Showing papers in "Applied physics in 1978"

Chevrel phases: Superconducting and normal state properties

Abstract: The many unusual properties of the ternary molybdenum chalcogenides of the type MMo6X8 (M=Metal, X=Chalcogen) are reviewed. Special emphasis is put on the superconducting properties like critical temperature, critical field and the problem of coexistence of magnetism and superconductivity.

Raman spectroscopy—A versatile tool for characterization of thin films and heterostructures of GaAs and AlxGa1−xAs

Abstract: It is shown that Raman spectroscopy can provide useful information on characteristic properties of thin crystalline films of compound semiconductors. Crystal orientation, carrier concentration, scattering times of charge carriers, composition of mixed crystals and depth profiles can be studied in thin layers and heterostructures of GaAs and AlxGa1−xAs. The advantages and disadvantages of Raman scattering compared to conventional characterization methods are discussed.

A new type of source generating misfit dislocations

Abstract: A new type of misfit dislocation multiplication is deduced from high-voltage electron micrographs of thin Ge layers on GaAs substrates. Two misfit dislocations with the same Burgers vectors on different glide planes cross and annihilate at the intersection point resulting in the formation of two angular dislocations. The tip of one of these dislocations may reach the growth surface by glide breaking into two separate dislocation segments. These segments may glide to form additional misfit dislocations, which may undergo the same multiplication process.

Laser dye stability. Part 5

Abstract: Photodegradation parameters that relate bleaching and absorption at the lasing wavelength λl have been examined for over 30 different coumarin and quinolone laser dyes in a number of solvents. Quinolone dyes were found to bleach faster than the coumarin dyes. The effect of chemical substituents was found to affect bleaching of the coumarin dyes only to a small (20%) extent in ethanol. The major effect of chemical substituents was in the conversion of a dye to products absorbing at λl. Effects of solvent, cover gas, and changes in fluorescent quantum yields are discussed. Of particular interest is the photodegradation parameterA, the ratio of the percent absorption at λl to the total input energy per dm3. Combined with τ, the total input energy per dm3 required for a laser to reach half its original intensity, it was found thatAτ=1.2±0.9 for all of the dyes independent of dye concentration in all of the solvents tested. It appears that where bleaching of the dye is only of the order of 10–20%, the absorption at λl is 1.2% when our dye laser has reached one-half of its initial output. It is consequently possible to estimate τ values of new dyes by the use ofA terms through the relationshipA 1τ1=A 2τ2 where τ1 of Dye 1 has been calibrated in the same dye laser system.

LiTaO 3 as holographic storage material

Abstract: The optical storage properties of LiTaO3:Fe are investigated and compared with those of the isomorphous compound LiNbO3:Fe. Absorption, photocurrent, photoconductivity and holographic measurements are reported. In the case of photovoltaic writing similar results for LiTaO3- and LiNbO3-crystals are obtained. However, in the case of photoconductive writing using external electric fields LiTaO3:Fe-crystals yield much better results due to large photoconductivity values. Considering the recording sensitivity and the extremely large storage time LiTaO3:Fe turns out to be one of the most promising materials for photorefractive storage of volume phase holograms.

Fluorescent solar energy collectors: Operating conditions with diffuse light

Abstract: The fluorescent energy conversion principle using several sheets of transparent material doped with fluorescent molecules to concentrate radiation is extended to include diffuse radiation. Two cases are treated here: diffuse radiation only and a composite spectrum consisting of 40% direct and 60% diffuse radiation simulating the average illumination of a flat exposure in central Europe. In both cases photovoltaic conversion efficiency is significantly higher than with the AM1 spectrum. This is due to the blue shift and narrow shape of the diffuse spectral distributions. With realistic boundary conditions the theoretical conversion efficiency is 1.56 times higher than for the AM1 case. The highest theoretical conversion efficiency is now 38%.

Solar cell materials and their basic parameters

Abstract: Parameters such as the energy gapE g , open circuit voltageV oc , short circuit current densityj sc , fill factor F.F., efficiency η, antireflection coating condition A.R. and illumination conditions of currently known photovoltaic diodes are reviewed and tabulated, followed by some critical comments.

Visual observation and optical cooling of electrodynamically contained ions

Abstract: Resonance fluorescence from as little as 10 to 20 barium ions, spatially confined in a miniaturized rf quadrupole ion trap, has been detected visually, photographically, and photoelectrically. Gross effects of optical sideband cooling of the ions were observed.

Particle Densities in a Decaying SF 6 Plasma

Abstract: Particle densities are determined for a decaying axially blown SF6 are between current zero and brekdown, some 100 μs later. During the time of interest, primarily only atoms, diatomic molecules and ions can be built up by reactions. Therefore, in a first approach equilibrium densities are derived for SF6 which has decomposed into its considemic and diatomic components. In a second approach reaction kinetics are considered. It turns out that the development of densities occurs in two steps. During the first approx. 100 μs there is a strong deviation from equilibrium. Afterwards the most abundant particles remain near their partial, diatomic equilibrium values. Near breakdown the degree of ionization is of the order of 1013 cm−3 with the most abundant ions S 2 + and F−. This density is sufficient to distort the applied electric field. Future theories of breakdown in hot gases must take into account this principal difference as compared with the conditions in cold gases.

Tunable UV-radiation by stimulated Raman scattering in hydrogen

Abstract: We observed tunable UV radiation down to 175 nm by SRS of the output of a frequency-doubled dye laser in H2. The high power output of the 4th anti-Stokes to 5th Stokes line generated by the frequency doubled dye laser and the 8th anti-Stokes to 3rd Stokes line generated by the undoubled dye laser represents a broadly tunable coherent radiation source between 189 nm and 2064 nm.

Focusing effects in the synthetic aperture radar imaging of ocean waves

Abstract: We derive the properties of the image obtained for an ocean wave whose cross-section may be given byσw(x,y,y) and surface profile byh(x,y,t).σw andh are functions representing the wave phenomena, but whose exact properties are determined by the ocean wave surface properties, for an ocean wavelength ofλw, heightH, and orbital frequency ω. We calculate the effect of defocusing of the wave image due to its temporal motion, and derive both the resolution of the radar system if no focus compensation is provided in the processor and the necessary distance the azimuth telescope has to be moved to provide diffraction-limited imaging. We illustrate these results for data taken by the JPL synthetic aperture radar over Hurricane Gloria on September 30, 1976, and the ERIM radar over Marineland, Florida, on December 15, 1975.

Collective and single-electron interactions of electron beams with electromagnetic waves, and free-electron lasers

Abstract: The field of radiation emission from electron beams is reviewed with special reference to work related to free-electron lasers. Different schemes of interaction in periodic structures, electromagnetic slow-wave structures, and in transverse confining force are distinguished. Various effects and devices such as traveling wave amplifiers, Smith-Purcell radiators, Cerenkov and bremsstrahlung-free electron lasers, cyclotron resonance masers, coherent bremsstrahlung and channeling radiation are discussed and the differences and relations among them are explained. A simple comprehensive model is developed to describe electron-beam interaction with an electromagnetic wave in periodic electromagnetic structures. The model is general enough to describe both collective and single-electron modes of interaction and quantum mechanical, classical and Fermi degenerate regimes. Simplified expressions are developed for the gain by stimulated emission of radiation and for gain conditions of the Smith-Purcell-Cerenkov type free-electron lasers under conditions of very thin electron beams and infinite interaction length.

Nitrogen-laser-pumped single-mode dye laser

Abstract: Single-mode operation of a N2-laser-pumped'dye laser has been obtained in a short laser cavity consisting of a diffraction grating used near grazing incidence and an uncoated resonant reflector. The dye laser pulse has a spectral linewidth of 420±30 MHz and a pulse width of 1.1±0.1 ns, giving a time-bandwidth of 0.46±0.07 close to the Fourier transform limit for Gaussian pulse.

Two-photon resonant four-wave mixing in atomic sodium vapour

Abstract: Using the 4d- and 5s-states of sodium, two different types of two-photon resonance enhanced four-wave mixing have been observed. In the first type 2vL =vIR +vpIR radiation near 2.34 μm and UV radiation near 320 nm has been detected. In the second typevP−vS =vL +vIR the UV radiation produced in the first parametric process acted as a pump for stimulated electronic Raman scattering via the 4p-state of sodium. Resonantly enhanced by the two-photon transition 3s–4s, radiation near 1.18 μm was produced in this scheme. The realisation of a tunable coherent IR source based on these four-wave mixing processes is discussed.

Structure of crystallized layers by laser annealing of 〈100〉 and 〈111〉 self-implanted silicon samples

Abstract: Channeling effect techniques with a 2.0 MeV He+ Rutherford backscattering and transmission electron microscopy were used to characterize the crystallized layers after Q-switched ruby laser irradiation of 4000 A thick amorphous layer on 〈100〉 and 〈111〉 underlined crystal substrates. At a laser energy density of 2.5 J/cm2 the crystal layer on the 〈111〉 specimen contains a large density of stacking-faults, that on 〈100〉 specimen contains a very small amount of screw dislocation lines. High quality single-crystal layers have been obtained after irradiation at 3.5 J/cm2. From a comparison with the growth rate and defect structure observed in thermally annealed implanted-amorphous layers, we propose that crystal growth by 50 ns pulse laser annealing occurs by melting the amorphous layer.

Intrinsic photorefractive effect of LiNbO3

Abstract: The photorefractive effect of undoped LiNbO3 crystals of high purity is studied by means of two-photon excitation of picosecond light pulses. We show that two-photon photorefractive recording is accompanied by characteristic changes of the optical absorption and electron spin resonance spectra due to the formation of color centers. The role of these centers for the photorefractive process in discussed.

Transmission spectra of bismuth trioxide thin films

Abstract: Transmission spectra of bismuth trioxide thin films, of tetragonal and ortorhombic structures, are studied. Experimental data on the wavelength dependence of the refractive index are presented. The dispersion of the refractive index follows a single-oscillator model. Optical energy gap of tetragonal phase is smaller than that of orthorhombic phase; at 300 K these are 2.6 eV and 2.85 eV and at 77K are 2.8 and 3.1 eV, respectively.

Coexistence of superconductivity and magnetic order

Abstract: Experimental work on systems showing evidence of the coexistence of superconductivity and magnetic order is reviewed. The presently known systems are shown to be “superconducting spin glasses” rather than “ferromagnetic superconductors”. In those systems where the magnetic order is expected to be long-ranged (up to now) superconductivity vanishes as soon as the magnetic order appears. But it is not excluded that magnetic order with a correlation length larger than the superconductive coherence length can coexist with superconductivity.

On the Influence of Reactive Gases on Sputtering and Secondary Ion Emission

Abstract: As a continuation of earlier sputtering yield measurements in an ion microprobe, the influence of oxygen and nitrogen on sputtering yield, ionisation efficiency and depth resolutions has been studied. For inert gas bombardment the yield of Ti and V falls sharply at a certain oxygen exposure. While this decrease in yield can be correlated with an increase in surface binding energy in the case of titanium, cone formation causes the yield to drop for oxygen exposed vanadium. In contrast, during nitrogen bombardment the only effect of oxygen exposure is a drastic increase of the ionisation efficiency; the sputtering yield or the depth resolution Δz/z is hardly influenced by oxygen coverage. As was observed earlier in the case of Cu−Ni layers, Δz is essentially constant for erosion depthsz≳800 A, thus yielding better resolution at large depths than is to beexpected from a sequential layer removal model. The extent of the transition zone Δz, is determined by surface topography and thus depends on the target composition as well as its structure.

A physical model for the stress-induced electromagnetic effect in metals

Abstract: Observations of the fracture-induced electromagnetic effect in metals have recently been reported by the author. This paper presents a physical model for explaining this new effect. The reasoning attributes the observed electromagnetic effect to dislocationelectron interactions in a metal. The frequency of the emitted radiation has been calculated from Bohr's correspondence principle and potential on the basis of Hertzian dipole radiation. The proposed physical model predicts emission of electromagnetic waves at distinct transition stages of elastic-plastic deformations. The paper also presents some of the experimental results, being obtained by the author, which confirm the practicability of this approach.

High-power tunable IR Raman lasers

Abstract: Physical principles, new ways and means of creation, schemes, characteristics and features of efficient high-power tunable pulse Raman lasers, operating in the near and middle ir are reviewed. The paper includes: tunable dye and Nd lasers as pump sources; promising active media and their optimal excitation methods; optical systems for producing spatially homogeneous pumping; the physics of Raman oscillators and their practical schemes, efficient high pulse energy liquid N2 and compressed H2 Raman oscillators, covering several bands in the range between 1.4 and 9.2 μm; the physics and construction of efficient tunable Raman amplifiers-convertors, amplifying in the saturation regime of spontaneously scattered or beforehand produced and collimated external Stokes signals, obtained in the spectral range between 0.83 μm and 18 μm. Raman laser using a, so-called, broadband pump where the linewidth of pumping light is broader than the spontaneous scattering linewidth, are also discussed. Features of both amplification and oscillation regimes of such broadband pumped Raman lasers are reported, and conditions for the efficient frequency conversion are determined.

Nonlinear wave propagation along periodic-loaded transmission line

Abstract: A high frequency transmission line periodically loaded with varactor diodes is presented to study nonlinear wave propagation. The nonlinearity and dispersion characteristics are experimentally and theoretically analysed. Experimental results on shock wave formation and harmonic frequency generation are found to be in good agreement with theoretical predictions. The influence of dispersion on second-harmonic frequency generation is particularly discussed yielding large values of the efficiency.

Surface polaritons in semi-infinite crystals

Abstract: Recent progress in the field of optical surface modes in semi-infinite crystals is reviewed. The basic equations for surface polaritons originating from phonons, plasmons, excitons and magnons are given. Their observation by ATR, the resulting information and possible applications are discussed.

The conditions for total reflection of low-energy atoms from crystal surfaces

Abstract: The critical angles for the total reflection of low-energy particles from Cu rows and {001} planes have been investigated, using the binary collision approximation computer simulation code MARLOWE. Breakthrough angles were evaluated for H, N, Ne, Ar, Cu, Xe, and Au in the energy range from 0.1 to 7.5 keV. In both the axial and the planar cases, recoiling of the target atoms lowers the energy barrier which the target surface presents to the heavy projectiles. Consequently, the breakthrough angles are reduced for heavy projectiles below the values expected either from observations on light projectiles or from analytical channeling theory.

Static characteristics of Josephson interferometers

Abstract: This investigation deals with the range in operating currents for which a Josephson interferometer, sometimes also referred to as Superconducting QUantum Interference Device (SQUID), may remain in the zero-voltage Josephson condition. An interferometer consists of one or more inductive loops each of which contains two Josephson junctions or other weak links. Two types of current are considered. Gate currentIgpasses the junctions in parallel. Control currentIcgenerates magnetic flux via inductive coupling in the loops. Zero-voltage operation is possible within certain areas of theIg,Icplane. These areas are manifestations of flux-quantum states and their boundary lines are referred to as static characteristics. In view of the nonlinearity of the constituting equations, not all their formal solutions are physically realizable. A stability analysis yields criteria which permit the identification of realizable operating conditions. The static characteristics comprise operating conditions where the limit of stability is reached. To obtain the static characteristics, linearized equations may be utilized if theLIo product, a measure for the size of an interferometer, is large compared to the flux quantumΦ0, whereL is the inductance per loop, andIo the maximum Josephson current per junction. As a general method of solving system of transcendental equations, continuation is discussed. The utilization of continuation for obtaining interferometer characteristics is explained. It is shown that some changes in the gate-current feed arrangement are equivalent to shearing the characteristics in theIg,Icplane. Analytical results are given on extrema, inflexion points, and singularities in the shape of cusps which conceptually relate to the existence and connectivity of flux-quantum states. Experimental static characteristics are presented on two-and four-junction interferometers. They are in agreement with characteristics computed on the basis of simple lumped circuit models. Relevant circuit parameters are obtained from the experimental characteristics.

Magnetostriction measurement by means of strain modulated ferromagnetic resonance (SMFMR)

Abstract: A novel method for measuring magnetostriction constants is presented. A strain, periodic in time, applied to the sample, causes a modulation of the ferromagnetic resonance line position. The height of the signal obtained after phase-sensitive detection is proportional to the strain modulation depth. The appropriate magnetostriction constant λ is obtained by comparing the height of the SMFMR signal with that of the FMR line, as recorded by means of magnetic field modulation. Features of the new technique are: An illustrative example (YIG layer on GGG substrate) is given.

Hf-sputtered indium oxide films doped with tin

Abstract: Indium oxide films doped with tin (ITO-films) have been hf-sputtered from an 80 at-%In2O3/20 at-%SnO2 target onto glass substrates. The sputter atmosphere contained mainly argon (10−2Torr) with addition of oxygen (0≦p O 2≦2·10−2Torr). The sputtered films aren-conductors. The conductivity and density of charge carriers depend on the oxygen content of the sputter gas. They could be varied by two orders of magnitude. In air or in oxygen atmosphere the films oxidize at the surface and for a certain depth beneath the surface, thus decreasing the conductivity. The Hall mobility of the sputtered films is smaller (≈10 cm2V−1 s−1) than one observes at ITO films produced by CVD sparaying or other methods. The conductivity of as sputtered films approached maximum values of about 1000Ώ−1cm−1.

Doppler-free polarization spectroscopy using linear polarized light

Abstract: Lines of theQ-branch of vibrational rotational spectra are distinguished fromP(R)-lines by light induced anisotropy in absorbing gases. We report, for the first time, on experiments using a linearly polarized saturation beam. A CO2-laser was employed to investigate the molecule SF6.

A prism anamorphic system for Gaussian beam expander

Abstract: Basic properties are given of the prism anamorphic expander of a Gaussian beam composed of a couple of prisms in tandem. Its shorter overall length for a given expansion, relative ease of optical adjustment, a sharp selection of polarization component, and rather small inclusive dispersion make it an excellent beam expander in a specific laser cavity. The design principle is presented of the prism system with a grating for an anastigmatic optical resonator for a dye laser of narrow linewidth and of short pulse duration.

Infrared-microwave two-photon spectroscopy with13C16O2 and12C18O2 lasers of the ν2-band of ammonia

Abstract: A tunable microwave frequency was added to, or subtracted from fixed frequency13C16O2 and12C18O2 laser lines using the nonlinearity of molecular absorptions. In this way the frequency of 30 transitions of the ν2-band of ammonia were measured with an accuracy of ±0.0005 cm−1. A further four transitions were measured with an accuracy of ±0.0001 cm−1 by saturating the two-photon transition and observing the Lamb dip. For laser lines up to 11 GHz off-resonant with in frared transitions Doppler-limited signals were observed with microwave power densities of only 10 mW/cm2 using a wide-band intracavity cell.