Refractivity

La rifrattività è il principale parametro che governa la propagazione di un’onda elettromagnetica nel mezzo. In questo lavoro verifichiamo in modo analitico la validità della formula approssimata della rifrattività dei plasmi che si formano in una miscela di gas a pressione  P > 3 bar sottoposta a scarica elettrica pulsata. Questo caso è di particolare rilevanza nei LASER a gas, in cui l’inversione di popolazione è ottenuta nei plasmi gassosi generati da scariche elettriche, ad esempio i LASER eccimeri e ad anidride carbonica. L’analisi quantitativa della rifrattività è propedeutica alla stima dei problemi causati dalle disomogeneità della scarica elettrica e del flusso di gas, che limitano la qualità ottica (e quindi la focalizzabilità e la massima intensità ottenibile) degli impulsi di luce LASER.

In recent years, the global positioning system (GPS) has been exploited via radio occultation techniques to obtain profiles of refractivity, temperature, pressure and water vapor in the neutral atmosphere and electron density in the ionosphere. The GPS/MET experiment, which placed a GPS receiver in a low-Earth orbit, provided a wealth of data which was used to test this concept and the accuracy of the retrievals. Several investigations have already demonstrated that the retrieval accuracies obtained with GPS/MET is already comparable, if not better, than the more traditional atmospheric sensing techniques (e.g., radiosondes). Even though the concept of atmospheric profiling via radio occultation is quite a simple one, care must be taken to separate the numerous factors that can affect the occulted signal. These include the motion of the satellites, clock drifts, relativistic effects, the separation of the ionosphere and the neutral atmosphere, and the contribution of the upper atmosphere where sensitivity of the GPS signal is weak. In addition, care must be taken to use proper boundary conditions, use proper smoothing intervals and interpolation schemes to avoid retrieving artificial atmospheric structures, and most importantly detect and correct phase measurement errors introduced by sharp refractivity gradients in the atmosphere. This work describes in some detail the several steps involved in processing such data. In particular, it describes a system that was developed at the Jet Propulsion Laboratory and used to process the GPS/MET data. Several examples of retrieved refractivity, temperature and water vapor profiles are shown and compared to analyses from the European Center for Medium-range Weather Forecast (ECMWF). Statistical comparisons of GPS/MET and ECMWF temperatures for data collected during June 21-July 4, 1995, indicate that differences are of order 1-2 K at northern latitudes where the ECMWF analyses are most accurate.

The present quantitative structure-activity relationship (QSAR) study attempts to explore the structural and physicochemical requirements of mannitol derivatives for HIV protease inhibitory activity using linear free energy related model of Hansch. QSAR models have been developed using electronic (Hammett sigma), hydrophobicity (pi), and steric (molar refractivity and STERIMOL L, B1, and B5) parameters of phenyl ring substituents of the compounds along with appropriate dummy variables. Whole molecular descriptors like partition coefficient (logP(calcd)) and molar refractivity (MR) were also tried as additional descriptors. Statistical techniques like stepwise regression, multiple linear regression with factor analysis as the data preprocessing step (FA-MLR), principal component regression analysis (PCRA), and partial least squares (PLS) analysis were applied to identify the structural and physicochemical requirements for HIV protease inhibitory activity. The generated equations were statistically validated using leave-one-out technique. The quality of equations obtained from stepwise, FA-MLR, PCRA, and PLS are of acceptable statistical range (explained variance ranging from 74.0% to 80.5%, while predicted variance ranges from 70.3% to 77.1%). The coefficient of molar refractivity shows that the activity decreases with increase in volume. Lipophilicity of the para substituents at Y position is conducive to the activity while lipophilicity of the para substituents at X position is detrimental to the activity. The coefficients of molar refractivity (mr(Y_p)) and STERIMOL parameters for para substituents at X and Y positions (B1(X_p) and B5(Y_p)) of the phenyl rings indicate that the width of the substituents at X position and the overall size of para substituents at Y position are the detrimental factors for the activity. The fluoro substituent at ortho position (Y) decreases the activity when compared to the corresponding unsubstituted congener. Presence of hydrogen bond donor groups at para position (Y) also reduces the activity. Additionally, presence of substituent at ortho position (X) and the presence of substituent at para position (Y) are conducive for the activity. Presence of fluorine at X and Y positions also increases the activity.

The optical-limiting behavior of a series of trans-alpha, omega-diphenyl polyene compounds was observed in solutions of chloroform. The influence of planarity, the substitution of donor and acceptor groups, and the extent of pi-electron delocalization on the nonlinear thresholds of the diphenyl polyenes in an optical-limiting geometry were examined. A saturation effect of optical-limiting nonlinear thresholds, consistent with the theoretical work, was observed at 10-11 pi-electron bonds. The temporal profiles of the transmitted laser pulses and the power dependence of the nonlinear thresholds as a function of spot size and wavelength were examined. These examinations led to the conclusion that the predominant nonlinear mechanism was quasi-steady-state self-focusing. Nonlinear thresholds an order of magnitude lower than the well-known self-focusing medium CS2 were observed. The series of polyenes consists of efficient broadband nonresonant optical-limiting materials.

We analyze codirectional and counterpropagating second harmonic generation, degenerate four‐wave mixing, and intensity‐dependent refractive index phenomena based on surface plasmon polaritons guided by thin metal films. Promising cross sections are predicted for three out of the four cases.

This report describes the experimental and analytical efforts performed to determine the constitutive parameters of a reinforced concrete pad on which an aircraft (the NASA Boeing 757) was parked while its internal electromagnetic environment was measured. This concrete pad is part of the Large Electromagnetic System-Level Illuminator (LESLI) test facility at the Phillips Laboratory, Kirtland Air Force Base, New Mexico. The relative dielectric constant, conductivity, index of refraction, and reflection coefficient have been determined over the frequency range of 0 to 300 MHz and are presented.

We demonstrate how slow group velocities of light, which are readily achievable in photonic-crystal systems, can dramatically increase the induced phase shifts caused by small changes in the index of refraction. Such increased phase sensitivity may be used to decrease the sizes of many devices, including switches, routers, all-optical logical gates, wavelength converters, and others. At the same time a

Enhanced two-beam mixing amplification at a wavelength of 1.06 mum obtained by using sinusoidal electrical fields at 7.7 MHz is reported. The peak measured gains are comparable with dc field enhanced gain but with smaller rms fields. At grating wavelengths >3 mum the ac enhanced gain is considerably higher than the dc enhanced gain. Uniform alternating electric fields are much easier to establish inside the material and result in a much more stable output than dc fields.

ABSTRACT: This study presents the complex index of refraction
and the complex permittivity of a magnetic ceramic material
made of copper, cobalt, and iron oxides. The index of refraction
and the extinction coefficient of the CuCo-ferrite exhibit an
almost frequency independent behavior and were averaged
to n = 3.62 ± 0.05 and k = 0.06 ± 0.02, respectively, over
the frequency range from 0.2 to 1 THz. The corresponding
complex permittivity was ε’ = 13.12 ± 0.35 for the real part
and ε’’ = 0.46 ± 0.15 for the imaginary one. The absorption
coefficient and the transmittance of the CuCo-ferrite were
also determined. The absorption coefficient exhibits a dip
at ~0.35 THz, which corresponds to a peak in transmittance at
this frequency. The impact of the observations on the potential
realization of novel THz electronic devices is discussed.