Heavy doping effects on donor-acceptor spectra of polar semiconductors at low temperatures

The heavy doping effects on donor-acceptor (DA)-pair spectra at small DA-pair distances R are investigated, basing on an effective screened impurity potential model given by Morgan and on the results of the R-dependent electron (hole)—longitudinal optical phonon interaction energy obtained by Kartheuser et al. It is suggested that the deviation of the electron—hole recombination energy by dense impurities (or at small R), found from the energy of the emitted photon for a zero-phonon transition, results from the band-gap narrowing effect.At large R, we have found that the energy gap of the pure ZnSe crystal is equal to 2.827 eV, in good agreement with observed results. Finally, at small R, our numerical results applied to ZnSe doped with ln and Li impurities are compared with other theories and with observed results.     

Molecular bond formation in Na*+N2 energy transfer: Crossed beam study of atomic alignment and orientation

We report the first full analysis of collisionally induced atomic alignment and orientation for a molecular collision process. In an experiment with crossed supersonic beams of N₀ and laser excited Na(3² P _3/2) we have studied the dependence of angular and energy resolved differential quenching cross sections as a function of the linear and circular polarization of the exciting laser light. The anisotropies observed in the linear polarization data range up to 2∶1 when corrected for electron and nuclear spin relaxation. The maximum effect is found at small scattering angles and intermediate energy transfer where the cross section is also largest. The atomic alignment angle most favourable for quenching relates to the scattering angle and can be understood in a model picture in such a way that the (NaN₂)^* molecular system is formed at internuclear distances as low asR=10a ₀. The circular asymmetry is small but with significant structure and is attributed to interaction on different potential surfaces atR>10a ₀. Full analysis of the four measurable parameter is given in terms of the density matrix in a frame withz-axis perpendicular to collision plane which allows a clear understanding of the properties of atomic reflection symmetry and coherence of the scattering process.     

Calculation of energies of radiative tunneling transitions between defects in alkali halides

The procedure for calculating the energies of radiative tunneling transition between spatially separated electron and hole centres in ionic crystals based on semiempirical INDO calculation of a pair of close defects and on a further account of electronic polarization of a crystal due to defects has been presented. Pairs of an electron, F, and hole (H, V_k, V₂) centres in KCl and LiF crystals are studied. The influence of close defect interaction and of polarization upon emitted photon energy has also been considered. It is shown that due to polarization this energy varies with a distance between defects in a more complicated way than $\text{e}{}^2\text{?R}$ shown in semiconductors.     

Polarization labeling spectroscopy of NO2

Polarization-labeling spectroscopy is used to simplify the visible spectrum of nitrogen dioxide. Nonlinear polarization effects are used to label and probe molecules in selected rotational-vibrational levels. Those labeled molecules can be detected by their depolarizing effect on a probe laser. The resulting spectra are considerably less complicated than normal absorption spectra. P- and R-branch transitions can be readily identified, and approximate excited-state rotational constants are determined. With more powerful lasers, it may be possible to use polarization labeling to unravel the very complicated vibrational structure of the NO₂ excited states.     

Laser-microwave double-resonance spectroscopy in CF3I with four CO2-laser lines

Double-resonance effects produced by three CO₂-laser lines, the 9.4-μmR(12),R(14), and R(18), have been newly observed. From an analysis of the observed laser-induced changes in intensity of various microwave transitions, the infrared transitions which were accidentally coincident with these three laser lines were determined. In every case an exact rotational assignment of the infrared transition was possible, but in some cases the assignment of the vibrational band involved was difficult. When the previously discussed (l) effects produced by the R(16) laser line are included these four consecutive laser lines pump a total of seven infrared transitions of CF₃I. It appears that all seven transitions belong to different fundamental, combination, and hot bands.     

Luminescence of direct-and indirect-gap electron-hole plasma in TlBr

Luminescence of TlBr crystals highly excited by a nanosecond pulsed-dye laser (3.4 eV) at the bath temperature ∼ 8.5 K was studied. Two emission lines labeled A (∼ 2.98 eV) and B (∼ 2.62 eV) were found, which show typical behavior of the electron-hole plasma recombination radiation. The A-line is assigned to the recombination of e-h pairs in the direct gap (X⁺₆−X^-₃) and the B-line to the simultaneous recombination in the indirect gap (X⁺₆−R^-₆). Condensation of carriers into an electron-hole liquid was not observed.     

Electroreflectance of strontium titanate

The room temperature electroreflectance (ER) measurements for cubic SiTiO₃ are reported with particular emphasis on experimental conditions important for consistent reproducible lineshapes. This study includes for the first time a determination of the position of zero band perturbation (flatband) by making use of the even field nature of the ER lineshapes. The ER spectra are dependent upon dc bias, indicating the importance of a knowledge of the flatband position. A Kramers-Kronig analysis is performed and the lineshapes are reported in terms of ΔR/R, Δε₁ and Δε₂. The results in terms of Δε₂ show that the strongest direct transition occurs near 3.8 eV. The weak structure in the 3.4 eV region is due to either indirect transitions or weak direct transitions. Our unpolarized flatband ER spectra give rise to structures which for (110) samples are resolved into contributions from [001] and [12̄10] polarized spectra. In addition to the large oscillations observed above the band gap, weaker oscillatory structure is observed in the range 1.5 to 2.8 eV in the polarized spectra of (110) faced samples. The oscillations for [11̄0] polarization are 180° out of phase with those for [001] polarization.     

Quadrupole hyperfine structure of CF3I investigated by Lamb-dip spectroscopy in the 10-μm region

Using a line-tunable CO₂ laser we observed the Lamp-dip spectrum of CF₃I in the region of the ν₁ band. Varying the temperature, measuring the saturation intensities, and applying the polarization spectroscopic technique we could obtain information about the assignment of the lines. The resolved quadrupole hyperfine splitting in the Q branch was used to determine the quantum number J with relatively high accuracy. In P and R branches the values J and K were roughly estimated. The influence of second-order Coriolis interactions on the hyperfine structure was discussed.     

Coherent transients observed by Stark switching in ruby having narrow optical hole burning

By using the Stark switching technique we observed new coherent transient signals in ruby at low laser intensities, which are unique in solids where T₁ ? T₂. It was confirmed that the existence of narrow optical hole burning before the switching plays an essential role in giving rise to the signals. The effects of the multiple optical hole burning were also observed in the time domain. Assuming the existence of the holes, the signals can well be interpreted by linear system analysis.     

Dual-wavelength diode-pumped laser operation of N p-cut and N g-cut Tm:KLu(WO4)2 crystals

Simultaneous continuous-wave laser oscillation at two wavelengths has been observed and studied in a diode-pumped monoclinic N _p-cut Tm:KLu(WO₄)₂ for different transmission of the output coupler. The maximum output power reached 1.15 W with a slope efficiency of 20.4 % with respect to the absorbed power for polarization parallel to the N _m optical axis. In an analogous N _g-cut crystal, the dual-wavelength laser operation is accompanied by polarization switching with increasing pump power and the switching point depends on the output coupling. The thresholds are slightly higher, and the slope efficiency reached a maximum of 25.5 % for polarization parallel to N _m at low pump levels, but at high pump levels, the oscillating polarization is parallel to N _p, reaching maximum output power of 3.09 W. Simple modelling with rate equations taking into account reabsorption losses explains qualitatively the complex behavior observed in the continuous-wave laser experiments with this anisotropic biaxial laser crystal.     

The multielectron dissociative ionization dynamics of N2 molecule in intense femtosecond laser fields with arbitrary polarization

The field-ionization Coulomb explosion model is extended to investigate the multielectron dissociative ionization process of N₂ molecule irradiated by an intense femtosecond laser field with an arbitrary polarization. The ionization process of N₂ molecule is found to be optimal at the critical internuclear distance R_c=7a.u., which is independent of the laser polarization state, the molecular explosion channel and the angle between the molecular axis and the direction of laser electric field. The kinetic energies of the ion fragments are identical in the cases of linear and circular polarizations at the same incident laser intensity. However,the probability of electron ionization is very sensitive to the above three parameters. At the critical distance R_c=7a.u. the angular dependence of the threshold intensity for the over-the-barrier ionization leads to the geometric alignment of molecules in the case of linear polarization. The threshold intensity in the case of circular polarization is apparently higher than that in the case of linear polarization, which can well explain the significant decrease of ionization in the case of circular polarization. The numerical calculations are compared with the experimental measurements.     

Spectral features of radiation from Nordström and Kerr-Newman white holes

Unlike the Schwarzschild white hole, Nordström and Kerr-Newman white holes cannot explode right down from the space time singularityR=0. For example a charged white hole has to commence explosion (i.e., comes into existence) with a radiusR ₀=R _c (2?R _c /R _b )^?1 whereR _c is the ‘classical radius’ andR _b is the final radius attained when the stationary state is reached. That means charged and rotating black holes also cannot hit the singularityR=0 and perish. Here the explosion is decelerated by the presence of charge and rotation and hence the radiation emitted would be not as energetic as in the Schwarzschild case where its energy is infinitely large for emission fromR=0.     

Quantum dynamics of strings in black hole space times

We quantize a bosonic string in the D-dimensional Schwarzschild geometry following the method recently proposed by the present authors. This allows us to take into account strong-curvature effects of the black hole. We start from the exact motion of the center of mass of the string, and compute the quantum fluctuations around it to first and second order. This provides the dominant term for physical magnitudes in an expansion in powers of √α' / R_S (√α' = l_pl = Planck length, R_S = Schwarzschild's radius). The mass spectrum and critical dimension are the same as in flat space-time but there is non-trivial elastic and inelastic scattering by the black hole. Ingoing and outgoing modes are introduced in a light-cone-gauge formalism. A linear transformation relating these modes desribes two main effects: (i) polarization changes and (ii) mixing of the particle and antiparticle modes reversing, at the same time, their right or left character.     

Dual band enhanced transmission through a subwavelength aperture based on two split-ring-resonators

Enhanced transmission through a subwavelength aperture is observed at two frequency bands with employment of two split-ring-resonators (SRR) of different sizes. Each of the SRR is excited to produce resonance and the electric field energy localized in its gap and split region can be coupled into a small hole with a radius of 2.3 mm around the respective resonance frequency. The simulation results show that the energy through the small hole is increased at 5.94 GHz (r/λ ₁=0.045) and 7.03 GHz (r/λ ₂=0.054), where 1042-fold and 88-fold enhancements are obtained, respectively, in comparison with the case of a single isolated hole. Moreover, it is found that placing two identical SRR structures in front of the hole can realize higher enhanced transmission with respect to the case of only one SRR utilized. The electric field coupling-enhancement mechanism is well described by studying the electric field distribution.     

Spatiotemporal polarization dynamics in a transverse multimode CO2 laser with optical feedback

We investigate the polarization dynamics in a quasi-isotropic CO₂ laser emitting on the TEM_01* mode subjected to an optical feedback. We observe a complex dynamics in which spatial mode and polarization competition are involved. The observed dynamics is well reproduced by a model that discriminates between the intrinsic asymmetry due to the kinetic coupling of molecules with different angular momenta and the anisotropy induced by the polarization feedback. We observe various dynamical regimes including chaotic dynamics and show that feedback changes these states from regular to chaotic and vice versa. Finally, the possible applications to polarization coding are discussed.     

Infrared laser magnetic resonance (LMR) spectroscopy of NO2 (ν2) with a CO2 laser

The infrared laser magnetic resonance spectra for the ν₂ band of NO₂ were observed by using a CO₂ laser. High-K vibration-rotation transitions from ^rR₆(N) to ^rR₁₁(N) (v₂ = 1 ← 0) were observed. The analysis yielded some molecular parameters including two g factors for the excited vibrational state (v₂ = 1).     

IR Laser Control of the Clustering of CF<Subscript>3</Subscript>Br Molecules during the Gas-Dynamic Expansion of a CF<Subscript>3</Subscript>Br/Ar Mixture: Bromine Isotope Selectivity

The infrared (IR) laser radiation control of the clustering of CF3Br molecules during the gas-dynamic expansion of a CF₃Br/Ar mixture at the exit from a nozzle is investigated. Prominence is given to studying the possibility of bromine-isotope-selective suppression of the clustering of CF₃Br molecules due to their resonance vibrational excitation in the gas-dynamic expansion zone near the nozzle. A continuous CO₂ laser is used in experiments to excite molecules and clusters in a beam, and a quadrupole mass spectrometer is used to detect them. The experimental setup and the experimental technique are described. The dependences of the efficiency of molecule clustering suppression on the exciting laser radiation parameters, the gas parameters (composition, pressure) above the nozzle, and the distance from the nozzle exit section to a molecule irradiation zone are obtained. Bromine-isotope-selective suppression of molecule clustering is shown to occur at the exit from the nozzle due to the resonance vibrational excitation of gas-dynamically cooled CF₃Br molecules. When CF₃Br/Ar mixtures are used at pressure ratios p(CF₃Br): p(Ar) = 1: 10 and 1: 30, the enrichment of a cluster beam by bromine isotopes are K_enr(⁸¹Br) ≈ 1.18 ± 0.09 and 1.12 ± 0.07 during the 9R(30) laser line (1084.635 cm^–1) irradiation of a jet. The clustering suppression selectivity is α ≈ 1.18 when the mixture at the pressure ratio p(CF₃Br): p(Ar) = 1: 10 is used. These results suggest that the proposed method can selectively control the clustering of the molecules containing the heavy element isotopes that have a small isotope shift in IR absorption spectra (OsO₄, WF₆, UF₆).     

A two dimensional theoretical model for describing gain coefficient in N2-lasers and the model validity for CVL and excimer lasers

Based on our previously reported measurements on the gain-value in a N₂- laser and numerical calculations, we introduce a method to obtain an analytical expression for the small signal gain, g₀, where the dependency of g₀ on the laser geometrical configuration, including electrodes length and gap separation, is demonstrated. For this study one- and two-dimensional approaches for the photon density have been applied independently to determine gain-parameter, where for explaining the observed dependency of the gain-parameter on the laser electrodes separation, d_AMP, which was found experimentally and explained by an empirical expression of the type g₀ = r + q/d_AMP, with r and q some constants, realization of introducing an extra dimension along the gap separation was unavoidable. For the electrodes length, l_AMP, we have already shown that an empirical equation of the type g₀ = m + n/l_AMP, with m and n some constants, is consistent with the measurements corresponding to N₂-lasers. With this realization, it is proved that the gain-parameter in N₂-lasers can be written as g₀^{above threshold} = m″ + g_0z(γ_L^z, z) + g_0y(γ_L^y, y), where it consists of two independent gain-values along the electrodes length (z) and gap separation (y) with the corresponding power losses given by γ_L^z and γ_L^y. m″ is a very small quantity showing that laser is operating slightly above the threshold. The results of this calculation are consistent with our recent measurements and also other reported N₂-laser gain values measured under moderate current density conditions. To check the validity of the model for other types of lasers, the reported gain-values for copper vapor lasers of different laser tube radii, R_AMP, and tube lengths, l_AMP, have been examined using the one-dimensional model of either g₀(R_AMP) or g₀(l_AMP) and the consistency with the observed measurements was found to be quite satisfactory. The model was also found to be valid for the excimer lasers of different types, different gas mixtures and pressures at a constant input operational voltage. Due to limited numbers of the reported experimental measurements, for the graphs preparation of g₀(l_AMP) in excimer lasers we used the observed data at V₀ = 30 kV and also some variations of the input voltages in the range of ΔV ≅ 20 kV have been adopted. The results for both cases were found to be consistent with the proposed one-dimensional model.     

Effects of oxygen partial pressure on the ferroelectric properties of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The Ba_0.8Sr_0.2TiO₃ thin films were grown on the Pt–Si substrate at 700 °C by using a pulsed laser deposition technique at different oxygen partial pressure (PO₂) in the range of 1–20 Pa and their properties were investigated. It is observed that the PO₂ during the deposition plays an important role on the tetragonal distortion ratio, surface morphology, dielectric permittivity, ferroelectric polarization, switching response, and leakage currents of the films. With an increase in PO₂, the in-plane strain for the BST films changes from tensile to compressive. The films grown at 7.5 Pa show the optimum dielectric and ferroelectric properties and also exhibit the good polarization stability. It is assumed that a reasonable compressive strain, increasing the ionic displacement, and thus promotes the in-plane polarization in the field direction, could improve the dielectric permittivity. The butterfly features of the capacitance–voltage (C–V) characteristics and the bell shape curve in polarization current were attributed to the domain reversal process. The effect of pulse amplitude on the polarization reversal behavior of the BST films grown at PO₂ of 7.5 Pa was studied. The peak value of the polarization current shows exponential dependence on the electric field.