Mid-infrared intersubband emission from optically pumped asymmetric coupled quantum wells

Mid-infrared optical emission due to intersubband transitions between excited conduction subbands of a coupled quantum well structure is studied. The emission process is based on optical pumping of free carriers from the ground subband into the third subband followed by a radiative transition from the third subband into the second subband and a fast phonon assisted relaxation into the ground subband. We have observed spontaneous emission at 14 μm that persists up to room temperature. Our results indicate that population inversion between conduction subbands and large stimulated gain can be achieved.     

相变及空位缺陷对高压下GeO_2光学性质的影响

本文采用第一性原理方法,在100 GPa的压力范围内,计算了GeO_2理想晶体和含锗、氧空位点缺陷晶体的光学性质.吸收谱数据表明,压力诱导的三个结构相变对GeO_2晶体的吸收谱均有影响:第一个相变将导致其吸收边蓝移,而第二和第三相变将使得其吸收边红移.锗和氧空位点缺陷的存在将导致GeO_2的吸收边红移,但氧空位点缺陷引起的红移更明显.尽管如此,分析发现,在100 GPa的压力范围内,压力、相变以及空位点缺陷等因素都不会导致GeO_2晶体在可见光区出现光吸收现象(是透明的).波长在532 nm处的折射率数据显示,在GeO_2的四个相区,其折射率均随压力增加而降低;而且,GeO_2的三个结构相变以及锗、氧空位点缺陷都会导致其折射率有所增大.本文预测,GeO_2有成为冲击光学窗口材料的可能.     

Density of states measured by scanning-tunneling spectroscopy sheds new light on the optical transitions in PbSe nanocrystals

The density-of-states function of individual colloidal PbSe nanocrystals varying in diameter between 3 and 7 nm is measured by resonant tunneling spectroscopy. It is in semiquantitative agreement with tight-binding calculations, but the energy separation between electron (hole) levels of S and P symmetry is systematically smaller than predicted by the theory. These results provide an explanation for the second and third excitonic optical transitions, which have been debated for a long time.     

Electromagnetically induced transparency in asymmetric double quantum wells

We show how to compute nonlinear optical absorption spectra of an Asymmetric Double Quantum Well (ADQW) in the region of intersubband electronic transitions. The method uses the microscopic calculation of the dephasing due to electron-electron and electron-phonon scattering rates and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. The polarization dephasing and the corrections to the Rabi frequencies due to the electron-electron interaction are also taken into account. For a proper choice of the QW widths and of the driving fields we obtain electromagnetically induced transparency. This transparency has a very narrow linewidth when a single driving field is applied resonant to the transition between the second and the third subband. In the case of two resonant driving fields or of a driving field resonant between the first and third subband we obtain a large transparency enhancement over the entire absorption spectrum. Results are given for GaAs/GaAlAs QWs and experiments are proposed. Received 21 June 2001 and Received in final form 21 January 2002     

Phase behavior of DODAB aqueous solution

Phase behavior of DODAB aqueous solution, prepared without sonication, was studied by adiabatic scanning calorimetry. Measurements revealed four phase transitions with the temperatures 35.2, 39.6, 44.6, and 52.4°C at heating and one transition at the temperature 40.4°C at cooling. The first three transitions at heating occur in unilamellar vesicles. The first and third transitions correspond to the subgel-gel and gelliquid phase transitions, corresponding enthalpy jumps are equal to 33 and 49 kJ/mol. The second transition appears after some aging and is similar to gel-ripple phase transition in a DPPC solution, with the enthalpy jump under the transition exceeding 7.4 kJ/mol. The transition occurs in unilamellar vesicles. The transition at the temperature 52.4°C occurs in another subsystem of the solution, which we believe to be multilamellar vesicles. The enthalpy jump at this transition is equal to 97 kJ/mol, and data analysis suggests that this is a subgel-liquid transition. The phase transition at cooling is the liquid-gel transition in unilamellar vesicles. During the measurements, a slow evolution of the solution occurs, consisting in a change of concentrations of unilamellar and multilamellar vesicles. This transformation mainly occurs at low temperatures.     

Expanding the thermodynamical potential and analysis of the possible phase diagram of deconfinement in the FL model

Deconfinement phase transition is studied in the FL model at finite temperature and chemical potential. At MFT approximation, phase transition can only be first order in the whole μ-T phase plane. Using a Landau expansion, we further study the phase transition order and the possible phase diagram of deconfinement. We discuss the possibilities of second order phase transitions in the FL model. From our analysis, if the cubic term in the Landau expansion could be cancelled by the higher order fluctuations, second order phase transition may occur. By an ansatz of the Landau parameters, we obtain a possible phase diagram with both the first and second order phase transitions, including the tri-critical point which is similar to that of the chiral phase transition.     

Phase transitions in KIO(3)

The high-pressure behavior of KIO(3) was studied up to 30?GPa using single crystal and powder x-ray diffraction, Raman spectroscopy, second harmonic generation (SHG) experiments and density functional theory (DFT)-based calculations. Triclinic KIO(3) shows two pressure-induced structural phase transitions at 7?GPa and at 14?GPa. Single crystal x-ray diffraction at 8.7(1)?GPa was employed to solve the structure of the first high-pressure phase (space group R3, a?=?5.89(1) ?, α?=?62.4(1)°). The bulk modulus, B, of this phase was obtained by fitting a second order Birch-Murnaghan equation of state (eos) to synchrotron x-ray powder diffraction data resulting in B(exp,second)?=?67(3)?GPa. The DFT model gave B(DFT,second)?=?70.9?GPa, and, for a third order Birch-Murnaghan eos, B(DFT,third)?=?67.9?GPa with a pressure derivative of [Formula: see text]. Both high-pressure transformations were detectable by Raman spectroscopy and the observation of second harmonic signals. The presence of strong SHG signals shows that all high-pressure phases are acentric. By using different pressure media, we showed that the transition pressures are very strongly influenced by shear stresses. Earlier work on low- and high-temperature transitions was complemented by low-temperature heat capacity measurements. We found no evidence for the presence of an orientational glass, in contrast to earlier dielectric studies, but consistent with earlier low-temperature diffraction studies.     

Singular renormalization group transformations and first order phase transitions (II). Monte Carlo renormalization group results

The first order phase transitions in the two-dimensional 10-state Potts model and in the two-dimensional Ising model with magnetic field are studied with Monte Carlo renormalization group methods. The deconfining phase transition of the four-dimensional U(1) lattice gauge theory is treated similarly. The results are not consistent with the standard discontinuity fixed point picture of first order phase transitions. In the U(1) case, where this possibility exists, they are not consistent with a second order phase transition either. The results show a discontinuous flow on the first order transition surface, which is a Monte Carlo renormalization group signal of singular RG transformations.     

Rydberg spectroscopy in an optical lattice: blackbody thermometry for atomic clocks

We show that optical spectroscopy of Rydberg states can provide accurate in situ thermometry at room temperature. Transitions from a metastable state to Rydberg states with principal quantum numbers of 25-30 have 200 times larger fractional frequency sensitivities to blackbody radiation than the strontium clock transition. We demonstrate that magic-wavelength lattices exist for both strontium and ytterbium transitions between the metastable and Rydberg states. Frequency measurements of Rydberg transitions with 10(-16) accuracy provide 10 mK resolution and yield a blackbody uncertainty for the clock transition of 10(-18).     

Theoretical studies of polarization dependent electro-optical modulation in lattice matched and strained multi-quantum well structures

We report on polarization dependent optical absorption for excitonic and interband transitions in lattice matched (GaAs/AlGaAs) and strained (biaxial tensile strain - GaAsP/AlGaAs; biaxial compressive strain - InGaAs/AlGaAs) multiquantum well structures in the presence of transverse electric fields. The hole states are solved by using the Kohn-Luttinger Hamiltonian and using an eigenvalue technique. The effect of heavy-hole and light-hole mixing due to the strain, electric field and quantization is studied. Under biaxial tensile strain the heavy-hole and light-hole transition can coincide, leading to interesting polarization dependent effects. Results are presented for excitonic and interband transitions.     

A study of the structural phase transition in strontium titanate single crystal by coherent and incoherent second optical harmonic generation

The surface phase transition in a SrTiO₃ crystal was studied by second optical harmonic generation. Nonlinear optical response singularities were observed at temperature T*=145 K, which was 40 K higher than the T_c structural phase transition temperature in the crystal volume. Nonlinear critical opalescence in the crystal volume caused by the presence of point defects was studied. The second harmonic field and the intensity of critical opalescence were calculated based on the phenomenological model of nonlinear optical processes with the use of the Landau theory of phase transitions.     

Phase transitions for an ideal Bose condensate trapped in?a?quartic potential

Based on the classification scheme of phase transitions, we study the phase transitions for an ideal Bose gas with a finite number of particles confined in a three-dimensional quartic trap. We show that the phase transition of an ideal Bose gas in the three-dimensional quartic trap is of third order for finite particle numbers, quite different from the fact that the phase transition is of first order in the thermodynamic limit. We discuss the effects of finite particle numbers on the nature of the phase transitions, and determine the dependence of transition temperature on particle number.     

Excitonic effects and optical properties of passivated CdSe clusters

We calculate the optical properties of a series of passivated nonstoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local-density approximation, and by solving the Bethe-Salpeter equation for optical excitations with the GW approximation for the self-energy. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level transition. In contrast, many-body methods predict a strong mixture of several different transitions, which is a signature of excitonic effects. The majority of the clusters have a series of dark transitions before the first bright transition. This may explain the long radiative lifetimes observed experimentally.     

Structural and electronic transitions in gadolinium iron borate GdFe<Superscript>3</Superscript>(BO<Superscript>3</Superscript>)<Superscript>4</Superscript> at high pressures

The optical properties and structure of gadolinium iron borate GdFe₃(BO₃)₄ crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ～26 GPa and ～43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ～2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ～0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO₃ crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.     

Particle number scale invariant feature of the states around the critical point of the first order nuclear shape phase transition

We study systematically the evolutive behaviors of some energy ratios,E2 transition rate ratios and isomer shift in the nuclear shape phase transitions.We find that the quantities sensitive to the phase transition and independent of free parameter(s) are approximately particle number N scale invariant around the critical point of the first order phase transition,similar to that in the second order phase transition.     

Optical studies of the ferroelastic phase transitions in KFe(MoO4)2

ABSTRACT

The ferroelastic phase transitions in KFe(MoO₄)₂ have been studied by means of polarized light microscopy. The crystal undergoes a sequence of ferroelastic phase transitions. It has been found that the second transition consists of two transitions separated by the temperature interval of about 0.4 K. Both these transitions are of the first order and are evidenced through a phase front passing, without the domain structure rebuilding. The disposition of optical indicatrix axes n_g, n_m has been established, and the birefringence has been measured in the plane (0001) in the temperature range covering all ferroelastic phases. From temperature studies of the morphic birefringence, a critical exponent of the order parameter has been estimated.     

Iodine swelling of polyacrylonitrile I. Effect of orientation and evidence for a three-phase structure

The swelling of polyacrylonitrile (PAN) film in aqueous I₂/KI (or I₃ ^?) solutions of different concentrations up to 2.0 molal has been investigated, using both biaxially oriented and unoriented film. The absorption curve (equilibrium absorption vs concentration) for oriented film shows three distinct steps, at concentrations of 0.25, 0.65, and 1.6 molal, leading to weight increases of the sample of approximately 250%, 2000%, and 3200%, respectively. In unoriented film, smooth progressive increases of absorption are observed in place of the first and third steps; only the second abrupt step remains. A significant difference in desorption behavior is seen in extraction experiments carried out above and below the second step. It is hypothesized that the first and third steps of swelling correspond to the penetration of two different “amorphous” phases in the polymer, which together with a crystalline phase would indicate a three-phase structure for PAN. The second step of swelling may represent a conformational transition of the molecules in the phase penetrated in the first step. The effects of iodine swelling and extraction on the dynamic mechanical spectrum of PAN were also investigated, and it seems possible to associate the swelling in the region of the first and third steps with the two peaks (or transitions) in the tan δ curve of PAN, which might be regarded as glass transitions of the two amorphous phases. An unusual behavior of sample length vs temperature was observed during the dynamic mechanical measurements in the temperature region of these two transitions. Stress-strain curves were also measured on samples containing different amounts of absorbed iodine.     

Comparative study of liquid-crystalline ordering in a monomer,linear polymer,and graft copolymer by the photon transmission technique

The photon transmission technique was used to study the phase transitions of a liquid crystalline acrylate monomer, 6-(4-cyanobiphenyl-4′oxy)hexyl acrylate (LC6), its homopolymer (PLC6) and its graft copolymer (GLC6) with polytetrahydrofuran grafts. The phase transitions were also confirmed by DSC and polarizing microscopy. We observed the phase transition sequence isotropic–nematic–smectic A–smectic C in the LC6 monomer. In PLC6 and GLC6 polymers, the nematic and smectic A phases appear dominant. The apparent nematic–smectic A transition is of first order in PLC6 and of second order in GLC6, with the transition temperature remaining the same. The effects of quenched random constraints introduced in GLC6 are consistent with the theory of quenched random interactions. The critical exponents were also evaluated.     

Two-step dissociation of a polaron in conjugated polymers

We have studied the electric-field-driven motion of a polaron by solving the time-dependent Schr\"{o}dinger equation nonadiabatically and the lattice equation of motion simultaneously. It is found that the polaron may experience two sequent transitions under high fields; one is the transition from the subsonic to the supersonic state, and the other from the supersonic to dissociated state. The acoustic mode is decoupled from the charge when the polaron moves at a speed faster than the sound speed, and then the optical mode is decoupled at the second transition to make the polaron dissociate completely.     

Structural phase transitions in CdTiO3

The reconstructive phase transition from the ilmenite-like CdTiO₃ modification to the perovskite modification is investigated thoroughly. It is revealed that the reconstructive transition is determined by size effects, results in the formation of the closest packing of the ilmenite CdTiO₃ structure, and is irreversible with a decrease in temperature. The perovskite CdTiO₃ modification undergoes displacive structural phase transitions at temperatures of 110, 220, and 380°C. The first displacive phase transition is isostructural, whereas the second and third transitions are associated with rotations of oxygen octahedra.