1,320 research outputs found
Optical diffraction of focused spots and subwavelength structures
We have developed a numerical diffraction tool for cases in which the incident field is a focused spot and the diffracting structure is a single structure or an aperiodic surface. Our approach uses the integral formulation to solve Maxwell’s equations and is different from previously published methods in its choice of basis function. We compared numerical results with experimental measurements of the far-field intensity for a focused spot incident on an aluminum grating, and the comparison was favorable. Finally, we predict the diffraction behavior of the proposed digital video disk format for the next generation of optical disk. Our analysis shows that the reflected signal for this format has a strong dependence on the polarization of the incident light
Polarization quadrature measurement of subwavelength diffracting structures
The amplitude and the phase of the diffracted far field depends on polarization when the diffracting structure is comparable to or less than the wavelength. When the far-field amplitude and the phase of one polarization with respect to the orthogonal polarization is measured, small changes in the structure can be measured. To make the far-field polarization measurements, we design a detector that measures the relative polarization amplitude and the phase in quadrature. We predict numerically and verify experimentally the polarization amplitude and the phase for an optical disc and a set of gratings with varying depth. Our results show that this measurement technique is sensitive to small variations in the diffracting structure and that it can be useful in applications such as critical dimension and overlay metrology in microelectronics fabrication
Slowly Rotating Relativistic Stars in Tensor-Vector-Scalar Theory
In order to examine the rotational effect around neutron star in
tensor-vector-scalar (TeVeS) theory, we consider the slowly rotating
relativistic stars with a uniform angular velocity. As a result, we find that
similar to the case in general relativity (GR), the angular momentum is
proportional to the angular velocity. Additionally, as the value of coupling
constant becomes higher, the frame dragging in TeVeS becomes quite
different distribution from that in GR, where we can also see the deviation
even in the interior of star. While with smaller value of , although the
frame dragging approaches to that expected in GR, the induced vector field due
to the rotation does not vanish and still exists. Thus, through the
observations associated with relativistic object, one could be possible to
distinguish the gravitational theory in strong field regime even in the case
that the value of coupling constant is quite small.Comment: Accepted in Phys.Rev.
A Metric for Rapidly Spinning Black Holes Suitable for Strong-Field Tests of the No-Hair Theorem
According to the no-hair theorem, astrophysical black holes are uniquely
characterized by their masses and spins and are described by the Kerr metric.
Several parametric deviations from the Kerr metric have been suggested to study
observational signatures in both the electromagnetic and gravitational-wave
spectra that differ from the expected Kerr signals. Due to the no-hair theorem,
however, such spacetimes cannot be regular everywhere outside the event
horizons, if they are solutions to the Einstein field equations; they are often
characterized by naked singularities or closed time-like loops in the regions
of the spacetime that are accessible to an external observer. For observational
tests of the no-hair theorem that involve phenomena in the vicinity of the
circular photon orbit or the innermost stable circular orbit around a black
hole, these pathologies limit the applicability of the metrics only to compact
objects that do not spin rapidly. In this paper, we construct a Kerr-like
metric which depends on a set of free parameters in addition to its mass and
spin and which is regular everywhere outside of the event horizon. We derive
expressions for the energy and angular momentum of a particle on a circular
equatorial orbit around the black hole and compute the locations of the
innermost stable circular orbit and the circular photon orbit. We demonstrate
that these orbits change significantly for even moderate deviations from the
Kerr metric. The properties of our metric make it an ideally suited spacetime
to carry out strong-field tests of the no-hair theorem in the electromagnetic
spectrum using the properties of accretion flows around astrophysical black
holes of arbitrary spin.Comment: 11 pages, 7 figures, accepted for publication in PR
Time and space integrating acousto-optic folded spectrum processing for SETI
Time and space integrating folded spectrum techniques utilizing acousto-optic devices (AOD) as 1-D input transducers are investigated for a potential application as wideband, high resolution, large processing gain spectrum analyzers in the search for extra-terrestrial intelligence (SETI) program. The space integrating Fourier transform performed by a lens channels the coarse spectral components diffracted from an AOD onto an array of time integrating narrowband fine resolution spectrum analyzers. The pulsing action of a laser diode samples the interferometrically detected output, aliasing the fine resolution components to baseband, as required for the subsequent charge coupled devices (CCD) processing. The raster scan mechanism incorporated into the readout of the CCD detector array is used to unfold the 2-D transform, reproducing the desired high resolution Fourier transform of the input signal
Temperature dependence of absorption in photorefractive iron-doped lithium niobate crystals
We present experimental data showing a significant dependence of light absorption on temperature in photorefractive LiNbO3:Fe crystals. The results are successfully explained by assuming that the widths of the Fe2+ absorption bands in the visible and in the infrared spectral region depend on temperature. The findings are of relevance for thermal fixing of holograms. Furthermore, a temperature-induced increase of the infrared absorption is promising for improved infrared recording
Neutron Stars in f(R) Gravity with Perturbative Constraints
We study the structure of neutron stars in f(R) gravity theories with
perturbative constraints. We derive the modified Tolman-Oppenheimer-Volkov
equations and solve them for a polytropic equation of state. We investigate the
resulting modifications to the masses and radii of neutron stars and show that
observations of surface phenomena alone cannot break the degeneracy between
altering the theory of gravity versus choosing a different equation of state of
neutron-star matter. On the other hand, observations of neutron-star cooling,
which depends on the density of matter at the stellar interior, can place
significant constraints on the parameters of the theory.Comment: Discussion extended, typos corrected, figures revised. Accepted for
publication in PR
Optical-radar imaging of scale models for studies in asteroid astronomy
During the past five years, delay-Doppler radar has become the primary technique for studying the structure of Earth-crossing asteroids. None of these objects has yet been visited by spacecraft, so ground-truth test cases are lacking. A laboratory system is described that provides optical-radar images at 0.1-mm resolution. These data are analogous to the highest-resolution asteroid radar images currently available and provided realistic test cases for developing signal-processing techniques. The system can be thought of as a 1/188,000 scale model of the Arecibo radar, or a 1/52,800 scale model of the Goldstone radar
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