71 research outputs found
Experimental and numerical studies of terahertz surface waves on a thin metamaterial film
We present experimental and numerical studies of localized terahertz surface
waves on a subwavelength-thick metamaterial film consisting of in-plane
split-ring resonators. A simple and intuitive model is derived that describes
the propagation of surface waves as guided modes in a waveguide filled with a
Lorentz-like medium. The effective medium model allows to deduce the dispersion
relation of the surface waves in excellent agreement with the numerical data
obtained from 3-D full-wave calculations. Both the accuracy of the analytical
model and the numerical calculations are confirmed by spectroscopic terahertz
time domain measurements.Comment: 3 pages, 3 figure
Gradient Index Metamaterial Based on Slot Elements
We present a gradient-index (GRIN) metamaterial based on an array of annular
slots. The structure allows a large variation of the effective refractive index
under normal-to-plane incidence and thus enables the construction of GRIN
devices consisting of only a small number of functional layers. Using full-wave
simulations, we demonstrate the annular slot concept by means of a 3-unit-cell
thin GRIN lens for the terahertz (THz) range. In the presented realizations, we
achieved an index contrast of Delta n = 1.5 resulting in a highly refractive
lens suitable for focusing THz radiation to a spot size smaller than the
wavelength.Comment: 4 pages, 5 figure
In-Plane Focusing of Terahertz Surface Waves on a Gradient Index Metamaterial Film
We designed and implemented a gradient index metasurface for the in-plane
focusing of confined terahertz surface waves. We measured the spatial
propagation of the surface waves by two-dimensional mapping of the complex
electric field using a terahertz near-field spectroscope. The surface waves
were focused to a diameter of 500 \micro m after a focal length of approx. 2
mm. In the focus, we measured a field amplitude enhancement of a factor of 3.Comment: 6 pages, 4 figure
Metamaterial near-field sensor for deep-subwavelength thickness measurements and sensitive refractometry in the terahertz frequency range
We present a metamaterial-based terahertz (THz) sensor for thickness
measurements of subwavelength-thin materials and refractometry of liquids and
liquid mixtures. The sensor operates in reflection geometry and exploits the
frequency shift of a sharp Fano resonance minimum in the presence of dielectric
materials. We obtained a minimum thickness resolution of 12.5 nm (1/16000 times
the wavelength of the THz radiation) and a refractive index sensitivity of 0.43
THz per refractive index unit. We support the experimental results by an
analytical model that describes the dependence of the resonance frequency on
the sample material thickness and the refractive index.Comment: 10 pages, 5 figure
Modification of spintronic terahertz emitter performance through defect engineering
Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for
the generation of THz radiation based on spin-to-charge conversion in the
layers. The key technological and scientific challenge of THz spintronic
emitters is to increase their intensity and frequency bandwidth. Our work
reveals the factors to engineer spintronic Terahertz generation by introducing
the scattering lifetime and the interface transmission for spin polarized,
non-equilibrium electrons. We clarify the influence of the electron-defect
scattering lifetime on the spectral shape and the interface transmission on the
THz amplitude, and how this is linked to structural defects of bilayer
emitters. The results of our study define a roadmap of the properties of
emitted as well as detected THz-pulse shapes and spectra that is essential for
future applications of metallic spintronic THz emitters.Comment: 33 pages, 13 figure
Highly Selective Terahertz Bandpass Filters Based on Trapped Mode Excitation
We present two types of metamaterial-based spectral bandpass filters for the
terahertz (THz) frequency range. The metamaterials are specifically designed to
operate for waves at normal incidence and to be independent of the field
polarization. The functional structures are embedded in films of
benzocyclobutene (BCB) resulting in large-area, free-standing and flexible
membranes with low intrinsic loss.
The proposed filters are investigated by THz time-domain spectroscopy and
show a pronounced transmission peak with over 80]% amplitude transmission in
the passband and a transmission rejection down to the noise level in the
stopbands. The measurements are supported by numerical simulations which
evidence that the high transmission response is related to the excitation of
trapped modes.Comment: 6 pages, 4 figure
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
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