16,848 research outputs found
A theoretical and experimental study of the ionosphere using radio signals from earth satellites progress report no. 9, 1 jan. - 30 jun. 1964
Morphology of ionosphere using radio signals from earth satellite
Optical surface waves in periodic layered medium grown by liquid phase epitaxy
Optical surface waves propagating along the surface of a multilayer stack have been observed. The multilayer stack is grown by liquid phase epitaxy. The transverse intensity distribution measured is found to agree with our theoretical calculation
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Length summation in noise
To investigate the effect of background noise on visual summation, we measured the contrast detection thresholds for targets with or without a white noise mask in luminance contrast. The targets were Gabor patterns placed at 3° eccentricity to either the left or right of the fixation and elongated along an arc of the same radius to ensure equidistance from fixation for every point along the long axis. The task was a spatial two-alternative forced-choice (2AFC) paradigm in which the observer had to indicate whether the target was on the left or the right of the fixation. The threshold was measured at 75% accuracy with a staircase procedure. The detection threshold decreased with target length with slope −1/2 on log-log coordinates for target lengths between 30′ and 300′ half-height full-width (HHFW), defining a range of ideal matched-filter summation extending up to about 200′ (or about 16× the center width of the Gabor targets). The summation curves for different noise contrasts were shifted copies of each other. For the threshold versus mask contrast (TvN) functions, the target threshold was constant for noise levels up to about −22 dB, then increased with noise contrast to a linear asymptote on log-log coordinates. Since the “elbow” of the target threshold versus noise function is an index of the level of the equivalent noise experienced by the visual system during target detection, our results suggest that the signal-to-noise ratio was invariant with target length. We further show that a linear-nonlinear-linear gain-control model can fully account for these results with far fewer parameters than a matched-filter model
Tunable transmission and bistability in left-handed bandgap structures
We study the defect-induced nonlinear transmission of a periodic structure
created by alternating slabs of two materials with positive and negative
refractive index. We demonstrate bistable switching and tunable nonlinear
transmission in a novel type of bandgap that corresponds to the vanishing
average refractive index, and compare the observed effects for two types of the
bandgaps.Comment: 3 pages, 5 figures; significant change
Bistable diode action in left-handed periodic structures
We study nonlinear transmission of an asymmetric multilayer structure created
by alternating slabs of two materials with positive and negative refractive
index. We demonstrate that such a structure exhibits passive spatially
nonreciprocal transmission of electromagnetic waves, the analogue of the
electronic diode. We study the properties of this left-handed diode and confirm
its highly nonreciprocal and bistable transmittance by employing direct
simulations.Comment: 4 pages, 5 figure
Transverse Bragg-reflector injection lasers
A GaAs-GaAlAs injection laser has been tested that confines light in the lateral dimension (normal to junction plane) by a multilayer Bragg reflector. In the past, light has been confined as a result of the higher-index guiding region and resulting evanescent fields
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