Semi-classical theory of laser cooling in two dimensions

We present a semi-classical theory of the light pressure force for atoms interacting with a two-dimensional laser field. Unlike previous 2D theory, ours is valid for general atomic level and laser field configurations. We show that striking new features appear in the velocity-dependent force arising from the multi-dimensionality. Finally, we describe in detail the novel numerical technique used in the calculation. Received 4 December 1998 and Received in final form 13 February 1999     

团簇淀积纳米结构薄膜的计算机模拟

对团簇的淀积方式作了简介。着重对团簇的淀积过程进行了计算机模拟,从中得出了淀积的尺寸分布与团簇在衬底上的徙动长度有关,而团簇的聚合长大存在一临界尺寸。并给出低能Ｐｂ团簇在碳膜表面的徙动长度和聚合的临界尺寸。     

流量大小对混合流场特性等的影响

在主、副气流各组份的流量比不变的情况下,改变输入氧碘化学激光器(COIL)光腔总气流流量,数值模拟给出了COIL拉伐尔喷管主、副气流混合状况和各物理量的分布等,通过对这些结果进行对比分析得出输入COIL光腔总气流流量增加三倍,氧碘气流的混合状况稍稍变差,但Mach数和密度等的分布和变化规律基本不变.     

Optical properties of amorphous films

We have analyzed the optical properties of a-Ge_30-xSb_xS₇₀ chalcogenide glass films (x=0,10,20 and 30 at%); the chalcogenide films were prepared by vacuum thermal evaporation. The optical-absorption data indicate that the absorption mechanism is non-direct transition. We found that the optical band gap, E_opt, decreases from 2.04±0.01 to 1.74±0.01 eV, whereas the refractive index increases with increasing Sb content. Data are analyzed by the Wemple equation, which is based on the single-oscillator model.     

Low-threshold optical power limiting of cw laser illumination based on nonlinear refraction in zinc tetraphenyl porphyrin

The increasing use of low power cw lasers in various applications calls for the design of optical limiters with low thresholds. To this end, the optical nonlinearity exhibited by zinc tetraphenyl porphyrin at low laser powers is utilized to design an optical limiter for low threshold operation. The basic parameter responsible for limiting action, the nonlinear refractive index of the medium, is measured using the Z-scan technique and found to have a value of at the helium–neon laser wavelength of 632.8 nm. The origin of nonlinearity is explained on the basis of the thermal lens model. It is shown that effective optical limiting at desired threshold values can be achieved by the optimal choice of aperture size and experimental geometry.     

Optical properties of individual manganese-doped quantum dots

The magnetic state of a single magnetic ion (Mn²⁺) embedded in an individual quantum dot is optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn²⁺ ion (S=) is analyzed in detail. The exciton–Mn²⁺ exchange interaction shifts the energy of the exciton depending on the Mn²⁺ spin component and six emission lines are observed at zero magnetic field. The emission spectra of individual quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton–Mn²⁺ exchange interaction (dependent on the Mn position) and the anisotropic part of the electron–hole exchange interaction (related to the asymmetry of the quantum dot).     

Segregation of H, C and B to Σ = 5 (0 1 3) α-Fe grain boundary: A theoretical study

The ASED-MO theory is used to study the effects of H and the HC and HB pairs in the electronic structure of a Fe grain boundary (GB). The results obtained for H in a GB model are consistent with its behavior as a chemical embrittler. The total energies calculated for FeH, FeC and FeB clusters indicate that all interstitials segregate to the GB. C has the lowest energy, followed by B, and could compete with other impurities for the site location on the GB.The results obtained for FeCH and FeBH are consistent with the observed behavior of C and B as cohesion enhancers. A strong repulsive interaction between C and H and B and H atoms is developed if they occupy the nearest interstitial site on the GB. When C or B are present, the total energies are similar to that obtained for the FeH cluster. This indicates that H is displaced from the capped trigonal prism (CTP). Also, we do not detect any CH or BH interaction.Density of states (DOS) and crystal orbital overlap population (COOP) curves are used to shed more light on the interstitial-Fe GB interaction. The existence of strong metal-metalloid bonds is shown, which are primarily due to Fe 3d, 4s and C (or B) 2s, 2p interactions.     

Spectroscopic studies on the effect of doping with CoBr2 and MgCl2 on some physical properties of polyvinylalcohol films

Filled and unfilled polyvinylalcohol (PVA) films were prepared by the casting technique. Films of equal amounts with various concentrations of two fillers (CoBr₂ and MgCl₂) were prepared. Spectroscopic, structural and some physical properties of these films were studied with different techniques. Fourier-transform infrared (FTIR) revealed that the syndiotacticity structure of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. X-ray diffraction (XRD) scans evidenced the presence of some semicrystalline structure of PVA films. The optical absorption spectra suggested the presence of an optical gap (E_g), which depends on filler concentration for all the filling levels. Differential thermal analysis (DTA) suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers act as plasticizers.     

All optical switching in semiconductor microresonators based on pattern selection

We present here a method for selecting optical patterns in a passive semiconductor microresonator, by using a spatial perturbation. A pattern is spontaneously generated in the system, and a switching beam causes this pattern to rotate even if the power in the switching beam is much lower than the power in the pattern. Thus, an all optical switch is realized, that operates at low light levels.