16 research outputs found
Comparison of imaging with sub-wavelength resolution in the canalization and resonant tunnelling regimes
We compare the properties of subwavelength imaging in the visible wavelength
range for metal-dielectric multilayers operating in the canalization and the
resonant tunnelling regimes. The analysis is based on the transfer matrix
method and time domain simulations. We show that Point Spread Functions for the
first two resonances in the canalization regime are approximately Gaussian in
shape. Material losses suppress transmission for higher resonances, regularise
the PSF but do not compromise the resolution. In the resonant tunnelling
regime, the MTF may dramatically vary in their phase dependence. Resulting PSF
may have a sub-wavelength thickness as well as may be broad with multiple
maxima and a rapid phase modulation. We show that the width of PSF may be
reduced by further propagation in free space, and we provide arguments to
explain this surprising observation.Comment: 17 pages,12 figure
Fourier Optics approach to imaging with sub-wavelength resolution through metal-dielectric multilayers
Metal-dielectric layered stacks for imaging with sub-wavelength resolution
are regarded as linear isoplanatic systems - a concept popular in Fourier
Optics and in scalar diffraction theory. In this context, a layered flat lens
is a one-dimensional spatial filter characterised by the point spread function.
However, depending on the model of the source, the definition of the point
spread function for multilayers with sub-wavelength resolution may be
formulated in several ways. Here, a distinction is made between a soft source
and hard electric or magnetic sources. Each of these definitions leads to a
different meaning of perfect imaging. It is shown that some simple
interpretations of the PSF, such as the relation of its width to the resolution
of the imaging system are ambiguous for the multilayers with sub-wavelenth
resolution. These differences must be observed in point spread function
engineering of layered systems with sub-wavelength sized PSF
Sub-wavelength diffraction-free imaging with low-loss metal-dielectric multilayers
We demonstrate numerically the diffraction-free propagation of sub-wavelength
sized optical beams through simple elements built of metal-dielectric
multilayers. The proposed metamaterial consists of silver and a high refractive
index dielectric, and is designed using the effective medium theory as strongly
anisotropic and impedance matched to air. Further it is characterised with the
transfer matrix method, and investigated with FDTD. The diffraction-free
behaviour is verified by the analysis of FWHM of PSF in the function of the
number of periods. Small reflections, small attenuation, and reduced Fabry
Perot resonances make it a flexible diffraction-free material for arbitrarily
shaped optical planar elements with sizes of the order of one wavelength.Comment: 5 pages, 4 figure
Layered and core-shell uniaxial absorbers
We derive periodic multilayer absorbers with effective uniaxial properties similar to a perfectly matched layer (PML). This approximate representation of a PML is based on effective medium theory, and we call it an effective medium PML (EM-PML). We also show that cylindrical core-shell nanostructures derived from flat multilayers also exhibit very good absorptive and reflective properties despite the different geometry
Two-dimensional imaging in hyperbolic mediaâthe role of field components and ordinary waves
We study full vector imaging of two dimensional source fields through finite slabs of media with extreme anisotropy, such as hyperbolic media. For this, we adapt the exact transfer matrix method for uniaxial media to calculate the two dimensional transfer functions and point spread functions for arbitrary vector fields described in Cartesian coordinates. This is more convenient for imaging simulations than the use of the natural, propagation direction-dependent TE/TM basis, and clarifies which field components contribute to sub-diffraction imaging. We study the effect of ordinary waves on image quality, which previous one-dimensional approaches could not consider. Perfect sub-diffraction imaging can be achieved if longitudinal fields are measured, but in the more common case where field intensities or transverse fields are measured, ordinary waves cause artefacts. These become more prevalent when attempting to image large objects with high resolution. We discuss implications for curved hyperbolic imaging geometries such as hyperlenses