InGaN multiquantum-well-structure laser diodes with GaN-AlGaNmodulation-doped strained-layer superlattices

InGaN multiquantum-well-structure (MQW) laser diodes with Al_0.14Ga_0.86N-GaN modulation doped strained-layer superlattice (MD-SLS) cladding layers grown on an epitaxially laterally overgrown GaN substrate was demonstrated to have a lifetime of more than 2300 h under the condition of room-temperature continuous-wave operation. The self-pulsation was observed with a frequency of 3.5 GHz. The relative intensity noise less than -145 dB/Hz was obtained even at the 6% optical feedback using the high-frequency modulation of 600 MHz. The threshold carrier density of the InGaN MQW-structure laser diodes was estimated to be 3×10¹⁹/cm³ using a carrier lifetime of 1.8 ns     

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     

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

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     

Ground-state emission and gain in ultralow-threshold InAs-InGaAsquantum-dot lasers

Emission spectra and modal optical gain are investigated in ultralow-threshold MBE-grown InAs-InGaAs quantum dot (QD) structures. The record lowest room-temperature inversion current is found to be ~13 A cm^-2. The rate-equation model is proposed describing the optical gain related to the ground-state (GS) transitions in QDs. The ground-state gain goes to the maximum value that corresponds to the total inversion of available levels. The gain cross section for the GS emission is estimated as ~7×10^-15 cm²     

A self-consistent model of electron emission from metals

We present a model of electron emission from metals, which takes into account deviation of the electron distribution function from the Fermi equilibrium form inside the cathode under the emission effect. Using the Grad method electron distribution function, the emission boundary is expressed consecutively in terms of its hydrodynamic moments: temperature, average velocity, heat flux density. These hydrodynamic moments are caused by the emission itself and thus need a self-consistent determination. New easy-to-use equations for the emission current density, emission heat flux, and total energy distribution function of the emitted electrons are proposed. Consideration was made for a wide range of temperatures (⩽3500 K), field strengths (⩽1.5×10⁸ V/cm) and work function (3.0 to 4.5 eV) of the emitter. A considerable deviation from the Fowler-Nordheim (FN) theory was found at high current density (>10 ⁹ A/cm²)     

Growth of InGaN self-assembled quantum dots and their applicationto lasers

We have successfully grown InGaN self assembled quantum dots (QD's) on a GaN layer, using atmospheric-pressure metalorganic chemical vapor deposition (MOCVD). The average diameter of the QD's was as small as 8.4 nm, and strong emission from the QD's was observed at room temperature. Next, we have investigated a structure in which InGaN QD's were stacked to increase the total QD density. InGaN QD's were formed even when the number of stacked layers was ten. As the number of layers increased, the photoluminescence (PL) intensity increased drastically. Moreover, we have fabricated a laser structure with InGaN QD's embedded into the active layer. A clear threshold of 6.0 mJ/cm² was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Above the threshold, the emission was strongly polarized in the transverse electric (TE) mode, and the linewidth of the emission spectra was reduced to below 0.1 nm (resolution limit). The peak wavelength was around 405 nm. These results indicate lasing action at room temperature     

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     

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     

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     

Growth of self-assembled InP quantum islands for red-light-emittinginjection lasers

In order to achieve laser emission in the visible part of the spectrum, we have investigated the growth of self-assembled InP quantum dots on GaInP by low-pressure metal-organic vapor phase epitaxy (MOVPE) using the Stranski-Krastanow growth mode. Unlike the well-established InAs-GaAs system, when InP is deposited on GaInP, typically, two types of coherently strained islands with different sizes are formed. A high density of small islands is favored when using growth conditions with a reduced surface diffusion, i.e., low temperatures, high growth rates, and substrates with high misorientation angles. After the deposition of 3.4 monolayers of InP at 580°C on GaAs-substrates with a surface angle of 15° to the next (111)B-plane, 2.10¹⁰ InP dots per square centimeter with an average height of 4 nm were assembled. The emission of these InP islands at 1.72 eV (4.2 K) shows an inhomogeneous broadening of 42 meV because of the size fluctuation of the quantum dots. At 90 K, lasing from self-assembled InP quantum islands was observed above a threshold current density of 288 A/cm². The detected laser line is located at 1.8 eV, about 80 meV higher than is the ground-state transition energy. We attribute this behavior to lasing from excited states in agreement with power-dependent photoluminescence experiments. For temperatures above 150 K, the threshold current density increases dramatically because of a thermally activated escape of carriers up to 4.9 k17/cm² at room temperature, where the characteristic temperature is 35 K. Injection lasers containing stacked InP quantum islands and AlGaInP barrier layers with a higher band offset may exhibit an improved temperature dependence     

Narrow-beam divergence 1.3-μm multiple-quantum-well laser diodeswith monolithically integrated tapered thickness waveguide

This paper describes the optimum design, fabrication, and performance of a 1.3-μm multiple-quantum-well (MQW) laser diode monolithically integrated with a tapered thickness spot-size transformer. The dependence of the lasing characteristics on the thickness distribution of the core layer and on the current injection profile of the device were analyzed. This integrated laser with its optimized structure performed at a low threshold current of 22.2 mA, even at 85°C. The integrated spot-size transformer effectively reduced the lateral and vertical far-field FWHM's to 8° and 9°, respectively. A very long lifetime of over 1×10⁵ h was estimated at 85°C and 8 mW under CW operation     

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。     

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     

Analysis of 6-nm AlGaAs SQW low-confinement laser structures forvery high-power operation

This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation. The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-μm-wide stripes was as high as 6.4 W before catastrophic optical degradation. If scaled to continuous-wave (CW) conditions, this value would be 800-1100 MW, which would mean a factor of 22.7 times more than reported for the best devices with normal design for threshold minimization. The absorption coefficient for the symmetrical structure is as low as 1.1 cm^-1, in spite of the low trapping efficiency of carriers in the quantum well (QW). The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions). Threshold current densities were 800 A/cm² for 5-mm-long devices in the symmetrical case and 2200 A/cm² in the asymmetrical one. The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed     

An ultra compact back-lighted thyratron for nanosecond switching applications

An ultra-compact back-lighted thyratron (BLT), which is also named as mini-BLT, with effective volume of 15 cm^{3 was developed to serve as a high voltage switch in compact transient plasma ignition systems. The mini-BLT can hold off 40 kV and was used to conduct a peak current up to 4.5 kA. A 30 ns switching delay with 2 ns jitter was achieved when triggered by a 70 mJ, 355 nm laser pulse. Delay and jitter increase as the photon energy of the trigger pulse decreases. The plasma density in the switch measured at a peak current of 3.2 kA is 9×1014 cm-3. The mini-BLT was successfully used as the switch in a 100 ns, 60 kV pulse generator for generation of streamers in a plasma ignition system.}     

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.     

High-performance 1.55-μm wavelength GaInAsP-InPdistributed-feedback lasers with wirelike active regions

High-performance 1.55-μm wavelength GaInAsP-InP strongly index-coupled and gain-matched distributed-feedback (DFB) lasers with periodic wirelike active regions mere fabricated by electron beam lithography, CH₄/H₂-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, whose index-coupling coefficient was more than 300 cm^-1. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width mere investigated both theoretically and experimentally. A record low threshold current density of 94 A/cm² among 1.55-μm DFB lasers was successfully obtained for a stripe width of 19.5 μm and a cavity length of 600 μm. Moreover, a record low threshold current of 0.7 mA was also realized at room temperature under CW condition for a 2.3-μm-wide buried heterostructure with a 200-μm-long cavity. Finally, we confirmed stable single-mode operation due to a gain-matching effect between the standing-wave profile and the wirelike active region     

Ultrafast all-optical spin polarization switch using quantum-welletalon

Ultrafast all-optical switch is proposed and demonstrated using picoseconds spin-polarization relaxation in a multiple-quantum-well (MQW) etalon structure. The decay time of the conventional all-optical switching in MQW etalon is restricted by the carrier lifetime, typically in nanoseconds. Using carrier spin relaxation, the polarization change of the probe beam has been demonstrated to be switched with a pulse width of 4 ps and a contrast of 4:1 at a pump pulse energy of 50 fJ/μ ². In the present device, the contrast is determined by the polarization rotation angle of the probe beam, and the polarization-rotation angle has been shown to be proportional to the total well thickness. It is predicted that a contrast can be improved over 13 dB by optimizing the MQW etalon structure, indicating potential applicability to ultrafast optical communication systems. The optimization would improve the transmission from a present value of about 1%