Nonequilibrium electron distributions and high-field electrontransport in an AlxGa1-xAs-based p-i-nnanostructure semiconductor-a picosecond Raman probe

Electron transport in an Al_xGa_1-xAs (x=0.3) based p-i-n nanostructure semiconductor under the application of an electric field has been studied at T=80 K by picosecond transient Raman spectroscopy. Single-particle excitations associated with spin-density fluctuations were used to directly measure electron distribution functions and drift velocities under various electric field intensities. Extremely nonequilibrium electron distributions were observed. Specifically, for an injected carrier density of n≅1×10¹⁸ cm^-3, a drift velocity V_d as high as 2.5×10⁷ cm/s was measured for an electric field intensity E=18 kV/cm. These experimental results are in good agreement with Ensemble Monte Carlo calculations     

Spectral analysis of the light emitted by streamers in hydrocarbonliquids

This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geometry. Experimental spectra of the light emitted by bush-like and filamentary streamers are composed of the H_α-Balmer line, the C₂ Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C₂ in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of C₂ and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited C₂ do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of H_α are in the range 4×10 ¹⁶ to 7×10¹⁶ cm^-3 for filamentary streamers and 2 to 6×10¹⁷ cm^-3 during the breakdown phase. For slow bush-like streamers, the electron density is not measurable     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

Optical and Photoluminescence Properties of Erbium-Doped Chalcogenide Glasses (GeGaS:Er)

We have examined the optical and photoluminescence (PL) properties of Er³⁺-doped GeGaS glasses of near-stoichiometric composition Ge₂₈Ga_6.2S_65.3:Er_0.5. We have also used powdered samples of various mean sizes (L) to examine the dependence of the 1.54 -mum PL emission spectrum and the PL decay time on the average sample size. Optical absorption spectra of Er³⁺ ions arising from transitions between different energy manifolds, such as 4 I_{15 /2} -⁴ I_13/2,⁴ I_{15 /2} -⁴ I_{11 /2} , etc., have been used to extract Omega₂, Omega₄, and Omega₆ values using the Judd-Ofelt analysis and a Judd-Ofelt radiative lifetime T_JO = 2.6 ms for the ⁴ I_{13 / 2} -⁴ I_{15 / 2} transition. The PL emission spectra and the decay time have been found to depend on the mean sample size. The spectra are broader and the decay times are longer for larger sample sizes, due to photon trapping occurring in the sample. The extrapolated decay time to zero particle size yields a decay time that matches the Judd-Ofelt radiative lifetime almost perfectly, and confirms the argument that the true PL lifetime needs to be measured in fine powders to avoid reabsorption effects. We have estimated the maximum emission cross section as 15.5 X 10^-21 cm².     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

Image quality analysis for dual energy subtraction imaging with afemtosecond laser-based hard X-ray source

We present a study of image quality for dual energy subtraction imaging using an iodinated contrast agent and a femtosecond laser-based hard X-ray source. The INRS CPA laser (400 fs pulse focused on solid targets in a 3 μm spot at 4 × 10¹⁸ W cm^-2) was used to create a bright hard X-ray source (conversion efficiency of 10^-5 in the characteristic K_∝ line emission, 12 μm X-ray source diameter). A model of image quality has been developed and been benchmarked with specific experiments using specially made angiography phantoms     

Breakdown behavior of SF6 gas-insulated systems at lowtemperature

The controversy surrounding low-temperature SF₆ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF₆ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24°C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43×10¹⁹ cm^-3 (equivalent to pressures of 105 and 624 kPa, respectively, at 24°C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF₆-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by ~10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30°C), take a constant value, and be fairly independent of density     

Loss and recovery of hydrophobicity, surface energies, diffusioncoefficients and activation energy of nylon

The loss of hydrophobicity of nylon 6/6 caused by immersion in saline water for up to 336 h at different conductivities (0.005 to 100 mS/cm) and different temperatures (0 to 98°C) and its subsequent recovery in air (during 4500 h) have been investigated. The hydrophobicity is determined by measuring the static contact angle &thetas; between the tangent to a droplet of distilled water and the horizontal surface. The changes in the surface roughness and in the weight of the specimens were determined and correlated with the changes in the contact angle. It has been found that &thetas; decreased with increasing conductivity and increasing temperature of the saline solution. After removal from the solution, the higher the conductivity and temperature, the longer it took for &thetas; to recover in air. &thetas; decreased from 70° to 54° after nylon was subjected for 521 h to a uniform field of 15 kV_dc/cm in air. The surface free energy of nylon was determined as a function of time of immersion, the conductivity and temperature of the solution and during the recovery in air. The surface energies calculated for the virgin specimen are in good agreement with the literature. The diffusion coefficient of water into nylon increased from 0.23×10^-12 m²/s at 23°C to 7.4×10^-12 m²/s at 75°C. The activation energy was determined to be 59.4±2.2 kJ/mol. For unaged nylon the surface energies were determined at 23°C to be γ_S=44.7 mJ/m², γ_SD=29.3 mJ/m², γ_SH=15.4 mJ/m², W_SL =97.7 mJ/m² and γ_SL=19.8 mJ/m²      

Charge trapping and conduction in pure and iodine-dopedbiaxially-oriented polypropylene

Study of charge trapping and conduction in pure and iodine doped biaxially oriented polypropylene (BOPP) is presented. Structural and chemical modifications induced by iodine were investigated using X-ray, optical and infrared methods. Optical spectra of doped BOPP show absorption at 290 nm from charge transfer complexes. X-ray examination revealed a decrease in crystallinity and crystallite size after doping. The effect of iodine on charge trapping was determined by thermally stimulated current technique. Deep traps (120°C peak) at crystalline-amorphous interfaces are destroyed by iodine, which provides new traps (68°C peak) with activation energy 0.9 eV. Pressure dependence of conductivity indicates ionic conduction in pure samples and electronic conduction in doped samples. Steady state currents in 0.5%wt iodine doped BOPP were measured for fields 1 to 5×10⁵ V cm^-1 and at elevated temperatures 22 to 50°C. Iodine enhances conductivity by ~700× in pure BOPP and the steady state conductivity shows a good fit of the 3-D Poole-Frenkel theory to the experiment. It is proposed that trapped electrons (arising due to donor-acceptor action) thermally released through PF lowering, predominantly contribute to the conduction     

基于中试平台的低低温电除尘器深度试验研究

基于50 000 m³/h实际烟气中试试验系统,采用常规采样枪+玻纤滤筒和一体化采样头+石英滤膜测定总尘,采用ELPI测定PM_2.5,采用自制的控制冷凝+异丙醇吸收系统测定SO₃,采用BDL型飞灰工况比电阻测试仪测定飞灰工况比电阻。试验结果表明,130℃、90℃、80℃时电除尘器出口烟尘浓度分别为11.7mg/m³、9.7 mg/m³、5.4 mg/m³,PM_2.5浓度分别为0.8 mg/m³、0.4 mg/m³、0.2 mg/m³,总尘及PM_2.5减排效果显著;电除尘器出口SO₃浓度分别为1.25 mg/m³、0.10 mg/m³、0.14 mg/m³,对应低低温电除尘系统的SO₃脱除效率分别为22.84%、96.15%、96.61%,低低温电除尘系统可脱除烟气中绝大部分SO₃;电除尘器入口飞灰工况比电阻分别为3.02×10¹³ Ω·cm、6.15×10¹² Ω·cm、5.24×10¹¹ Ω·cm。     

1.047-μm Yb:Sr5(PO4)3F energystorage optical amplifier

The pumping and gain properties of Yb³⁺-doped Sr₅ (PO₄)₃F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm² based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF₄ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0×10^-20 and 1.5×10^-20 cm ² for π and σ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3×10^-20 and 1.4×10^-20 cm² for π and σ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25×25×175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (<~1%/cm) and adequately large laser damage thresholds at 1064 nm (~20 J/cm² at 3 ns). Overall, diode-pumped Yb:S-FAP amplifiers are anticipated to offer a viable means of amplifying 1.047-μm light, and may be particularly well suited to applications sensitive to overall laser efficiencies, such as inertial confinement fusion energy applications     

Liquid microvortex generated around a laser focal point in anintense high-frequency electric field

A microvortex was discovered in liquid films when subjected to the combined action of a focused laser beam and a high-frequency electric field. The laser beam was perpendicular to both the surface of the liquid film and the direction of the electric field. The size of this opto-electrostatic microvortex (OEMV) in various liquids (water, ethanol, acetone, nitrobenzene) varied with the experimental conditions, but stayed in the 10-μm range. The velocity of the liquid flow in the vortex could be controlled by the intensity and frequency of the electric field (5.0×10⁴ to 0.8×10⁶ V/m, 2×10⁴ to 1×10⁶ Hz), by the laser power (0.01 to 2 W) and by the laser power density in the focal point. No microvortex was observed in benzene and chloroform. As an application of this phenomenon, single DNA molecules could be transported along the OEMV     

非平衡等离子体特征量对NO脱除效率的影响

采用数值模拟的方法开展了非平衡等离子体脱除烟气中NO的研究,考察了非平衡等离子体放电中的2个主要特征量（电子数密度n_e和电子平均能量 ）对NO脱除效率的影响。计算结果表明,NO脱除效率随电子数密度n_e和电子平均能量 的变化而有不同程度的变化,相比较而言,电子数密度量级变化对NO脱除效率的影响更大。在NO初始分子浓度（粒子数密度）为10¹⁶ cm^-3情况下,电子数密度较高（10¹⁷ cm^-3）时,即使电子平均能量 低至4 eV,也可获得高达99%的NO脱除效率。当电子数密度和电子平均能量保持不变时,NO脱除效率随着NO初始分子浓度的增加而降低。     

Composition Optimization and Phase Transformation of Si-Nanocrystal-Doped $hbox{SiO}_{x}$ for Enhancing Luminescence From MOSLED

Near-infrared (NIR) photo- and electroluminescence (PL and EL) of Si nanocrystals buried in Si-rich SiO_x, film, and their correlation with the structural phase transformation and the varied oxygen composition of SiO_x, are investigated. By detuning the N₂O flowing ratio (Y_{N 2 O} = [N₂O/(N₂O + SiH₄)] times 100%) from 93% to 80% during plasma-enhanced chemical vapor deposition growth, the oxygen composition ratio of the Si-rich SiO_x, can be adjusted from 1.64 to 0.88. The grazing incident X-ray diffraction and X-ray photoelectron spectroscopy spectra indicate that the SiO_x, transforms its structural phase from Si + SiO₂ isomer to Si + SiO + SiO₂ isomer. With O/Si ratio >1.24, the SiO_x, matrix becomes SiO₂ isomer, whereas the SiO_x, structure approaches SiO phase at O/Si ratio that is nearly 1.0. The formation of SiO matrix in SiO_x, grown at Y_{N 2 O} below 85% reduces the precipitated Si nanocrystal density from 2.8 times 10¹⁸ to 7 times 10¹⁶ cm^-3, and monotonically attenuates the NIR PL by one order of magnitude. Such a structural phase transformation from SiO₂ to SiO in SiO_x with lower O/Si ratio causes the degradation in EL power conversion efficiency and external quantum efficiency (EQE). Maximum EL power of 0.5 muW and EQE of 0.06% are obtained from MOSLED made on SiO_x, with optimized O/Si ratio of 1.24.     

Wurtzite GaN-based heterostructures by molecular beam epitaxy

Progress in plasma and reactive molecular beam epitaxy (PMBE and RMBE) grown n- and p-type GaN and GaN-AlGaN-based epitaxial films and optoelectronic devices is reviewed. The growth of GaN by RMBE (PMBE) is achieved by employing ammonia gas (plasma activated nitrogen) as the nitrogen source with resultant growth rates of about 2 μm/h (⩾1 μm/h). The structural, electrical, and optical properties of binary and ternary (Al,Ga)N and (In,Ga)N layers point to high quality. The GaN layers with Mg as the dopant atoms are p-type without any postgrowth treatment, but the hole concentrations are limited to mid 10¹⁷ cm^-3 although reports in the low 10¹⁸ cm^-3 dot the literature. The background carrier concentration, mobility, optical characteristics and ability to dope p-type depend significantly on the substrate temperature and V-III ratio employed, AlGaN-GaN, and GaN-InGaN electroluminescent devices have been realized but lack commercial quality. The AlGaN-GaN photodiodes by RMBE exhibited a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than three orders of magnitude for wavelengths longer than 390 nm. A reverse bias of -10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The noise equivalent noise power near zero bias is below the detection limit of the measurement setup. At a reverse bias of 28 V, the total noise equivalent power is 2.06×10^-11 W     

Winter lightning on Japan Sea coast-development of measuring systemon progressing feature of lightning discharge

With the aid of recent optoelectronic techniques, the authors have developed a measuring instrument named ALPS (Automatic Lightning Discharge Progressing Feature Observation System) to determine the progress of lightning flashes. The progress velocities of stepped leaders for lightning in winter seasons were observed using the ALPS in the coastal area of the Japan Sea. The progress velocity of an individual step of a stepped leader can be expressed by a lognormal distribution and its mean value is 1.6×10⁶ m/s for upward leaders and 4.0×10⁶ m/s for downward leaders. The mean progress velocity of a total leader process is given as 0.8~2.7×10⁵ m/s for upward leaders and 2.9×10⁵ m/s for downward leaders     

Electrical surface resistance, hydrophobicity and diffusionphenomena in PVC

Polymeric materials are used increasingly in both outdoor and indoor insulation and it has become imperative to find a convenient and a practical method to evaluate the performance of the insulation in situ. The hydrophobicity of a polluted surface particularly in the presence of moisture, determines the level of the leakage current which may result in a flashover and an outage of the power system. However, it is difficult to measure the hydrophobicity of insulators in the field, and therefore the measurement of the electrical surface resistance in situ has been suggested as an alternative method that might give information on the surface state. In the present study, polyvinylchloride (PVC) was used to study the characteristics of the surface resistance, the change of the hydrophobicity and the relationship between both of them in the presence of salt-fog. The dependence of the surface resistance on factors such as the duration of the wetting in salt-fog, the recovery during drying time, the length of the specimens, the level of the applied dc stress used to measure the resistance and the ac stress is reported. The surface free energy per unit area of PVC, during exposure to salt-fog, was calculated using the harmonic-mean method and was found to be consistent with the changes in both the surface resistance and the hydrophobicity of the surface. The surface tension γ_s, increased from 43.1×10^-3 J/m² for the virgin specimen to 76.8×10^-3 J/m² after complete wetting in un-energized salt-fog. The diffusion coefficients of a saline solution having a conductivity of 1 mS/cm into the pvc were found to increase from 2.8×10^-15 m²/s at 74°C to 1.6×10^-14 m² /s at 98°C     

Carrier mobility in LDPE at high temperature and pressure

Current peaks due to transient SCLC (space charge limited current) were observed in LDPE (low-density polyethylene) at 70°C. From the time at which the current peak occurs, carrier mobilities ranging from 5×10^-15 to 7×10^-14 m²/Vs were obtained. The mobility values obtained for LDPE depend on both applied field and hydrostatic pressure and verify hopping mechanism. The presence of space charge has been confirmed by analyzing discharge currents measurements