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  • 1
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 92-96 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A normal-incidence modulation mechanism is proposed which uses the Stark effect to induce Γ-L transitions in asymmetrically stepped AlSb/InAs/GaSb/AlSb quantum wells (QWs). A significant feature of this structure is the unusual band alignments which localize two deep wells for the Γ and L bands in adjacent layers, i.e., the Γ-valley minimum is in the InAs while the L-valley minimum is in the GaSb. In contrast to a square QW, where the Stark shifts for both Γ and L subbands are in the same direction, the two step wells for Γ and L valleys in the proposed structure are oppositely biased in the presence of an electric field. Therefore, the first Γ and L subbands move toward each other, making the Γ-L crossover occur more efficiently. Near this point, most of the Γ electrons transfer to the L valleys, where they are allowed to make intersubband transitions under normally incident radiation. As a result, the device switches from being transparent to normal-incidence light to strongly absorbing it. The calculations indicate that excellent on/off ratios can be achieved in this structure operating at T≤150 K with electric fields on the order of ∼100 kV/cm for any infrared wavelength within the range of 3–20 μm.
    Materialart: Digitale Medien
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  • 2
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4628-4634 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Photocurrent excitation spectra of xenon and of the xenon–methane absorber–perturber system were studied in cells equipped with LiF windows cooled to ≈−30 °C. The cooling was needed to shift the LiF cutoff so as to enable measurements at photon energies up to the first ionization potential of Xe (12.13 eV). The spectra observed in methane–xenon mixtures were essentially similar to those in neat xenon, demonstrating that the Hornbeck–Molnar process (excitation of a xenon atom–excimer formation–ionization of an excimer) is feasible even in the presence of the molecular perturber CH4 at pressures up to ≈ 105 Pa. The primary effect of CH4 is to decrease the photocurrent, since photons in the energy range in question are absorbed by the methane molecules as well, without photoionizing them. The dependence of the current on methane concentration showed that the above mechanism is not the only one by which methane quenches the current. Moreover, the degree of quenching of the current by methane is dependent on the photon energy. The analysis of this dependence led to the assumption that an excited xenon atom and a methane molecule may form an excimer. Previous results on the Ar:Xe and Kr:Xe systems support this assumption. Peak inversions observed at high pressures in neat xenon, but absent in the CH4:Xe system, are discussed on the basis of prevailing models.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 303-307 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Oscillatory magnetoresistance (Shubnikov–de Haas) measurements have been used to determine free-carrier effective masses in HgTe-CdTe superlattices. Measurements on an n-type superlattice yield an electron mass that is in excellent agreement with theoretical results from a tight-binding band-structure calculation. The p-type data are more complex, showing evidence for a light-hole mass at low magnetic fields and a much heavier mass at fields above 2.0 T. This finding is also in agreement with the predictions of band-structure theory.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 843-851 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A novel, active, mode-locking scheme for producing single, line-tunable, high-power ultrashort CO2-laser pulses is described. Using an auxiliary grating-tuned cw CO2 laser for injection locking on the 9R(16) line, the Q-switched, mode-locked, and cavity-dumped 10-atm CO2 laser produces single, detection-limited (〈500 ps) pulses of 5-mJ energy at λ=9.29 μm. Details are given on the traveling-wave GaAs Pockels cells, both of which are controlled by a single, ultraviolet (UV) triggered spark gap. The time-locked output pulse is delayed by precisely 300.7 ns with 〈±50 ps jitter against the UV triggered spark gap, so the laser pulse is synchronizable to an external event to 〈±250 ps. To drive the transverse-electric (TE) 10-atm amplifier section, instead of the conventional Marx bank or inductance-capacitance (LC) generator, a new type of circuit is used. The automatically preionized, double-sided LC inversion circuit uses only one spark gap, and a detailed description with operating characteristics is provided.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 1803-1808 
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: In this paper, we describe a new, entirely computer-controlled apparatus designed for degradation studies of photochromic compounds. Photodegradation can be performed in three different ways using, in addition to the usual flash mode, the cyclic and continuous modes which more closely simulate daylight exposure conditions. In each mode, the "fatigue resistance'' parameter is computed. Moreover, the spectrokinetic parameters of photomerocyanine species can be obtained from the kinetic mode. Due particularly to the fully computer-controlled experimental setup, the between-day reproducibility of the initial absorbance and fatigue resistance determinations are better than 4% and 6%, respectively. Owing to the modular design of the menu-driven software, written in C language, the apparatus is very flexible and easy to use.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 839-853 
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: Pulsed transverse electric (TE) glow discharges used to produce population inversions in gas lasers at 1–20 atm require high-speed, high-voltage, high-current capacitor banks. In this article we first review and compare the circuits in common use to produce the required fast, self-sustained glow discharges in CO2, excimer, and N2 lasers. The parameters and circuit types given will be useful to future designers of pulsed gas lasers to design and to optimize any proposed circuit in order to yield any desired operating conditions and parameters. The n-stage circuit types presented are capacitor-transfer (C-C), L-C inversion, Marx bank, and fourfold L-C inversion, with their double-sided counterparts, and also their transmission-line counterparts, such as the Blumlein circuit. A one-parameter differential equation is developed which describes the general series C-L-R circuit. Electrical pulses measured on various lasers are compared with the differential equation solutions for various bank impedances and load resistances. We then present the design and measurements of a new automatically preionized, double-sided, L-C inversion circuit for an 800-mJ, TE CO2 laser, and compare it to previous systems. Methods to lengthen the discharge pulse, and some novel active mode locking techniques are discussed. We compare the L-C inversion circuit, the fourfold L-C inversion circuit, the conventional Marx bank, and the "double Marx bank'' for lasers and other applications, such as pulsed particle accelerators. Finally, we discuss capacitor bank interconnections, electro-optic probe systems, and various triggering methods.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 64 (1988), S. 1557-1559 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A detailed description of the kinematics governing a nuclear reaction analysis is presented for exothermic reactions. In particular, the peculiar behavior demonstrated when the emitted particle energy decreases with increasing bombarding energy is emphasized. The method is applied to 16O(d,p)17O and D(d,p)T reactions from SiO2 and D2O targets, respectively. Moreover, an algorithm was written to compute reaction cross sections as a function of energy or concentration as a function of depth from the measured reaction yields.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 1047-1054 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A comprehensive photo-Hall investigation of compensation densities in n-type Hg1−xCdxTe (0.224〈x〈0.235) is reported. The technique consists of measuring the low-temperature electron mobility as a function of optically excited carrier density. Pulses of 25-μs duration from a CO2 laser yield excess electron-hole plasma densities of between 1013 and 1016 cm−3. At low excitation levels the mobility increases due to the neutralization of ionized acceptors by photoexcited holes, while at higher excitation levels the mobility slowly decreases due to electron-hole scattering. By comparing to a detailed theory for electron transport in photoexcited narrow-gap semiconductors, the density of compensating acceptors NA can be accurately determined. The theory is generalized to account for multi-ion scattering, carrier heating, surface conduction, and inhomogeneous excitation. Experimental refinements significantly extending the range of narrow-gap n-type samples for which compensation densities may be determined are discussed. The present study demonstrates the broad applicability of the technique by determining NA for samples with compensation ratios between 0.09 and 0.96 and with both single and double acceptors.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 4892-4896 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report on the design and operation of an optimized version of a Q-switched, mode-locked, and cavity-dumped ruby-laser oscillator. The modulator window is much narrower than that assumed in conventional active mode-lock theory, and is shown to yield much shorter pulses than the latter in cases where the number of round trips is restricted. To allow a high-power pulse (≈1 GW) to evolve in the oscillator, and to allow simple synchronization to a (∼100 ns fixed delay) CO2 laser, a limit of 23 round trips was chosen, but similar limits may be imposed by lasers having short-gain duration as in an excimer laser. Details are given on the single spark gap switching element and Pockels cells, with an analysis of their expected switching speeds, in order to establish the effectiveness of the modulator, as compared to conventional sinusoidally driven active mode lockers. Single pulses of 50–70 mJ are reliably cavity-dumped after only 100-ns delay (23 round trips) with pulse length adjustable from 50–100 ps with ±5-ps stability. Relative timing between the main (CO2) and probe (ruby) pulses allows a measurement accuracy of ±50 ps to be attained.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 4638-4644 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A more rigorous numerical method for determining bulk semiconductor transport properties from photo-Hall data is presented and is used to determine compensation densities in narrow-gap HgCdTe. Previous studies have treated carrier density inhomogeneities by means of a two-layer approach, assuming a photoexcitation layer of constant thickness associated with either the absorption depth or the ambipolar diffusion length. Here, we show that this approximation can lead to significant error. For arbitrary optical intensities a more detailed integration over depth is required, properly accounting for the variation in the optical and transport of the material with carrier density and optical intensity. In the present technique, the spatial profile of the carrier density, n(z), is calculated as a function of optical intensity Φ0. The electron mobility μ is expanded in known functions of the carrier density. By using this n(Φ0,z) and μ(n), integral expressions for the net conductivity and Hall coefficient are then evaluated. The unknown coefficients in the mobility expansion are varied parametrically to obtain the best fits to the measured Hall coefficient and conductivity as a function of optical flux. From this fit, the electron mobility is determined as a function of carrier density. Detailed sample calculations are performed for the case of narrow-gap, n-type Hg1−xCdxTe at low temperatures assuming 10.6 μm CO2 laser excitation. Analysis of the photo-Hall data using the improved method leads to a reliable determination of compensation densities in this material.
    Materialart: Digitale Medien
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