Temperature dependent optical properties of stacked InGaAs/GaAs quantum rings

In this paper we describe the results of temperature dependent photoluminescence intensity and decay time measurements of In(Ga)As/GaAs quantum rings where the depth of barrier is varied from sample to sample. The activation energy found for the reduction of the exciton decay time as a function of the temperature is approximately half the value of the thermionic escape energy of excitons. The temperature dependant behaviour is ascribed to the carriers lost via the excited state to the WL.

The time resolved PL study indicates that thermal escape mechanisms is not so affected by reducing the spacer thickness, but it's influenced essentially by the excited state recombination.     

Quantum Tunneling of the Magnetization In Mn12-Ac

We present new experiments on the zero-field magnetic relaxation in a single crystal of Mn12-acetate. This study shows the important role played by the interactions between the spins and their environment. It suggests the existence of two relaxation regimes. Above 2.0 K, the relaxation time follows the Arrhenius law. The time decay is not strictly exponential due to dipolar interactions. At lower temperatures, the relaxation time depends weakly on the temperature. A square root decay is observed, as recently predicted by a theoretical model in which the tunneling is assisted by dipolar and hyperfine interactions.     

Light-induced transmission changes in liquid crystal displays foradaptive pattern projection

The changes in the transmission and contrast properties of a super-twisted nematic liquid crystal display panel, induced by the light impinging on the panel itself, have been studied. Upon illumination by laser radiation with power density close to that used in practice, the transmittance of the panel increases or decreases according to the brightness voltage that sets the transmittance level. The dynamics of the transmittance change are typically bi-exponential, with a shorter decay time on the order of a fraction of a second and a longer decay on the order of tens of seconds. The observed changes were interpreted and modeled by considering local temperature changes in the crystal because of the light impinging on it. The temperature changes produce a shift of the transmittance curve and a change of slope in its central region. The presence of the dip after the Frederickzs region accounts for the anomalous behavior of the transmittance in that portion of brightness voltages     

Bi3+、Ce3+离子极大增强铁石榴石磁光法拉第效应微观机制研究

采用单离子晶格模型,以磁光量子理论计算了Bi 3+、Ce3+离子替代的稀土铁石榴石晶体中Bi3+、Ce3+离子的晶场能级,波函数及能态间电偶极跃迁几率,进而计算了Bi3+、Ce3+离子对法拉第旋转角θF的贡献,获得了与实验符合较好的计算结果,加深了对Bi3+、Ce 3+离子极大增强铁石榴石磁光效应微观机制的认识。,The split energy levels,occupation probabili ties,wave function and electrical dipole transition induced by crystal field and exchange interaction for the Ce3+ ions in Ce3+-substituted and the Bi3+ ions in Bi3+-substituted iron garnets at room temperature bas ed on quantum theory has been calculated.Furthermore,the calculation of the Fara day rotation contributed by the Ce3+ and Bi3+ ions has also been car ried out.The crystal field and the super-exchange interaction are two crucial f actors in determining the Faraday rotation.The theoretical results are in good t hose of experiments.     

Optical properties of three sectors in a zinc-oxide single crystal grown under hydrothermal process

We evaluated photoluminescence, radioluminescence, transmittance, and decay time at room temperature at each sector of zinc oxide (ZnO) single crystals grown utilizing the hydrothermal process. The −c-sector wafer, grown on the oxygen face of the c-plane seed crystal, has a short decay time and a high light yield and shows properties different from those of other sectors. We also fabricated a radiation sensor by combining the −c-sector chip cut from a wafer and a multipixel photon counter (MPPC). X-rays were detected using sensor with high sensitivity. At room temperature, typical decay time for the −c sector at the near band edge was 700 ps, and the range of relative light yield was thirty- to forty-times larger than that of the +c sector. Therefore, the properties of the −c-sector wafer at the near band edge will be suitable for high-speed radiation sensors in the near future.     

Evaluating the depth of open cracks in concrete from parameters of electric response to elastic-impact excitation

We have studied the character of decay of the spectral components of electric response to elastic impact excitation of a concrete sample with artificial cracks of various depths. A criterion for determining the depth of surface cracks in concrete is proposed based on results of time-frequency analysis of the response signal.     

Optical properties of pure Ytterbium Alluminium perovskites

We present the first report about growth and optical properties of pure Ytterbium Alluminium perovskites single crystals (YbAlO₃). The sample crystal structure was studied and assigned by means of Raman spectroscopy while the photoluminescence measurements from Yb³⁺ charge-transfer state show a broad ultraviolet emission bands with nanosecond lifetimes at room temperature. Yb emissions are also studied as a function of temperature revealing an abrupt quenching in the 180-240 K range. The fast time decay and the high material density suggest Ytterbium Alluminium perovskites crystal as a good candidate for the development of fast scintillators for high energy physics applications.     

光纤电压互感器蝇电光晶体对测量精度的影响

光纤电压互感器中电光晶体ＢＧＯ（Ｂｉ４Ｇｅ３Ｏ１２）的缺陷,杂质及内部残余应力引起自然双折射,降低仪器的电压测量精度。从理论上分析了晶体在不同方向应力时不同通光方向上的双折射,又从热光效应的原理出发,推导出调制度的变化与温度、电压之间的定量关系,得出调制深度对温度的变化率与调制度本身成正比、与外施电压成正比的结论。因此为提高仪器的温度稳定性,保证必要的灵敏度,必须使施加到电光晶体上的电压合理。ＢＧ     

Scintillation characteristics of Cs2LiCeBr6 crystal

This work investigates the scintillation properties of Cs₂LiCeBr₆ crystal as a new material for radiation detection. This scintillation material is grown by the vertical Bridgman method. Under X-ray excitation the sample crystal shows a broad cerium based emission band between 390 and 450 nm wavelength range. Energy resolution for 662 keV γ-rays is measured to be 7.4% (FWHM). At room temperature Cs₂LiCeBr₆ crystal exhibits three exponential decay time components. The fast and major component of scintillation time profile of Cs₂LiCeBr₆ emission decays with a 86 ns time constant. Absolute light yield for the sample crystal is estimated to be 27,000 and 29,000 photons/MeV using APD and photomultiplier tube, respectively. The sample crystal shows good proportionality of 5% in the measured energy range from 31 to 1333 keV. This study showed that this new scintillation crystal can be a good candidate for radiation detection and medical imaging. The sample crystal is highly hygroscopic.     

Photoinduced spontaneous and stimulated emission in sexithiophene single crystals

Temperature dependent steady-state and transient photoluminescence (PL) measurements were carried out on sexithiophene (6T) single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The first energy set is attributed to transitions to the lower level of the Davydov splitting of the 1B_u excited state of the isolated molecule, whereas the two other sets are ascribed to the crystal defects. An analysis of the PL emission and excitation spectra allowed us to establish a general exciton energy scheme of sexithiophene crystal. At low excitation levels, the transient PL follows a single exponential decay, with a decay time of 1.8 ± 01 ns. When the excitation energy is increased, a second, much faster component is added. Concurrently, the PL spectrum narrows to a single line centered at the origin of the low energy set of the steady state PL. The width of the emission line is 13 cm⁻¹ at 10 K, and tends to widen and shift towards lower energies when the temperature is raised. All these observations are accounted for in term of stimulated emission.     

Energy deposition during molecular depth profiling experiments with cluster ion beams

The role of the location of energy deposition during cluster ion bombardment on the quality of molecular depth profiling was examined by varying the incident angle geometry. Cholesterol films approximately 300 nm in thickness deposited onto silicon substrates were eroded using 40-keV C60(+) at incident angles ranging from 5 degrees to 73 degrees with respect to the surface normal. The erosion process was evaluated by determining at each incident angle the total sputtering yield of cholesterol molecules, the damage cross section of the cholesterol molecules, the altered layer thickness within the solid, the sputter yield decay in the quasi-steady-state sputter regime, and the interface width between the cholesterol film and the silicon substrate. The results show that the total sputtering yield is largest relative to the product of the damage cross section and the altered layer thickness at 73 degrees incidence, suggesting that the amount of chemical damage accumulated is least when glancing incident geometries are used. Moreover, the signal decay in the quasi-steady-state sputter regime is observed to be smallest at off-normal and glancing incident geometries. To elucidate the signal decay at near-normal incidence, an extension to an erosion model is introduced in which a fluence-dependent decay in sputter yield is incorporated for the quasi-steady-state regime. Last, interface width calculations indicate that at glancing incidence the damaged depth within the solid is smallest. Collectively, the measurements suggest that decreased chemical damage is not necessarily dependent upon an increased sputter yield or a decreased damage cross section but instead dependent upon depositing the incident energy nearer the solid surface resulting in a smaller altered layer thickness. Hence, glancing incident angles are best suited for maintaining chemical information during molecular depth profiling using 40-keV C60(+).     

Band gap study on zinc-doped cadmium sulphide

Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.     

Variable optical attenuator with a polymer-stabilized dual-frequency liquid crystal

A transmission-type variable optical attenuator (VOA) based on a polymer-stabilized dual-frequency liquid crystal (PSDFLC) is demonstrated at gamma = 1.55 microm. The VOA is highly transparent in the voltage-off state but scatters light in the voltage-on state. By using a birefringent beam displacer incorporated with half-wave plates, we can obtain a VOA that is polarization independent and that exhibits a 31 dB dynamic range. The polymer networks and dual-frequency effect together reduce the response time (rise + decay) of a 16 microm PSDFLC cell to 30 ms at room temperature and at a voltage of 24 Vrms.     

Solid Hydrogen for Nonlinear Optics

We recently developed a new method of preparing a parahydrogen crystal which is suitable for experiments on nonlinear optical processes. The crystal was grown slowly from pressurized liquid–phase in order to avoid internal stress from the thermal constriction, and was cooled down to 4.2 K. The obtained crystal was uncracked and perfectly transparent with a high damage threshold. To evaluate the quality, the vibrational coherence decay was measured using the time–resolved coherent anti–Stokes Raman spectroscopy (CARS) technique. The decay process showed a nonexponential behavior, with an asymptotic limit characterized by a single time constant of 2.6 s, which corresponds to a linewidth of less than 0.1 MHz. This decay time is extremely slow compared to the previously published work.     

Auger-free luminescence of the BaF2:Sr, BaF2:MgF2 and CsBr:LiBr crystals under excitation of VUV photons and high-energy electrons

The emission spectrum, the time dependence of the luminescence, the excitation spectrum and the reflectance spectrum have been measured for the pure BaF₂ crystal, the BaF₂:Sr crystals with Sr⁺⁺ concentrations of 2 and 5 mol%, the BaF₂:MgF₂ crystal with a mole mixing ratio of 1:2, and the pure CsBr crystal and the CsBr:LiBr crystal with a mole mixing ratio of 1:1. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF₂, the BaF₂:Sr and the BaF₂:MgF₂ crystals have the Auger-free luminescence having decay times around 0.90 ns. The slow and fast luminescence components of the BaF₂ crystal are suppressed by introduction Sr⁺⁺ and Mg⁺⁺ ions. It is, however, clearly seen that the suppression of the slow component is more effectively occurring than that of the fast component, especially in the BaF₂:MgF₂ crystal. By analyzing the Auger-free luminescence spectrum, it is shown that the valence band widths are 2.7 ± 0.3 eV, 2.7 ± 0.3 eV and 4.2 ± 0.3 eV and the band-gap energies are 11.1 ± 0.3 eV, 11.1 ± 0.4 eV and 11.9 ± 0.4 eV for BaF₂, BaF₂:Sr and BaF₂:MgF₂ crystals, respectively. The introduction of LiBr into CsBr has resulted in a significant enhancement in the luminescence intensity, attaining 40 times larger intensity than that of CsBr crystal at room temperature, due to the suppression of the non-radiative relaxation of the outer-most core holes due to the absorption induced by the Urbach effect. The observations of the increases in the band gap-energy, the exciton energy and the decay time of the CsBr:LiBr crystal compared with those of the CsBr crystal support the argument given in the present study regarding the suppression of the Urbach effect in the CsBr:LiBr crystal. The decay times observed are 0.20 ± 0.05 ns and 0.90 ± 0.06 ns for CsBr and CsBr:LiBr crystals, respectively.     

Light induced enhancement of transient heat transfer from a solid into liquid helium I

We report preliminary studies of a new effect: enhancement of transient heat transfer from a bismuth crystal into liquid helium by a light pulse. We found that when a single crystal of bismuth is rapidly heated above a certain threshold temperature in a bath of liquid helium by a step-function electric current, the application of a light pulse of intensity 1 to 200 mW cm^?2 and duration 2 μs to 1 ms causes a decrease of up to 75% in the total crystal superheat temperature within 3 ms. The threshold temperature is less than the homogeneous nucleation temperature of liquid helium. The marked increase in heat transfer across the solid-liquid interface, associated with the observed rapid cooling, is believed to be caused by increased bubble activity due to the light induced rapid nucleation of bubbles near the interface. Although the exact nature of the interaction mechanism involved is unclear at this time, several hypotheses are presented and discussed.     

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for ¹²C and ¹³C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from ⁷Li to ⁶Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.     

4πγ井型NaI(Tl)晶体探测器性能研究和初步实验结果

介绍了中国计量科学研究院用于测量放射性核素活度的4πγ井型NaI(Tl)晶体装置。通过分析测量得到的60Co放射源发射的γ射线能谱发现,采用4πγ井型NaI(Tl)晶体对衰变纲图较为复杂的核素进行测量时存在相加峰效应。当放射源从井型NaI(Tl)晶体井口附近逐渐移至井底的过程中,相加峰的计数率逐渐增加,从而使得对60Co特征γ射线全能峰探测效率逐渐降低;此外,由于在井口附近探测器对放射源所张立体角相对较小,对γ射线的探测效率也较低;放射源在井型晶体内的位置发生变化也会对探测器的能量刻度带来影响。     

Equilibrium phase diagram and kinetics of isotopical phase separation of 3%3He in HCP4He mixture

The isotopical phase separation in a weak solid mixture of³He in⁴He is investigated by 250 kHz pulsed NMR under a pressure of 3.7 MPa in the temperature range 100 – 240 mK. The equilibrium concentrations of³ He in HCP phase are found to be in a good agreement with classical regular solution theory for phases with different crystal structures. The time constant of decay at steplike cooldown along the separation line are shown to monotonously decrease. The linear dependence of the time on the concentration may point towards an important role of quantum diffusion.