Controlling spontaneous emission and threshold-less laser oscillation with optical microcavities

We describe the alteration of spontaneous emission of materials in optical microcavities having dimensions on the order of the emitted wavelength. Particular attention is paid to one-dimensional optical confinement structures with pairs of planar reflectors (planar microcavities). The presence of the cavity causes great modifications in the emission spectrum and spatial emission intensity distribution accompanied by changes in the spontaneous emission lifetime. Experimental results are shown for planar microcavities containing GaAs quantum wells or organic dye-embedded Langmuir-Brodgett films as light emitting layers. Also discussed are the laser oscillation properties of microcavities. A remarkable increase in the spontaneous emission coupling into the laser oscillation mode is expected in microcavity lasers. A rate equation analysis shows that increasing the coupling of spontaneous emission into the cavity mode causes the disappearance of the lasing threshold in the input-output curve. Experimentally verification is presented using planar optical microcavities confining an organic dye solution. The coupling ratio of spontaneous emission into a laser mode increases to be as large as 0.2 for a cavity having a half wavelength distance between a pair of mirrors. At this point, the threshold becomes quite fuzzy. Differences between the spontaneous emission dominant regime and the stimulated emission dominant regime are examined with emission spectra and emission lifetime analyses.     

Ferromagnetic domain structures and nanoclusters in Nd(1/2)Sr(1/2)MnO3

Magnetic domain structures of Nd(1/2)Sr(1/2)MnO3 were investigated by means of low-temperature Lorentz electron microscopy. On cooling, magnetic domain walls started to appear at 250 K, and they were oriented straight along the [100] and [110] directions. With a further decrease in temperature, the volume of each magnetic domain increased with discontinuous domain-wall jumps. A characteristic granular image was observed at around 140 K, near the charge-ordering transition temperature. We consider that this originated from ferromagnetic nanoclusters that appeared in the antiferromagnetic matrix.     

Interaction of polymers with fullerene C60

The fundamentals of the method for estimating the fullerene-binding power of polymer macromolecules and supramolecular formations are developed for fullerene C₆₀. Polymers with covalently attached anthracene groups (one group per macromolecule) whose luminescence decreases in the presence of fullerene in solution in direct proportion to the stability of the fullerene-polymer complex are used. The effect of the anthracene mark on the fullerene-binding power of a luminescent-marked polymer is taken into account or discarded on the basis of an analysis of the interaction between fullerene and low-molecular models of luminescent marks.     

Relaxation of vibrationally excited reaction products in a crystal lattice

The relaxation of vibrationally excited nitrogen molecules in the matrix of silver azide is studied. The effective rate constants for the interaction of an excited nitrogen molecule with free charge carriers (1.7 × 10^–10 cm³ s–1) and for the generation a hole from the level of the produced defect at the expense of the vibrational energy of the molecule (3 × 10¹⁰ s^–1) are estimated. An associative–dissociative mechanism of the deactivation is proposed, which consists in the capture of an electron onto the level of the produced defect with the subsequent emission of an electron into the conduction band at the expense of the vibrational energy of the excited molecule. The effective rate constants for electron emission from the excited level of a hydrogen-like defect at values of the principal quantum number of 3 and 4 are estimated as 1.8 × 10⁹ and 2.8 × 10⁹ s^–1, respectively. Based on the processes considered, an expression for the probability of chain propagation is obtained.     

波束控制算法在FPGA中的实现

从工程实践角度出发,提出了一种对某雷达波束控制算法进行优化改进的方法。利用定点乘加器完成浮点运算,并在FPGA中进行了仿真及实现。在保证精度的前提下,提高了算法运行效率,缩短了系统配相时间,完成了硬件平台的更新。     

DDS在复杂信号产生中的应用研究

讨论直接数字频率合成(DDS)在复杂信号产生方面的应用,介绍了使用DSP实现对DDS控制产生复杂信号的一种新方法。     

Scattering of Solar and Atmospheric Background Radiation from a Target

Light scattering property of environment is very important in theoretical study and application of the remote sensing. What's more, it is valuable for infrared radiation, imaging, and the detection of target and tracking. In this paper, solar and atmospheric background radiation, and their scattering property from target are discussed. BRDF (Biodirectional Reflectance Distribution Function) is a very important quantity that shows the radiation and reflection feature of target. According to electromagnetic radiant and scattering theories, the relationship between laser radar scattering cross section (LRCS) and BRDF is introduced. LOWTRAN model is an effective method of calculating the spectral distribution of solar and atmospheric radiation. Here it is applied to compute solar and atmospheric background radiation scattered from a target. The relative equations are deduced. Thus, the spatial and spectral distribution of scattering light is given. As a special example, for the Lambert's surface, the equations are simplified. As a result, the spatial and spectral distributions scattering radiation of solar and atmospheric background from a rough painted surface are present. The scattering of solar radiation plays a primary role in MIR region, but scattering of atmospheric background radiation is higher in LIR region. At the same time, there is obviously specular reflectance for solar radiation due to coherent scattering from rough surface.     

A Simple Analytical Model for Expected Frame Waiting Time Evaluation in IEEE 802.11e HCCA Mode

IEEE 802.11e standard defines two access modes to provide Quality of Service support for 802.11-based Wireless LANs: Enhanced Distributed Channel Access and HCF Controlled Channel Access (HCCA). Only HCCA can guarantee bandwidth per flow which is suitable for various multimedia application requirements. However, the reference scheme HCCA is designed without awareness of variable bit rate (VBR) transmission. Several enhancement techniques have been proposed to address the VBR problem, but there are few research results for a model to capture the characteristics of HCCA. In this paper, an analytical model for the expected packet waiting time is proposed. The model can capture the system behavior per session leading to suitable admission control process for VBR support. The proposed model has been validated using the Network Simulator. The results confirm that the reference admission control scheme with mean data rate acceptance cannot guarantee the required bound on waiting time. Using the model, suitable parameters can be investigated for admission control with VBR support. The model can also be applied with polling based technologies, such as WiMAX.