Experimental preparation of entangled Bell’s vacuum-single exciton and vacuum-biexciton states for pair centers of neodymium ions in a crystal

The process of low temperature laser excitation of neodymium ion M pair centers in CaF₂ crystals at the ⁴I_9/2-⁴G_5/2 optical transition is analyzed. It is shown that maximally entangled Bell’s vacuum-single exciton and vacuum-biexciton states are experimentally prepared when irradiating these crystals by nanosecond laser pulses.     

Hole Spin Relaxation in an Ultrathin InAs Monolayer

We investigate the spin relaxation time of holes in an ultrathin neutral InAs monolayer （1.5 ML） and compare with that of electrons, using polarization-dependent time-resolved photoluminescence （TRPL） experiments. With excitation energies above the GaAs gap, we observe a rather slow relaxation of holes （τ1h = 196± 17 ps） that is in the magnitude similar to electrons （t1e= 354 ± 32 ps） in this ultrathin sample. The results are in good agreement with earlier theoretical prediction, and the phonon scattering due to spin-orbit coupling is realized to play a dominant role in the carrier spin kinetics.     

Observation of ultra-broadband terahertz emission from ZnTe films grown by metaloganic vapor epitaxy

ZnTe films were grown on (0001) sapphire substrates by the metalorganic vapor phase epitaxy (MOVPE) method. Single crystalline (111) ZnTe epitaxial layers were confirmed by x-ray diffraction, reflection high-energy electron diffraction, and cathodoluminescence measurements. Emission of THz radiation with a spectral distribution up to 40 THz was clearly observed from the ZnTe film with a thickness of 10 μm. The results show that MOVPE is a promising growth method for obtaining high-quality ZnTe epitaxial films on sapphire substrates, which paves the way for obtaining thinner ZnTe films to provide a flatter frequency response in THz device applications.     

Interband quantum kinetics with static disorder scattering I: Direct CPA solution for a rectangular-pulse-excited semiconductor

For a two band model semiconductor alloy with the disorder potential concentrated to the conduction band, the photoexcitation by a long rectangular pulse represents a case soluble in the coherent potential approximation. Explicit analytic expressions for the transient electron distribution are derived using the nonequilibrium Green functions. The evanescent coherent component is gradually superseded by the incoherent distribution whose saturation value is obtained using the Ward identity.     

Calculus removal on a root cement surface by ultrashort laser pulses

Ultrashort-pulse-laser ablation of dental calculus (tartar) and cement is performed on root surfaces. The investigation shows that the threshold fluence for ablation of calculus is a factor of two to three times smaller than that of a healthy root cement surface. This indicates that ultrashort laser pulses may provide an appropriate tool for selective removal of calculus with minimal damage to the underlying root cement. Future application of an in situ profiling technique allows convenient on-line monitoring of the ablation process.     

Ab initio study of electronic structures and magnetism in ZnMnTe and CdMnTe diluted magnetic semiconductors

In this work, we aimed to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of ZnMnTe- and CdMnTe-diluted magnetic semiconductors (DMSs) in the ferromagnetic phase, and with 25% of Mn. The calculations are performed by the recent ab initio full potential augmented plane waves plus local orbitals (FP−L/APW+lo) method within the spin-polarized density-functional theory and the local spin density approximation. We have determined the exchange splittings produced by the Mn d states: Δ_x(d) and Δ_x(pd), and we found that the effective potential for the minority spin is more attractive than that for the majority spin. Also, we show the nature of the bonding from the charge spin-densities calculations, and we calculate the exchange constants N₀α and N₀β, which mimics a typical magneto-optical experiment. The calculated total magnetic moment is found to be equal to 5μ_B for both DMSs. This value indicates that every Mn impurity adds no hole carriers to the perfect ZnTe and CdTe crystals. Furthermore, we found that p–d hybridization reduces the local magnetic moment of Mn and produces small local magnetic moments on the nonmagnetic Te, Zn and Cd sites.     

Energy levels of vanadium ions in CdTe

In CdTe doped with vanadium the photoluminescence due to the ³ T ₂(F) ³ A ₂(F) transition of V³⁺(d ²) is detected. Its decay time is determined as (630±20) s, a result comparable to the analogous emissions in various host lattices. Further emissions around 5000 cm^–1 and 9000 cm^–1 are caused by charge-transfer transitions or bound-exciton decay. Excitation and sensitization spectra yield information on the positions of the energy levels within the gap, which are discussed using two different models. At T=4.2 K, the distance of the V²⁺/V³⁺ donor level is 7300 cm^–1 and 5700 cm^–1 referred to the valence and the conduction band edges, respectively. The absence of V²⁺(d ³) centres is tentatively ascribed to the existence of deeply bound excitons.     

Influence of magnetic quantization on the Einstein relation in non-linear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestion for experimental determination

In this paper, we have investigated the Einstein relation for the diffusivity-to-mobility ratio (DMR) under magnetic quantization in non-linear optical materials on the basis of a newly formulated electron dispersion law by considering the crystal field constant, the anisotropies of the momentum-matrix element and the spin-orbit splitting constant, respectively, within the frame work of k·p formalism. The corresponding result for the three-band model of Kane (the conduction electrons of III-V, ternary and quaternary compounds obey this model) forms a special case of our generalized analysis. The DMR under magnetic quantization has also been investigated for II-VI (on the basis of Hopfield model), bismuth (using the models of McClure and Choi, Cohen, Lax and parabolic ellipsoidal, respectively), and stressed materials (on the basis of model of Seiler et al.) by formulating the respective electron statistics under magnetic quantization incorporating the respective energy band constants. It has been found, taking n-CdGeAs₂, n-Hg_1−xCd_xTe, p-CdS, and stressed n-InSb as examples of the aforementioned compounds, that the DMR exhibits oscillatory dependence with the inverse quantizing magnetic field due to Subhnikov de Haas (SdH) effect with different numerical values. The DMR also increases with increasing carrier degeneracy and the nature of oscillations are totally dependent on their respective band structures in various cases. The classical expression of the DMR has been obtained as a special case from the results of all the materials as considered here under certain limiting conditions, and this compatibility is the indirect test of our generalized formalism. In addition, we have suggested an experimental method of determining the DMR for degenerate materials under magnetic quantization having arbitrary dispersion laws. The three applications of our results in the presence of magneto-transport have further been suggested.     

Binding energy of hydrogen-Cd vacancy complex in CdTe

The structural and electronic properties of hydrogen-Cd vacancy complex in CdTe have been studied by using density functional theory. Three typical complexes between hydrogen and Cd vacancy were found in our present study. The stablest complex P₂ was predicted by using the formation energy and the binding energy. We found that the formation of the complex is exothermic and its binding energy is extremely large. The analysis of the transition energy level indicates that the forming of the complex turns the double acceptors caused by Cd vacancy to be a single one. The position of the single acceptor deeper than the shallow acceptor derived from the cation vacancy. At the same time, the forming of the complex directly increases the lifetime of the minority carrier from decreasing its concentration. The mechanism of the hydrogen passivation effect was also briefly discussed in our present study.     

Analysis of hydroxyapatite laser ablation plumes in a water atmosphere

4 m/s, shows the development of a shock wave in its front, resulting from the interaction between the species released from the target and the background gas. The water vapor slows down this component up to values of about 3×10³ m/s following a behavior that can be well described by the dynamics of a spherical shock wave. The low intensity of emission of the second component has not allowed us to analyze its dynamics. The third and slowest component expands at a constant velocity of 5×10² m/s and is constituted by hot particulates leaving the target. Spectra recorded in the shock front have shown the presence of emission lines arising from Ca I, Ca II, P I and some impurities, and two strong emission bands that can be assigned to some sort of calcium oxide. Received: 7 November 1997/Accepted: 17 February 1998     

Optical phonon modes in graded III-V nitride quantum wells

The dispersion relation for optical phonon modes in graded wurtzite AlN/GaN and AlN/InN quantum wells is calculated taking into account the existence of interfacial transition regions. We make use of a model based on the macroscopic theory developed by Loudon, known as the continuum dielectric model. The optical phonon modes are modelled considering only the electrostatic boundary conditions (neglecting retardation effects), in the absence of charge transfer between ions. We show that the graded interfaces strongly shift the frequencies of the phonon modes of the otherwise abrupt nitrides quantum wells.     

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd_xZn_1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (r_HB=2700 ns⁻¹) is two orders of magnitude more efficient than in the low QD-density samples (r_HB=33 ns⁻¹). Radiative recombination rates are similar in both types of samples, r_R=1-1.3 ns⁻¹. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.     

Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

Effect of isotopic mass on the photoluminescence spectra of β zinc sulfide

Zinc sulfide is a wide bandgap semiconductor which crystallizes in either the wurtzite modification (α-ZnS), the zincblende modification (β-ZnS) or as one of several similar tetrahedrally coordinated polytypes. In this work, we report a photoluminescence study of different samples of isotopically pure β-ZnS crystals, and crystals with the natural isotopic abundances, at 15 and 77 K. The derivatives of the free and bound exciton energies on isotopic mass have been obtained. They allow us to estimate the contribution of the zinc and sulfur vibrations to the bandgap renormalization energy by electron-phonon interaction. A two-oscillator model based on the zinc and sulfur renormalization energies has been used to account for the temperature dependence of the bandgap energy in ZnS. The results are compared with those found for other tetrahedrally coordinated semiconductors.     

Shell thickness dependence of exciton trapping in colloidal core/shell nanorods

We quantitatively investigated, by time-resolved photoluminescence (PL) spectroscopy, the shell thickness dependence of exciton trapping and its effects on the PL quantum yield (QY) in colloidal CdSe/CdS/ZnS core/shell quantum rods. The defects passivation, due to a thin shell (0.6 monolayer), leads to a 2 times reduction of the trapping from both emitting and high-energy excited states, thus explaining the observed 4.3 times increase of the PL QY. Moreover, the QY decrease in the thick shell (1.3 monolayers) sample is fully explained in terms of increased trapping from the emitting states, which is ascribed to new defects caused by the strain relaxation at the core-shell interface.     

Energy gaps in a spacetime crystal

This Letter presents an analysis of the band structure of a spacetime potential lattice created by a standing electromagnetic wave. We show that there are energy band gaps. We estimate the effect, and propose a measurement that could confirm the existence of such phenomena.     

Structural, Optical and Electrical Properties of Hydrogen-Doped Amorphous GaAs Thin Films

Amorphous CaAs films are deposited on substrates of quartz glass and sificon by rf magnetron sputtering technique in different gas ambient. First, the amorphous structure of the prepared samples is identified by x-ray diffraction. Second, analysis by radial distribution function and pair correlation function method is established to characterize the microstructure of the samples. Then, the content and bond type of hydrogen are analysed using Fourier transform infrared absorption spectroscopy. It is found that the bonded hydrogen content increases with increasing partial pressure PH of H2. However, the hydrogen content saturates at PH 〉 1 × 10^-1 Pa. Hydrogen addition shills the optical absorption edge to higher energy, decreases the dark conductivity and improves the photo-sensitivity. The optical gap, dark conductivity and photo-sensitivity of the films are dependent on the bonded hydrogen content. These results demonstrate that hydrogen has obvious passivation effects on rf sputtered amorphous GaAs thin films.     

Localization of nonequilibrium current carriers close to Cr ions in ZnSe:Cr single crystals

The photoluminescence (PL) of Cr-doped ZnSe single crystals is investigated in a temperature interval from 83 up to 297 K and in a wavelengths region from 440 up to 2700 nm. The doping was carried out during a high-temperature annealing of ZnSe crystals in CrSe vapors and in chrome chlorides medium. It is revealed that the doping results in an appearance of both luminescence bands located at 0.54, 0.97, and 2.15 μm and edge luminescence bands located at 454, 457, and 460 nm at 83 K. It is shown that the PL bands located at 457 and 460 nm are caused by the radiative recombination with the participation of holes located on hydrogen-like orbits close to Cr⁺ centers, having a binding energy of 99 meV. The excitons bound with centers responsible for the radiation located at 0.54 μm and having a binding energy of 65-68 meV are considered. The energy of a lattice relaxation at recharge of centers responsible for green radiation is estimated and equals 40-170 meV.     

Formation Mechanisms of Electrical Conductivity and Optical Properties of ZnO:N Film Produced by Annealing Treatment

The effects of annealing on the chemical states of N dopant, electrical, and optical properties of N-doped ZnO film grown by molecular beam epitaxy (MBE) are investigated. Both the as-grown ZnO:N film and the film annealed in N₂ are of n-type conductivity, whereas the conductivity converts into p-type conductivity for the film annealed in O₂. We suggest that the transformation of conductivity is ascribed to the change in ratio of the N molecular number on O site (N₂)_O to the N atom number on O site (N_O) in ZnO:N films under the various annealed atmosphere. For the ZnO:N film annealed in N₂, the percentage content of (N₂)_O is larger than that of N_O, i.e.the ratio >1, resulting in the n-type conductivity. However, in the case of the ZnO:N film annealed in O₂, the percentage content of (N₂)_O is fewer than that of N_O, i.e., the ratio <1, giving rise to the p-type conductivity. There is an obvious difference between low-temperature (80K) PL spectra of ZnO:N film annealed in N₂ and that of ZnO:N film annealed in O₂. An emission band located at 3.358eV is observed in the spectra of the ZnO:N film after annealed in N₂, this emission band is due to donor-bound exciton (D⁰X). After annealed in O₂, the PL of the donor-bound exciton disappeared, an emission band located at 3.348eV is observed, this emission band is assigned to acceptor-bound exciton (A⁰X).