92 research outputs found
Time-reversal in dynamically-tuned zero-gap periodic systems
We show that short pulses propagating in zero-gap periodic systems can be
reversed with 100% efficiency by using weak non-adiabatic tuning of the wave
velocity at time-scales that can be much slower than the period. Unlike
previous schemes, we demonstrate reversal of {\em broadband} (few cycle) pulses
with simple structures. Our scheme may thus open the way to time-reversal in a
variety of systems for which it was not accessible before.Comment: Accepted for publication in Phys. Rev. Letter
Time varying gratings model Hawking radiation
Diffraction gratings synthetically moving at trans-luminal velocities contain
points where wave and grating velocities are equal. We show these points can be
understood as a series of optical event horizons where wave energy can be
trapped and amplified, leading to radiation from the quantum vacuum state. We
calculate the spectrum of this emitted radiation, finding a quasi-thermal
spectrum with features that depend on the grating profile, and an effective
temperature that scales exponentially with the length of the grating, emitting
a measurable flux even for very small grating contrast.Comment: 13 pages, 4 figure
Ray-optical negative refraction and pseudoscopic imaging with Dove-prism arrays
A sheet consisting of an array of small, aligned Dove prisms can locally (on the scale of the width of the prisms) invert one component of the ray direction. A sandwich of two such Dove-prism sheets that inverts both transverse components of the ray direction is a ray-optical approximation to the interface between two media with refractive indices +n and –n. We demonstrate the simulated imaging properties of such a Dove-prism-sheet sandwich, including a demonstration of pseudoscopic imaging
Design of Electromagnetic Cloaks and Concentrators Using Form-Invariant Coordinate Transformations of Maxwell's Equations
The technique of applying form-invariant, spatial coordinate transformations
of Maxwell's equations can facilitate the design of structures with unique
electromagnetic or optical functionality. Here, we illustrate the
transformation-optical approach in the designs of a square electromagnetic
cloak and an omni-directional electromagnetic field concentrator. The
transformation equations are described and the functionality of the devices is
numerically confirmed by two-dimensional finite element simulations. The two
devices presented demonstrate that the transformation optic approach leads to
the specification of complex, anisotropic and inhomogeneous materials with well
directed and distinct electromagnetic behavior.Comment: submitted to "Photonics and Nanostructures", Special Issue "PECS
VII", Elsevie
Optical design of reflectionless complex media by finite embedded coordinate transformations
Transformation optics offers an unconventional approach to the control of
electromagnetic fields. A transformation optical structure is designed by first
applying a form-invariant coordinate transform to Maxwell's equations, in which
part of free space is distorted in some desired manner. The coordinate
transformation is then applied to the permittivity and permeability tensors to
yield the specification for a complex medium with desired functionality. The
transformation optical structures proposed to date, such as electromagnetic
"invisibility" cloaks and concentrators, are inherently reflectionless and
leave the transmitted wave undisturbed. Here we expand the class of
transformation optical structures by introducing finite, embedded coordinate
transformations, which allow the electromagnetic waves to be steered or
focused. We apply the method to the design of several devices, including a
parallel beam shifter and a beam splitter, both of which exhibit unusual
electromagnetic behavior as confirmed by 2D full-wave simulations. The devices
are designed to be reflectionless, in accordance with a straightforward
topological criterion.Comment: submitted to the journal on Sep 10 2007, abstract changed to make it
more accessible, keywords adde
Overcoming losses in superlenses with synthetic waves of complex frequency
Superlenses made of plasmonic materials and metamaterials have been exploited
to image features of sub-diffractional scale. However, their intrinsic losses
impose a serious restriction on the imaging resolution, which is a
long-standing problem that has hindered wide-spread applications of
superlenses. Optical waves of complex frequency exhibiting a temporally
attenuating behavior have been proposed to offset the intrinsic losses in
superlenses via virtual gain, but the experimental realization has been missing
due to the challenge involved in preparing the illumination with temporal
decay. Here, by employing multi-frequency measurement, we successfully
implement a synthetic optical wave of complex frequency to experimentally
observe deep-subwavelength superimaging patterns enabled by the virtual gain.
Our work represents a practical approach to overcoming the intrinsic losses of
plasmonic systems for imaging and sensing applications.Comment: 17 pages, 3 figure
Second harmonic generation at a time-varying interface
Time-varying metamaterials rely on large and fast changes of the linear
permittivity. Beyond the linear terms, however, the effect of a
non-perturbative modulation of the medium on harmonic generation and the
associated nonlinear susceptibilities remains largely unexplored. In this work,
we study second harmonic generation at an optically pumped time-varying
interface between air and a 310 nm Indium Tin Oxide film. We observe an
enhancement of the modulation contrast at the second harmonic wavelength, up to
93% for a pump intensity of 100 GW/cm, leading to large frequency
broadening and shift. We demonstrate that, in addition to the quadratic
dependence on the fundamental field, a significant contribution to the
enhancement comes from the temporal modulation of the second order nonlinear
susceptibility, whose relative change is double that of the linear term.
Moreover, the spectra resulting from single and double-slit time diffraction
show significantly enhanced frequency shift, broadening and modulation depth,
when compared to the infrared fundamental beam, and could be exploited for
optical computing and sensing. Enhanced time-varying effects on the harmonic
signal extends the application of materials to the visible range and calls for
further theoretical exploration of non-perturbative nonlinear optics.Comment: 19 pages, 3 figures, 8 supplementary figure
Theory of wave-front reversal of short pulses in dynamically-tuned zero-gap periodic systems
Recently, we have shown that the wave-front of short pulses can be accurately
and efficiently reversed by use of simple one-dimensional zero-gap photonic
crystals. In this Article, we describe the analytical approach in detail, and
discuss specific structures and modulation techniques as well as the required
steps for achieving complete time-reversal. We also show that our scheme is
only very weakly sensitive to material losses and dispersion
Scattering of elastic waves by periodic arrays of spherical bodies
We develop a formalism for the calculation of the frequency band structure of
a phononic crystal consisting of non-overlapping elastic spheres, characterized
by Lam\'e coefficients which may be complex and frequency dependent, arranged
periodically in a host medium with different mass density and Lam\'e
coefficients. We view the crystal as a sequence of planes of spheres, parallel
to and having the two dimensional periodicity of a given crystallographic
plane, and obtain the complex band structure of the infinite crystal associated
with this plane. The method allows one to calculate, also, the transmission,
reflection, and absorption coefficients for an elastic wave (longitudinal or
transverse) incident, at any angle, on a slab of the crystal of finite
thickness. We demonstrate the efficiency of the method by applying it to a
specific example.Comment: 19 pages, 5 figures, Phys. Rev. B (in press
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