Sinusoidal gratings often in form of Gabor patches are used in popular contrast sensitivity tests. They're presented in one, three or six positions. Increasing the number of positions allows for decrease of the impact of the residual astigmatism and other non-rotationally symmetric aberrations of the eye on the results, but at the same time it makes the examination longer. Patients may get weary and answer incorrectly. The purpose of this study is to check whether using rotationally symmetric targets modulated by a Bessel profile leads to a reduction of the examination time while preserving the low sensitivity to astigmatism. Two types of patterns of different spatial frequencies, that are static Gabor patches and targets modulated by a Bessel profile, were presented to subjects as contrast sensitivity test targets. They were presented monocularly in an increasing contrast procedure to avoid afterimages effect. Forty subjects were healthy, without refractive errors or with the best possible vision correction and visual acuity 1.0 or higher. As a result, contrast sensitivity thresholds was measured and compared. The usability of Bessel modulated tests was confirmed.
We present a numerical study of the dispersion characteristic modification in a nonlinear photonic crystal fibre (PCF) infiltrated with organic solvents. The PCF is made of PBG08 glass and was developed in the stack-and-draw process. The PBG08 glass has a high refractive index (n < 2.0), high nonlinear refractive index (n2 = 4.3×10−19 m2/W) and good rheological properties that allow for thermal processing of the glass without crystallization. In the numerical study 18 different solvents were used. The dispersion, mode area, and losses characteristics were calculated. The zero dispersion wavelength (ZDW) of the fibre can be shifted towards longer wavelengths by approx. 150 nm by using Nitrobenzene as infiltrating liquid and by a smaller value using other liquids. At the same time the mode area of the fundamental mode increases by approx. 5 to 15% depending on the wavelength considered. The confinement losses increase significantly for six analysed liquids by a few orders of magnitude up to 102 dB/m. Our approach allows to combine high nonlinearities of the soft glass with the possibility to tune zero dispersion wavelength to the desired value.
To achieve high non-linearity in photonic crystal fibers a high nonlinear coefficient of the glass is required accompanied by high coupling efficiency and flat dispersion profile of the fiber with the specific zero dispersion wavelength. In this paper, we present a deterministic method that allow step-by-step design of photonic crystal fibers with desired zero dispersion wavelength, modality and coupling efficiency due to sequential engineering of geometrical parameters of microstructured fibers with nanostructured cores. The fiber consists of inclusions of low refractive index material, embedded in a host glass of higher refractive index, where a single central micro-rod is omitted. In its place an additional nano-inclusion is located of a given diameter. The choice of the glass determines the nonlinear coefficient of the fiber and fabrication possibilities as well. Zero dispersion wavelength is varied by the change of the lattice constant of the cladding. High filling factor in the cladding leads to a large number of propagating high order modes, which can be selectively cut off, when the filling factor of the outer part of the cladding is reduced. The diameter of the nano-inclusion in the core is responsible for the fundamental mode area, which influences directly the coupling efficiency. Several designed structures were modeled numerically and developed to confirm the design method.
In this paper we report a two octave spanning supercontinuum generation in the range 750-3000 nm with a newly
developed photonic crystal fiber. The fibre is fabricated using an in-house synthesized lead-bismuth-galate glass PBG08
with optimised rheological and transmission properties in the range 500-4800 nm. The photonic cladding consists of 8
rings of air holes with a fibre core diameter of 3 μm and a lattice constant of 2.2 μm. The dispersion characteristic is
determined mainly by the material dispersion and the first ring of holes in the cladding with a filling factor of 0.68. The
filling factor of the remaining 7 rings is 0.45 which allows single mode performance of the fibre in the infrared range.
The fibre has a zero dispersion wavelength of 1490 nm which allows the use of 1550 nm wavelength as an efficient
pump in the anomalous dispersion regime. The 2 cm long sample of photonic crystal fiber is pumped in the femtosecond
regime with a pulse energy of 10 nJ at a wavelength of 1550 nm. A flatness of 5 dB is observed in the spectral range 950-2500 nm.
The development of all-solid photonic crystal fibers for nonlinear optics is an alternative approach to the air-glass solid
core photonic crystal fibers. The use of soft glasses ensures a high refractive index contrast (>0.1) and a high nonlinear
coefficient of the fibers. In addition, the manipulation of the subwavelength structure of the core of a photonic crystal
fiber allows significant modification of its dispersion characteristics and efficient generation of supercontinuum with
various femtosecond and nanosecond sources. The development of all-solid photonic crystal fiber allows very accurate
control of all the parameters of the developed fiber in very good agreement with the design criteria.
In this paper, we report on the dispersion management capabilities in all-solid photonic crystal fibers with nanostructured
cores using thermally matched glasses, which can be jointly processed using the stack-and-draw fiber fabrication
technology. We consider a photonic crystal fiber made of the high index lead-silicate glass SF6 and the in-house
synthesized low index silicate glass NC21. The NC21 glass plays the role of low index inclusion in the photonic cladding
and a nano-inclusion in the core of the fiber. The final dispersion profile of the photonic crystal fiber is determined by
the low index nano-inclusion in the core with diameter in the range 100-500nm. The dispersion profiles are modeled for
a theoretical structure and for the developed fiber. Supercontinuum generation is expected and numerically confirmed for
the developed fiber in the range 1150-1500nm with flatness below 1dB. The fiber is dedicated for supercontinuum
generation with 1550nm laser sources.
We present the development of a large core multimode photonic crystal fibre with hyperspectral transmission that covers
the visible, near and (in part) mid infra-red wavelength ranges (400-6500 nm). We have optimised the composition of a
heavy metal-oxide glass based on the PbO-Bi2O3-Ga2O3 system modified with Nb2O5, Ta2O5, SiO2, GeO2, BaO, CdO, Na2O
and K2O. The optimised glass shows good transmission up to 6 μm as well as good rheological properties that permits
multiple thermal processing steps in an optical drawing tower without crystallisation. The selected glass is synthesized inhouse
and has been used for fibre development. We have fabricated a multi-mode photonic crystal fibre with an effective
mode area of 295 μm2. The photonic cladding is composed of 8 rings of air holes with a fill factor of 0.46. The transmission of a hyperspectral spectrum is experimentally verified using a broadband source. The attenuation of the fibre and its
sensitivity to bending losses is presented.
In the paper metamaterial nanotips with multi-frequency local field enhancement are proposed and studied
theoretically and numerically. The nanotips are in form of pyramids built from silver nanoplates embedded in
a dielectric block. The pyramids are layered structures with subsequent plates parallel to the pyramid base.
Thickness of nanoplates and their transversal size is chosen to efficiently convert the light impinging on the
pyramid base into hot spots near the pyramid apex, and also to support large number of plasmonic resonances
which allow for multi-frequency enhancement of the light intensity. Geometrical parameters are designed to cover
the visible light range.
In this paper we study the propagation of light through silver-dielectric metamaterial layered prism which
operates in the canalization regime. The prism is illuminated with TM-polarized light and is designed using the
effective medium theory as strongly anisotropic and impedance matched to air. The structure has an infinite
value of the effective permittivity in the direction perpendicular to layer surfaces. Therefore it is able to couple a
broad spectrum of incident spatial frequencies, including evanescent waves, into propagating modes. As a result,
subwavelength resolution at the output interface of the structure is observed. Further the device is characterised
with the transfer matrix method (TMM), and investigated with Finite Difference Time Domain method (FDTD).
Two parameters of the prism are studied, namely the angle of incidence and the apex angle, to obtain the best
In this paper we present technical details of a metal nanolens in the form of a free standing silver film with no hole on the
optical axis and double-sided concentric corrugations. In a numerical experiment we analyze the nanolens performance,
that is transmission and focusing of radially polarized beams of different full widths at half maximum and wavelengths
from the visible range. Corrugations of the front surface couple incident light to surface plasmons and those on the back
surface allow efficient reradiation. The silver lens of thickness 100 nm has five concentric corrugations of periodicity
500 nm with groove depth and width equal 40 nm and 100 nm, respectively. Focusing properties of such a structure are
analyzed and optimized for wavelengths in the range from 400 to 600 nm. At intensity transmission of 10-25% of
incident light achievable focal spot areas reach down to 0.15λ2. For different illumination parameters the nanolens has
focal lengths from 1 to 2 wavelengths. Without contribution of evanescent waves it focuses a far-field source into a farfield
spot. The nanolens acts like a refractive optical system of high numerical aperture close to unity. Nanolenses of this
kind can be used as light couplers in nanooptics.
Propagation of continuous-wave (CW) Gaussian beams through a RHM-LHM interface as well as a metamaterial
slab or a metallo-dielectric stack is subject to both spatial filtering and dispersive reshapement. Spatial filtering
is polarisation-dependent and results from diraction, multiple reflections, and multiple positive or negative
refractions on the layer boundaries. Structures with a broad and flat transmission spatial spectrum are capable
of imaging with resolution exceeding the diraction limit. Due to strong Drude-type dispersion and absorption of
the layers, it is possible to identify higher order dispersion eects taking place within very short distances. This
opens the possibility of designing novel filters for joint reshaping of the spatial and temporal pulse envelopes.
In the present work, we analyse the transmission of time-modulated CW Gaussian beam through structures
with dispersive and lossy layers. Depending on beam divergence and the attenuation strength of the layers,
the dominant eect may be identified as one-dimensional propagation with absorption and dispersion, negative
refraction with dispersive modulation of the wavefront, or two-dimensional diractive focusing. We propose
a theoretical model to explain the interplay of the observed spatial and dispersive phenomena. In the near
diraction field pulses are reshaped and broadened. The slopes of the pulses become aected with side-lobes, and
with a probable presence of superluminal eects. It is shown that the second-order dispersion eects do not yet
provide a suciently accurate model for the pulse evolution. The simulations rely on a rigorous quasi-analytical
planewave-based model of propagation. Transfer Matrix method (TMM) is used for numerical calculations.
Two-dimensional imaging through a layered metallic flat lens involves coupling of the TE and TM polarisations
that appear at the same time in the 2D spatial spectrum of the incident image. In effect the modulation transfer
function and the impulse response that characterise 2D imaging through a metallic multilayer both have a matrix
form and cross-polarisation coupling is observed for most spatially modulated beams with a linear or circular
incident polarisation. Our present analysis is focused on these 2D cross-polarisation effects. In particular
we investigate the role of singularities in the MTF and their relation to the regularisation problems for the
respective 2D point spread functions. The analysis is based on transfer matrix method without the quasi-static
approximation or scalar field approximation.
We analyse the silver multilayer superlens as a modulating photonic device for spatial reshapment of circularly
polarised light beams. The silver flat lens of Pendry and its generalisations to multilayer structures have been
primarily introduced for in-plane perfect near-field imaging that includes a limited range of evanescent spatial
harmonics of the field. In this paper, using the fully vectorial transfer matrix method we analyse the two-dimensional
impulse response of the imperfect superlens for spatial and temporal filtering of the optical signal
and on this basis investigate the way it modifies the shape of the beam.
We present a review of recent achievements in nanoscale optical devices based on energy transport with surface
plasmon-polaritons and localized surface plasmons. Chains of metal subwavelength-size particles and stripes are used to
build straight waveguides, s-bends, y-junctions and beam shaping devices. Strong enhancement of near-field in
nanogaps between particles leads to efficient light emission from such nanoantennas. Development of surface plasmon
nanoptics stimulates further progress in near-field imaging. To improve resolution of scanning near-field optical
microscope (SNOM) it is necessary to improve light throughput in tapered metal-coated SNOM probes. This is
achievable due to resonant surface plasmons that propagate in corrugated probes.
Using the transfer matrix method we analyse the transmission of time-modulated Gaussian beam through the
RHM-LHM layered structures with dispersive, lossy, and magnetic layers. The modulated parts of the beam
are subject to a complex temporal and spatial shape transformation, which may be characterised in terms of
spectral and spatial filtering, while the layered structure itself is entirely described with the respective transfer
function or equivalently with the impulse response. Due to strong dispersion and absorption of the LHM layers,
it is possible to identify higher order dispersion effects taking place within very short distances. This opens the
possibility of designing novel filters capable of complex reshaping of the beam envelope.
Wave fronts of a Gaussian light pulse and a CW Gaussian beam refracted at the boundary of right- and left-handed
media are modified due to material dispersion. Group velocity of a negatively refracted pulse is parallel to the direction
of energy transport and anti-parallel to phase velocity. Positively refracted group front moves sideways with respect to
negatively refracted phase front. We analyze modifications of light pulses of different spectra in effective media with
different frequency dispersion curves. Analysis of dispersion effects is important because of possible applications in
band separation and pulse compression in metamaterial devices. In 2D FDTD simulations we analyze evolution of CW
Gaussian beams and short Gaussian pulses in an effective double negative material with dependence of permittivity and
permeability functions on frequency corresponding to Drude material-dispersion model.
We present recent achievements in fabricating a two-dimensional (2D) photonic crystal in the form of a bundle of parallel micro- or nanowires embedded in glass matrix. The method is similar to that of sequential thinning used for fabrication of photonic crystal fibers. We discuss technological issues that aim at preservation of regularity of photonic crystal lattice and uniformity of wire diameters. Proper selection of a melting point of metal alloy and the range of temperatures of glass viscosity leads to reduction of regularity losses resulting from sequential processes of drawing. Measured distributions of crystal lattices, wire diameters and shapes of wires are used to simulate photonic band structure of fabricated crystals. This work is directed toward fabrication of a photonic crystal showing the negative refraction in the near infrared and visible spectral range.
The idea of a substance with simultaneously negative values of dielectric permittivity ε and magnetic permeability μ
presented by Veselago in 1968 has been brought to reality. Firstly, negative permittivity ε(ω) of a three dimensional
photonic structure composed of thin metal wires was experimentally demonstrated in the GHz range. Secondly, a
concept of split ring resonator has appeared and a structure composed of such metal resonators was shown to have
negative permeability μ. Consequently, in a so called double negative, both ε(ω) and μ(ω) < 0, composite material made
of cells consisting of a split ring resonator and a wire unnatural phenomenon of negative refraction was experimentally
observed in the microwave spectral region. Recently, perfect lenses made of metamaterial with negative refraction index,
photonic crystal or metal slabs were used to focus light below the diffraction limit of resolution. Electromagnetic
transport of energy in plasmon waveguides made of subwavelength metallic elements offers a great potential value for
nanoscale photonic devices of the future.
We examine the propagation of energy along chains of silver nanoelements oriented perpendicularly to the flow of light
and ordered in several ways. The first chain is composed ofvertical silver nanorods arranged in a hexagonal lattice. The
second one consists of vertical elongated nanoplates that form a herring-bone pattern. In the third, distribution of
vertically oriented nanoplates recalls footsteps. The chains are embedded in a medium with refractive index n = 1 and
1.5. Incident polarized Gaussian beams propagate along chains of nanoelements and have electric field components
oriented transversally with respect to the vertical nanoelements. Transport of energy is investigated with the Finite
Difference Time Domain (FDTD) method for visible and infrared range ofwavelengths, where the Drude model is valid.
Propagation constants and attenuation factors are calculated. Losses are due to absorption in metal and light scattering on
structure elements. In the analyzed structures, energy is transported due to localized surface plasmons-polaritons, where
the amplitude of optical fields is locally enhanced by orders of magnitude. This property might be useful in the
construction of nanoscale photonic devices. The smaller the metallic elements are, the stronger is the concentration of
energy. Waveguides of that form may be used for creating a medium with novel effective electromagnetic properties.
Interest in photonic nanodevices motivates search for efficient transport of energy in plasmon waveguides. Chains of silver nanoelements guide light in channels of below-the-diffraction-limit size due to surface plasmon coupling. We calculate attenuation factors in chains with several geometries of nanoplates using the Finite Difference Time Domain (FDTD) method for visible and near infrared range of wavelengths, where the Drude model of dispersion is valid. Nanoplates considered in simulations are 1 micrometer high, 50 nm thick and 380 nm long and are embedded in a medium with refractive index reaching n = 1.5. Advantages of proposed waveguides are connected with their small size and possible tuneability by adjustment of geometrical parameters. However, the waveguides highly attenuate signals due to radiation into the far field and internal damping. For the optimum considered geometry and 595 nm wavelength, the energy transmission of 2 micrometers long chain of parallel nanoplates reaches 39%.
A metal-in-dielectric metamaterial structure different from that composed of split-ring-resonators and wire units was proposed. The metamaterial layer is composed of randomly distributed parallel pairs of nanowires of subwavelength size that form electromagnetically active units. It was predicted that the metamaterial should exhibit macroscopic negative refraction. In a recent paper fabrication of the metamaterial in the form of periodic array of parallel golden nanorods with trapezoidal cross section was reported and a negative refractive index of n = -0.3 was observed at a wavelength 1.5 μm (200 THz).
In this paper we simulate response of a single pair of nanowires to near-infrared illumination and observe surface plasmon resonances using FDTD method. We simulate light propagation through the metamaterial slab made of one, two and three layers. In each layer the nanowires cover 10% of the surface. In simulations made for a single layer medium, negative refraction is observed for wavelengths from 1.55 to 2.1 μm, with Δλ/λ ≈ 0.3. When the number of layers increases, the range of negatively refracted wavelengths becomes narrower. For a narrow range of wavelengths that are close to the resonant frequency the intensity transmission of three layers reaches −7dB for the angle of incidence of 10°. Then layers with two orientations of nanowires are considered. In the first stack of layers all nanowires are oriented in parallel. This configuration assures plasmon resonances for both the electric and magnetic components of electromagnetic wave in all layers. In the second stack, nanowires in two subsequent layers are oriented perpendicularly. In the second layer, the plasmon resonance for the electric component of light is due to the oblique incidence of light. For a small angle of incidence of a near infrared narrow Gaussian beam we calculate two characteristics: the attenuation vs. wavelength and the lateral shift of the beam on the plane-parallel slab vs. wavelength. For a narrow range of wavelengths simulations show negative refraction of a beam incident the plane of the nanowires and a corresponding shift in the far field.
We present a method for elimination of global intensity deformation in gray-scale images using contrast control based on image resolution manipulation. During the process, a Gaussian pyramid representation of an input image is constructed by means of low-pass filtering and sampling of successive pyramid levels, where the input image
constitutes the first (zero) level of the pyramid. In the second step, a Laplacian pyramid is built, through subtracting
successive levels of the Gaussian pyramid. Then, all levels in the Laplacian pyramid are expanded to the original image size and added with weights, to reconstruct the image. An algorithm and a computer routines library written in object programming language C++ are developed.
We present a method for elimination of global intensity deformation in greyscale and color images of any size using contrast control. The method is based on image resolution manipulation and uses a representation of an image in the form of a difference-of-low-pass pyramid (DoLP). In the first step, a Gaussian pyramid representation of the input image is prepared through low-pass filtration and sampling of successive pyramid levels. In the second step, the DoLP pyramid is built, and finally all levels in the DoLP pyramid are expanded to the original image size and added with weights, to reconstruct the image. Proper choice of the weights is crucial for efficient elimination of global intensity deformation and leads to contrast enhancement at certain levels of the pyramid. Color images and images of a size different than 2N+1 x 2N+1, where N = 2, 3, ?., require additional processing. They are converted to and from hue-saturation-value (HSV) color space model and geometrically transformed, which can be performed using two proposed methods. An algorithm and a computer routines library written in object programming language C++ are developed. The proposed method is useful in digital archiving of airborne, scanned, photo-copied and optical camera-made photographs, degraded due to ageing processes.
Our aim is to build a digital elevation model (DEM) for the basin of Rega River, a tributary of the Baltic Sea, on a 0.5 x 0.5 m grid. It is based on hand-drawn topographical maps in 1:10,000 scale scanned with 508 dpi accuracy. Then a digital terrain model (DTM) results from integration of DEM with remotely sensed data (space and airborne images) and detailed geodata. In this paper, we describe algorithms for noise removal, thinning and continuing contour lines, and interpolation of elevation data used to process the topographical maps.