Energy Sublevels Research Articles

Optimization of AlInGaAs/InGaAs/InAs strain compensated triangular quantum wells grown by gas source molecular beam epitaxy for laser applications in 2.1–2.4 μm range

A group of AlInGaAs/InGaAs/InAs strain compensated triangular quantum well samples have been grown by using gas source molecular beam epitaxy (GSMBE), and their properties are investigated by X-ray diffraction and photoluminescence (PL) measurements. Through the adjustment of the growth temperature and barrier width, the quality of the quantum wells has been improved distinctly. The maximal PL peak wavelength of 2.38 μm at 300 K has been reached. The X-ray diffraction measurements show good structural properties and the full-width at half-maximum (FWHM) of PL spectrum is 17 meV at 12 K and 33 meV at 300 K. For the sample with larger well width, the transition involving the second sub-energy levels occurs.

A proposal for enhancement of optical nonlinearity in GaN/AlGaN centered defect quantum box (CDQB) nanocrystal

In this paper, effect of an introduced cubic defect on electrical and optical properties of cubic quantum dot is studied. Self-consistent solution of the Schrödinger–Poisson equations is used for evaluation of the proposed complex quantum dot structure. Optical properties (quadratic electro-optic effect and the resonant third-order nonlinear susceptibility associated with intersublevel transitions) of the introduced structure are also investigated using density matrix method. The effects of size, height and position (on or off-center) of introduced defect on energy levels, carrier density, matrix element and optical nonlinearity of centered defect quantum box (CDQB) are examined. It is shown that increasing the defect size and height, lead to a considerable enhancement in magnitude of the dipole transition matrix element, third-order susceptibility of THG and QEOE and also red shift in resonance frequency. We show that the CDQB has higher nonlinearity (at least 14 times) and tunable susceptibilities, due to increase of the matrix element and modified energy sublevels, compared with quantum box structure without defect. Also, it is shown that the enhancement of optical nonlinearity is approximately independent of defect position which is so excellent from practical implementation point of view.

Influence of V/III ratio on the structural and photoluminescence properties of In0.52AlAs/In0.53GaAs metamorphic high electron mobility transistor grown by molecular beam epitaxy

A series of metamorphic high electron mobility transistors (MMHEMTs) with different V/III flux ratios are grown on GaAs (001) substrates by molecular beam epitaxy (MBE). The samples are analysed by using atomic force microscopy (AFM), Hall measurement, and low temperature photoluminescence (PL). The optimum V/III ratio in a range from 15 to 60 for the growth of MMHEMTs is found to be around 40. At this ratio, the root mean square (RMS) roughness of the material is only 2.02 nm; a room-temperature mobility and a sheet electron density are obtained to be 10610.0cm2/(V·s) and 3.26 × 1012cm−2 respectively. These results are equivalent to those obtained for the same structure grown on InP substrate. There are two peaks in the PL spectrum of the structure, corresponding to two sub-energy levels of the In0.53Ga0.47As quantum well. It is found that the photoluminescence intensities of the two peaks vary with the V/III ratio, for which the reasons are discussed.

Defect-induced enhancement of absorption coefficient and electroabsorption properties in GaN/AlGaN centered defect quantum box (CDQB) nanocrystal

In this paper, effect of an introduced cubic defect on electrical and optical properties of cubic quantum dot is studied. Self-consistent solution of the Schrödinger–Poisson equations for evaluation of the proposed complex quantum dot is used. Optical properties (absorption and electroabsorption properties associated with intersublevel transition) of the proposed structure are also investigated using density matrix method. Effects of defect size on energy levels, carrier density, matrix element and optical linear absorption coefficient of centered defect quantum box (CDQB) are examined. It is shown that with increasing the defect size a considerable enhancement in magnitude of the absorption coefficient and also red-shift in resonance frequency are achievable. We show that the CDQB has higher absorption peak (at least 80 times) and tunable absorption spectra, due to increase of the matrix element and modified energy sublevels, compared quantum box structure without defect. Also, it is shown that the defect enhances electroabsorption properties (modulation bandwidth and the maximum variation of absorption peak with external field) of the quantum box structure.

GaN Doped with Neodymium by Plasma-Assisted Molecular Beam Epitaxy for Potential Lasing Applications

AbstractWe provide an investigation of in situ doping of GaN with the RE element Nd by plasma assisted-molecular beam epitaxy (PA-MBE). GaN epilayers are grown on c-plane sapphire and free standing GaN substrates and the Nd doping is controlled by an effusion cell. The ideal growth conditions for Nd incorporation maintaining crystal quality in GaN were investigated. The optical absorption characteristics indicate that the GaN:Nd epilayer remains transparent at the Nd emission wavelength of interest. For the highest Nd effusion cell temperatures, Rutherford backscattering and secondary ion mass spectrometry data indicate ˜5 at. % in epilayers grown on c-plane sapphire. X-ray diffraction found no evidence of phase segregation up to ˜1 at. % Nd. The highest luminescence intensities correspond to a doping range 0.05-1 at. %, with the strongest emission occurring at 1.12 eV (1107 nm). We also present the Stark energy sublevels of Nd3+ ions in GaN as determined by luminescence spectra. Photoluminescence excitation spectra reveal an optimal excitation energy of 1.48 eV (836 nm). We correlate the photoluminescence spectra with transitions from the 4F3/2 excited state to the 4I9/2, 4I11/2, and 4I13/2 multiplets of the Nd3+ ion for above (325nm) and below (836nm) bandgap excitation. Spectral correlation of the Nd emission multiplets in addition to site-selective spectroscopy studies using combined excitation-emission spectroscopy with confocal microscopy indicate enhanced substantial doping at the Ga site compared to other techniques (ion implantation and co-sputtering).

Hydrogen Bonds between Nitrogen Donors and the Semiquinone in the Qi-site of the bc1 Complex

The ubisemiquinone stabilized at the Qi-site of the bc1 complex of Rhodobacter sphaeroides forms a hydrogen bond with a nitrogen from the local protein environment, tentatively identified as ring N from His-217. The interactions of 14N and 15N have been studied by X-band (approximately 9.7 GHz) and S-band (3.4 GHz) pulsed EPR spectroscopy. The application of S-band spectroscopy has allowed us to determine the complete nuclear quadrupole tensor of the 14N involved in H-bond formation and to assign it unambiguously to the Nepsilon of His-217. This tensor has distinct characteristics in comparison with H-bonds between semiquinones and Ndelta in other quinone-processing sites. The experiments with 15N showed that the Nepsilon of His-217 was the only nitrogen carrying any considerable unpaired spin density in the ubiquinone environment, and allowed calculation of the isotropic and anisotropic couplings with the Nepsilon of His-217. From these data, we could estimate the unpaired spin density transferred onto 2s and 2p orbitals of nitrogen and the distance from the nitrogen to the carbonyl oxygen of 2.38+/-0.13A. The hyperfine coupling of other protein nitrogens with semiquinone is <0.1 MHz. This did not exclude the nitrogen of the Asn-221 as a possible hydrogen bond donor to the methoxy oxygen of the semiquinone. A mechanistic role for this residue is supported by kinetic experiments with mutant strains N221T, N221H, N221I, N221S, N221P, and N221D, all of which showed some inhibition but retained partial turnover.

Energy band structure and spectral gain characteristics of dilute-nitride zinc blende InGaNAs quantum wells embedded in GaAs and GaNAs barriers

The spectral gain characteristics of dilute-nitride zinc blende InxGa1−xNyAs1−y quantum wells embedded in GaNy1As1−y1 barriers have been investigated experimentally and theoretically. Two samples, both with the gain peak at 1300nm, were studied for comparison. One has a high nitrogen concentration in the quantum well with the surrounding barriers being pure GaAs. The other has a lower and uniform nitrogen concentration in the quantum well and the barriers (GaNAs barriers). Measurements show the redshift of the gain peak induced by the incorporation of nitrogen and difference in the spectral gain characteristics. The energy band structures and spectral gain characteristics are analyzed theoretically using the standard eight-band k∙p theory. It is shown that the introduction of nitrogen atoms in the GaAs barriers reduces the barrier height for the central quantum well so that the energy sublevels in the conduction band becomes condensed. The condensation of the conduction-band energy sublevels reduces the peak gain and makes the gain spectrum narrower, in agreement with measurements.

Multiple frequency radiofrequency-optical resonance in a four-level model of rubidium atoms with optical pumping

We consider a four-level model for alkali metal atoms with optical pumping by nonresonant light under conditions when magnetic dipole transitions are induced between energy sublevels of the hyperfine structure in the ground state. We present the dependences of the observed signal as a function of the frequency detuning of the applied rf fields relative to the resonant value, calculated in the density matrix formalism. We note the absence of a light shift in the radiofrequency-optical resonance signal, independent of the amplitude of the rf field and the optical and thermal relaxation rates. We show that when using a modulation technique for phase detection of the signal, its maximum discrimination ability is observed under conditions for simultaneous modulation of the pump light intensity and the frequency of the rf field, which in principle does not occur in the classical two-level model for optically oriented atoms in magnetic resonance.

Use of fluorescence spectroscopy of hydrogen atoms and BH molecules for measurements of electric fields in a plasma

Two laser-aided techniques for electric field (EF) measurements in a plasma have been realized. The first technique is based on the use of Stark splitting of high-lying levels of atomic hydrogen, whereas the second technique is based on the use of Stark mixing of the Λ-doublet energy sublevels of boron monohydride molecules. Both diagnostic techniques have been applied to the measurements of EFs in a hollow cathode discharge. The results of the EF measurements obtained by both techniques are in good agreement. The strength of the EF in a hollow cathode discharge was in the range of 1000–2600 V cm−1 for the dark space region and in the range of 200–300 V cm−1 for the negative glow region.

Enhanced photocatalytic activity of Ce 3+–TiO 2 for 2-mercaptobenzothiazole degradation in aqueous suspension for odour control

A series of cerium ion-doped titanium dioxide (Ce 3+–TiO 2) catalysts with special 4 f electron configuration was prepared by a sol–gel process and characterized by Brunauer-Emmett-Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce 3+–TiO 2 catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce 3+–TiO 2 catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs. The experimental results verified that both the adsorption equilibrium constant ( K a) and the saturated adsorption amount ( Γ max) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti 3+, Ce 3+, and Ce 4+ ions reside in the Ce 3+–TiO 2 catalysts. The results of DRS analysis indicated that the Ce 3+–TiO 2 catalysts had significant optical absorption in the visible region between 400 and 500 nm because electrons could be excited from the valence band of TiO 2 or ground state of cerium oxides to Ce 4 f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4 f level) in Ce 3+–TiO 2 might be a critical reason to eliminate the recombination of electron-hole pairs and to enhance the photocatalytic activity.

Current and capacitance characteristics of a metal–insulator–semiconductor structure with an ultrathin oxide layer

By self-consistently solving the Schrödinger and Poisson equations, we have studied theoretically the current and capacitance characteristics of a metal–oxide–semiconductor structure consisting of an ultrathin oxide layer embedded between an n-silicon substrate and n+-polycrystalline silicon gate. It has been shown that due to the finite doping level in the polycrystalline silicon gate and the complication of the energy sublevels in the accumulation layer, self-consistent calculation of Schrödinger and Poisson equations is of vital importance. Decreasing the oxide layer thickness drastically increases the conduction current density from the substrate to the polycrystalline silicon gate. The capacitance increases monotonically as a function of the gate bias, it gradually saturates. The theoretical results explain well our experimental data.

Photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic HEMTs with different thickness of spacer layer

The photoluminescence spectra of the single δ-doped AlGaAs/InGaAs/GaAs pseudomorphic HEMTs with different thickness of spacer layer were studied. There are two peaks in the PL spectra of the structure corresponding to two sub-energy levels of the InGaAs quantum well. It was found that the photoluminescence intensity ratio of the two peaks changes with the spacer thickness of the pseudomorphic HEMTs. The reasons were discussed. The possible use of this phenomenon in optimization of pseudomorphic HEMTs was also proposed.

Magnetooptical Spectroscopy of the 5D4 → 7F5 Radiative Transition of the Non-Kramers Tb3+ Ion in an Yttrium-Aluminum Garnet

Spectra of luminescence and magnetic circular polarization of luminescence corresponding to the 5 D 4 → 7 F 5 radiative transition of the Tb3+ ion in an yttrium-aluminum garnet are studied experimentally. In the context of crystalline field theory, the energy spectrum of the 7 F 5 multiplet of the Tb3+ ion in the garnet is calculated. The structure of internal energy levels of the 7 F 5 multiplet of the Tb3+ ion is refined based on a comparison of the calculated results with the experimental data, and the diagram of magnetooptically active radiating transitions between 5 D 4 and 7 F 5 multiplet energy sublevels is constructed.

Model of temperature dependence for gain shape of erbium-doped fiber amplifier

The problem of modeling the temperature dependence of erbium-doped fiber amplifier (EDFAs) is important for multichannel optical WDM systems. A physical model is presented in this paper, which could be used to predict the gain change under temperature variations for such systems. Some of the input parameters for the model are the erbium energy sublevel density, excitation coefficients from lower sublevels to upper sublevels of erbium ions, and electron distribution over energy levels. It is difficult to measure these parameters. In order to use the model for gain shape calculations, some simplifications are demonstrated. These simplifications lead to two numerical models, which are shown to be consistent with experimental data with reasonable accuracy, and are based only on two spectral measurements for different temperatures. Both numerical models were tested for the signal band and the 980 nm pump band of a typical erbium-doped fiber.

Relaxation-stimulated resonances in Mössbauer absorption spectra under radiofrequency magnetic field excitation

A new type of resonant phenomena in the Mössbauer spectroscopy of soft magnetic materials in radiofrequency (rf) magnetic fields is predicted for the rf field frequencies related to those of components of the magnetic hyperfine structure via the parametric resonance condition. Resonant effects of this kind are still unknown in physics; they are realized not at frequencies of transitions between the energy sub-levels of the ground or excited nuclear states, which take place in conventional nuclear magnetic resonance, but at frequencies which are a combination of the frequencies of hyperfine transitions. In this case a separation between the real energy levels can exceed the value of the `resonant' frequency by about 10 orders of magnitude and the resonant rf field sets as if in coherency between these energy levels. Such resonances can only be observed when relaxation processes play an essential role, which permits them to be defined as the relaxation-stimulated resonances. The simplest relaxation model of so-called one-way and localized relaxation is proposed. Within this model an analytical expression for Mössbauer absorption spectra is derived, based on which the specific features of the new resonant effects are analysed in detail.

Second order mesoscopic electric susceptibility in AlxGa1−xAs/GaAs quantum wells

It has been demonstrated that it is possible to obtain frequency doubling in the infrared by using two quantum wells so that light is absorbed at frequency f by electron up-transitions between adjacent energy sublevels while down-transition between nonconsecutive levels provides frequency doubling. The barrier heights were the same all over the structure. In this paper we numerically study what might be gained by using a third compound in the barrier. This would allow tailoring of not only the barrier width but also the barrier height. Results are for the AlxGa1−xAs/GaAs heterostructure material and predict that depending on the frequency to be tuned, and hence on the well structure, an improvement of fifteen to forty percent on the second order susceptibility might be possible.

Excitons in nanocrystals

The excitation energies can be described in the frame of the effective mass approximation when the crystals are not too small. The lowest energy exciton state is split in several sublevels by the electron—hole exchange interaction and by the crystal field in wurtzite-type semiconductors. We determine the oscillator strength of the transitions. The optical transition to the lowest energy sublevel is forbidden by spin conservation law. The intensity of the transition to the upper sublevel in cubic nanocrystals is proportional to the number of nanocrystals per unit volume in the strong confinement regime and to the volume fraction occupied by the nanocrystals in the weak confinement regime. We show that, in a cubic or a wurtzite-type nanocrystal, all the sublevels are shifted to higher energy by the polarization of the medium and that the intensity of the transition to the lowest energy optically allowed sublevel in a wurtzite-type nanocrystal is very much decreased by the polarization of the medium. The energy relaxation of excited electron—hole pairs is made possible by the coupling to acoustical and optical phonons. We conclude that no “phonon bottleneck” exists in the relaxation. We show that Auger effect is responsible for “photodarkening” of the samples under illumination. It is the source of the random telegraph signal observed in the luminescence of a single nanocrystal.

Optical pumping in a L-system by parametric luminescence light

We examine the optical pumping effect in an ensemble of three-level atoms with a Λ configuration of the energy sublevels excited by parametric luminescence light in the squeezed state. We derive quantum kinetic equations that describe the evolution of the density matrix of atoms irradiated by low-intensity squeezed light with a finite-width spectrum. In particular, we show that because of the quantum statistical properties of the squeezed light there can be a redistribution of atoms among the lower energy sublevels, despite the equality of the intensities of the spectral components of the light that resonantly excites optical transitions in the Λ-system. The relation of the optical pumping effect to the correlation and spatial-temporal spectral properties of squeezed light is discussed in detail. Finally, we show that the effects are closely linked to the finiteness of the width of the squeezed-light spectrum.

A proposal to measure atomic alignment and orientation parameters using laser photon deflection

In this paper, it is pointed out that the optical force which an atom experiences when excited by near-resonant laser light can under certain conditions depend not only on the atomic energy sublevel populations, but also on the horizontal coherences between the sublevels. A theoretical investigation of this behaviour is presented and a novel technique is proposed for applying photon deflection of atoms to the measurement of atomic collision alignment and orientation parameters.

A study of the relative emission intensity and polarization for Nd3+ ions in an NdAl3(BO3)4 crystal

On the basis of both the crystal-field theory and the Judd-Ofelt method, the relative intensities and polarization characteristics of the individual transitions between the manifolds 4F3/2 to 4I11/2 and 4F3/2 to 4I9/2 for Nd3+ ions in an NdAl3(BO3)4 crystal were studied by experimental fluorescence spectroscopy and branching ratios. The theoretical results are in agreement with those of laser and fluorescence experiments. In this study, the crystal-field wavefunctions were obtained by fitting the Stark energy sublevels with a constraint condition imposed on the electrostatic ratios of the crystal-field parameter belonging to the same order of k, in a similar manner to that proposed by the present authors previously and referred to as the quasi-three-parameter method. It was shown that the minimum obtained in this way was the only solution corresponding to physical reality.