nm and maximum bistability loop width ≂70% are also obtained. Moreover, due to fast carrier escape times, the symmetric E‐SEED exhibits useful CRs≳2 even at continuous‐wave intensities ≳70 μW/μm2.