Stimulated Raman scattering of picosecond light pulses in hydrogen, deuterium, and methane

Experimental results are presented on stimulated Raman scattering of short pulses of approximately 100 ps duration in H2, D2, and CH4, both in capillary waveguides and in a tight focusing geometry. Experimentally determined thresholds are in good agreement with calculation. Low thresholds (< 20 muJ) are observed in CH4and preliminary results using a mode-locked dye laser as pump indicate a useful source of tunable short pulse radiation in the near infrared.     

Simultaneous HCN and H2O laser emission in mixtures containing H, C, N, and O

Simultaneous laser emission at 337 and 118 μ has been observed in a pulsed discharge through a mixture of N2, O2, and either CH4or (C2H5)2O. Earlier observation of a laser line at 469 μ is rejected and explained.     

Pulsed high-power far-infrared gas lasers: Performance and spectral survey

We describe the practical performance of a pulsed mirrorless far-infrared gas laser pumped by a TEA CO2laser. The system is designed to serve as a high-power source for experiments in far-infrared nonlinear optics and saturation spectroscopy of solids. Out of numerous candidates, 11 gases were selected for a careful survey: CH3F,¹³CH3F, NH3, D2O, D2¹⁸O, D2S, HDS, CH3Cl, CH3Br, CH3I, and CH3CN. With an available pump energy of up to 10 J, a total of 203 far-infrared lines with pulse energies up to 12 mJ were found, covering the wavelength range from 42 μm to 1.2 mm. 78 of these lines had not been observed before. The position of further 39 lines is at variance with former literature reports.     

Collisional narrowing in the optically pumped CH3OH and CH3F lasers

The gain linewidth of the optically pumped CH3F laser is observed to narrow and rebroaden with the addition of He. In addition, we observe the same effect in the CH3OH laser with the addition of the polyatomic buffer gases SF6and CS2. These results offer conclusive evidence of the Dicke narrowing phenomena in these inverted pure rotational transitions. The effect is observed using a high harmonic mixing technique in a Schottky barrier diode.     

Laser initiated explosions and chemiluminescence

Explosions can be induced in mixtures of SF6with appropriate fuels, by a single pulse of a free running CO2laserapprox 1.5 mus [full width at half maximum (FWHM)]. A typical threshold for initiation of reaction was somewhat above 4 J for a 1:1 mixture of SF6and SiH4, at a total pressure of 10 torr, with the unfocused beam illuminating 8 cm²of cell area. High levels of luminosity were generated covering both the near ultraviolet, visible, and infrared. The spectral and temporal characteristics of the following fuels and fuel-oxidizer combinations (with SF6present) are summarized: the most prominent emitter from SiH4is S*2, B³Sigma-_{u}; Sn(CH3)4+ N2O [SnO*; SnF*] ; Pb(CH3)4+ N2O [PbO*; PbF*; PbS*] ; Bi(CH3)3+ N2O [BiF*]; B2H6with and without N2O [BO*; BO*2; S*2]. Several atomic lines were also recorded. Estimates of the energy density deposited by the laser pulse suggest that initiation of rapid reaction was primarily due to attack of superexcited SF6by highly reducing radical species.     

Gain in CW laser pumped FIR laser gases

This study is an investigation of the FIR radiation amplification in CW laser excited gases so essential for a complete quantitative understanding of the FIR laser cycle. The small-signal FIR gain coefficients have been measured as functions of the relevant parameters for two transitions in CH3OH and three in CH2F2. The measurements, demonstrating the influence of coherent pumping have been compared to quantum-mechanical calculations of a three-level system, interacting resonantly with two coherent fields. The good agreement found between theory and measurements indicates the correctness of the theory in predicting the gain properties of the CW FIR lasers. Also FIR gain saturations have been measured directly and exhibit the expected features.     

1C4--Infrared difference frequency generation

Several molecular vibrational frequencies have been generated by beating together a laser and laser-stimulated Raman emission. A Nd³⁺/glass laser was used to excite Raman emission from C6H6, C6D6, and CH3. NO2. The difference frequencies were generated in a single crystal of CdS and appeared at 992, 944, and 927 cm^-1, respectively. A difference frequency has also been detected at 944 cm^-1following mixing in CdSe. The data presented for HgS indicates that phase-matched outputs can be generated over a limited range of frequencies, provided that the Raman emission is excited with a Nd³⁺/glass laser. Experimental details are given and the results are discussed.     

Pulsed discharge initiated chemical lasers -- Part II: HF laser emission from NF3and N2F4systems

A transverse, multiple-arc pulsed discharge has produced laser emission in NF3and N2F4mixed with H2, CH4, C2H6, HCl, HBr, and natural gas.P_{10}, P_{21}, P_{32}, and P43HF transitions were observed. The peak powers measured ranged from ∼8.5 to 25 kW with typical pulsewidths ofsim0.3 mus.     

High-efficiency driven Q switching of the CO2laser using the stark effect in molecular gases

High-efficiency drivenQswitching of the CO2laser has been observed by the use of the Stark effect in CH3Cl, CH3F, CH3CHF2, CH2= CF2, and C2H3Cl on various lines in both the 9.4- and 10.4-μ bands. The effect is observed with both pulsed and sinusoidal electric fields applied to the intracavity modulator.     

Submillimeter laser action of CW optically pumped CD2Cl2, CH2DOH, and CHD2OH

More than 30 laser lines, several of them with an intensity in the order of mW have been obtained in the submillimeter wavelength range from 100 to 900 μm by optically pumping the deuterated molecules of dichloromethane (CD2Cl2) and methanol (CD2HOH, CH2DOH) with a step tunable CO2laser.     

New lines in the UV: SRS of excimer laser wavelengths

The UV excimer lasers ArF, KrCl, KrF, and XeCl were utilized to create new families of UV lines by multiple orders of broad-band nonresonant stimulated Raman scattering (SRS) in the media H2, D2, CH4, and LN2. Mixed-media and excited-state SRS were also studied.     

Millimeter and submillimeter-wave laser action in symmetric top molecules optically pumped via perpendicular absorption bands

Ninety-nine new far infrared (FIR) laser lines from 227 μm to 1.965 mm have been observed in CH3CN, CH3CCH, CH3Cl, CH3Br,and CH3I by optically pumping these gases with CO2-laser pulses of 150-μs duration.     

Pulsed high-power mid-infrared gas lasers

We present a survey in the spectral region of 200-700 cm^-1(14-50 mum) of pulsed mirrorless gas lasers pumped by a TEA CO2laser. 55 lines are found covering the wavelength range 15.8-57 cm^-1in the gases NH3,¹⁵NH3, D2O, CH3OH, CD3OH, and CD3OD. Most of these lines are reported for the first time. One of the strongest lines (26.4 μm in NH3) emits a considerable pulse energy of 100 mJ. No lines were found in C2H5OH and C3H6O.     

Application of laserQswitching for infrared spectroscopy

PassiveQswitching of IR lasers (N2O as well as CO2) has been applied for the detection of coincidences within a few 100 MHz between laser lines (νL) and molecular transitions (νM). Polar (¹²CH3F,¹³CH3F, C2H5OH, and C6H5CH3) as well as nonpolar molecules (SiH4, GeH4, CCl4, C2H4, C3H4, and C6H6) have been studied. The phenomenon depends strongly on the frequency mismatchDeltanu = nu_{M} - nu_{L}. 81combinations between laser lines and molecular absorption lines were found to produce theQ-switch effect.     

Transition metal-gate MOS gaseous detectors

MOS capacitors with gates of palladium, platinum, and nickel are shown to be sensitive detectors of H2, CH4, C4H10, and CO gases. 1-MHz and 10-HzC-Vcharacteristics change by - 1240 and - 215 mV when the Pd-gate MOS capacitor is exposed to H2gas at 760 and 2 × 10^-8torr, respectively. Platinum-gate MOS capacitors exhibit approximately one-half the change of Pd-gate devices. Nickel shows a response of - 120 and - 140 mV in 1 atm of H2find CO gas, respectively. When the capacitors are operated in a constant-capacitance mode by varying the bias, absorption time characteristics are obtained. Pd-gate devices absorb H2gas in 190-200 ms at 760 torr of H2but require 200 min to reach 63 percent of the total change at 2 × 10^-8torr H2pressure. Both Pt- and Ni-gate devices are slower than Pd. The detection mechanism is attributed to the change in work function upon hydrogen absorption. This is established by demonstrating no change either in the density of interface states or in the distribution across the gap or in the number of states at midgap upon absorption. Also, no accumulation of additional charges in the SiO2or Si3N4dielectric is found upon hydrogenation of devices with either Pd or Pt gates.     

Optical breakdown in gases of medium ionization potential

Breakdown threshold with ruby laser excitation has been determined for the gases Cl2, Br2, I2, ICl, NO, NO2, Hg, and C6H6, using argon as a reference. Our results indicate that threshold depends mainly on the ionization potential and that effects due to absorption in the visible region are secondary.     

Infrared third-harmonic generation in molecular gases

We report the first observation of third-harmonic generation (THG) in molecular gases. Using a CO2TEA laser and choosing molecules with vibrational transitions that contribute to a resonance enhancement of the third-order susceptibility tensor we have studied absolute conversion efficiencies, spectral response, and pressure dependences for THG in SF6, BCl3, and CO and compared the results to theoretical predictions.     

Assignments of the high power optically pumped CW laser lines of CH3OH

Thirteen far IR laser lines in CH3OH, optically pumped by a CO2laser, have been assigned toa- andb-type transitions in ν5CO-stretch band. Identification procedures used were: a frequency match between the CO2pump and CH3OH absorption, frequency match of laser lines and CH3OH ν5transitions, polarization of far IR radiation, cascade and competition effects between far IR laser lines, and frequency consistencies. An improved set of molecular constants for ν5transitions were also obtained.     

Photonitration of hydrocarbons with lasers

The photonitration of isobutane (i-C4H10) has been investigated in the 458-515 nm region with an argon-ion laser as the radiation source, t-nitrobutane (t-C4H9NO2) was the major product. Its rate of production was linear with laser intensity and increased with increasing photon energy. Computer modeling of possible reaction steps indicated that the reaction is initiated by the direct abstraction of hydrogen from i-C4H10by electronically excited NO2.     

An optical-fiber-based gas sensor for remote absorption measurement of low-level CH4gas in the near-infrared region

A long-distance low-loss silica optical-fiber link connected to a compact absorption cell was employed to realize a fully optical highly sensitive fiber gas sensor for low-level CH4gas in conjunction with a highly radiant InGaAsP light-emitting diode (LED) at 1.66 μm. By using a dielectric interference filter which replaces a monochromator to simplify the system configuration and operation, a 2-km-long silica fiber link was demonstrated to be capable of achieving reproducibly the detection sensitivity of nearly 700 ppm for CH4gas in air, i.e., 1.3 percent of the lower explosion limit of CH4density. This high sensitivity verifies a potential for major applications to strategic points within the environment, like industrial and mining complexes as well as urban and residential areas. This result also indicates that the purely optical gas-sensor system based on ultralow loss optical-fiber networks, incorporating LED's or laser diodes, can be extensively utilized for the real-time remote measurement and surveillance of a number of dangerous, explosive, and toxic gases in the near-infrared region.