The Dicke model in quantum optics: Dicke model revisited

A short review of recent developments of the Dicke model in quantum optics is presented. The focus is on the model in a cavity at zero temperature and in the rotating wave approximation. Topics discussed include spectroscopic structures, the giant quantum oscillator, entanglement and phase transitions.     

Diffusion-drift model of the transport of charge carriers and photons in injection lasers

A mathematical model is proposed that can be used for numerical analysis of the dynamics of processes in injection lasers with allowance for their structural features; nonuniform spatial distributions of electrons, holes, and photons; and various mechanisms of radiative recombination. Results of numerical modeling of double-heterostructure injection lasers performed using the proposed model are compared to the results obtained using the equations of laser kinetics. Boundaries of applicability of these models are considered.

     

On a Modification of the Dicke Model

The modification of the Dicke model for the case of long-wave photons, describing the direct dipole-dipole interaction and dipole-phonon interaction, is constructed. It is shown that the phonon contribution can lead to a change in phase transition order and plays the role of a supplementary ordering factor.     

Novel nano-structured glasses containing semiconductor quantum dots: controlling the photoluminescence with phonons and photons

Effects of temperatures and excitation intensities on the photoluminescence properties of PbS quantum dots precipitated in the glass were investigated. Peak wavelength of the near-infrared photoluminescence shifted towards the short wavelength side with an increase in temperature and excitation intensity. The largest shift in the peak wavelength of the photoluminescence bands was approximately 90 nm. The temperature coefficient of band gap energy (deduced from the photoluminescence wavelength) of quantum dots varied from 230 to 28 μeV/K under the excitation intensity of 50–600 mW. The integrated photoluminescence intensity also showed similar dependencies on temperature and excitation intensity. The shifts of the photoluminescence with changes in the temperature and excitation intensity were associated with the trapping and re-activation of charge carriers at defect sites located at the QDs/glass interface and inside the glass matrix.     

Theoretical study on controllability of quantum state energy in an InGaAs/GaAs quantum dot buried in InGaAs

We theoretically studied the relationship between quantum energy states and structural parameters of an InGaAs/GaAs quantum dot (QD) buried in strained InGaAs, emitting at 1.1 to 1.4 em. The crystal distortion of the buried QD structure in three dimensions was computed based on the three-dimensional finite element method. Under the computed strain fields, the Schr?dinger equation was solved to obtain wavefunctions and eigenenergies. By calculating the dependence on structural parameters, we investigated the controllable range of the ground state energy and the energy separation between the ground state and the first excited state. We found that the energy separation exhibited a maximum value as a function of QD composition, enabling us to identify the composition of the QD structure. The effects of the burying layer composition and QD diameter were also investigated to expand the controllable range of the state energy. We also showed that the wavefunction symmetry was improved by burying the QD in the InGaAs layer. Our results will be useful in developing advanced devices for optical telecommunications and quantum information technology.     

Determining the internal quantum efficiency of PbSe nanocrystal solar cells with the aid of an optical model

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1+/-0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60+/-5% and 80+/-7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE<100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.     

Effect of the vertical correlation of quantum dots on diffuse X-ray scattering

The effect of the vertical correlation of quantum dots (QDs) on diffuse X-ray scattering from multilayer structures has been studied in the framework of a statistical theory of X-ray diffraction. A model of the long-range structural order for the spatial distribution of vertically stacked QDs is considered. Diffuse X-ray scattering in superlattices with QDs has been numerically simulated. The obtained results are applied to a quantitative analysis of heterostructures with QDs using experimental data of high-resolution X-ray diffractometry.     

Rough model of a quantum liquid

As applied to condensed helium and hydrogen, a theory of quantum effects is suggested, one that is insensitive to the type of statistics of particles—the theory of quantum nondegenerate liquids. Evidently such a theory assumes that the mass of the atom and its polarization are small. For a zero-approximation, a system of hard spheres is chosen, and the attractive forces and the softness of the atom are considered to be small effects. The correlations between thermodynamic and kinetic characteristics of liquid He³ and He⁴ are obtained. On the basis of the experimental data for He⁴, and an isotropic law of the corresponding states for quantum systems with a large value of the de Boer parameter, all thermodynamic functions of liquid He³ are defined and tabulated in the regions of temperatures and molar volumes where they have not yet been measured. A rough model of a quantum liquid is suggested, based on the assumption of a dominating contribution by the diffusion excitations to the free energy of a dense condensed medium. The equation of state of the quantum liquid is derived. On the basis of the experimental data for He⁴, the equation of state of a quantum liquid consisting of hard spheres is compared with the similar equations obtained by a numeric computer simulation. The energy of the ground state of spin-oriented condensed hydrogen isotopes is defined.     

Quantifying the correlation between spatially defined oxygen gradients and cell fate in an engineered three-dimensional culture model

A challenge in three-dimensional tissue culture remains the lack of quantitative information linking nutrient delivery and cellular distribution. Both in vivo and in vitro, oxygen is delivered by diffusion from its source (blood vessel or the construct margins). The oxygen level at a defined distance from its source depends critically on the balance of diffusion and cellular metabolism. Cells may respond to this oxygen environment through proliferation, death and chemotaxis, resulting in spatially resolved gradients in cellular density. This study extracts novel spatially resolved and simultaneous data on tissue oxygenation, cellular proliferation, viability and chemotaxis in three-dimensional spiralled, cellular collagen constructs. Oxygen concentration gradients drove preferential cellular proliferation rates and viability in the higher oxygen zones and induced chemotaxis along the spiral of the collagen construct; an oxygen gradient of 1.03 mmHg mm⁻¹ in the spiral direction induced a mean migratory speed of 1015 μm day⁻¹. Although this movement was modest, it was effective in balancing the system to a stable cell density distribution, and provided insights into the natural cell mechanism for adapting cell number and activity to a prevailing oxygen regime.     

Experimental study of a model digital space optical communication system with new quantum devices

We report a digital space optical communication system with new features both in the transmitting and in the receiving ends. The diode laser source is stabilized to within ±100 kHz by locking its frequency to the transmission peak of a Faraday anomalous dispersion optical filter (FADOF). The optical filter in the receiver uses two FADOF's that are linked to eliminate the multipeak structure and achieve a single-peak bandwidth of ~1 GHz. The detection sensitivity of this system is 23 times higher than that of a system with a traditional interference filter.     

Observation of the fractional quantum Hall effect in an oxide

The quantum Hall effect arises from the cyclotron motion of charge carriers in two-dimensional systems. However, the ground states related to the integer and fractional quantum Hall effect, respectively, are of entirely different origin. The former can be explained within a single-particle picture; the latter arises from electron correlation effects governed by Coulomb interaction. The prerequisite for the observation of these effects is extremely smooth interfaces of the thin film layers to which the charge carriers are confined. So far, experimental observations of such quantum transport phenomena have been limited to a few material systems based on silicon, III-V compounds and graphene. In ionic materials, the correlation between electrons is expected to be more pronounced than in the conventional heterostructures, owing to a large effective mass of charge carriers. Here we report the observation of the fractional quantum Hall effect in MgZnO/ZnO heterostructures grown by molecular-beam epitaxy, in which the electron mobility exceeds 180,000 cm(2) V(-1) s(-1). Fractional states such as ν = 4/3, 5/3 and 8/3 clearly emerge, and the appearance of the ν = 2/5 state is indicated. The present study represents a technological advance in oxide electronics that provides opportunities to explore strongly correlated phenomena in quantum transport of dilute carriers.     

A quantitative study of the quantum interferometer in pure magnesium

The quantum interferometer oscillations observed in the transverse magnetoresistance of Mg at liquid helium temperatures forJ[11 0] andH[1 00] have been experimentally investigated in detail. A new method of analyzing these quantum interference oscillations has been developed which permits direct line shape comparison between the experimentally observed magnetoresistance oscillations and theoretical calculations based on the stacked-mirror model of quantum transport. By using the stacked-mirror model in conjunction with the line shape comparison technique, virtually all of the approximations made in the initial studies of the quantum interferometer have been eliminated. Distinct and striking line shape changes in the magnetoresistance oscillations, critically dependent on the angle between the direction of the applied magnetic field and the basal plane of the Mg crystal, have also been observed and analyzed. This phenomenon provides direct evidence for the existence of long-range quantum phase coherence of the electron states in the Mg crystal over dimensions of 0.3 mm, corresponding to a quantum state lifetime of 3.5×10^–10 sec. Experimental values for the three magnetic breakdown parameters that characterize the quantum interferometer in pure Mg areH ₁(k _H =0)=3000±75 G,H ₂(k _H =0)=10,000±1500 G, andH ₃(k _H =0)=1000±250 G. Experimental evidence is also presented which shows a magnetic field dependence of the quantum state lifetime for some of the crystals studied.Presented as a thesis to the Department of Physics, The University of Chicago, in partial fulfillment of the requirements for the Ph.D. degree.Supported by NSF Grant GS-39932.     

Improved model of a superconducting quantum magnetometer

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 47–49, June, 1989.     

Designing spatial correlation of quantum dots: towards self-assembled three-dimensional structures

Buried two-dimensional arrays of InP dots were used as a template for the lateral ordering of self-assembled quantum dots. The template strain field can laterally organize compressive (InAs) as well as tensile (GaP) self-assembled nanostructures in a highly ordered square lattice. High-resolution transmission electron microscopy measurements show that the InAs dots are vertically correlated to the InP template, while the GaP dots are vertically anti-correlated, nucleating in the position between two buried InP dots. Finite InP dot size effects are observed to originate InAs clustering but do not affect GaP dot nucleation. The possibility of bilayer formation with different vertical correlations suggests a new path for obtaining three-dimensional pseudocrystals.     

Determination of the optical properties of a two-layer tissue model by detecting photons migrating at progressively increasing depths

We have investigated a method for solving the inverse problem of determining the optical properties of a two-layer turbid model. The method is based on deducing the optical properties (OPs) of the top layer from the absolute spatially resolved reflectance that results from photon migration within only the top layer by use of a multivariate calibration model. Then the OPs of the bottom layer are deduced from relative frequency-domain (FD) reflectance measurements by use of the two-layer FD diffusion model. The method was validated with Monte Carlo FD reflectance profiles and experimental measurements of two-layer phantoms. The results showed that the method is useful for two-layer models with interface depths of >5 mm; the OPs were estimated, within a relatively short time (<1 min), with a mean error of <10% for the Monte Carlo reflectance profiles and with errors of <25% for the phantom measurements.     

The microdosimetry of low-energy photons in radiotherapy

Low energy photons are more and more in use in clinical practice, for treatment in radiotherapy as well as for imaging purposes. Their relative biological effectiveness is however still debated. In this paper, some microdosimetric parameters have been calculated for different sources: (125)I, (103)Pd, (131)Cs, an electronic brachytherapy source and various clinical mammography X-ray qualities. These parameters have been used to deduce the quality factors as defined in ICRU 40.     

Potential barrier study at the InGaAs/InAs quantum dot interface in asymmetric multi quantum well structures

The photoluminescence and its temperature dependences have been investigated for the InAs quantum dots embedded in the asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs quantum wells (dot-in-a-well, DWELL structures) as a function of the In content x (x = 0.10-0.25) in the capping In_xGa_1−xAs layer. The study of PL temperature dependences in the range of 80-120 K has revealed the potential barrier for electrons at the capping In_xGa_1−xAs/InAs QD interface. The value of mentioned barrier has varied in QD structures with the different In_xGa_1−xAs capping layer compositions and it was estimated in the studied asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs DWELL structures. It is shown that the barrier for electrons equal to 48 and 24 meV appears in the InGaAs conduction band at the formation of InGaAs QW/InAs QD interface for the In compositions of x = 0.10 and 0.15, respectively. This barrier has not been detected in DWELLs with the In compositions x = 0.20 and 0.25. The energy gap offsets at the InAs/In_xGa_1−xAs interface in studied structures has been estimated and discussed as well.     

The correlation model of acoustic emission in fine diamond turning

The process of cutting of an aluminum alloy has been monitored by the acoustic emission method. Single-crystal natural and synthetic diamond tools have been used in the investigation. The comparative analysis has demonstrated that the correlation model of acoustic emission, which was put forward by Pan and Dornfeld, is valid for the conditions of fine diamond turning of aluminum alloys.     

Exact solutions of a linear model of quantum solids

The ground state energyE of a linear quantum crystal can be calculated exactly by the transfer integral method, if the total wave function is assumed to be a product of single-particle functions ? and Jastrow factorsf between nearest neighbors only. The variation ofE with respect tof leads to a Schrödinger equation containing an effective potential, which consists of the bare interaction between two particles and a part due to the influence of all other particles. This equation is coupled to the transfer integral equation. For any given ? the coupled system of equations can be solved in a self-consistent way. The one- and two-particle distribution functions and the effective potential can also be computed exactly. In a similar way for any givenf the optimal ? can be determined.     

Two-lifetime model calculations of the quantum interference dominated transverse magnetoresistance of magnesium

The magnetoresistance of extremely pure, strain-free magnesium is dominated by the transport properties of a narrow slab of coupled orbits in the geometry in whichJ [11 0] andH lies in the (11 0) plane. In samples whose quantum state lifetime _s exceeds 10^–10 sec the conductivity of the coupled orbit system is controlled by _s as well as the large-angle scattering lifetime ₁. The magnetoresistance then exhibits complicated behavior, oscillating with |H| and varying rapidly with field orientation. The behavior withH aligned to within about 0.001° of [10 0] is due to manifold multiply connected open orbits, whereas forH tilted away from [10 0] by more than this the behavior is due to extended, multiply connected closed orbits. Detailed models for both these regimes are developed, taking into account the interference effects resulting from the feedback paths. Within these models the transport properties are calculated exactly, taking into account to all orders the amplitude propagation of wave packets within the network. Results are compared to experimental data and previous calculations, and the limitations of the model are discussed.Work supported in part by NSF grant DMR 7910533.