Saturation behavior of SF6at high pressure and laser intensity

An experimental study of the saturable absorption of SF6at high pressure with high incident laser flux is reported. Experiments with a high-pulse-power CO2laser show 15 orders of magnitude change in transmittance of SF6(17 torr pressure) at theP(16) CO2line and apparent saturation of the residual absorption atP(20) andP(22) CO2wavelength. Comparison of these results with behavior predicted by a four-level model yields reasonable agreement over certain intensity ranges.     

Nanosecond pulse transmission of buffered SF6at 10.6 µm

The 10.6 μm transmission properties of SF6were measured using nanosecond-duration pulses for various buffer gases and buffer gas pressures up to 3 atm. The addition of a buffer gas strongly increased the SF6absorption at moderate energy fluences without significantly altering the high fluence pulse transmission. For pressures above 1 atm, the transmission behavior appeared independent of the pulse duration over the range of parameters investigated in this experiment. The implication of these results for CO2laser applications will be discussed.     

Saturation of infrared absorption in SF6

An analysis of the saturable absorption of SF6at 10.59 μ is presented. A model for the absorber is developed and experiments have been performed to determine the value of the saturation parameter. It is shown that at pressures above 0.1 torr, the saturation parameter varies linearly with the pressure and is governed by the relaxation time of the lowest level ν6. Effects of temperature on the absorption are discussed.     

Pulse-probe measurements in the v3band of SF6at low temperature and low pressure

To improve understanding of multiple-photon absorption by polyatomic molecules, we have performed a series of experiments wherein a pulsed CO2laser irradiated 0.02-0.08 torr samples of SF6at 145 K. A CW probe laser monitored the time response of the induced absorption or transmission at many CO2laser lines that are in or near the ν3absorption band of SF6. The experiments covered a 40-fold fluence range and probe times out to 4 ms. We conclude that the absorbed laser radiation produces a nonthermal vibrational-energy distribution and that intermolecular vibrational-energy transfer is important at early times in redistributing the absorbed energy. We also discuss the influence of other processes on the induced spectrum.     

Excited-state absorption studies of Cr4+ ions in severalgarnet host crystals

An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr⁴⁺ :YAG, Cr⁴⁺:GGG, and Cr⁴⁺:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr⁴⁺:YAG at λ=1064 nm, yielding the higher excited-state lifetime as τ*=(0.55±0.1) ns. The regular transmission bleaching curve was measured in Cr⁴⁺:GGG, for the first time in this material, yielding σ_ga=(58±5)×10^-1 cm², and σe_s=(13±2)×10^-19 cm² at λ=1064 nm, ESA spectra were measured for the three materials between ~700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths     

Infrared pulse transmission in SF6-He mixtures

The transmission ofQ-switched CO2laser pulses through SF6-He mixtures is studied experimentally and compared with predictions generated by computer calculations based on a four-state model for SF6. The results show that for saturating fields the factor determining the rate of absorption is the rotational energy transfer process that feeds the levels interacting with the laser. This conclusion has implications for models of passiveQswitching in molecular lasers and infrared-infrared double resonance.     

CARS thermometry of polyatomic gases: SF6

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to study the temperature behavior of SF6gas. In the temperature range 200-1000 K, evaluation of CARS bandwidths and lineshifts resuited in a relatively precise temperature determination. The advantages of a CARS-based SF6thermometer are discussed in detail.     

Nonlinear optical properties of matrix isolated SF6

The properties of a new nonlinear material for the infrared (IR) based on matrix isolated SF6are described. Fabrication dependent saturation parameters for the homogeneously broadened KCl:SF6composite system range from 20 MW . cm^-2to 100 MW . cm^-2. Applications of this material to passive mode locking of the CO2laser are described.     

60-J pulses from an electron-beam-initiated SF6-C2H6chemical laser

A 5-kJ electron-beam pulse has been used to drive an SF6-C2H6- fueled chemical laser to produce 60-J 600-ns pulses with 100-ns rise time.     

Subnanosecond pulses from a KrF laser pumped SF6Brillouin amplifier

Experimental measurements are presented of pulse compression in a backward Brillouin amplifier using SF6gas pumped by 0.9 GHz linewidth KrF laser radiation. Average pulse durations of 390 ps and energy extraction effieiencies of 40 percent from 24 ns pump pulses have been demonstrated at pressures of 15.3 and 12.6 atm, respectively.     

Behavior of first- and second-positive emission in the N2/SF6laser

Experimental results for the behavior of the emission of the discharge-excited first- and second-positive bands of N2and flow-tube measurements for the deactivation of theA^{3}Sigmamin{u}max{+}andB^{3}Pi_{g}levels of N2by SF6are presented. The results of both these experiments are used to explain the operating mechanisms of the N2/SF6laser.     

Investigations of the properties of SF6as an arc quenching medium

The results of experiments on stationary and nonstationary arcs in SF6are summarized. High temperature gas properties, like the electrical and thermal conductivity as well as the theoretically predicted plasma demixing effects, are determined by electrical and spectroscopic measurements. Investigations on interrupted dc arcs give insight into the energy transport mechanism of the arcs. The transient temperature behavior of gas blast interrupted arcs is measured. Finally, the application of the investigations to circuit breaker arcs is discussed. The following principal results have been found: From the lower time constant of blown N2arcs in comparison to SF6arcs above 10 000 K it follows that the good quenching properties of SF6must be due to processes taking place below 10 000 K. This agrees also with the time-constant measurements in the interrupted cascade arc: at lower temperature--below approximately 8000 K-- the conductance decay in N2is very much slower than in SF6. Further, these measurements revealed that the steep descent in SF6is not caused by electron attachment but is due to energy transport mechanisms. Experimental work about the plasma properties on steady-state arcs, including demixing effects, is in good agreement with theory. Finally the ac experiments show that the temperature profile of the blown ac arcs has a nearly rectangular shape. Variations in current affect mainly the diameter of the arc whereas the temperature variations are fairly small.     

CW CO2at atmospheric Pressure

The operation of a CW CO2laser is reported in which a flow-conditioning technique is used to produce a uniform discharge at atmospheric pressure with a specific input in excess of 500 J/g. In this technique the premixed gas is introduced radially at sonic velocity through a narrow gap adjacent to the upstream electrode. A theoretical and experimental examination of the conditions for the appearance in the discharge of fine filaments, which can be regarded as incipient arcs, gave an insight into the method of operation of the technique. A laser is described that uses a cascade of six 0.2-m-long 19-mm-ID discharge tubes in which the gas flow, discharge, and optical axes are along the length of the tubes. The output power of 530 W at about 5-percent efficiency was limited by the difficulty of power extraction from the low-gain, high-saturation-flux medium.     

Some numerical solutions of the boundary layer equations for an SF6arc

An approximate integral formulation of the thermal boundary layer equation is derived for a steady arc in a nozzle. This differential equation defines the constriction of the arc radius in terms of a heat transfer parameter γ which represents the ratio of all radial heat losses to axial convection. These radial losses include both optically thin radiation and turbulent heat transfer as estimated from a mixing length model. Numerical solutions of the continuity, energy, and momentum equations have been obtained within an arc radius determined from the above equation. These solutions show the effects of various degree of arc constriction upon the velocity and temperature profiles and the arc time constant. The calculations make use of the thermodynamic, transport, and radiative properties of SF6evaluated for a throat pressure of 10 atm for temperatures up to 100 000°K. Solutions have been obtained for arc currents of 100 and 1000 A. It is found that a 100-A arc will have a time constant of several microseconds if the arc radius is of the order of 0.5 mm. Voltage gradients corresponding to this degree of constriction are approximately 200 V/cm.     

Ba原子6s2 1S0 - 6s6p3P1跃迁Hanle效应实验研究

Ba原子是光频标的候选者之一,对其进行有效的激光冷却与囚禁需要相关能级的寿命和跃迁几率的信息。Ba原子激发态6s6p 3P1能级在激光冷却实验中很重要,通过Hanle效应实验测量了这一能级的寿命和自发辐射率,从理论和实验上研究了探测激光有限线宽和光强对Ba原子基态6s2 1S0与激发态6s6p 3P1之间跃迁（波长791 nm）的Hanle效应的荧光信号的影响。在考虑了激光线宽和光强因素后所得到的激发态6s6p 3P1的能级寿命和自发辐射率与其他方法给出的结果很好符合。     

C-MOS/SIS—Using selective SF6etching of {1102} sapphire

A process for selectively etching holes in {1102} sapphire using SF6in H2is described. SiO2, Si3N4, and combinations thereof are studied as possible etchant masks. Refilling the holes with epitaxial silicon produces an SIS (silicon-in-sapphire) wafer wherein the silicon islands are imbedded into the sapphire substrate. The electrical characteristics of C-MOS/SIS transistors are similar to those of conventionally processed SOS devices.     

Efficient V-V energy transfer in a multiphoton excited mixture of SF6and CF4

Spectral and temporal measurements of infrared fluorescence (IRF) in SF6, CF4, and mixtures of SF6and CF4gases excited by a pulsed CO2laser are reported. Using the 944.2 cm^-1[P(20)] laser line for excitation and measuring the IRF spectra between 700 and 1500 cm^-1, a strong red shifted IRF peak of the ν3mode in pure SF6was found. No IRF was observed under these conditions in pure CF4. In a 13:10 mixture of SF6and CF4, two strong IRF peaks of the ν3modes, shifted to the red relative to the room-temperature fundamentals at 948 cm^-1and 1283.2 cm^-1, were observed. The peaks are almost equal in height and since both molecules have almost the same ν3band intensity, the present result suggests that efficient intermolecularV-Venergy transfer occurs from excited SF6to cold CF4molecules. The temporal behavior of the IRF signals under high excitation (langlenrangle approx 6) in pure SF6exhibits two relaxation times, one of bulk cooling withtau_{1} approx 1ms and the other of aV-Tnature withPtau_{2} approx 20 mus . torr. In the mixture, an additional relaxation of the intermolecularV-Venergy transfer process is observed withPtau_{3} approx 10 mus . torr. The red shift dependence of the IRF peaks on the degree of excitation was also measured and used to examine population distributions, specific IRF transitions, and anharmonicity parameters.     

Plasma etching of polysilicon and Si3N4in SF6with some impact on MOS device characteristics

Accurate delineation of the circuit materials polycrystalline silicon ("poly"), and silicon nitride are important requirements of most SFC process sequences. We have investigated the use of SF6as an active species in the parallel-plate plasma etching of these materials. For the etching of poly there is good selectivity (better the 50:1) with respect to the etch rates of SiO2and positive photoresist. This process has been used in the fabrication of MOS transistor with 3-µm poly-gate lengths and threshold voltages vary by less than 0.05 V both across a wafer and from wafer to wafer. Etching of nitride is less selective and less isotropic than that of poly.     

Time-resolved output spectrum from a hydrogen fluoride laser using mixtures of SF6and HI

The time-resolved spectrum from a transverse-discharge hydrogen fluoride (HF) laser using a mixture of SF6and HI is reported. Because this spectrum matches that from a high-pressure H2- F2laser, and because the SF2-HI mixture is chemically stable, this laser should be a suitable and convenient source for probing H2- F2amplifiers.     

Ho3+laser action in YAlO3at 2.119 µ

Pulsed laser action at 2.119 μ has been observed from Ho³⁺in a crystal of YAlO3sensitized with Er³⁺and Tm³⁺. A linearly polarized output of 1.2 J was obtained at 77°K. Measurements of the anisotropy of the stimulated-emission cross sections are reported.