Showing papers on "Physical optics published in 2003"

Observation of geometric phases for mixed states using NMR interferometry.

Abstract: Examples of geometric phases abound in many areas of physics. They offer both fundamental insights into many physical phenomena and lead to interesting practical implementations. One of them, as indicated recently, might be an inherently fault-tolerant quantum computation. This, however, requires one to deal with geometric phases in the presence of noise and interactions between different physical subsystems. Despite the wealth of literature on the subject of geometric phases very little is known about this very important case. Here we report the first experimental study of geometric phases for mixed quantum states. We show how different they are from the well-understood, noiseless, pure-state case.

Channeling of neutral particles in micro- and nanocapillaries

Abstract: After a brief review of the main areas of research in X-ray optics and an analysis of the development of capillary optics, a general theory of radiation propagation through capil- lary structures is described in both geometrical optics and wave optics approximations. Analysis of the radiation field structure inside a capillary waveguide shows that wave propagation in channels can be of a purely modal nature, with the transmitted energy mostly concentrated in the immediate neighborhood of the capillary inner walls. A qualitative change in radiation scattering with decreasing channel diameter — namely, the transition from surface channeling in microcapillaries to bulk channeling in nanocapillaries — is discussed.

Scattering matrices for large ice crystal particles.

Abstract: The problem of light scattering by ice crystal particles whose sizes are essentially larger than the incident wavelength is divided into two parts. First, the scattered field is represented as a set of plane-parallel outgoing beams in the near zone of the particle. Then, in the far zone the scattered field is represented as a result of both diffraction and interference of these beams within the framework of physical optics. A proper ray-tracing algorithm for calculation of the amplitude (Jones) scattering matrix is developed and applied. For large particles, a number of reduced Mueller matrices are introduced and discussed, since the pure Mueller matrix obtained from the Jones matrix becomes a rather cumbersome and quickly oscillating value. Backscattering by hexagonal ice crystals, including polarization properties, is considered in detail.

Bessel-Gauss resonator with spherical output mirror: geometrical- and wave-optics analysis.

Abstract: A detailed study of the axicon-based Bessel-Gauss resonator with concave output coupler is presented. We employ a technique to convert the Huygens-Fresnel integral self-consistency equation into a matrix equation and then find the eigenvalues and the eigenfields of the resonator at one time. A paraxial ray analysis is performed to find the self-consistency condition to have stable periodic ray trajectories after one or two round trips. The fast-Fourier-transform-based Fox and Li algorithm is applied to describe the three-dimensional intracavity field distribution. Special attention was directed to the dependence of the output transverse profiles, the losses, and the modal-frequency changes on the curvature of the output coupler and the cavity length. The propagation of the output beam is discussed.

A radar cross-section model for power lines at millimeter-wave frequencies

Abstract: The knowledge of radar backscatter characteristics of high-voltage power lines is of great importance in the development of a millimeter-wave wire detection system. In this paper, a very high-frequency technique based on an iterative physical optics approach is developed for predicting polarimetric radar backscattering behavior of power lines of arbitrary strand arrangement. In the proposed scattering model the induced surface current is obtained using the tangent plane approximation in an iterative manner where the first-order current, obtained from the incident wave, is used as the source for the second-order current and so on. The approximation is valid for frequencies where the cable strand diameter is on the order of or larger than the wavelength. It is shown that the copolarized backscatter is dominated by the contribution from the first-order PO currents, whereas the cross-polarized backscatter is generated by the second- and higher order PO currents. Using this model, the effects of radar antenna footprint, surface irregularities, and cable sag (when suspended between towers) on radar backscatter are studied. To verify the validity of the proposed model, theoretical results are compared at 94 GHz with experimental results and are found to be in good agreement.

A study of the higher-order small-slope approximation for scattering from a Gaussian rough surface

Abstract: Results from the first three terms of the small-slope approximation (SSA) for incoherent electromagnetic scattering from a penetrable randomly rough interface are discussed. Surface roughness is characterized as a Gaussian random process with an isotropic Gaussian correlation function. Sample results illustrate parameter spaces for which each correction term is appreciable. Reduction of the SSA to the physical optics theory is also discussed for both perfectly conducting and dielectric surfaces.

Laser Beam Shaping

Abstract: Laser beam shaping is the process of redistributing the irradiance and phase of a beam of optical radiation. This process of controlling optical beams is an enabling technology, which is used in a number of sectors of scientific, engineering, and industrial R&D. The shape of a laser beam generally refers to its irradiance profile, while the phase of a beam generally affects its propagation characteristics. Earlier work of Frieden and Kreuzer during the 1960s articulated well the goals of some contemporary laser beam shaping applications. Namely, geometrical optics was used to determine the configuration of a twoelement optical system that would transform an input plane wave with a Gaussian irradiance profile into an output plane wave with uniform irradiance. For some contemporary applications, physical optics must be used for the optical design of the laser beam shapers. A general presentation of the theory and techniques of laser beam shaping is given in a book edited by Dickey and Holswade. Laser beam shaping has become an important component of many laser-based applications, such as materials processing, medical applications, lithography, optical data storage, laser printing, isotope separation, optical data processing, and laboratory research. The first conference on laser beam shaping was held in 2000 at the Annual Meeting of the SPIE. This Laser Beam Shaping conference included a discussion of the fundamental limits of any beam shaping technique and the foundation for successful beam shaping design, including geometric techniques and optimization-based techniques. Applications included high-power laser fiber injection, UV and deep-UV homogenizers, micromachining in the electronics industry, and beam shaping techniques for laser printing. The Laser Beam Shaping II conference continued to bring together both workers in the field and potential users of the technology. Theory and design was again addressed, but the papers were primarily in the area of de-

Optics, light and lasers : the practical approach to modern aspects of photonics and laser physics

Abstract: Preface. 1 Light rays. 1.1 Light rays in human experience. 1.2 Ray optics. 1.3 Reflection. 1.4 Refraction. 1.5 Fermat's principle: the optical path length. 1.6 Prisms. 1.7 Light rays in wave guides. 1.8 Lenses and curved mirrors. 1.9 Matrix optics. 1.10 Ray optics and particle optics. Problems for chapter 1. 2 Wave optics. 2.1 Electromagnetic radiation fields. 2.2 Wave types. 2.3 Gaussian beams. 2.4 Polarization. 2.5 Diffraction. Problems for chapter 2. 3 Light propagation in matter. 3.1 Dielectric interfaces. 3.2 Complex refractive index. 3.3 Optical wave guides and fibres. 3.4 Functional types and applications of optical fibres. 3.5 Photonic materials. 3.6 Light pulses in dispersive materials. 3.7 Anisotropic optical materials. 3.8 Optical modulators. Problems for chapter 3. 4 Optical images. 4.1 The human eye. 4.2 Magnifying glass and eyepiece. 4.3 Microscopes. 4.4 Telescopes. 4.5 Lenses: designs and aberrations. Problems for chapter 4. 5 Coherence and interferometry. 5.1 Young's double slit. 5.2 Coherence and correlation. 5.3 The double-slit experiment. 5.4 Michelson interferometer: longitudinal coherence. 5.5 Fabry-Perot interferometer. 5.6 Optical cavities. 5.7 Thin optical films. 5.8 Holography. 5.9 Laser speckle (laser granulation). Problems for chapter 5. 6 Light and matter. 6.1 Classical radiation interaction. 6.2 Two-level atoms. 6.3 Stimulated and spontaneous radiation processes. 6.4 Inversion and amplification. Problems for chapter 6. 7 The laser. 7.1 The classic system: the He-Ne laser. 7.2 Mode selection in the He-Ne laser. 7.3 Spectral properties of the He-Ne laser. 7.4 Applications of the He-Ne laser. 7.5 Other gas lasers. 7.6 Molecular gas lasers. 7.7 The workhorses: solid-state lasers. 7.8 Selected solid-state lasers. 7.9 Tunable lasers with vibronic states. 7.10 Tunable ring lasers. Problems for chapter 7. 8 Laser dynamics. 8.1 Basic laser theory. 8.2 Laser rate equations. 8.3 Threshold-less lasers and micro-lasers. 8.4 Laser noise. 8.5 Pulsed lasers. Problems for chapter 8. 9 Semiconductor lasers. 9.1 Semiconductors. 9.2 Optical properties of semiconductors. 9.3 The heterostructure laser. 9.4 Dynamic properties of semiconductor lasers. 9.5 Laser diodes, diode lasers, laser systems. 9.6 High-power laser diodes. Problems for chapter 9. 10 Sensors for light. 10.1 Characteristics of optical detectors. 10.2 Fluctuating opto-electronic quantities. 10.3 Photon noise and detectivity limits. 10.4 Thermal detectors. 10.5 Quantum sensors I: photomultiplier tubes. 10.6 Quantum sensors II: semiconductor sensors. 10.7 Position and image sensors. Problems for chapter 10. 11 Laser spectroscopy. 11.1 Laser-induced fluorescence (LIF). 11.2 Absorption and dispersion. 11.3 The width of spectral lines. 11.4 Doppler-free spectroscopy. 11.5 Transient phenomena. 11.6 Light forces. Problems for chapter 11. 12 Photons - an introduction to quantum optics. 12.1 Does light exhibit quantum character? 12.2 Quantization of the electromagnetic field. 12.3 Spontaneous emission. 12.4 Weak coupling and strong coupling. 12.5 Resonance fluorescence. 12.6 Light fields in quantum optics. 12.7 Two-photon optics. 12.8 Entangled photons. Problems for chapter 12. 13 Nonlinear optics I: optical mixing processes. 13.1 Charged anharmonic oscillators. 13.2 Second-order nonlinear susceptibility. 13.3 Wave propagation in nonlinear media. 13.4 Frequency doubling. 13.5 Sum and difference frequency. 13.6 Optical parametric oscillators. Problems for chapter 13. 14 Nonlinear optics II: four-wave mixing. 14.1 Frequency tripling in gases. 14.2 Nonlinear refraction coefficient (optical Kerr effect). 14.3 Self-phase modulation. Problems for chapter 14. Appendix. A Mathematics for optics. A.1 Spectral analysis of fluctuating measurable quantities. A.2 Poynting theorem. B Supplements in quantum mechanics. B.1 Temporal evolution of a two-state system. B.2 Density-matrix formalism. B.3 Density of states. Bibliography. Index.

Fast radiation pattern evaluation for lens and reflector antennas

Abstract: A novel algorithm referred to as the fast physical optics (FPO) for computing the radiation patterns of nonplanar aperture antennas over a range of observation angles is presented. The computation is performed in the framework of the conventional physical optics approximation appropriate for the high frequency regime. The proposed algorithm is directly applicable to reflector and lens antennas as well as to radomes. The method comprises two steps. First, a decomposition of the aperture into subdomains and computation of the pertinent radiation pattern of each subdomain. Second, interpolation, phase-correction and aggregation of the radiation patterns into the final pattern of the whole aperture. A multilevel algorithm is formulated via a recursive application of the domain decomposition and aggregation steps. The computational structure of the multilevel algorithm resembles that of the FFT while avoiding its limitations.

Nonlinear Optics, Quantum Optics, and Ultrafast Phenomena with X-Rays

Abstract: 1 X-Ray Sources.- 1 Introduction.- 2 X-Ray Tubes.- 3 Laser-Driven Sources.- 4 Synchrotrons and Storage Rings.- 5 Pulse Slicing and Ultrafast Thomson Scattering.- 6 Energy-Recovering Linacs.- 7 X-Ray Free-Electron Lasers.- 7.1 The Physics of the FEL Process.- 7.2 Hard X-Ray FEL Facilities in Planning.- 7.3 The Quantum FEL.- 7.4 Lasing without Inversion.- 8 Comparison of Sources.- 2 Nonlinear Optics of Free Electrons.- 1 Introduction.- 2 Relativistic Electrons in Electromagnetic Waves.- 2.1 Single Plane Wave Packet.- 2.2 Multiple Parallel Plane Waves.- 2.3 Multiple Plane Waves, Nonrelativistic Approximation.- 2.4 Relativistic Electrons in Two Plane Wave Packets.- 2.4.1 Discussion.- 2.5 Laser Acceleration of Electrons.- 3 Dynamical Diffraction.- 1 Introduction.- 2 Linear Perfect Crystal Theory.- 2.1 Perfect Lattice, Fourier and Bloch Sums.- 2.2 The System of Linear Equations.- 2.3 The Dispersion Surface.- 2.4 Phase and Group Velocity, Beam Direction.- 2.5 Extinction and Boundary Conditions.- 3 Extended Takagi-Taupin Theory.- 3.1 Disturbed Lattice, Fourier and Bloch Sums.- 3.2 The System of Differential Equations.- 3.3 Comparison with the Takagi-Taupin Theory.- 3.3.1 Differential Equations.- 3.3.2 Generalized Wave Fields.- 3.4 Comparison with Kato's Eikonal Theory.- 3.5 Numerical Solution of the Differential Equations.- 3.6 The Dispersion Surface.- 3.6.1 Propagation of the Field Amplitudes.- 3.7 Beams, Adiabatic Change and Interbranch Scattering.- 3.8 Obtaining Qualitative Information.- 3.8.1 Example: Optical Phonons, Frequency Shifts.- 3.8.2 Example: Static Distortion, Guided Waves.- 3.9 From Boundary to Transition Conditions.- 3.10 Summary and Discussion.- 4 Nonlinear Dynamical Diffraction from Free Electrons.- 4.1 Multiple Bloch Waves.- 4.2 The System of Nonlinear Equations.- 4.3 An Example: Parametric Down Conversion.- 5 Appendix.- 5.1 Dynamical Diffraction in Macroscopic Form.- 5.2 The Longitudinal Current.- 5.3 Applicability of Macroscopic Electromagnetism.- 5.4 The Position of a Tie Point in Reciprocal Space.- 5.5 The Direction of the Poynting Vector.- 5.6 Details of Derivations.- 5.6.1 Amplitude Ratio, Equation (3.15).- 5.6.2 Equation (3.27).- 5.6.3 An Integral.- 4 Ultrafast Diffractive X-Ray Optics.- 1 Introduction.- 2 Laser-Induced Changes in Crystal Diffractive Properties.- 3 Bragg Reflection.- 4 Laue Transmission.- 4.1 Redirection of the Poynting Vector.- 4.2 An X-Ray Optical Femtosecond Streak Camera.- 4.2.1 Grazing Incidence.- 4.2.2 Swept Laser Excitation.- 4.2.3 An Example.- 4.2.4 Discussion.- 4.3 An Ultrafast Phase Retarder.- 4.4 Spectral Concentration of X-Rays.- 4.5 A Fast Borrmann Shutter.- 5 Parametric Down Converion.- 1 Introduction.- 1.1 Nonlinear Medium.- 1.2 Wave Vector and Frequency Matching.- 1.3 Strength of the Effect.- 2 Experiments.- 2.1 The Classical Experiment by Eisenberger and McCall.- 2.2 The First Synchrotron Results, Yoda et al.- 2.3 Energy Discrimination and Time Correlation.- 2.4 High Event Rate.- 2.5 High Pump Photon Energy - 98.9 keV.- 2.6 Suppression of the Pump Photons with a Mirror.- 2.7 Small Angles.- 2.7.1 The First Small-Angle Experiment.- 2.7.2 APS, 1-ID.- 2.7.3 APS, 7-ID.- 2.7.4 Suppression of Down Conversion at Small Angles.- 2.8 Wave Vector Matching by Dynamical Diffraction.- 3 Potential Applications.- 3.1 Tests of the Quantum Theory.- 3.2 Sub-Poisson Absorption Spectroscopy.- 3.3 Integration into a Beam Line.- 4 Experimental Issues.- 4.1 Background Suppression.- 4.2 Electric Noise.- 4.3 Stray Radiation.- 4.4 Energy Resolution.- 4.5 Time Resolution.- 4.6 Time Structure of the Source.- 4.7 Choice of Sample Material.- 5 Summary.- 6 Appendix.- 6.1 The Virtual Power Density of Vacuum Fluctuations.- 6.2 Cross Section.- 6.3 Amplitude Growth.- 6.4 Wave Vector Matching.- 6.4.1 Without Dynamical Diffraction.- 6.4.2 With Dynamical Diffraction of the Pump Only.- 6.5 Electronics.- 6.5.1 The Correlation Circuit.- 6.5.2 The Event Logger.- 6 Laser Pump, X-Ray Probe Spectroscopy on GaAs.- 1 Introduction.- 2 Physics Background.- 3 The Experiment.- 4 Results.- 5 Discussion.- 6 Experimental Issues.- 6.1 Monochromatization.- 6.2 Electronic Noise.- 7 Potential Applications.- 7.1 Spectroscopy with an Absolute Energy Reference.- 7.2 A Femtosecond Detector and X-Ray/Laser Correlator.- 7 Ultrafast structural changes induced by femtosecond laser pulses.- 1 Introduction.- 2 Theory.- 2.1 Lattice motion: molecular dynamics simulations.- 2.2 Potential energy surface: laser induced electron dynamics.- 2.2.1 Summary of the numerical approach.- 2.2.2 Pair correlation function.- 3 Ultrafast nonequilibrium graphitization of diamond.- 4 Ablation mechanisms in graphite.- 5 Nonequilibrium melting and ablation of carbon.- 6 Ablation of silicon.- 7 Laserinduced melting of a C60 molecular crystal.- 8 Fragmentation of nanotubes.- 9 Summary.- 8 Ultrafast Lattice Dynamics.- 1 Introduction.- 2 Experimental Setup.- 2.1 The Advanced Light Source.- 2.2 X-ray Time-Structure.- 2.3 Laser Synchronization.- 2.4 Streak Camera.- 3 Theory of Time-Resolved X-Ray Diffraction.- 4 Wave-vector Matching Considerations.- 4.1 Symmetric Case.- 5 Generation of coherent displacements.- 6 Extension to Finite Electron-Phonon Coupling Times.- 7 Experimental Results.- 7.1 Slow (Nanosecond) Time-scale Measurements.- 7.2 50 ps Resolution Pump-Probe Experiments.- 7.3 Streak Camera Results.- 8 Extraction of Electron-Phonon Coupling Times.- 9 High Fluence Results.- 10 Coherent Control.- 10.1 Introduction.- 10.2 Experimental Results.- 11 Control of the Diffraction Efficiency of a Crystal.- 12 Conclusion.- 9 Seeing Sound: Measuring acoustic pulse propagation with x-rays.- 1 Introduction.- 2 Ultrafast Strain Generation.- 2.1 Thermo-elastic model.- 2.2 Plasma Diffusion.- 3 The X-ray Source.- 3.1 Bunch Timing.- 4 Ultrafast Laser.- 4.1 T:sapphire oscillator.- 4.2 Chirped Pulse Amplification.- 4.3 Laser/X-ray Timing.- 5 Time-resolved x-ray Bragg diffraction.- 5.1 Dynamical diffraction calculations.- 5.2 Acoustic Pulse Evolution.- 5.3 Acoustic Reflections.- 5.3.1 Acoustic Dispersion.- 6 Time-resolved Laue diffraction.- 6.1 Pump-Probe X-ray Anomalous Transmission.- 6.2 Multiple crystal model.- 6.3 Acoustic Reflections.- 6.4 Acoustic Collisions.- 7 Summary and Acknowledgements.- 10 Time-dependent dynamical diffraction theory for phonon-type distortions.- 1 Introduction.- 2 The generalised Takagi-Taupin equation.- 3 Comparison with classical Takagi-Taupin theory.- 4 Coherent phonons and selection rules.- 5 Perturbative analytical solution.- 6 Numerical solution of the generalised Takagi-Taupin equation.- 7 High spatial frequency phonons.- 11 Nonlinear Response Functions for X-Ray Laser Pulses.- 1 Introduction.- 2 Nonlinear response functions: general formalism.- 3 Applications.- 4 Event rates and cross sections.- 5 Conclusions.- 6 Acknowledgments.- 7 Appendix.- References.

Evolution from a Fraunhofer to a Pearcey diffraction pattern

Abstract: A cylindrical lens of small aperture produces in its focal plane a row of wave dislocations (phase singularities or optical vortices) as part of the Fraunhofer diffraction pattern for a slit. On the other hand, when the aperture is large, aberration produces a cusp caustic at the focus and its associated diffraction, namely, the Pearcey pattern. Passing smoothly from one extreme to the other shows that the Fraunhofer dislocations move to become the dislocations outside the caustic while the dislocations inside the caustic are created successively in pairs. The pairs are accompanied by phase saddles, as expected from previous work. Similar sequences would be expected for all the higher diffraction catastrophes. In this way the movement of the structurally stable dislocations forms a link between the structurally unstable line or point focus of engineering optics and the stable caustics of catastrophe optics.

Electromagnetic realization of orders-of-magnitude tunneling enhancement in a double well system.

Abstract: We report on tunneling enhancement in a periodically perturbed double well system. The double well system was realized by a structure of two optical waveguides. The transfer of light power from one waveguide to the another as induced by the periodic variations of the waveguide geometry was investigated. Our experimental measurements show that, in the presence of periodic perturbation, this transfer of light power can be enhanced by more than 500 times. We use an analogy between electromagnetic wave optics and the quantum wave phenomena to provide an experimental support to the theoretical model of tunneling enhancement of a quantum particle, facilitated by its interaction with auxiliary quantum states.

Near-field optical investigation of three-dimensional photonic crystals.

Abstract: We show that the coupling of light from an external pointlike light source into a three-dimensional photonic crystal depends on the relative launching position with respect to the crystal lattice as well as on the frequency of light. The results are obtained with a near-field technique which is used to acquire optical information beyond the diffraction limit and to access optical details within the unit cell of the crystal. The experiments are performed at frequencies near the second-order L-gap. As a result, the changes in the shape of the near-field pattern are explained by the photonic properties of the crystal.

Asymmetric optical radiation pressure effects on liquid interfaces under intense illumination

Abstract: Deformations of horizontal liquid interfaces by optical radiation pressure are generally expected to display similar behavior, whatever the direction of propagation of the exciting laser beam is. We found that this expectation is borne out as long as the cw laser illumination is moderate in strength. However, we found that, as a striking contrast for high field strengths, either a large stable tether can be formed or a breakup of the interface can occur, depending on whether the laser beam is directed upward or downward. Physically, the reason for this asymmetry can be traced to whether total reflection can occur. We present two simple theoretical models, one based on geometrical optics, the other on wave optics, that are able to illustrate the essence of the effect. In the case leading to interface disruption our experimental results are compared with those obtained by Zhang and Chang for water droplets illuminated by intense laser pulses [Opt. Lett.13, 916 (1988)]. A key point in our experimental investigations is to work with a near-critical liquid–liquid interface. The surface tension therefore becomes significantly reduced, which thus enhances the magnitude of the stationary deformations induced.

Wide-band EMC analysis of on-platform antennas using impedance-matrix interpolation with the moment method-physical optics method

Abstract: An efficient method for the broadband EMC analysis of on-platform antennas is proposed, where the hybrid moment method-physical optics (MM-PO) formula is employed to generate the impedance matrix. The impedance matrix is calculated at relatively large frequency intervals and interpolated to approximate its values at intermediate frequencies. Numerical examples, including the coupling and radiation pattern evaluation of on-platform antennas, are presented. Through these examples, it is shown that the proposed method can greatly reduce the computing time in the wide-band computation compared with the traditional MM-PO method.

Application of Gaussian-ray basis functions for the rapid analysis of electromagnetic radiation from reflector antennas

Abstract: A highly efficient Guassian beam (GB) method was recently developed to provide a relatively rapid analysis of large reflector antennas. This GB method is very efficient because it represents the feed radiation in terms of a set of very few rotationally symmetric GBs and provides an essentially closed form solution for the reflection and diffraction of each GB incidence on the reflector surface. In contrast, conventional approaches evaluate the radiated field via a numerical integration of the physical optics integral over the large reflector surface; such numerical techniques are time consuming for large reflectors. However, the fast GB method becomes less accurate if each GB illuminating spot on the reflecting surface is comparable to the overall reflector size, since its closed-form solution requires a small spot size. This limitation of the GB approach happens particularly in situations where the feed is located relatively far from the reflector, since the GBs are not able to remain narrow in the angular and spatial domains simultaneously, thereby creating too large a spot size on the reflector surface. The above limitation of the GBs is overcome here by introducing a Gaussian-ray basis function (GRBF), a hybridisation of a geometrical optics (GO) ray tube and a GB. Owing to its similarity with a GB in the amplitude variation transverse to the ray, its incorporation into any existing GB code is straightforward. Numerical results are presented to demonstrate the accuracy and robustness of the new GRBF approach.

Equivalence between focussed paraxial beams and the quantum harmonic oscillator

Abstract: The paraxial approximation to the scalar Helmholtz equation is shown to be equivalent to the Schrodinger equation for a quantum harmonic oscillator. This equivalence maps the Gouy-phase of classical wave optics onto the time coordinate of the quantum harmonic oscillator and also helps us understand the qualitative behavior of the field and intensity distributions of focused optical beams in terms of the amplitude and probability distributions of quantum harmonic oscillators and vice versa.

Physical optics models for the backscatter response of road-surface faults and roadside pebbles at millimeter-wave frequencies

Abstract: Theoretical models, based on the physical optics (PO) approximation, are presented to predict the backscatter response of road-surface faults and roadside pebbles. Two types of surface faults are considered, cracks and potholes. By applying the PO model, the backscattering coefficients of a road-surface crack are approximated by the radar cross section (RCS) per unit length of a lossy dielectric cylinder. For a road-surface pothole with a simple geometry, its RCS is estimated by the coherent sum of the backscattered fields from the pothole edges with significant backscatterers. The backscatter response of roadside pebbles is a combination of the surface scattering from surface roughness and the volume scattering from a layer of rock particles. The surface scattering is depicted by the integral equation method, a simplified second-order iterative physical optics approximation. The hybrid scattering model based on the theory of vector radiative transfer is employed to predict the volume scattering. The validity of the theoretical models is examined by comparing the simulation results with experimental data from the University of Michigan, where backscatter measurements at W-band frequencies were conducted on road-surface faults and roadside pebbles at near grazing incidence angles (74/spl deg/-88/spl deg/) (Sarabandi, K. et al., ibid., vol.45, p.1679-88, 1997; Li and Sarabandi, ibid., vol.47, p.851-61, 1999).

Physics-based polarimetric BRDF models

Abstract: Through the use of measurements and analysis we have devised a series of physics-based analytic models for the BRDF that describe the differential polarization of light scattered from a random rough surface. These models incorporate both intrinsic (refractive index) and extrinsic (statistical moments of the surface height variations) properties of the surface as well as wavelength dependence. Detailed surface statistics are acquired with a stylus scanner. Physical optics theory relates these statistics (and thus the complex coherence factor of the fields at the surface) to the far-field intensity of the scattered light. An outcome of these models is the ability to predict the spectrally varying differential polarization of emittance. Excellent agreement between measured and modeled BRDF’s is demonstrated.

Line integral representation of physical optics scattering from a perfectly conducting plate illuminated by a Gaussian beam modeled as a complex point source

Abstract: An exact line integral representation of the physical optics fields scattered by a perfectly conducting plate illuminated by a vector complex point source is derived. Complex point sources provide, in the paraxial region, an accurate model of Gaussian beams, and can thus be used to efficiently represent the pattern of directive sources. Numerical results are shown to illustrate the exactness of the procedure and the computation time saving is investigated by comparison with surface integration. Moreover, by analyzing the scattering from a plate illuminated by a corrugated circular horn, the feasibility and accuracy of the proposed approach for modeling scattering due to real sources are demonstrated.

Quasi-ray Gaussian beam algorithm for short-pulse two-dimensional scattering by moderately rough dielectric interfaces

Abstract: We consider short-pulse (SP) time-domain (TD) two-dimensional (2-D) scattering by moderately rough interfaces, which separate free space from a slightly lossy dielectric half-space, and are excited by one-dimensional (1-D) SP-TD aperture field distributions. This study extends to the SP-TD in our previous investigation of time-harmonic high frequency 2-D scattering of Gabor-based quasi-ray Gaussian beam fields excited by 1-D aperture field distributions in the presence of moderately rough dielectric interfaces (Galdi et al.). The proposed approach is based on the Kirchhoff physical optics (PO) approximation in conjunction with the Gabor-based quasi-ray narrow-waisted Gaussian pulsed-beam (PB) discretization (Galdi et al.), which is applied to the SP-induced equivalent magnetic surface currents on the interface that establish the TD reflected/transmitted fields. We show that, for well-collimated truncated SP incident fields, the PO-PB synthesis of the reflected/transmitted fields yields an approximate explicit physically appealing, numerically efficient asymptotic algorithm, with well-defined domains of validity based on the problem parameters. An extensive series of numerical experiments verifies the accuracy of our method by comparison with a rigorously-based numerical reference solution, and assesses its computational utility. The algorithm is intended for use as a rapid forward solver in SP-TD inverse scattering and imaging scenarios in the presence of moderately rough dielectric interfaces.

Rigorous model of the scattering of a focused spot by a grating and its application in optical recording

Abstract: We describe a rigorous model for the scattering of a three-dimensional focused spot by a one-dimensional periodic grating. The incident field is decomposed into a sum of quasi-periodic fields, and the scattering of each of these is computed inside one unit cell of the grating. The model is applied to the simulation of the readout of a DVD disk. The polarization dependence of the reflected near and far fields is studied, and, for a TM-polarized incident spot, plasmons are observed in the reflected far-field intensity.

Diffraction loss in radiometry.

Abstract: Diffraction loss in radiometry has gained in importance recently because of an increased interest in longer wavelengths and the continuous improvement in experimental accuracy. The deviation from geometrical optics now contributes significantly to the errors of experiments. Previous research has concentrated on geometries classified as F1 and F2, leaving an intermediate case yet to be investigated. This intermediate case has some interesting behavior, as it is in this envelope of geometries that it is possible to have zero diffraction loss. We designate this intermediate geometric regime as F3. We introduce a numerical regime to calculate diffraction loss for intermediate geometries, which is also highly efficient for the F1 and F2 regimes.

"True" physical optics for the accurate characterization of antenna radomes and lenses

Abstract: In this contribution we propose to adopt the exact Green's functions of the dielectric slab, assuming the source at finite distance, to evaluate the equivalent currents which are then used in a PO like surface integral procedure to evaluate the scattering from a generically curved stratified dielectric structure. We denominate this technique "true" physical optics.

Improvement of prompt response from impulse radiating antennas by aperture trimming

Abstract: Reflector impulse radiating antennas (IRA) traditionally have been constructed by terminating a self-reciprocal, transverse electromagnetic (TEM) transmission-line feed structure into a paraboloidal reflector. The section of the paraboloid used is usually circular in cross-section, with the outer boundary coinciding with the circle of symmetry of the TEM feed. The reflector converts the spherical TEM mode on the feed line into an approximate plane wave in the near field by geometric optics. The prompt radiated electric field in the direction of focus is given in the physical optics approximation in terms of the integral of the electric field of the TEM mode over the aperture plane inside the reflector boundary. Balanced feed structures have TEM modes that provide both positive and negative contributions to this integral in the aperture plane. Determination of the contour where the principal component of the electric field in the TEM mode is zero identifies portions of the aperture that contribute destructively to the integral. These portions are removed, thereby increasing the prompt radiated field without altering the feed structure or the applied voltage waveform. Furthermore, decreasing the size of the TEM feed relative to the aperture size, followed by appropriate aperture trimming, allows an even greater increase in radiated field. Results are presented that predict an increase in prompt radiated fields for all electrode configurations. Improvements are largest for electrode angles that are large (with respect to the vertical). The trends predicted by the numerical results are verified by an experiment conducted on a time-domain antenna range.

Asymptotic method for analysis of RCS of arbitrary targets composed by dielectric and/or magnetic materials

Abstract: A method for the analysis and prediction of monostatic radar cross-section (RCS) of targets of complex geometry is presented. These targets can be formed by multilayer structures of dielectric and magnetic materials. The geometry representation of the targets is given as a collection of NURBS (nonuniform rational B-spline) surfaces. Physical optics (PO) is used to obtain the scattered field of each surface. The PO integral is expressed as a function of the parametric co-ordinates of the surface and is solved using Gordon's method for planar surfaces and the stationary phase method (SPM) for curved ones. Fresnel coefficients are included in the PO approach to take into account the effect of the RAM material.

Interference and diffraction in capillary x‐ray optics

Abstract: The optical guiding of X-ray radiation by capillary waveguides is analysed by using the Fresnel-Kirchhoff diffraction theory and the method of images. The guiding of the continuous, short-pulse, coherent, partially coherent and incoherent waves is presented as the diffraction and interference of several X-ray beams produced in free space by the Fresnel source of the waveguide. Using the Fresnel source properties, the necessary conditions for influence of the diffraction and interference on the output characteristics of the capillary X-ray optics in the near- and far-field diffraction zones are derived. The experimental data and computer simulations presented confirm the wave optics behaviour of X-rays under such conditions.

Modelling the diffraction of light by structures with spatial periodicity of the optical parameters of the substance and of the surface relief

Abstract: This paper discusses the transmission of light through a layered birefringent structure with periodic variation of the refractive indices and surface microrelief. A method is proposed for calculating the complex amplitude of the light wave as it passes through the successive layers of the structure and the diffraction efficiency for normal incidence of the light. The results of the computations are compared with those of a direct integration of Maxwell?s equations by the finite-difference method. The satisfactory agreement of the results of the computations makes it possible to use the new method to design new optical elements and devices.

Dielectric spherical lens antenna for wireless millimeter‐wave communications

Abstract: A simplified theoretical model based on geometrical optics (GO) in combination with physical optics (PO) in its vector form is used to analyze the performance of a homogeneous dielectric-sphere lens fed by a feed horn. The proposed model provides physical insight and sufficient accuracy for the initial design cycle. The computed results are also compared to measured data acquired from a prototype model and the agreement is satisfactory. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 39: 28–33, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11117

A closed form, physical optics expression for the radar cross section of a perfectly conducting flat plate over a dielectric half‐space

Abstract: [1] The physical optics approximation is employed in the derivation of a closed form expression for the radar cross section (RCS) of a flat, perfectly conducting plate of various shapes, located over a dielectric, possibly lossy half-space. The half-space is assumed to lie in the far field region of the plate. The well-known “four-path model” is invoked in a first-order approximation of the half-space contribution to the scattering mechanisms. Numerical results are compared to a reference, Moment Method solution, and the agreement is investigated, to assess the accuracy of the approximations used. The analytical expressions derived can facilitate very fast RCS calculations for realistic scatterers, such as ships in a sea environment, or aircraft flying low over the ground.