Multiscale analysis of subwavelength imaging with metal-dielectric multilayers

Imaging with a layered superlens is a spatial filtering operation characterized by the point spread function (PSF). We show that in the same optical system the image of a narrow sub-wavelength Gaussian incident field may be surprisingly dissimilar to the PSF, and the width of PSF is not a straightforward measure of resolution. FWHM or std. dev. of PSF give ambiguous information about the actual resolution, and imaging of objects smaller than the FWHM of PSF is possible. A multiscale analysis of imaging gives good insight into the peculiar scale-dependent properties of sub-wavelength imaging.Comment: 3 pages, 5 figures

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

Multilayer metamaterial absorbers inspired by perfectly matched layers

We derive periodic multilayer absorbers with effective uniaxial properties similar to perfectly matched layers (PML). This approximate representation of PML is based on the effective medium theory and we call it an effective medium PML (EM-PML). We compare the spatial reflection spectrum of the layered absorbers to that of a PML material and demonstrate that after neglecting gain and magnetic properties, the absorber remains functional. This opens a route to create electromagnetic absorbers for real and not only numerical applications and as an example we introduce a layered absorber for the wavelength of $8$~$\mu$m made of SiO$_2$ and NaCl. We also show that similar cylindrical core-shell nanostructures derived from flat multilayers also exhibit very good absorptive and reflective properties despite the different geometry

Differential real-time single-pixel imaging with Fourier domain regularization -- applications to VIS-IR imaging and polarization imaging

The speed and quality of single-pixel imaging (SPI) are fundamentally limited by image modulation frequency and by the levels of optical noise and compression noise. In an approach to come close to these limits, we introduce a SPI technique, which is inherently differential, and comprises a novel way of measuring the zeroth spatial frequency of images and makes use of varied thresholding of sampling patterns. With the proposed sampling, the entropy of the detection signal is increased in comparison to standard SPI protocols. Image reconstruction is obtained with a single matrix-vector product so the cost of the reconstruction method scales proportionally with the number of measured samples. A differential operator is included in the reconstruction and following the method is based on finding the generalized inversion of the modified measurement matrix with regularization in the Fourier domain. We demonstrate $256 \times 256$ SPI at up to $17~$Hz at visible and near-infrared wavelength ranges using two polarization or spectral channels. A low bit-resolution data acquisition device with alternating-current-coupling can be used in the measurement indicating that the proposed method combines improved noise robustness with a differential removal of the direct current component of the signal

Rigorous optical modelling of long-wavelength infrared photodetector with 2D subwavelength hole array in gold film

The quantum efficiency of an InAs/InAsSb type-II superlattice (T2SL) high operating temperature (HOT) long-wavelength infrared (LWIR) photodetector may be significantly improved by integrating a two-dimensional subwavelength hole array in a metallic film (2DSHA) with the detector heterostructure. The role of the metallic grating is to couple incident radiation into surface plasmon polariton (SPP) modes whose field overlaps the absorber region. Plasmon-enhanced infrared photodetectors have been recently demonstrated and are the subject of intensive research. Optical modelling of the three-dimensional detector structure with subwavelength metallic components is challenging, especially since its operation depends on evanescent wave coupling. Our modelling approach combines the 3D finite-difference time-domain method (FDTD) and the rigorous coupled wave analysis (RCWA) with a proposed adaptive data-point selection for calculation time reduction. We demonstrate that the 2DSHA-based detector supports SPPs in the Sommerfeld-Zenneck regime and waveguide modes that both enhance absorption in the active layer

Some considerations on the transmissivity of trirefringent metamaterials

Nonlocal effects in metal–dielectric (MD) periodic nanostructures may typically be observed when the plasmonic particles and gaps are on the scale of a few tens of nanometers, enabling under certain conditions (succinctly for epsilon near zero) a collimated beam to split into three refracted signals. We developed a method for precisely evaluating the categorized transmissivity in an air/trirefringent metamaterial interface, which uses a fast one-dimensional Fourier transform and finite element solvers of Maxwell’s equations. In periodic arrays of MD nanofilms, it is proved a tunable transmissivity switch of the multirefracted beams under varying angle of incidence and wavelength, while keeping reduced levels of reflectivity. Low-loss trirefringent nanomaterials may enable the development of ultracompact and tunable light splitters and modulators.Ministerio de Economía y Competitividad (MINECO) (TEC2013-50416-EXP)

Plasmon-enhanced high operating temperature infrared photodetectors

Plasmonic enhancement has a great potential for performance improvement of high operating temperature (HOT) photodetectors, especially those optimized for long-wavelength infrared (LWIR). Conventional HOT photodetectors exhibit poor quantum efficiency (QE) due to short carrier diffusion lengths of narrow bandgap semiconductors and relatively low absorption coefficients within the LWIR range. Plasmon-driven subwavelength light confinement enables high absorption even in a very thin absorber that provides efficient carrier collection, boosting the detector QE. We propose a photovoltaic detector equipped with a two-dimensional subwavelength hole array (2DSHA) in gold metallization on InAs/InAsSb type-II superlattice (T2SL) heterostructure. Our numerical study utilizing the finite-difference time-domain (FDTD) method predicts five times increased absorption in comparison with a conventional, back-side illuminated device. The simulated behavior of the plasmonic structure was confirmed experimentally by transmittance measurements, which revealed resonant features corresponding to various plasmonic modes