First measurement of the Λ→nγ branching ratio

In an experiment performed at the CERN-SPS charged hyperon beam, we have observed the radiative decay Λ→nγ, using well identified Λ from the decays of Ο^? of 116 GeV/c momentum. Neutrons and photons from the Λ decays were observed in liquid argon and lead glass detectors. From a sample of 31 candidates, containing an estimated background of 7.3 events, the decay branching ratio was measured to be Γ(Λ→nγ)/Γ(Λ→all)=(1.02±0.33)×10^?3.     

Radiative decay time of a nongray medium with arbitrary configurations

Based on the principle of conservation of energy, a simple analytical expression is presented for the radiative decay time of a nongray medium with arbitrary configurations. It is expressed explicity in terms of the mean beam length and the total band absorptance. In the high-pressure limit, where the decay time based on the harmonic thermal perturbation is available for two configurations, comparison is made with the present decay time. Good agreement is found.     

Radiative decay times for thermal pertubations in a nongray medium

The radiative decay time of harmonic thermal perturbations in a nongray medium of infinite extent is obtained in closed form for two specific band absorption models. These models are the frequently used gray band and the exponential band, the latter being considered more realistic for molecular gases. It is found that the decay time at the boundary of a semi-infinite medium can be obtained in terms of that in an infinite medium. The decay time for combined thermal radiation and conduction is also discussed. The difference in radiative decay rates for a medium with gray bands and one with exponential-tailed bands is marked; in an infinite medium at large Bouguer number, the former falls to zero while the latter rises to a maximum.     

THERMAL RADIATION PROPERTY OF A THREE DIMENSIONAL PHOTONIC CRYSTAL BASED ON MULTIPLE SCATTERING METHOD

We have studied absorption and thermal radiation for a three dimensional lossy photonic crystal of dielectric spheres by using a multiple scattering method. It is shown that such simple structures have excellent selective thermal radiative characteristics in different photonic bands. There is stronger thermal radiation at high frequency than that at low frequency. In comparison with the uniform slab without photonic crystal structure, in the photonic band gap, the thermal emission intensity is greatly suppressed and very weak, but not zero due to the penetration depth. Under the same lattice structure, the thermal emission of photonic crystal varies with the change of the radius of the spheres.     

In插入层对硅衬底外延InN晶体质量和光学特性的影响

在Si（111）衬底上分别预沉积0,0.1,0.5,1 nm厚度的In插入层后,采用等离子辅助分子束外延法制备了纤锌矿结构的InN材料,结合X射线衍射（XRD）、扫描电子显微镜（SEM）、吸收谱及光致发光谱研究了不同厚度的In插入层对外延InN晶体质量和光学特性的影响。XRD和SEM的测试结果表明,在Si衬底上预沉积0.5 nm厚的In插入层有利于改善外延InN材料的形貌,提高材料的晶体质量。吸收谱和光致发光谱测试表明,0.5 nm厚In插入层对应的InN样品吸收边蓝移程度最小,光致发射谱半峰宽最窄,并且有最高的带边辐射复合发光效率。可见,引入适当厚度的InN插入层可以改善Si衬底上外延InN材料的晶体质量和光学特性。     

Prompt isothermal decay of thermoluminescence in MgB4O7:Dy,Na and LiB4O7:Cu,In dosimeters

According to standard delocalized kinetic models of thermoluminescence (TL), when an irradiated sample is held at a high temperature T, the isothermal TL signal will decay with a characteristic thermal decay constant λ which depends strongly on the temperature T. This prediction of standard delocalized kinetic theory is investigated in this paper by studying two TL dosimeters, MgB₄O₇:Dy, Na and LiB₄O₇:Cu, In (hereafter MBO and LBO correspondingly). In the case of LBO it was found that the thermal decay constant λ of the main dosimetric TL peak follows exactly the predictions of standard delocalized kinetic theory. Furthermore, the thermal activation energy of the main peak evaluated by the isothermal decay method is in full agreement with values obtained from initial rise and glow curve fitting methods. However, in the case of MBO it was found that the thermal decay constant λ varies little with the isothermal decay temperature T. In order to explain these unusual results for MBO, the TL glow curves and isothermal decay curves were analyzed using analytical expressions derived recently from a radiative tunneling recombination model. Based on the different behavior of the two TL dosimeters, it is suggested that the isothermal decay of TL at high temperatures can be used to discriminate between radiative delocalized recombination and radiative localized recombination processes.     

Simulation of bulk-absorption thermal lensing in transmissive optics of gravitational waves detectors

This paper presents finite-element modelling simulations of thermal lensing and thermal lens compensation in transmissive optics for gravitational wave detectors. We compare the current candidate test mass materials, fused silica and sapphire, in terms of sample geometry and time-dependent phenomena. For both materials, the thermal-lensing time constant is a few minutes, yet the core temperature needs several tens of minutes to stabilize. Thermal lens compensation using simple radiative heating is limited in temperature by absorption in the test mass. This effect limits the maximum allowed absorption for sapphire to 10–20 ppm/cm. For reasonable parameters, optical path length compensation within 1 nm can be achieved over a beam radius of 5 mm. If the optical absorption of the transmissive optics is too high, compensation can be achieved by means of a separate compensation plate. PACS 42.25.Bs; 42.70.Ce     

Optical properties of Er:SrMoO4 single crystal

Er³⁺:SrMoO₄ crystal of high optical quality was grown by the Czochralski method. The room temperature polarized absorption and emission spectra together with the lifetime decay curve were measured. Based on the Judd-Ofelt theory, three intensity parameters, radiative transition rates, radiative lifetimes and fluorescent branching ratios, were obtained. Emission cross-section and gain cross-section around 1.54 μm were also obtained.     

Heat and mass transfer in a gas in a capillary induced by light with nonuniform intensity distribution over the beam cross section

An analysis is presented of the heat and drift fluxes induced by velocity-selective light absorption in a single-component gas in a capillary tube. The light intensity distribution across the beam is assumed to have a Gaussian profile. Kinetic equations are solved numerically to calculate flux profiles and kinetic coefficients quantifying the contributions of surface and collisional mechanisms to light-induced transfer as functions of the Knudsen number, the ratio of the rate of radiative decay of the exited level and intermolecular collision frequency, accommodation coefficient, and the ratio of the tube radius to the light beam radius.     

Effects of polarization on radiation absorption in one-dimensional semitransparent slab exposed to collimated irradiation

The internal distribution of radiative absorption in one-dimensional semitransparent slab exposed to collimated irradiation is determined by the ray tracing method, and the detailed computation formulae for the distribution of radiative absorption are deduced. It is found that the ray will be polarized by specular reflection of slab boundaries even if the collimated irradiation is unpolarized, and the extent of polarization increases with the internal reflection times. The effects of polarization on the radiative absorption are analyzed and the radiative absorption distributions are compared with those obtained from the case in which the effects of polarization are omitted. The results show that the large differences between reflectances of perpendicular and parallel polarized components are the main causes resulting in errors of radiative absorption distribution. The polarization of incident beam has significant influences on the radiative absorption. Even for an unpolarized beam irradiated near Brewster's angle, omitting the effects of polarization will result in large errors, and the errors increase with the refractive index.     

Pecularities in Optical Properties of ns2 Ions

The behavior of an octahedral center created in alkali chloride and bromide single crystals by doping with divalent lead is analyzed using the experimental results from absorption and luminescence measurements. We focus on the questions of local distortion both in ground and excited states, the regularities in A_T and A_X emissions and differences in the decay shape of high energy emission.     

Phonon coherent states in transient phenomena

Phonon coherent states (PCS) are shown to be involved in electronic absorption and emission of light and in non radiative decay of molecules in gases and of centers in solids. The relations giving the absorption and emission coefficients and the non radiative decay rate constant are discussed.     

Radiative decay of Wannier excitons in thin crystal films

The radiative decay of the Wannier exciton in thin crystal films is studied by the method of Heitler and Ma in the resonance approximation. It is shown that, while the broken crystal symmetry opens up the possibility of radiative decay, the correlation brought about by the interaction of the lattice atoms with a common radiation field leads to a superradiative enhancement of the decay rate. However, when it is compared with the corresponding case of a Frenkel exciton, the decay rate of the Wannier exciton is reduced due to the averaging of the dipole transition matrix element over the various possible distances between the widely separated electron and hole.     

Frequency and density dependent radiative recombination processes in III–V semiconductor quantum wells and superlattices

In this paper we review the radiative recombination processes occurring in semiconductor quantum wells and superlattices under different excitation conditions. We consider processes whose radiative efficiency depends on the photogenerated density of elementary excitations and on the frequency of the exciting field, including luminescence induced by multiphoton absorption, exciton and biexciton radiative decay, luminescence arising from inelastic excitonic scattering, and electron-hole plasma recombination.

Semiconductor quantum wells are ideal systems for the investigation of radiative recombination processes at different carrier densities owing to the peculiar wavefunction confinement which enhances the optical non-linearities and the bistable behaviour of the crystal. Radiative recombination processes induced by multi-photon absorption processes can be studied by exciting the crystal in the transparency region under an intense photon flux. The application of this non-linear spectroscopy gives direct access to the excited excitonic states in the quantum wells owing to the symmetry properties and the selection rules for artificially layered semiconductor heterostructures.

Different radiative recombination processes can be selectively tuned at exciting photon energies resonant with real states or in the continuum of the conduction band depending on the actual density of photogenerated carriers. We define three density regimes in which different quasi-particles are responsible for the dominant radiative recombination mechanisms of the crystal: (i) The dilute boson gas regime, in which exciton density is lower than 10¹⁰ cm^-2. Under this condition the decay of free and bound excitons is the main radiative recombination channel in the crystal. (ii) The intermediate density range (n < 10¹¹ cm^-2) at which excitonic molecules (biexcitons) and inelastic excitonic scattering processes contribute with additional decay mechanisms to the characteristic luminescence spectra. (iii) The high density range (n ?10¹² cm^-2) where screening of the Coulomb interaction leads to exciton ionization. The optical transitions hence originate from the radiative decay of free-carriers in a dense electron-hole plasma.

The fundamental theoretical and experimental aspects of the radiative recombination processes are discussed with special attention to the GaAs/Al _x Ga_1-x As and Ga _x In_1-x As/Al _y In_1-y As materials systems. The experimental investigations of these effects are performed in the limit of intense exciting fields by tuning the density of photogenerated quasi-particles and the frequency of the exciting photons. Under these conditions the optical response of the quantum well strongly deviates from the well-known linear excitonic behaviour. The optical properties of the crystal are then no longer controlled by the transverse dielectric constant or by the first-order dielectric susceptibility. They are strongly affected by many-body interactions between the different species of photogenerated quasi-particles, resulting in dramatic changes of the emission properties of the semiconductor.

The systematic investigation of these radiative recombination processes allows us to selectively monitor the many-body induced changes in the linear and non-linear optical transitions involving quantized states of the quantum wells. The importance of these effects, belonging to the physics of highly excited semiconductors, lies in the possibility of achieving population inversion of states associated with different radiative recombination channels and strong optical non-linearities causing laser action and bistable behaviour of two-dimensional heterostructures, respectively.     

Active correction of thermal lensing through external radiative thermal actuation

Absorption of laser beam power in optical elements induces thermal gradients that may cause unwanted phase aberrations. In precision measurement applications, such as laser interferometric gravitational-wave detection, corrective measures that require mechanical contact with or attachments to the optics are precluded by noise considerations. We describe a radiative thermal corrector that can counteract thermal lensing and (or) thermoelastic deformation induced by coating and substrate absorption of collimated Gaussian beams. This radiative system can correct anticipated distortions to a high accuracy, at the cost of an increase in the average temperature of the optic. A quantitative analysis and parameter optimization is supported by results from a simplified proof-of-principle experiment, demonstrating the method's feasibility for our intended application.     

基于光学浮标的赤潮生消过程半分析监测方法

赤潮爆发时水体叶绿素a质量浓度升高,引起浮游植物吸收系数、光束总吸收系数等水体固有光学性质(IOP)的变化,并导致水体表观光学性质(AOP)的改变。海洋光学浮标可实现水体表观光学性质的定点连续时间序列观测,基于此发展相应的模型方法有望实现赤潮生消全过程的监测。利用一次赤潮生消过程的海洋光学浮标数据,发展了一种赤潮半分析监测方法。该方法首先由光学浮标数据得到的水体光谱漫衰减系数Kd(λ)和遥感反射率rrs(λ),结合经验确定的水下光场平均余弦进行水体光束总吸收系数a(λ)的半分析估算,然后再半分析反演浮游植物吸收系数aph(λ)和叶绿素a质量浓度。经检验,该方法估算a(675),aph(675)和叶绿素a质量浓度的中值相对误差分别为8.6%,34.9%和38.9%。将本方法与半分析方法(QAA)和统计回归方法进行了对比分析,本方法的优势在于反演精度较高,所采用的经验参数大都源自辐射传输理论计算、不依赖于浮标数据且对反演结果的影响有限。     

Relaxation mechanisms of the Zeeman sublevels of the phosphorescent triplet state of pyrazine at 1·6K

The observed change in the phosphorescence decay of the spin-aligned triplet state of pyrazine at 1·6K upon application of a magnetic field is examined both experimentally and theoretically. Experimentally, the decay curve is resolved into the three decay components of the three triplet sublevels in the field range of 0–6000 G. Both the decay constants and the fraction of total initial intensity of each decay component are determined for each field strength. Theoretically, the observed change in the decay characteristics is assumed to be solely due to the Zeeman mixing of the zero-field levels in the absence of spin-lattice relaxation processes. This has the effect of distributing the radiative strength of the strongly radiative zero-field level among the other two weakly radiative zero field levels. A simple approximate calculation gives excellent and encouraging agreement with experimental quantities in the range of validity of the approximations made (below 1000 G). A rigorous calculation was then performed using the exact solution of the secular equation resulting from the Hamiltonian including the spin-spin and Zeeman interactions. Due to the difficulty in finding a host of known crystal structure in which pyrazine can dissolve substitutionally and uniquely, the comparison between theory and experiment is made using a polycrystalline sample. A special averaging procedure is used in calculating a theoretical decay curve for each magnetic field strength used. The theoretical decay curve is then decomposed into three components employing the same computer programme used for decomposing the experimental decay curve. The agreement between the observed and the more rigorous calculated decay quantities is excellent up to 2000 G, but observed disagreement appears above 3000 G and increases with field strength. The calculated lifetimes above 3000 G are found to be longer than the observed ones. The difference is due to other relaxation mechanisms at these high fields, for example, in spin-lattice relaxation processes between the Zeeman sublevels of the lowest triplet state.     

Intensities and spectral features of the ~4I_(13/2)-~4I_(15/2) potential laser transition of Er~(3+) centers in CaF_2-CeF_3 disordered crystal

A CaF_2-CeF_3 disordered crystal containing 1.06% of Er~(3+) ions was grown by the temperature gradient technique.Optical absorption and emission spectra recorded at room temperature and at 10 K, luminescence decay curve recorded at room temperature, and extended x-ray-absorption fine structure spectra were analyzed with an intention to assess the laser potential related to the ~4I_(13/2)→~4I_(15/2) transition of Er~(3+). In addition, the thermal diffusivity of the crystal was measured at room temperature. The analysis of room-temperature spectra revealed that the ~4I_(13/2) emission is long-lived with a radiative lifetime value of 5.5 ms, peak emission cross section of 0.73 × 10~(-20) cm~2, and large spectral width pointing at the tunability of the emission wavelength in the region stretching from approximately 1480 nm to 1630 nm. The energies of the crystal field components for the ground and excited multiplets determined from low-temperature absorption and emission spectra made it possible to predict successfully the spectral position and shape of the room-temperature ~4I_(13/2)→~4I_(15/2) emission band. Based on the correlation of the optical spectra and dynamics of the luminescence decay, it was concluded that in contrast to Yb~(3+) ions in heavily doped CaF_2 erbium ions in the CaF_2-CeF_3 crystal reside in numerous sites with dissimilar relaxation rates.     

多次漫反射法测量粉末中的杂质吸收光谱及其计算机模拟

本文提出了用多次漫反射法来测定粉末样品的杂质吸收光谱,其结果与同材料的透明片状样品的杂质吸收光谱基本一致,为了验证更普遍的对不同折射率粉末情况,文中采用假设模型,以计算机模拟方法,其所得的结果说明,当颗粒足够小,色散又不太大时,所得的粉末样品与透明片状样品的吸收光谱一致,只差一个未知的吸收光程,如再与另一特性参数(寿命或量子效率)测量相结合,即可用来确定稀土杂质在材料中的辐射跃迁几率等物理参数。     

Comparison of two experiments on radiative neutron decay

Over 10 years ago we proposed an experiment on measuring the characteristics of radiative neutron decay in papers [1, 2]. At the same time we had published the theoretical spectrum of radiative gamma quanta, calculated within the framework of the electroweak interactions, on the basis of which we proposed the methodology for the future experiment [3, 4]. However, because we were denied beam time on the intensive cold neutron beam at ILL (Grenoble, France) for a number of years, we could only conduct the experiment in 2005 on the newly opened FRMII reactor of Technical University of Muenchen. The main result of this experiment was the discovery of radiative neutron decay and the measurement of its relative intensity B.R. = (3.2 ± 1.6) × 10⁻³ with C.L. = 99.7% for radiative gamma quanta with energy over 35 kev [5, 6]. Over a year after our first announcement about the results of the conducted experiment, “Nature” [7] published a letter asserting that its authors have also measured the branching ratio of radiative neutron decay B.R. = (3.13 ± 0.34) × 10⁻³ with c.l. = 68% and gamma quanta energy from 15 to 340 kev. This article aims to compare these two experiments. It is shown that the use of strong magnetic fields in the NIST (Washington, USA) experiment methodology not only prevents any exact measurement of the branching ratio and identification of radiative neutron decay events, but also makes registration of ordinary neutron decay events impossible.