Recently, there has been significant progress made in the attainment of very high c.w. power levels from optically pumped FIR lasers. These successes have relied on just a few power enhancement techniques which have been applied in the presence of high pump power. Power levels approaching 1 Watt have been observed on a few particularly strong lines.
The discovery of line-tunable cw NH3 lasers has led to new sources of powerful coherent radiation in the 11 to 14 μm region. In this paper we describe recent progress in the development of these lasers, and present results obtained with a variety of laser devices. Compact line-tunable cw NH3 lasers have operated with pump powers of less than 1 W from conventional CO2 lasers, while larger devices give output powers as high as 5.5 W with conversion efficiencies of 33%. on more than 80 different NH3 transitions has been obtained by pumping both 14NH3 and 5NH3. The frequencies of many of these transitions have been accurately measured by heterodyne spectroscopy.
Using two isotopic species, 12CH3F and 13CH3F in a Raman FIR laser kilowatt level pulses have been generated which can be tuned continuously over more than half of the range from 145 to 610 µm. Laser emission spectra and tuning curves are presented, and several molecular processes are described which produce distinctive features in the observed data.
A brief introduction to the theory of free electron laser operation is presented. Emphasis is on summarizing the characteristics needed for operation in the 10 - 1000 μm region of the spectrum. A survey of current far infrared free electron laser experiments is also included.
The status of gyrotrons operating at millimeter and submillimeter wavelengths is reviewed. Results for fundamental and harmonic emission are compared. Prospects for extending gyrotron operation to higher frequency and power are described. High frequency gyrotrons may have applications in plasma heating, plasma diagnostics, radar, spectroscopy and other areas.
A review is presented of recent advances in heterodyne detector and receiver technology for the 100 to 1000 µm wavelength region. The discussion is confined to the following four detector types: (1) the Schottky diode, (2) the bulk InSb detector, (3) the Ge:Ga photoconductor and (4) the SIS (luasiparticle mixer.
The interest given to GaAs Schottky diodes having small junction sizes has been continuously growing during the last decade to realize fast devices for detection, mixing, harmonic generation or sideband conversion in the terahertz domain. The studies carried out at LDIR on those devices cover both experimental and theoretical aspects. The following topics are considered here: Videodetection up to 10 THz with diodes having circular or non-circular geometries, tunable sideband generation from a laser source by using the power back radiated by a wire antenna connected to the diode, and detectormixer block development to improve the electromagnetic coupling efficiency between the submillimeter wave radiation and the antenna.
This paper summarizes theoretical work that was performed at MPB Technologies Inc. to explain the behaviour of a certain type of non-linear detector. A theoretical model of the rectifying mechanism taking place at the Schottky junction of a quasioptical Schottky diode detector has been developed and compared to various experiments of FIR and microwave detection. An excellent fit has been obtained between this model and the experimental results for both video and heterodyne detection. This study has resulted in a good understanding of the detector behaviour. The existence of an optimum diode biasing current has been shown. A generalized theoretical curve of the saturation power has been calculated for the complete SMM wavelength range. This curve defines the linear dynamic range of any type of Schottky diode detector device. The theory has been also extended to higher order terms to describe the non-linear effects observed at high biasing currents and for strong incident radiation.
A detailed review of the principles governing the performance of point-contact diodes is presented. The requirements necessary for these devices to be used as harmonic generators and mixers in the submillimetre and infrared region are emphasized, and some new developments concerning the physical mechanisms responsible for the operation of the diode are reported. Special attention is given to the importance of surfaces and interfaces in these devices.
Far infrared photoconductive detection is considered in the wavelength range 30 to 1000)π m. High sensitivity direct detection in a low photon flux background, as typically met in spaceborne astrophysics measurements, is emphasized. Recent advances in extrinsic germanium photoconductors, particularly stressed Ge:Ga are reported. Wideband photoconductive detection with Hgi-xCdxTe and narrow band magnetically tunable photodetection based on InSb and GaAs are reviewed. Prospects for new devices and supporting technologies are discussed.
Pyroelectric vidicons and newly developped Pyroelectric arrays give a NETD around 0.5 K for 25 Hz chopping frequency. However while Pyroelectric vidicons operate with 200 TV lines, pyroelectric arrays are limited to 64 elements in the present state of the art. This number of elements will probably increase dramatically in the next few years, and new solutions are proposed.
The combination of weak source intensity and the availability of relatively insensitive detectors have prevented effective exploitation of Far IR spec troscopy using the classical dispersive techniques.
Far infrared measurements between 10 and 250 cm-1 characterize and probe II-VI semiconductor systems of current scientific and technical interest including: the leading infrared detector material Hg1-xCdxTe; the substrate materials CdTe and Cd1-xZnxTe; the magnetic semiconductor Hgl-xMnxTe; and the new HgTe-CdTe superlattice. The advantages, the theory and the technique of these measurements are reviewed and applications to these materials are described.
Far infrared (FIR) spectroscopy has been demonstrated to be a powerful tool for studies of the electronic properties of semiconductors, particularly in investigation and characterization of shallow impurities. Recent advances in growth techniques have permitted the fabrication of repeated semiconductor heterostructures (quantum wells and superlattices) with interface abruptness on an atomic scale. For the case of Quantum Wells, simple considerations indicate that the electronic states of shallow impurities in such confining structures depend strongly on the width of the wells and/or the impurity location along the growth direction. In the present paper, recent experimental work on shallow donor impurities in GaAs/AlGaAs multiple-quantum-well (MQW) structures is reviewed. Absorption and photoconductivity studies were carried out with a FIR Fourier Transform Spectrometer in magnetic fields up to 9T. MQW samples nominally doped with Si donors in the well centers and at the well edges have been investigated for QW widths between 80Åand 450Å. Results are generally in very good agreement with recent theoretical calculations. Possible use of such measurements to determine the impurity distribution along the growth direction is discussed.
A system for simultaneously measuring the far-infrared absorption and near-infrared photoluminescence of excitons bound to defects in semiconductors is described. It is based on the integration of a FTIR spectrometer, a cryostat containing both the sample and a cooled bolometer, a laser to generate the excitons, and a grating spectrometer. Modes of operation are described and typical spectra are presented, along with a brief discussion of their significance.
The dominant contributions to the far infrared dielectric loss in glasses are reviewed. Special emphasis is placed on correlating the dielectric loss data with information obtained from other low energy probing techniques, such as inelastic neutron scattering, ballistic phonon transport, as well as Raman and Brillouin scattering.
The organic charge transfer salts and the heavy fermion systems are recently discovered superconductors that may involve entirely new mechanisms of superconductivity. Many competing interactions vie with superconductivity for the lowest energy state: magnetic fluctuations, charge and spin density waves. The characteristic energies of these excitations are in the far infrared. This paper reviews a selection of recent work in far infrared spectroscopy on some unconventional superconductors.
High-resolution Fourier transform spectra of methyl alcohol have been investigated in the FIR and IR regions from 40-300 cm and 950-1100 cm-1, respectively. A principal aim was to confirm and extend identifications of transitions contributing to the rich FIR laser emission known to be obtained by optically pumping methanol vapour with CO2 lasers. The general structures of the energy levels and spectra are discussed, and results and analysis are presented for the normal isotopic species as well as the C-13 and 0-18 isotopic forms. The technique of forming closed frequency combination loops is described, with application to the rigorous confirmation of transition assignments and improvement of frequency accuracies of reported FIR laser lines. Also outlined are some recent novel assignments of high-frequency TEA-laser-pumped FIR laser lines as torsional refilling transitions within the vibrational ground state.
A large commercial Fourier transform spectrometer has been used in conjunction with three absorption cells which have low loss at long wavelengths. Absorption paths of 15 μm to 20 m are available. A number of spectra are described which illustrate the performance of the system.
We have developed a six-beam submillimeter wave interferometer at 214 μm to be installed on the Varennes tokamak for plasma shape and density fluctuation measurements (ne 3x1019 m-3, Te 450 eV). The number of chords and their distribution in the plasma cross-section are chosen for optimum density profile reconstruction through Abel inversion. Each channel consists of a frequency-modulated Mach-Zehnder interferometer derived from the same dualbeam submillimeter source. This source is an optically pumped twin CH2F2 laser featuring three stabilizing feedback loops: Stark-cell tuning of the pump CO2 laser, beat frequency control of the two output carriers and power locking of the submillimeter wave cavities. At a total pump power of 22 W, each laser delivers 25 mW of radiation at 214 μm. The beat frequency is maintained to 1 MHz with a long term stability of ± 15 kHz. Using corner-cube Schottky diodes optimized for 250 μm, the interferometer attains a line-integrated density resolution of 5x1016 m-2, a factor of 300 below the line density corresponding to the center chord passing through the plasma axis. The fringe-free line-integrated density signals as well as the amplified fluctuation signals are produced by a 0-8n electronic phase detector. Its 200-kHz output bandwidth makes the interferometer suitable for density fluctuation correlation studies in the .001 sn/n range. The extension of the present interferometer to polarimetry is discussed.
χSecond and third harmonic generation of far-infrared radiation in bulk solids is reported The far-infrared source is a step-tunable gas laser emitting in the range 8.2 to 57 cm-1. Single pulses with power of 100 KW and duration of 40 ns are focused on dielectrics or semi-conductors. Harmonic power is selectively transmitted through perforated metal cutoff filters and detected with a GaAs photoconductor. Harmonic powers up to 100 W are obtained. Quantitve measurementllow to determine absolute values of the nonlinear susceptibilities: )0 /(2w,w,w) and )0 1(3w,w,w,w). In noncentrosymmetric infraredactive crystals )0(2 ) exhibits a lattice contribution which comes in addition to the electronic contribution usually dealt with in near-infrared and visible nonlinear optics. The lattice contcligution is resonant near the optical phonon or Reststrahlen frequency. We have measured )0 ion the low-frequency wing of the lattice resonance of LiTa03 and found it consistent with theory. Free cmriers in semiconductors exhibit a surprisingly large third-order nonlinear susceptibility x iprovided the frequency is low enough. Typically x iincreases by 7 orders of mmitude when the wavelength increases from 10 μm to 500 μm. We have absoluly determined )0 lby measuring the efficiency of frequency tripling at 20 cm-1. We found xk /proporti9941 to the density n of free electrons or holes for n < 1017 cm-3. The absolute value is )0 " 10-4 e.s.u. for n = 1017 cm-3 in a variety of semiconductors. The theoretical analysis allows to distinguish between two contributions, (i) due to the nonparabolicity of the carrier dispersion and (ii) due to the energy dependence of carrier scattering. Altogether the free carrier nonlinearity offers applications of efficient four-wave mixing employing at least one high-intensity far-infrared beam.
The use of submillimetre (SMM) waves for non destructive testing of paper has already been demonstrated. This report presents the basic results wich led to the use of SMM waves to monitor the fibre orientation anisotropy in paper. The measurement of moisture content and basis weight is also covered. An experiment to test the effect of moisture content on the polarizing properties of paper is presented. Measures were made for three wavelengths (70, 96 and 118 microns) and for moisture content covering the region 0 to 40 %. The effect of bound water to cellulose surface will be introduced to explain the results.
A standard three components mixture model for the dielectric constant is compared to the measured transmittance and reflectance of FIR radiation through a wet cellulose matrix. This model reproduces, at least qualitatively, the peculiar behaviour of the reflectance versus water content curve. However, the absorption coefficient for water content above 5% can only be reproduced if we postulate that the water adsorbed between 5% and 10% in volume possesses an anomalously high absorption coefficient. This is interpreted in terms of the flickering clusters model for water and the structure breaking role of the cellulose surface.
With the advance of far-infrared laser applications the need arises to vary the radiation intensity over a large dynamic range. An example is laser radar where a return loss of 50 or even 100 db has to be achieved without distorting the wavefront. First we review the more common techniques used to attenuate FIR laser beams, and then describe a new step attenuator that uses diffracting metal mesh screens to attenuate the laser radiation. Broadband performance over nearly one decade of frequency is achieved. The design can handle even higher peak and average FIR powers than are available at present, while maintaining beam quality in all respects, including arbitrary input polarization.