Molecular Gas Lasers Research Articles

Laser-based analysis of carbon isotope ratios.

A laser technique for analysis of carbon-13:carbon-12 ratios with the specificity of laser resonance spectroscopy and the sensitivity and accuracy typical of isotope ratio mass spectrometers is reported. The technique is based on laser optogalvanic effect spectroscopy, in which an electrical (galvanic) signal is detected in response to the optical stimulation of a resonance transition in a gas discharge species. Carbon dioxide molecular gas lasers are used, with the probed transitions being identical to the lasing transitions. Measurements for carbon dioxide samples with 100-second averaging times yield isotopic ratios with a precision of better than 10 parts per million.

Far-infrared second-harmonic generation in GaAs/AlxGa1-xAs heterostructures: Perturbative and nonperturbative response.

We report measurements of far-infrared (FIR) harmonic generation from GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heter- ostructures. The samples studied were a modulation-doped ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/GaAs heterojunction and a sample with ten modulation-doped half-parabolic quantum wells. The samples were driven with intense far-infrared radiation from a molecular gas laser at 29.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the University of California--Santa Barbara free-electron laser at 51.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The FIR radiation was polarized parallel to the growth direction. Second harmonics of the FIR were detected from both the semi-insulating GaAs substrate and from the confined electrons. For the heterojunction sample, the second-harmonic power generated by the electrons depended quadratically on fundamental power at low power, as expected from time-dependent perturbation theory. However, this dependence became subquadratic at higher powers, indicating a nonperturbative response. At high FIR powers, electrons were also ionized from the heterojunction and half-parabolic wells. For the heterojunction at f=29.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the perturbative regime, the surface second-order susceptibility was computed to be ${\mathrm{\ensuremath{\chi}}}_{\mathit{S}}^{(2)}$=1.0\ifmmode\pm\else\textpm\fi{}0.75\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{esu}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{3}$. This value agrees, within experimental error, with a simple model of the heterojunction as a triangular quantum well. The second-order polarizability of a conduction electron in the heterojunction is nine orders of magnitude larger than that of a valence electron in pure GaAs.

Discharge ignition phenomena in CO2 and N2O lasers with preliminary filling of the interelectrode gap by electrons

A comprehensive model is presented that describes preionization and discharge ignition processes in large-aperture molecular-gas lasers. By initially filling the interelectrode gap with electrons and establishing a space-charge screen at the cathode, electron avalanche begins adjacent to the anode. Cathode-directed ionization waves develop and ignite the discharge. Dynamic profiling of the electric-field distribution and Penning ionization of a readily ionizable additive combine to decrease both the ignition and quasi-steady-state voltages.

The Berkeley tunable far infrared laser spectrometers

A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10–100 cm−1 and exhibits a maximum absorption sensitivity near one part in 106. Each laser line can be tuned by ±110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper.

Generation of very short far-infrared pulses by cavity dumping a molecular gas laser

Very short far-infrared (FIR) pulses have been generated by cavity dumping an optically pumped molecular gas laser using optical switching techniques. The active element used is an intracavity semiconductor, which is placed under the Brewster angle with respect to the optical axes. This element is made highly reflective by the sudden increase of the free-carrier concentration, induced by above band-gap illumination. In this way high power (∝1 kW) FIR pulses with a duration less than 10 ns can be produced.

Waveguide-type molecular gas laser with extra-axial optical pumping

Waveguide-type molecular gas laser with extra-axial optical pumping

Fir spectroscopy on shallow donor states in GaAs:Si

Far infrared spectroscopy experiments on the shallow donor states of GaAs:Si are presented. Transitions between shallow donor levels are induced by F.I.R. radiation from an optically pumped molecular gas laser. The measurements are performed by measuring the electrical conductivity of the GaAs:Si sample at fixed laser frequencies by sweeping an external applied magnetic field up to 12 T. In total about 30 different transitions are observed using radiation between 60 μm and 1.8 mm. In low magnetic fields and at high frequencies a series of photoconductivity signals are observed, which we ascribe to transitions from the ground state towards bound donor levels with hydrogen quantum numbers n, l=n−1, and m=n−1.

Cyclotron resonance in the n-inversion layer of InSb grain boundaries

We report the first observation of the cyclotron resonance of electrons in a grain boundary inversion layer. We provide transmission measurements on a p-type InSb bicrystal at discrete wavelengths between λ = 70.6 μm and 251 μm using an optically pumped molecular gas laser. The observed cyclotron resonance spectra consisting of the cyclotron resonance lines of at least three electric subbands do not allow a direct determination of the cyclotron masses of the different electtic subbands because of the large line-width of several lines.

Bidirectional ring laser: Stability analysis and time-dependent solutions.

We provide a comprehensive analysis of the standard model of the bidirectional ring laser in which only one mode can be supported in each direction and in the limit that the polarization can be eliminated adiabatically. The interaction between the two counterpropagating modes can be derived and it is most naturally viewed as a coupling between them via scattering from a spatial grating formed in the population inversion. If the grating is a sufficiently small modulation of the spatial average of the population inversion, it can be approximated by a sinusoidal function. A systematic derivation of the model with only a sinusoidal grating for the homogeneously broadened case is presented that reveals and corrects errors in several previously published analyses. The stability of the steady-state solution is analyzed. The bidirectional steady-state solution is unstable and the unidirectional steady-state solutions may be stable or unstable depending on the parameters. The well-established result of bistability between the two modes when the cavity is tuned to resonance is recovered, a result that persists, in part, even when the losses between the two modes are different.With detuning, the unidirectional steady-state solutions can be destabilized, creating regions in the parameter space where only time-dependent solutions are found. For parameters characteristic of solid-state or molecular gas lasers, the instability can occur for very small detunings and even very close to the lasing threshold. Asymptotic limits of the instability boundaries for these parameters are presented. Additional results are derived formally for inhomogeneous broadening with a Lorentzian line shape and for arbitrary inhomogeneous linewidths. Explicit analytic results are presented in the limits of very small and very large inhomogeneous linewidths compared with the homogeneous linewidth. Time-dependent solutions in the homogeneously broadened case for relaxation rates appropriate to ${\mathrm{CO}}_{2}$ lasers show that there are broad regions in the parameter space of gain and detuning for which the behavior is dynamically chaotic in the form of nearly ``square-wave'' alternation between the counterpropagating modes of the laser with irregular switching times. In other regions of that parameter space we find more irregular pulsations and pulse alternations. Regions of periodic pulsations also have been found.

Mid-infrared optically pumped molecular lasers

Optically pumped molecular gas lasers have proved to be a convenient and versatile source of coherent mid-infrared (MIR) radiation. The present state of development and understanding of these lasers is reviewed in this article. Emphasis has been placed on recent results with a view to complement the earlier reviews of the field.

Cyclotron Resonance in the n‐Inversion Layer of InSb Grain Boundaries

Abstract We report the first observation of the cyclotron resonance of electrons in a grain boundary inversion layer. We provide transmission measurements on a p-type InSb bicrystal at discrete wavelengths between λ = 70.6 μm and 251 μm using an optically pumped molecular gas laser. The observed cyclotron resonance spectra consisting of the cyclotron resonance lines of at least three electric subbands do not allow a direct determination of the cyclotron masses of the different electtic subbands because of the large line-width of several lines.

First observations of time-resolved submillimetre photoconduction in indium phosphide

Abstract The time-resolved submillimetre photoconduction of high purity (μ 77 ≈ 10 m 2 V −1 s −1 ) indium phosphide samples has been investigated at different magnetic fields using a pulsed molecular gas laser (λ = 118.8 μm). The effective lifetime of the electrons in the conduction band is found to depend weakly on the magnetic field. When the sample dc bias is increased above the threshold for impact ionisation a long ‘tail’ is observed extending to 20 μs after the end of the FIR pulse. A simple rate equation model is developed to explain this behaviour.

Investigation of electron-beam-excited molecular gas lasers

Physical processes are investigated in active media of high-pressure gas lasers operating on electronic transitions of molecules. Lasing was obtained in the VUV (λ = 172 nm) region of the spectrum by exciting compressed xenon and an Ar:Xe mixture. The effect of the temperature and pressure on the kinetics of the processes in the active laser media Ar:N2, Xe:02, and Ar:Kr:F2 are investigated. A new class of excimers, consisting of one halide and two inert atoms, is observed for the first time ever, and the temperature dependence of the spontaneous and laser emissions of XrF* is investigated; this dependence is due to the redistribution of the energy drawn from the electron beam among the excimers KrF* and Kr2F*.

Laser Stark spectroscopy

Abstract This paper discusses the development of laser Stark spectroscopy (or laser electric resonance spectroscopy) in which transitions between molecular energy levels are tuned into resonance with fixed frequency molecular gas lasers. Emphasis is laid on the fundamentals of the method, and its extensions to sub-Doppler resolution, including optical-optical double resonance and level crossing. Examples of the use of these methods are chosen from three main areas; the study of the vibrational dependence of molecular dipole moments, the elucidation of the effects of collisional energy transfer, and the detection of the spectra of semi-stable molecules.

Materials processing by high-repetition-rate pulsed excimer and carbon dioxide lasers

Recently created pulsed gas lasers with high pulse repetition rate produce radiation in the IR [ V. Yu. Baranov , in Molecular Gas Lasers. Physics Applications, VelikhovE. P., Ed. ( Mir, Moscow, 1981), pp. 252– 255] and the UV ( V. Yu. Baranov , in Proceedings, International Conference on Lasers, 14–18 Dec. 1981, pp. 968– 974) regions of the spectrum. These lasers are widely used in research on the selective excitation of matter. This field of study includes laser isotope separation { G. I. Abdushelashvili , Kvantovaya Elektron. (Moscow)9, 743 ( 1982) [ Sov. J. Quantum Electron.12, 459 ( 1982)]} and laser-induced chemical reactions [ V. Yu. Baranov , Preprint IAE 3693/13 ( Moscow, 1982)]. But there are few publications dealing with uses of high-repetition-rate lasers for materials processing, although this problem undoubtedly is of considerable scientific interest.

Kinetic Description of a Dynamically Coupled Free-Electron and Molecular Gas Laser

The parametric interaction of longitudinal and transverse waves is studied for a system of a molecular medium and a relativistic electron beam. The system offers interesting possibilities for generation of coherent radiation.

Infrared multiphoton isomerization and fragmentation reactions of organic molecules

Abstract

Rate equation model for mid IR OPML having common pump and upper lasing level; Application to 12.8 µm emission from NH<inf>3</inf>

A five-state kinetic model has been used to obtain an analytical expression for the power output of an optically pumped molecular gas laser having a common pumped and upper lasing level. The theoretical estimates for the various operational parameters of an optically pumped NH 3 laser (like operating pressure range, optimum operating pressure, variation of these with NH 3 gain length, etc.) obtained from this model are in good quantitative agreement with experimental results.

Far-infrared raman laser with high intensity laser pumping

Theoretical and experimental results are presented for a pulsed far-infrared (FIR) molecular gas laser with high intensity laser pumping. In these FIR lasers, high intensity pumping is found to produce stimulated Raman emission at very large offsets (up to 30 GHz) from resonance with the intermediate state. A theoretical, density matrix model is developed for these lasers to account for simultaneous Raman emission on rotational levels in the ground and excited vibrational states (double Raman resonance). This theoretical approach is necessary in the case of off-resonant, high intensity pumping. Theory predicts the FIR emission frequency, the FIR laser gain, and the pump threshold intensity as a function of pump laser frequency. Experimental results are obtained on P -, Q -, and R -branch transitions in12CH 3 F and13CH 3 F using a single-mode, grating tuned CO 2 TEA pump laser with an intensity of up to 40 MW/cm2. Good agreement is obtained between theory and experiment for the observed values of FIR emission frequency and pump threshold intensity. These results indicate that a widely tunable ( 150-1200 \mu m), pulsed FIR CH 3 F laser could be constructed with a tunable, multiatmospheric CO 2 pump laser of modest power (about 2-5 MW).

Line narrowed TEA CO2 laser for optical pumping of molecular gas lasers

Line narrowed TEA CO2 laser for optical pumping of molecular gas lasers