Time-resolved spectroscopic study of high-pressure self-sustaineddischarge-pumped atomic xenon lasers

To explore the lasing kinetics of UV-preionized, self-sustained discharge-pumped atomic xenon (5d→6p) lasers, the time-resolved spectroscopy of the laser output from the multiline laser resonator is reported. The dilutents used were Ar and He. Increasing Xe concentration shortened the 1.73-μm laser pulse duration and decreased the total (multiline) laser output energy, because increased Xe metastable state population contributes to the increase of the 6p state population (lower laser level) by electron-impact excitation and radiation trapping during discharge pumping. High-excitation-rate pumping resulted in the decrease of the laser output power of 1.73- and 2.63-μm lines. Increasing the total gas pressure leads to high-efficiency operation due to modest-excitation-rate pumping at high pressures     

Kinetic processes in electron beam-excited XeF (C40aA) laser media

Fundamental processes affecting the operation and performance of electron beam-excited XeF(C→A) laser media have been analyzed and modeled. Emphasis has been placed on conditions typical of high current density (~250A cm^-2), short pulse (~10 ns FWHM) e-beam excitation of high pressure (~6 atm) multicomponent mixtures comprised of Ar-Kr-Xe-NF₃-F₂ . Computation of the temporal evolution of excited and ionized species for such circumstances has permitted identification of the factors controlling XeF(C) formation and loss, and has resulted in the identification of the primary transient species that absorb radiation in the blue-green spectral region. The data so obtained serve to explain measured XeF(C→A) properties, particularly net gain, under conditions for which the C→ A laser energy density and efficiency values are comparable to those of the UV XeF(B→X) laser     

Energy loading effects in the scaling of atomic xenon lasers

The intrinsic power efficiency of the atomic xenon (5d→6p) infrared (1.73-3.65-μm) laser is sensitive to the rate of pumping due to electron collision mixing of the laser levels. Long-duration pumping at moderate power deposition may therefore result in higher energy efficiencies than pumping at higher powers. The consequences of high energy deposition (hundreds of joules per atmosphere) during long pumping pulses (hundreds of microseconds) on the intrinsic power and energy efficiency and optimum power deposition of the atomic xenon laser are examined. The dominant effect of high energy loading, gas heating, causes an increase in the electron collision mixing of the laser levels. The optimum power deposition for a given gas density therefore shifts to lower values with increasing gas temperature     

InGaAs/InAlAs/InP collector-up microwave heterojunction bipolartransistors

Collector-up InGaAs/InAlAs/InP heterojunction bipolar transistors (HBTs) were successfully fabricated, and their DC and microwave characteristics measured. High collector current density operation (J_c>30 kA/cm²) and high base-emitter junction saturation current density (J₀>10^-7 A/cm²) were achieved. A cutoff frequency of f _t=24 GHz and a maximum frequency of oscillation f _max=20 GHz at a collector current density of J₀ =23 kA/cm² were achieved on a nominal 5-μm×10-μm device     

The sublinear relationship between index change and carrier densityin 1.5 and 1.3 μm semiconductor lasers

The carrier-induced index change was measured using a novel injection-reflection technique in combination with differential carrier lifetime data. The observed relation between index change and injected carrier density at bandgap wavelength is nonlinear and is approximately given by δn_act=-6.1×10^-14 ( N)^0.66 for a 1.5-μm laser and δn _act=-1.3×10^-14 (N)^0.68 for a 1.3-μm laser. The carrier-induced index change for a 1.3-μm laser at 1.53-μm wavelength is smaller and is given by δn _act=-9.2×10^-16 (N)^0.72      

High-speed InP/InGaAs heterojunction bipolar transistors

Very-high-performance common-emitter InP/InGaAs single heterojunction bipolar transistors (HBTs) grown by metalorganic molecular beam epitaxy (MOMBE) are reported. They exhibit a maximum oscillation frequency (f_T) of 180 GHz at a current density of 1×10⁵ A/cm². this corresponds to an (R_BC_BC)_eff=f _T/(8πf²_max) delay time of 0.12 ps, which is the smallest value every reported for common-emitter InP/InGaAs HBTs. The devices have 11 μm² total emitter area and exhibit current gain values up to 100 at zero base-collector bias voltage. The breakdown voltage of these devices is high with measured BV_CEO and BV_CEO of 8 and 17 V, respectively     

Hot-carrier current modeling and device degradation insurface-channel p-MOSFETs

The channel field and substrate current models developed for n-MOSFETs are applicable to p-MOSFETs. The impact ionization rate extracted for holes is found to be 8×10⁶ exp (-3.7×10⁶/E), where E is the electric field. The lucky electron approach was used to model the gate current of surface-channel (SC) p-MOSFETs successfully. Device degradation in p-MOSFETs is due to trapped electrons in the oxide. p-MOSFET lifetime has good correlation with gate current in SC p-MOSFETs. The correlation is better than with substrate current. I_G can be larger in a buried-channel (BC) p-MOSFET than in a comparable SC n-MOSFET. This makes the SC MOSFET a much more reliable device. Device lifetime of a p-MOSFET under pulse stress can be predicted from DC stress data for inverterlike waveforms. For other waveforms, there is an extra degradation probably caused by the excess hot carriers generated during the gate turn-off transient     

Characteristics including electron velocity overshoot for0.1-μm-gate-length GaAs SAINT MESFET's

Short-channel effects, substrate leakage current, and average electron velocity are investigated for 0.1-μm-gate-length GaAs MESFETs fabricated using the SAINT (self-aligned implantation for n⁺-layer technology) process. The threshold-voltage shift was scaled by the aspect ratio of the channel thickness to the gate length ( a/L_g). The substrate leakage current in a sub-quarter-micrometer MESFET is completely suppressed by the buried p layers and shallow n⁺-layers. The average electron velocity for 0.1- to 0.2-μm-gate-length FETs is estimated to be 3×10⁶ cm/s from the analysis of intrinsic FET parameters. This high value indicates electron velocity overshoot. Moreover, a very high f_T of 93.1 GHz has been attained by the 0.1-μm SAINT MESFET     

Analyses of mode partition and mode hopping in semiconductor lasers

Mode power fluctuations in semiconductors laser due to mode partition and mode hopping are discussed. The power dropout probability P_e in the mode partition was measured for a wide range, 1.6×10^-6⩽P_e⩽1, which decreased by increasing I/I_th (I and I_th are the DC injection current and its threshold value respectively). The duration time t_d of the power dropout expressed at t_d=3.7×10 ⁴⁸ exp[-118 (I/I_th)] for 1.065⩽I/I_th⩽1.104. Power fluctuations exhibited specific characteristics around the threshold, which were similar to the critical slowing down in the phase transition phenomenon. An increase in the variance of the power fluctuations was observed when the laser oscillating condition was converted from mode partition to mode hopping. The unified stochastic model based on the Fokker-Planck approach described well both mode partition and mode hopping     

Simulations of HEMT DC drain current and 1 to 50 GHzS-parameters as a function of gate bias

The usefulness over an extended range of a high-electron-mobility transistor (HEMT) model previously validated for a 1-25-GHz S-parameter model is shown. Experimental and simulation results for the DC drain current and 1-50-GHz S-parameters of a pseudomorphic 0.32-μm gate AlGaAs-InGaAs-GaAs HEMT are presented. The model predicts the device's DC current and S-parameters as functions of the applied gate bias with good accuracy. The core of the model is directly dependent on the HEMT wafer structure and the physical gate length. As part of the modeling procedure, a value of (1.77±0.07)×10⁵ m-s^-1 is found, confirming the results of other research, for the electron velocity in undoped pseudomorphic In_0.15Ga_0.85As under ≈0.3-μm gates     

Measurement of the wavelength-dependent photoionization cross section of Se*(1S) in the spectral region of 170-175 nm

The photoionization cross section of excited atomic selenium, Se(¹S0), obtained by photodissociation of OCSe, has been measured in the wavelength region of 170-175 nm using a tunable xenon laser. A minimum value, at 172 nm, was found to be1.2 times 10^{-20}cm², approximately ten times less than a recently calculated value. The photodissociation cross section for OCSe, at the same wavelength, was found to be0.8 times 10^{-16}cm², in good agreement with published work. The effect of an election cooling buffer gas upon the electron production from excited selenium was also investigated.     

Differential gain in bulk and quantum well diode lasers

Differential gain (g^') of bulk and single-quantum-well (SQW) lasers was determined from threshold current density and differential quantum efficiency measurements. The threshold measurement technique was used to show that g^' is a function of cavity length (L) in SQW lasers and independent of L in bulk lasers. It was found that g^' of long SQW lasers (1000 μm) is about 7×10^-16 cm² , approximately two times that of bulk lasers. At short cavity lengths (250 μm), g^' is about the same for both laser types     

50 J discharge-pumped XeCl laser

An X-ray preionized XeCl laser with a wide aperture of 10×10 cm² is described. The density for preionization electrons is estimated from an X-ray energy distribution to be greater than 6×10⁹ cm^-3 under operational gas conditions. In high pressure operation at 4500 torr, sufficient preionization has led to successful discharge-breakdown at a low E/N of 6.4×10^-17 V-cm², resulting in a high electrical efficiency of 3.1% with an output energy of 17.6 J. An output energy of 50 J in an 85-ns (FWHM) optical pulse has been extracted from an active volume of about 10:1 for a pulse-forming-line (PFL) voltage of 400 kV. The effects of impedance transformation of a pulse-transmission-line (PTL) following the PFL have been investigated using two types of PTL with output impedances of 0.26 and 0.48 Ω     

A simple interferometric method for monitoring mode hopping intunable external-cavity semiconductor lasers

The mode-hopping events in a 1.3-μm grating-tuned external-cavity laser are analyzed on the basis of interferometric measurements. The average frequency of mode hopping for a 7.5-cm external-cavity laser is estimated and is expressed as f_c =2.7×10⁶×exp [-1.7/(I/I _th-1)] (hertz) for 0.07⩽(I/I_th -1)⩽0.8 (I=injection current and I_th =threshold current). The mode-hopping monitoring signal was negatively fed back to the laser using an automatic control circuit which maintained single-mode operation while the wavelength of the grating external-cavity laser was tuned     

Dependence of the frequency shift of the optical-microwave doubleresonance signal in the Cs D2 line on the pumpingfrequency and power of a GaAs semiconductor laser

The OMDR (optical-microwave double resonance) effect in the Cs D₂ line was studied for realizing a gas-cell-type Cs atomic frequency standard. A glass cell containing Cs with buffer gases (Ar/N₂=1.26, total pressure=39 torr) was placed in a TE₀₁₂ mode microwave cavity at a temperature of 45°C and was pumped using a GaAs semiconductor laser frequency locked to an external interferometer tuned to the 6P_3/2 (F=2,3,4)←6 S_1/2(F=3) transition. The OMDR signal appearing at the resonance to the F=4←3 hyperfine transition of the 6S_1/2 state shifted with detuning of the laser frequency and with change of the laser and microwave powers. The dependence of the shift on these variables around an optimum operating condition was obtained as, Δν_MW[Hz]=-(0.31±0.02) {1+(0.44±0.15) (ΔP_L/P_L)} Δν_L [MHz]-10(ΔV_MW/V _MW)     

Yttrium oxide/silicon dioxide: a new dielectric structure forVLSI/ULSI circuits

Electrical characteristics of Al/yttrium oxide (~260 Å)/silicon dioxide (~40 Å)/Si and Al/yttrium oxide (~260 Å)/Si structures are described. The Al/Y₂O₃/SiO₂/Si (MYOS) and Al/Y₂ O₃/Si (MYS) capacitors show very well-behaved I-V characteristics with leakage current density <10^-10 A/cm² at 5 V. High-frequency C- V and quasistatic C-V characteristics show very little hysteresis for bias ramp rate ranging from 10 to 100 mV/s. The average interface charge density (Q_f+Q _it) is ~6×10¹¹/cm² and interface state density D_it is ~10¹¹ cm^-2-eV^-1 near the middle of the bandgap of silicon. The accumulation capacitance of this dielectric does not show an appreciable frequency dependence for frequencies varying from 10 kHz to 10 MHz. These electrical characteristics and dielectric constant of ~17-20 for yttrium oxide on SiO₂/Si make it a variable dielectric for DRAM storage capacitors and for decoupling capacitors for on-chip and off-chip applications     

Quantum-mechanical constraints on electron-beam brightness

Bounds on the brightness and quality of fermion beams are derived from Pauli's exclusion principle and the finite density of quantum eigenstates for free fermions. These bounds are applied to electron-beam sources to derive limitations on the quality (γJ/Δγ_z) of the electron beams they generate. In the absence of electromagnetic fields, the quality of a relativistic electron beam of current density J and kinetic energy (γ-1)mc² cannot exceed (Jγπec)^1/2λ_c ^-3/2, where λ_c is the Compton wavelength. The normalized brightness cannot exceed 2Δγeccλ_c^-3, where Δγmc² is the electron energy spread, and will be 1/π times the beam quality for a beam of maximal quality. It is concluded that the qualities of electron beams produced by existing linear accelerators for free-electron lasers could be increased by six to eight orders of magnitude before reaching the quality limit     

Saturation studies of the E-beam sustained discharge atomic xenonlaser

In an electron beam sustained discharge xenon laser the discharge energy deposition has been varied in order to investigate the saturation effect on the xenon laser. The current density of the electron beam is varied separately in the range of 0.1-2.7 A/cm² to obtain optimized discharge excitation conditions as a function of electron beam current density and gas pressure. An optimal fractional ionization f=3.5-4×10^-5 is found, independent of the electron beam parameters. The synergy of electron beam and discharge excitation has resulted in a maximum specific energy of 15 J/l at a total gas pressure of 9 bar     

Effects of helium addition to Ar-Xe mixtures in high-pressureatomic-transition xenon lasers

The output energy and spectral distribution of the atomic-transition xenon laser have been investigated with ternary mixtures based on Ar-Xe to explore kinetic issues and to enhance particular lines of the xenon laser pumped by an e-beam and e-beam sustained discharge. Addition of helium into the Ar-Xe mixtures increased the 2.03 μm line energy by a factor of 3-5 while suppressing the 1.73-μm line even during single-line oscillation. The maximum output energy obtained at 2.03 μm was 1.2 J/L and the intrinsic efficiency was ~1%. Effects of neon or krypton addition have also been studied with e-beam sustained discharge pumping     

Generation of short pulses of coherent electromagnetic radiation ina free-electron laser amplifier

Theoretical and experimental studies of the evolution of a frequency-chirped pulse under the influence of both phase and gain dispersion effects induced by the free-electron laser interaction are presented. For the experimental parameters used (electron beam voltage V=150 kV, wiggler periodicity l_w=3.5 cm, gain ~10 dB, input pulse width Δt~200 ns, frequency w ₀/2π=10 GHz, and frequency chirp α/2π~5 MHz/ns), pulses of a few nanoseconds were generated after an interaction length of 2.30 m, in good agreement with theoretical expectations