Nonlinear luminescence of high-concentration rhodamine solutions: On the problem of pulsed laser cooling

The conditions of occurrence of collective photoinduced luminescence in model solutions are determined. The transmittance and luminescence of concentrated solutions of rhodamine 101 under excitation by laser pulses in the Stokes and anti-Stokes spectral ranges are studied. It is shown that, at high pump energies, collective photoinduced luminescence, which competes with induced radiation, plays a significant role in luminescence generation. Temperatures of heating and cooling of solutions are estimated. The photodynamics of cooling of solutions of complex molecules under their pulsed (Stokes and anti-Stokes) excitation is analyzed.     

The influence of strain on the photoluminescence from pure CsI

Studies of the influence of strain on the photoluminescence from pure CsI at room temperatures showed that the luminescence from strained CsI at room temperature before cooling (RTBC) is similar to that of the underformed CsI at room temperature after cooling to lower temperatures (RTAC). We also observed the step-like dependence of the luminescence intensity of the strained sample on the exciting wavelength. Several interpretations of the results are suggested.     

局域化电子态集体的荧光

载流子局域化对固体材料的光电性质有着深刻的影响。长期以来，人们就发现许多固体材料的发光异常行为与固体中电子态的局域化密切相关．文章介绍了作者最近所发展的一个局域态荧光模型．借助于一个新推导出来的局域化电子态的分布函数，文章作者发展出了该模型．该模型不但定量地解释了局域态集体荧光的温度依赖的异常现象，而且清晰地揭示了异常现象背后的载流子热动力学物理．文章还介绍了该模型在一些材料的荧光数据分析中的应用。     

反斯托克斯荧光制冷的研究进展与综述

反斯托克斯荧光制冷 (Anti StokesFluorescentCooling)也被称为激光制冷 (LaserCooling)。自 1 995年以来 ,该项研究取得了飞速的发展。目前 ,人们利用激光制冷的方法已经得到了比家用冰箱冷冻室还低的温度 ,并能够利用半导体量子阱材料得到低于液氮温区的降温。由于这项技术具有全光性 ,它的制冷器具有体积小、重量轻、无电磁辐射、无振动、无噪声等特点 ,因此也就具有了非常诱人的应用前景和符合军事、空间、集成光学、微电子、医学等领域的特殊要求 ,而被国外研究者所重视。做为一项基本技术 ,激光制冷研究的突破必然会导致许多对温度有特殊要求的高技术实用化 ,推动那些领域向前发展。本文详细地介绍了反斯托克斯荧光制冷研究的历史和最新进展 ,详细地介绍了该项研究中的方法和理论。着重介绍了激光制冷的热力学限制、发光过程的热力学理论 ,探讨激光制冷产生的机制和制冷理论。最后 ,对激光制冷器的发展前景和设计依据进行了讨论 ,尝试性地探讨了线圈型制冷器、用于芯片的制冷器和单分子 光子泵型制冷器的应用考虑。     

DYNAMICS OF CARRIER CAPTURE IN AlGaAs/GaAs MULTIPLE QUANTUM WELLS

Dynamics of carrier relaxation and capture in AlGaAs/GaAs multiple quantum wells (MQW) at 80 K is studied using picosecond luminescence and femtosecond absorption saturation measurements. Carriers generated in the wells and in the barriers scatter initially out of the excited states to a quasi-equilibrium state in 35 and 400 femtoseconds, respectively, before they are captured into the bound states of the quantum wells. Carrier capture occurs during carrier cooling and recombination. A carrier capture time of 25 ps has been deduced from time-resolved luminescence.     

Hot carrier cooling in gaas quantum wells

The cooling of a hot electron-hole plasma in undoped, p-doped, and n-doped GaAs/AlGaAs quantum wells of three different thicknesses (3, 9, and 20 nm) is investigated by picosecond luminescence spectroscopy. The energy loss of holes due to the Fröhlich interaction is at low excitation densities independent of well width and close to the value obtained by a simple theory. The rate strongly decreases with increasing excitation density. For electrons, the energy loss is even at low densities strongly reduced compared to the simple theory of the Fröhlich interaction. The reduction of the energy loss at high densities is independent of dimensionality and well width and not caused by screening or degeneracy effects. The energy loss due to acoustic deformation potential scattering depends on well width.     

Anti-Stokes luminescence in heavily doped semiconductors as a mechanism of laser cooling

The anti-Stokes luminescence is a mechanism of the optical refrigeration in semiconductor light sources. The heavily doped semiconductors are considered as a material for the laser cooling. The limitation of this mechanism appears to be connected with a transition from the non-degenerate to degenerate occupation. This transition occurs at higher pumping rate (along with the transition to the optical gain and lasing) and at lower temperature. Thus, the limit for the laser cooling can be indicated. The minimal obtainable temperature is about 60–120 K depending on the doping level. The laser cooling of a semiconductor is impeded by the difficulty of extracting the spontaneous emission from a radiating body that is characterized by large angle of the total internal reflection.     

Direct observation of electron-to-hole energy transfer in CdSe quantum dots

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots. Time-resolved luminescence and terahertz spectroscopy reveal that the rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes the first direct, quantitative measurement of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 +/- 0.15 ps time scale.     

单分子-光子制冷泵的热力学行为(Ⅰ)

建立了一般意义上的单分子－光子泵模型,并用计算机模拟了在不同能量的光子激发下单个分子作为制冷泵时的量子跃迁过程,研究了单分子－光子泵处于基态和激发态时的热激发过程以及在发生荧光辐射时的声子参与情况,得出了基态具有较强的制冷能力的结论。基态和激发态在光吸收和发射过程中热力学效应是不同的,因此在选择反斯托克斯芝光制冷材料时,发光中心的基态能级劈裂更具重要性。     

Effects of Coupling Lens on Optical Refrigeration of Semiconductors

Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling emciencies of different coupling mechanisms and of different lens materials. A GaAs/GalnP heterostructure coupled with a homo-epitaxial GalnP hemispherical lens is recommended.     

Quantum-kinetic theory of hot luminescence from pulse-excited semiconductors

A theory of time-resolved luminescence from photoexcited semiconductors is presented. It combines quantum kinetics of hot-carrier relaxation and quantum theory of spontaneous emission. Model calculations show the "transfer" of photoluminescence from the initial signal at the pump frequency via subsequent phonon replicas until the buildup of luminescence at the excitonic resonance. Time-resolved photoluminescence is predicted to be a sensitive measure of electron-LO-phonon quantum kinetics and bottleneck effects.     

激子旋转弛豫对低掺杂卟啉侧链聚合物荧光衰变过程的影响

利用激子旋转扩散理论研究了一类低掺杂卟啉侧链聚合物中卟啉侧链基团的旋转对其发光动力学过程的影响.研究表明,卟啉侧链基团的旋转行为是导致激发态无辐射能量弛豫的重要途径.基团旋转越容易,能量弛豫速度越快,这可导致一个快速的荧光衰变动力学过程.在卟啉低掺杂浓度和聚合物分子链间距离较大的情况下,卟啉侧链基团的旋转成为影响荧光寿命和发光效率的主要因素.对实验测得的两种样品的荧光弛豫过程进行了拟合,理论结果与实验结果符合较好. 关键词： 激子旋转弛豫 瞬态荧光 卟啉侧链聚合物     

Photoluminescence and terahertz emission from femtosecond laser-induced plasma channels

Luminescence as a mechanism for terahertz emission from femtosecond laser-induced plasmas is studied. By using a fully microscopic theory, Coulomb scattering between electrons and ions is shown to lead to luminescence even for a spatially homogeneous plasma. The spectral features introduced by the rod geometry of laser-induced plasma channels in air are discussed on the basis of a generalized mode-function analysis.     

Research on Intracavity Laser Cooling of Solid

A theoretical study of intra-cavity laser cooling by anti-Stokes luminescence in a rare-earth doped glass is performed. Compared with cooling in an external cavity by multipassing the radiation, intra-cavity cooling has the advantage of high pumping power and high-absorbed power. However, one must ensure that the cavity can still form a laser by locating the material in the cavity. A model is developed to evaluate the enhancement factor and the absorbed power. The results show that for a low optical density, especially when the sample length is less than 2ram, the intracavity configuration is a very efficient method for laser cooling. The diode laser, which may become the best candidate for our model, is briefly discussed.     

白色荧光粉CaF2∶Yb3+/Eu3+/La3+的制备及其上转换发光特性

采用改进的两步高温固相熔融法制备了Yb^3+、Eu^3+、La^3+共掺杂CaF 2的上转换荧光粉。基于荧光猝灭原理,通过改变La^3+掺杂浓度来调节CaF 2∶Yb^3+/Eu^3+材料的发光性能,并在980 nm近红外光激发下,获得了该材料的白色上转换发光(UCL)。在该发光体系中,Yb^3+不仅起到了敏化Eu^3+的作用,同时,Yb^3+二聚体(Yb^3+-dimer)自身合作发出波长范围480~540 nm的绿色荧光。而白光三基色中的绿光正是来自Yb^3+二聚体的合作发光。Eu^3+则作为激活剂,同时发出红色和蓝色荧光。荧光寿命测试结果表明Yb^3+-dimer与Eu^3+之间存在有效的能量传递。值得注意的是,在980 nm激光激发下,1%La^3+掺杂的样品表现出最佳的红、绿、蓝三基色光比列,实现了材料的上转换白光发射,其色度坐标为(0.311,0.340)。     

Crystal LiNbO<Subscript>3</Subscript>-Ho<Superscript>3+</Superscript>: Material for optical cooling

The possibilities of LiNbO₃-Ho³⁺ crystals for optical cooling based on the anti-Stokes luminescence in the wavelength range 2000-2200 nm are investigated. The efficiency and cooling temperature under the continuous wave (CW) excitation at the wavelengths 2035-2071 nm by ~100 W power are estimated. It is shown that under the CW excitation at 2035 nm wavelength the maximum cooling temperature is equal to 2.5 K, and at 2071 nm wavelength is equal to 10.9 K.     

Transient resonant light scattering under random perturbations

A theory of light scattering for transient phenomena is formulated; a general scattering formula is obtained for pulse excitations. For a general stochastic model of random perturbations on a matter system, the formula is exactly expressed in terms of continued fractions. A detailed analysis is given for time-resolved spectra. It is explicitly shown that a change from “scattering” to luminescence can be observed separately as a function of time.     

激光制冷中能级间距的选择

激光制冷问题的核心是材料的选择。荧光中心能级的间距是其中的一个关键指标。确定合理的能级间距有助于选择合适的激光制冷材料。能级间距决定了对激光制冷至关重要的两个因素:量子效率和无辐射跃迁速率。如果单纯地从制冷功率的角度来看,能级间距越大量子效率越高,也越有利于荧光制冷。但当能级间距宽到某一值后,制冷功率基本上保持不变。如果从热一光转换效率的观点来考察激光制冷的效率问题,能级间距宽度的合理取值就应该小得多。如果在选择激光制冷材料时,确定制冷功率具有第一位的重要性,那么,能级间距选择在5000cm^-1左右的宽度是比较合适的、这不仅可以确定较大的热－光转换效率,同时也基本上保证了很高的制冷功率。     

Specific Features of Photoluminescence and Nanosecond Relaxation of Photocurrent in CuIn5S8 Crystals upon Strong Optical Excitation

Optics and Spectroscopy - Intense luminescence and nanosecond relaxation of photocurrent upon strong optical excitation have been observed in CuIn5S8 crystals grown by slow melt cooling at a...     

Laser cooling of quasi-free atoms in a nondissipative optical lattice

A quasi-classical theory of laser cooling is applied to the analysis of cooling of unbound atoms with the angular momenta 1/2 in the ground and excited states in a one-dimensional nondissipative optical lattice. In the low-saturation limit with respect to the pumping field, the mechanisms of cooling can be interpreted within the framework of an effective two-level system of ground-state sublevels. In the limit of weak Raman transitions, the mechanism of cooling of unbound atoms is similar to the Doppler mechanism known in the theory of a two-level atom; in the limit of strong transitions, the mechanism of cooling is analogous to the well-known Sisyphys mechanism. In the slow-atom approximation, analytical expressions are obtained for the friction (drag) coefficient and the induced and spontaneous diffusion, and the kinetic temperature is estimated.