Rafal Kotynski

Asymmetric transmission of terahertz radiation through a double grating

We report on experimental evidence of unidirectional transmission of terahertz waves through a pair of metallic gratings with different periods. The gratings are optimized for a broadband transmission in one direction, accompanied with a high extinction rate in the opposite direction. In contrast to previous studies, we show that the zero-order nonreciprocity cannot be achieved. Nonetheless, we confirm that the structure can be used successfully as an asymmetric filter.

Comparison of imaging with sub-wavelength resolution in the canalization and resonant tunnelling regimes

We compare the properties of sub-wavelength 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, regularize 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 and 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.

Phase and group modal birefringence of triple-defect photonic crystal fibres

We discuss the phase and group modal birefringence in photonic crystal fibres (PCFs) with an elongated core. Owing to large form birefringence, these two types of birefringence in such PCFs may not only have opposite signs, but their absolute value can also differ by several orders of magnitude. We also show that PCFs offer the unique possibility of having a large phase birefringence and a negligible polarization mode dispersion at the same time. Using a fully vectorial mode solver, we show how these parameters can be tailored by a proper choice of the geometry of the PCF. We demonstrate both numerically and experimentally the strong wavelength dependence of phase and group modal birefringence in triple-defect photonic crystal fibres (PCFs).

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–Pérot resonances make it a flexible diffraction-free material for arbitrarily shaped optical planar elements with sizes of the order of one wavelength.

Optimized low-loss multilayers for imaging with sub-wavelength resolution in the visible wavelength range

We optimize the effective skin-depth and resolution of Ag-TiO2, Ag-SrTiO3, and Ag-GaP multilayers for imaging with sub-wavelength resolution. In terms of transmission and resolution, the optimized multilayers outperform simple designs based on combined use of effective medium theory, impedance matching and Fabry–Perot resonances. For instance, an optimized Ag-GaP multilayer consisting of only 17 layers, operating at the wavelength of 490 nm and having a total thickness equal to one wavelength, combines 78% intensity transmission with a resolution of 60 nm. It is also shown that use of the effective medium theory leads to sub-optimal multilayer designs with respect to the trade-off between the skin depth and resolution already when the period of the structure is on the order of 40 nm or larger.

Dual nonlinear correlator based on computer controlled joint transform processor : digital analysis and optical results

Abstract The hybrid optoelectronic processor, presented in this paper, realizes the dual nonlinear correlation (DNC) in a set-up based on a two-step nonlinear joint transform correlator architecture. In the first step three power spectrum distributions (input scene power spectrum, reference target power spectrum, and the joint power spectrum) necessary for the nonlinear processing are captured with a CCD camera. Nonlinear modification of the joint power spectrum, which does not have to be symmetrical in the input and reference channels, is introduced digitally. In the second step, the modified joint power spectrum is Fourier transformed optically. Numerical analysis of this processor shows a crucial influence of the dynamic range and the limited number of grey levels of the CCD camera during image acquisition in the first step, on the output signal parameters and the discrimination capability of the set-up. Optical results of recognition obtained for noise-free segmented input scenes show that the set-up ...

Two-dimensional point spread matrix of layered metal–dielectric imaging elements

We describe the change of the spatial distribution of the state of polarization occurring during two-dimensional (2D) imaging through a multilayer and in particular through a layered metallic flat lens. Linear or circular polarization of incident light is not preserved due to the difference in the amplitude transfer functions for the TM and TE polarizations. In effect, the transfer function and the point spread function (PSF) that characterize 2D imaging through a multilayer both have a matrix form, and cross-polarization coupling is observed for spatially modulated beams with a linear or circular incident polarization. The PSF in a matrix form is used to characterize the resolution of the superlens for different polarization states. We demonstrate how the 2D PSF may be used to design a simple diffractive nanoelement consisting of two radial slits. The structure assures the separation of nondiffracting radial beams originating from two slits in the mask and exhibits an interesting property of a backward power flow in between the two rings.

Sensitivity of imaging properties of metal-dielectric layered flat lens to fabrication inaccuracies

We characterize the sensitivity of imaging properties of a layered silver-TiO2 flat lens to fabrication inaccuracies. The lens is designed for approximately diffraction-free imaging with subwavelength resolution at distances in the order of a wavelength. Its operation may be attributed to self-collimation with a secondary role of Fabry-Perot resonant transmission, even though the first order effective medium description of the structure is inaccurate. Super-resolution is maintained for a broad range of overall thicknesses and the total thickness of the multilayer is limited by absorption. The tolerance analysis indicates that the resolution and transmission efficiency are highly sensitive to small changes of layer thicknesses.

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.

Optical correlator with dual nonlinearity

Abstract A generalization of the main linear and nonlinear correlation methods, the dual nonlinear correlation (DNC), is introduced. Various filters (CMF, POF, IF, FPF, PPC, OF) appear to be special cases of the DNC. The convolution formula describing the peak shape and the intermodulation effects for the noise-free multi-object scenes is discussed. Optimization of the DNC according to the peak-to-correlation energy criterion is performed. Superior performance of the PPC is presented. The influence of noise on the intermodulation effects is studied. Hybrid, optoelectronic realization of the DNC based on joint transform correlator architecture with nonlinear preprocessing which allows the introduction of various correlation methods without any fiters is proposed.