Spontaneous and stimulated light emission due to radiative recombination in forward biased lead telluride P-N junctions

<p>Since the discovery in 1962 of laser action in semiconductor diodes made from GaAs, the study of spontaneous and stimulated light emission from semiconductors has become an exciting new field of semiconductor physics and quantum electronics combined. Included in the limited number of direct-gap semiconductor materials suitable for laser action are the members of the lead salt family, i.e . PbS, PbSe and PbTe. The material used for the experiments described herein is PbTe . The semiconductor PbTe is a narrow band- gap material (E_g = 0.19 electron volt at a temperature of 4.2°K). Therefore, the radiative recombination of electron-hole pairs between the conduction and valence bands produces photons whose wavelength is in the infrared (λ ≈ 6.5 microns in air).p> <p>The p-n junction diode is a convenient device in which the spontaneous and stimulated emission of light can be achieved via current flow in the forward-bias direction. Consequently, the experimental devices consist of a group of PbTe p-n junction diodes made from p –type single crystal bulk material. The p - n junctions were formed by an n-type vapor- phase diffusion perpendicular to the (100) plane, with a junction depth of approximately 75 microns. Opposite ends of the diode structure were cleaved to give parallel reflectors, thereby forming the Fabry-Perot cavity needed for a laser oscillator. Since the emission of light originates from the recombination of injected current carriers, the nature of the radiation depends on the injection mechanism.p> <p>The total intensity of the light emitted from the PbTe diodes was observed over a current range of three to four orders of magnitude. At the low current levels, the light intensity data were correlated with data obtained on the electrical characteristics of the diodes. In the low current region (region A), the light intensity, current-voltage and capacitance-voltage data are consistent with the model for photon-assisted tunneling. As the current is increased, the light intensity data indicate the occurrence of a change in the current injection mechanism from photon-assisted tunneling (region A) to thermionic emission (region B). With the further increase of the injection level, the photon-field due to light emission in the diode builds up to the point where stimulated emission (oscillation) occurs. The threshold current at which oscillation begins marks the beginning of a region (region C) where the total light intensity increases very rapidly with the increase in current. This rapid increase in intensity is accompanied by an increase in the number of narrow-band oscillating modes. As the photon density in the cavity continues to increase with the injection level, the intensity gradually enters a region of linear dependence on current (region D), i.e. a region of constant (differential) quantum efficiency.p> <p>Data obtained from measurements of the stimulated-mode light-intensity profile and the far-field diffraction pattern (both in the direction perpendicular to the junction-plane) indicate that the active region of high gain (i.e. the region where a population inversion exists) extends to approximately a diffusion length on both sides of the junction. The data also indicate that the confinement of the oscillating modes within the diode cavity is due to a variation in the real part of the dielectric constant, caused by the gain in the medium. A value of τ ≈ 10p>-9p> second for the minority- carrier recombination lifetime (at a diode temperature of 20.4°K) is obtained from the above measurements. This value for τ is consistent with other data obtained independently for PbTe crystals.p> <p>Data on the threshold current for stimulated emission (for a diode temperature of 20. 4°K) as a function of the reciprocal cavity length were obtained. These data yield a value of J’_th = (400 ± 80) amp/cmp>2p> for the threshold current in the limit of an infinitely long diode-cavity. A value of α = (30 ± 15) cmp>-1p> is obtained for the total (bulk) cavity loss constant, in general agreement with independent measurements of free- carrier absorption in PbTe. In addition, the data provide a value of n_s ≈ 10% for the internal spontaneous quantum efficiency. The above value for n_s yields values of t_b ≈ τ ≈ 10p>-9p> second and t_s ≈ 10p>-8p> second for the nonradiative and the spontaneous (radiative) lifetimes, respectively.p> <p>The external quantum efficiency (n_d) for stimulated emission from diode J-2 (at 20.4° K) was calculated by using the total light intensity vs. diode current data, plus accepted values for the material parameters of the mercury- doped germanium detector used for the measurements. The resulting value is n_d ≈ 10%-20% for emission from both ends of the cavity. The corresponding radiative power output (at λ = 6.5 micron) is 120-240 milliwatts for a diode current of 6 amps. p>

Decoding cosets of first-order Reed-Muller code

<p>Proper encoding of transmitted information can improve the performance of a communication system. To recover the information at the receiver it is necessary to decode the received signal. For many codes the complexity and slowness of the decoder is so severe that the code is not feasible for practical use. This thesis considers the decoding problem for one such class of codes, the comma-free codes related to the first-order Reed-Muller codes.p> <p>A factorization of the code matrix is found which leads to a simple, fast, minimum memory, decoder. The decoder is modular and only n modules are needed to decode a code of length 2p>np>. The relevant factorization is extended to any code defined by a sequence of Kronecker products.p> <p>The problem of monitoring the correct synchronization position is also considered. A general answer seems to depend upon more detailed knowledge of the structure of comma-free codes. However, a technique is presented which gives useful results in many specific cases.p>

Measurements of mantle velocities of P waves with a large array

A large array has been used to investigate the P-wave velocity structure of the lower mantle. Linear array processing methods are reviewed and a method of nonlinear processing is presented. Phase velocities, travel times, and relative amplitudes of P waves have been measured with the large array at the Tonto Forest Seismological Observatory in Arizona for 125 earthquakes in the distance range of 30 to 100 degrees. Various models are assumed for the upper 771 km of the mantle and the Wiechert-Herglotz method applied to the phase velocity data to obtain a velocity depth structure for the lower mantle. The phase velocity data indicates the presence of a second-order discontinuity at a depth of 840 km, another at 1150 km, and less pronounced discontinuities at 1320, 1700 and 1950 km. Phase velocities beyond 85 degrees are interpreted in terms of a triplication of the phase velocity curve, and this results in a zone of almost constant velocity between depths of 2670 and 2800 km. Because of the uncertainty in the upper mantle assumptions, a final model cannot be proposed, but it appears that the lower mantle is more complicated than the standard models and there is good evidence for second-order discontinuities below a depth of 1000 km. A tentative lower bound of 2881 km can be placed on the depth to the core. The importance of checking the calculated velocity structure against independently measured travel times is pointed out. Comparisons are also made with observed PcP times and the agreement is good. The method of using measured values of the rate of change of amplitude with distances shows promising results.

Part I: Total yield measurements for the ^(21)Ne(α,n)^)(24)Mg reaction. Part II: A study of the ^(12)C(He,p)^(14)N reaction

<p>PART Ip> <p>The total cross-section for the reaction p>21p>Ne(α, n)p>24p>Mg has been measured in the energy range 1.49 Mev ≤ E_cm ≤ 2.6 Mev. The cross-section factor, S(O), for this reaction has been determined, by means of an optical model calculation, to be in the range 1.52 x 10p>12p> mb-Mev to 2.67 x 10p>12p> mb-Mev, for interaction radii in the range 5.0 fm to 6.6 fm. With S(O) ≈ 2 x 10p>12p> mb-Mev, the reaction p>21p>Ne(α, n)p>24p>Mg can produce a large enough neutron flux to be a significant astrophysical source of neutrons.p> <p>PART IIp> <p>The reactionp>12p>C(p>3p>He, p)p>14p>N has been studied over the energy range 12 Mev ≤ E_lab ≤ 18 Mev. Angular distributions of the proton groups leading to the lowest seven levels in p>14p>N were obtained.p> <p>Distorted wave calculations, based on two-nucleon transfer theory, were performed, and were found to be reliable for obtaining the value of the orbital angular momentum transferred. The present work shows that such calculations do not yield unambiguous values for the spectroscopic factors.p>

Inelastic K+p reactions at incident momenta from 1.37 GeV/c to 2.17 GeV/c

<p>A number of recent experiments have suggested the possibility of a highly inelastic resonance in Kp>+p>p scattering. To study the inelastic Kp>+p>p reactions, a 400 K exposure has been taken at the L.R.L. 25 inch bubble chamber. The data are spread over seven Kp>+p> momenta between 1.37 and 2.17 GeV/c.p> <p>Cross-sections have been measured for the reaction Kp>+p>p → pK°π+ which is dominated by the quasi-two body channels K∆ and K*N. Both these channels are strongly peripheral, as at other momenta. The decay of the ∆ is in good agreement with the predictions of the rho-photon analogy of Stodolsky and Sakurai. The data on the K*p channel show evidence of both pseudo scalar and vector exchange.p> <p>Cross-sections for the final state pKp>+p>π+π- shows a strong contribution from the quasi-two body channel K*∆. This reaction is also very peripheral even at threshold. The decay angular distributions indicate the reaction is dominated as at higher momenta by a pion exchange mechanism. The data are also in good agreement with the quark model predictions of Bialas and Zalewski for the K* and ∆ decay.p>

偏振分配角θp对高功率激光三倍频波形的影响

使用四阶龙格-库塔算法数值求解Ⅱ类-Ⅱ类偏振失配方案的三倍频稳态耦合波方程组。针对“神光Ⅱ”激光驱动器的三倍频系统，在二倍频匹配角吼和三倍频匹配角只均处于最佳匹配情况下，以1°为单位改变偏振分配角郇的偏离量，得到对应的三倍频波形特性的变化。研究表明，偏振分配角θp处于最佳理论匹配位时，对应的三倍频波形半峰全宽最窄，且波形上升沿抬起也最快；θp从最佳匹配位增加比从最佳匹配位减小对应的半峰全宽变宽更快，对应的上升沿抬起更慢。根据所得理论结果，在实验中调节偏振分配角θp，达到了改变三倍频输出波形特性，进而实现八

Mortalidade por asma no município do Rio de Janeiro no período de 2000 2009: análise de causas múltiplas

O estudo das causas múltiplas de óbitos permite conhecer a extensão real das estatísticas de mortalidade, minimizando a subestimação dos dados de mortalidade por asma. O objetivo desta pesquisa foi avaliar a tendência das taxas de mortalidade por asma informada em qualquer linha ou parte do atestado médico da declaração de óbito, no município do Rio de Janeiro, no período de 2000-2009. Os dados foram obtidos no Sistema de Informações de Mortalidade (SIM), no período de 2000 a 2009, nas Declarações de Óbitos (DO) registradas com CID-10 J45 e J46, de residentes do município do Rio de Janeiro, com um ano ou mais de idade. Foram calculadas taxas de mortalidade padronizadas por idade, nas seguintes faixas etárias: 1-4 anos, 5-34 anos, 35-59 anos, 60 e mais anos, considerando-se asma como causa básica e como causas múltiplas, segundo gênero para cada ano do período. Para análise de dados foi utilizado a técnica de regressão linear. No período de 10 anos a asma foi causa básica em 67,2% dos óbitos que mencionaram asma. A subestimação da mortalidade por asma como causa básica, foi igual a 48,7%. A taxa de mortalidade padronizada por asma como causa básica declinou de 2000 a 2009 de 2,22 para 1,72/100.000 habitantes em 2009, (β= -0.06, p=0.017) e como causas múltiplas passou de 3,45 para 2,82/100.000 habitantes (β= -0.11, p=0.005). A análise segundo gênero evidenciou um declínio mais acentuado entre os homens, cuja taxa de mortalidade por asma como causa básica padronizada passou de 1,58/100.000 em 2000 para 0,59/100.000 em 2009 (β= -0.08, p=0.007); como causa múltipla a taxa diminuiu de 2,49/100.000 em 2000 para 1,11/100.000 em 2009 (β= -0.14, p<0.00001). Entre as mulheres a taxa de mortalidade passou de 2,79/100.000 em 2000 para 2,72/100.000 em 2009 como causa básica e de 4,29/100.000 em 2000 para 4,32/100.000 em 2009. A regressão linear segmentada, realizada em dois períodos, de 2000 a 2004 e 2004 a 2009, não foi estatisticamente significativa (2000 a 2004: β= -0,16, p=0,131 e 2004 a 2009: β= 0,04, p=0,630). Do total de óbitos nos quais a asma foi mencionada como causa múltipla 2,8% ocorreram na idade de 1 a 4 anos e 61% na faixa de 60 anos e mais. Quando a asma foi causa básica, as causas associadas mais frequentes foram as doenças do aparelho respiratório e nos óbitos em que foi classificada como causa associada destacaram-se como causas básicas as doenças do aparelho respiratório e circulatório. A magnitude das taxas de mortalidade por asma foi sempre maior nas mulheres comparado aos homens. A série histórica mostrou tendência ao declínio nas taxas de mortalidade, segundo causas básicas e múltiplas, com declínio entre os homens e estabilidade entre as mulheres. A mortalidade por asma foi subestimada quando considerada apenas como causa básica, o que poderia ser evitado com a utilização da metodologia de causas múltiplas nas estatísticas de mortalidade da asma.

Solutions for New Terrestrial Broadcasting Systems Offering Simultaneously Stationary and Mobile Services

221 p.