Radiative properties of cirrus clouds in the infrared (8–13 μm) spectral region

Atmospheric radiation in the infrared (IR) 8–13 μm spectral region contains a wealth of information that is very useful for the retrieval of ice cloud properties from aircraft or space-borne measurements. To provide the scattering and absorption properties of nonspherical ice crystals that are fundamental to the IR retrieval implementation, we use the finite-difference time-domain (FDTD) method to solve for the extinction efficiency, single-scattering albedo, and the asymmetry parameter of the phase function for ice crystals smaller than 40 μm. For particles larger than this size, the improved geometric optics method (IGOM) can be employed to calculate the asymmetry parameter with an acceptable accuracy, provided that we properly account for the inhomogeneity of the refracted wave due to strong absorption inside the ice particle. A combination of the results computed from the two methods provides the asymmetry parameter for the entire practical range of particle sizes between 1 and 10,000 μm over the wavelengths ranging from 8 to 13 μm. For the extinction and absorption efficiency calculations, several methods including the IGOM, Mie solution for equivalent spheres (MSFES), and the anomalous diffraction theory (ADT) can lead to a substantial discontinuity in comparison with the FDTD solutions for particle sizes on the order of 40 μm. To overcome this difficulty, we have developed a novel approach called the stretched scattering potential method (SSPM). For the IR 8–13 μm spectral region, we show that SSPM is a more accurate approximation than ADT, MSFES, and IGOM. The SSPM solution can be further refined numerically. Through a combination of the FDTD and SSPM, the extinction and absorption efficiencies are computed for hexagonal ice crystals with sizes ranging from 1 to 10,000 μm at 12 wavelengths between 8 and 13 μm.

Calculations of the cirrus bulk scattering and absorption properties are performed for 30 size distributions obtained from various field campaigns for midlatitude and tropical cirrus cloud systems. Ice crystals are assumed to be hexagonal columns randomly oriented in space. The bulk scattering properties are parameterized through the use of second-order polynomial functions for the extinction efficiency and the single-scattering albedo and a power-law expression for the asymmetry parameter. We note that the volume-normalized extinction coefficient can be separated into two parts: one is inversely proportional to effective size and is independent of wavelength, and the other is the wavelength-dependent effective extinction efficiency. Unlike conventional parameterization efforts, the present parameterization scheme is more accurate because only the latter part of the volume-normalized extinction coefficient is approximated in terms of an analytical expression. After averaging over size distribution, the single-scattering albedo is shown to decrease with an increase in effective size for wavelengths shorter than 10.0 μm whereas the opposite behavior is observed for longer wavelengths. The variation of the asymmetry parameter as a function of effective size is substantial when the effective size is smaller than 50 μm. For effective sizes larger than 100 μm, the asymmetry parameter approaches its asymptotic value. The results derived in this study can be useful to remote sensing studies of ice clouds involving IR window bands.     

Megawatt peak power 8–13 μm CdSe optical parametric generator pumped at 2.8 μm

Infrared pulses, continuously tunable in the 8–13 μm range, and with up to 1 MW peak power, have been achieved using single-stage frequency conversion in a CdSe travelling-wave optical parametric generator, pumped by 100 ps pulses from an actively mode-locked, Q-switched and cavity dumped 2.8 μm Cr,Er:YSGG laser. The external quantum conversion efficiency reached 10%.     

Multi-order diffractive optical elements for achromatic interconnections at 1.30 μm and 1.55 μm

We propose a class of diffractive components allowing free-space optical systems to operate at the two telecommunication wavelengths simultaneously. These are fifth order diffractive components working at the sixth order at 1.30 μm and at the fifth order at 1.55 μm. Simulation results showing the link efficiency between two single-mode fibres as a function of the wavelength are presented. The width of the two transmission windows depends on the architecture of the whole system, which must be designed in accordance with technological realizability.     

Development of a 4–15 μm infrared GaAs hyperspectral QWIP imager

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a four band, 640 × 512, 23 μm × 23 μm pixel array which we have subsequently integrated with a linear variable etalon (LVE) filter providing over 200 spectral bands across the 4–15.4 μm wavelength region. This effort was a collaboration between NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory (JPL) and the Army Research Laboratory (ARL) sponsored by the Earth Science Technology Office of NASA. The QWIP array was fabricated by graded molecular beam epitaxial (MBE) growth that was specifically tailored to yield four distinct bands (FWHM): Band 1; 4.5–5.7 μm, Band 2; 8.5–10 μm, Band 3; 10–12 μm and Band 4; 13.3–14.8 μm. Each band occupies a swath that comprises 128 × 640 elements. The addition of the LVE (which is placed directly over the array) further divides the four “broad” bands into 209 separate spectral bands ranging in width from 0.02 μm at 5 μm to 0.05 μm at 15 μm. The detector is cooled by a mechanical cryocooler to 46 K. The camera system is a fully reflective, f/4.2, 3-mirror system with a 21° × 25° field of view. The project goals were: (1) develop the 4 band GaAs QWIP array; (2) develop the LVE and; (3) implement a mechanical cryocooler. This paper will describe the efforts and results of this undertaking with emphasis on the overall system characteristics.     

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55 μm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by 60 nm Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about 6 V. Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at 1.3 μm. When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and 5 mA/W at 1.3 and 1.55 μm, respectively.     

Designs of broadband quantum-well infrared photodetectors and 8–12 μm test device results

Several designs of broadband quantum-well infrared photodetectors are considered. The performance characteristics are analyzed by modeling. Test devices designed to cover the 8–12 μm wavelength region are fabricated and characterized. The results show that a broad device can be realized and their performance is in accordance with expectation.     

“Proper Time” Method Calculation of the “One Loop” Thermodynamical Potential for QED in External Electromagnetic Field

The extension of the “proper time” method to the statistical field theory is obtained. The procedure is used to calculate the “one loop” contribution to the thermodynamical potential in electrodynamics, when the electron positron gas is located in a combination of an inhomogeneous electrostatic field and an uniform magnetic one. Also, a “proper time” representation of the temperature Green function for this problem is given.     

“T-odd effects” in unpolarized Drell–Yan scattering

We consider the leading twist “T-odd” contributions as the dominant source of the cos2 azimuthal asymmetry in unpolarized di-lepton production in Drell–Yan scattering. This asymmetry contains information on the distribution of quark transverse spin in an unpolarized proton. In a parton-spectator framework we estimate these asymmetries at 50 GeV center of mass energy. This azimuthal asymmetry is interesting in light of proposed experiments at GSI, where an anti-proton beam is ideal for studying the transversity properties of quarks due to the dominance of valence quark effects.     

π-electron density dependence of third-order optical nonlinearities in poly(1,4-phenylene vinylene) derivatives

The third-order nonlinear optical properties of poly(1,4-phenylene vinylene) (PPV), poly(2-bromo-1,4-phenylene vinylene) (BrPPV) and poly(2-chloro-1,4-phenylene vinylene) (ClPPV) were studied by forward degenerate four-wave mixing (DFWM). They have large and ultrafast third-order optical nonlinear responses, which mainly arise from the distortion of the large π-conjugated electronic charge distribution in the backbone. It was found that there was a significant contribution to π-electron density and thus χ⁽³⁾ values from the substituents at the phenylene ring.     

GaInAsSb/AlGaAsSb lasers in the wavelength range between 2.73 μm and 2.93 μm

Compressively strained multiple quantum well lasers in the GaInAsSb/AlGaAsSb material system are reported. Indium concentrations between 40% and 47.5% were chosen for the GaInAsSb quantum wells. Compressive strains varied between 1.16% and 1.43%. The lasers worked continuous wave at room temperature up to a wavelength of 2.81 μm. For a laser with 2.93 μm wavelength continuous wave operation was found up to a temperature of −23°C. This laser worked in pulsed operation at 15°C.     

“Hot Potato Voice” in Peritonsillitis: A Misnomer

The "hot potato voice" is widely recognized as a symptom of peritonsillar cellulitis or abscess; yet there have been no studies assessing the resonance characteristics of the vocal tract in peritonsillitis. Analysis was undertaken of formant frequencies in the articulation of the vowels /i:/. /a:/ and /u:/ in six subjects with peritonsillitis and compared with articulation once the peritonsillitis had settled. Significant variation was found in F1 when articulating /i:/ and in F2 when articulating /a:/, which are explainable by dyskinesis of the peritonsillar musculature. These findings were compared with six subjects articulating the same vowels with and without a hot potato in their mouth. Variation was found in both F1 and F2 when articulating /i:/, which can be related to interference of the potato with movement of the anterior tongue. The changes in the vocal tract differ in these two cases and the title "hot potato voice" in peritonsillitis is a misnomer.     

On the “quantum rainbow” in surface scattering

In this letter we propose a new method to compute the scattering amplitudes of atoms from a corrugated wall model of a crystal surface. The method method requires reasonable computing time and converges rapidly to the solution of the scattering amplitudes (within 0.01% in each beam and satisfying unitarity) for any corrugation such that kζ₀ < 4.5 and ζ₀ < 0.35 Å, where k is the magnitude of the incident wave vector and ζ₀ the amplitude of the corrugation. These conditions for kζ₀ and ζ₀ are satisfied for any crystal surface and light scattering atoms (H and He) at thermal energies which makes the method appropriate for the system He, LiF for which results are presented comparing them with recent experimental data.     

Observations of ω0/2 harmonic emission from 0.35μm laser-irradiated plasmas

Observations of ω₀/2 harmonic emission from both spherical and plane targets irradiated by 0.35 μm, 800 ps laser pulses have been obtained with simultaneous high spectral and temporal resolutions of 16 Å and 20 ps respectively. The ω₀/2 harmonic emission spectrum is interpreted as providing a direct measurement of the frequency of the ω₀/2 plasma waves and hence can be used to estimate the electron temperature.