Broadband layered absorbers are analysed theoretically and experimentally. A genetic algorithm is used to opti- mize broadband and wide-angle of incidence metal-dielectric layered absorbers. An approximate representation of the perfectly matched layer with a spatially varied absorption strength is discussed. The PML is realised as a stack of uniform and isotropic metamaterial layers with permittivieties and permeabilities given from the effective medium theory. This approximate representation of PML is based on the effective medium theory and we call it an effective medium PML (EM-PML).1 We compare the re ection properties of the layered absorbers to that of a PML material and demonstrate that after neglecting gain and magnetic properties, the absorber remains functional.
In photovoltaic devices, metal nanoparticles embedded in a semiconductor layer allow the enhancement of solar-toelectric energy conversion efficiency due to enhanced light absorption via a prolonged optical path, enhanced electric fields near the metallic inclusions, direct injection of hot electrons, or local heating. Here we pursue the first two avenues. In the first, light scattered at an angle beyond the critical angle for reflection is coupled into the semiconductor layer and confined within such planar waveguide up to possible exciton generation. In the second, light is trapped by the excitation of localized surface plasmons on metal nanoparticles leading to enhanced near-field plasmon-exciton coupling at the peak of the plasmon resonance. We report on results of a numerical experiment on light absorption in polymer- (fullerene derivative) blends, using the 3D FDTD method, where exact optical parameters of the materials involved are taken from our recent measurements. In simulations we investigate light absorption in randomly distributed metal nanoparticles dispersed in polyazomethine-(fullerene derivative) blends, which serve as active layers in bulkheterojunction polymer solar cells. In the study Ag and Al nanoparticles of different diameters and fill factors are diffused in two air-stable aromatic polyazomethines with different chemical structures (abbreviated S9POF and S15POF) mixed with phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl ester (PC<sub>71</sub>BM). The mixtures are spin coated on a 100 nm thick Al layer deposited on a fused silica substrate. Optical constants of the active layers are taken from spectroscopic ellipsometry and reflectance measurements using a rotating analyzer type ellipsometer with auto-retarder performed in the wavelength range from 225 nm to 2200 nm. The permittivities of Ag and Al particles of diameters from 20 to 60 nm are assumed to be equal to those measured on 100 to 200 nm thick metal films.
We report on measurements of optical, morphological and electrical properties of silver nanolayers. The Ag films of thickness from 10 to 500 nm are deposited in e-beam evaporator. Fused silica and sapphire substrates are used with nominal root-mean-square (RMS) roughness equal 0.3 and 0.2 nm, respectively. Silver is deposited either directly on substrates or on Ge, Ni, or Ti wetting interlayer. The refractive index <i>n</i> and the extinction coefficient κ of Ag films are derived from spectroscopic ellipsometry and reflectance measurements carried in air in the spectral range from 0.6 to 6.5 eV (2200 – 193 nm) using a rotating analyzer ellipsometer (V-VASE, J.A. Woollam Co.). Surface roughness is measured using AFM (Ntegra NT-MDT) under tapping mode in air with sharp etalon probes and 5:1 aspect ratio. Ag layers of 10 and 30 nm thickness have nearly the same RMS roughness when deposited at temperatures from 180 to 350 K. The lowest RMS=0.2 nm is achieved for 10 nm film Ag/Ge evaporated at 295 K. The sheet resistance of the Ag films is measured using two methods: the van der Pauw method with the electrical contacts located on perimeters of the samples and four probes contacting the samples at points lying in a straight line. Specific resistivity of Ag films on fused silica change from <10<sup>9</sup> to 1.80 [μΩ∙cm] when thickness increases from 10 to 500 nm. Specific resistivity of 10, 30 and 50 nm thick Ag films on 1 nm Ge wetting layer are equal 14.01, 7.89, and 5.58 [μΩ∙cm], respectively, and are about twice higher than those of Ag films on Ti or Ni interlayers.
This research is motivated by our interest in fabrication of plasmonic single metal layer and metal-dielectric multilayer nanolenses with resolution exceeding the diffraction limit. Nanolayers of noble metals are evaporated in an e-beam physical vapour deposition machine on smooth substrates at temperatures controlled in the range 90÷300 K. For dielectric nanolayers ion assisted deposition is used. Thin films of Ag are deposited on polished fused silica and sapphire substrates. To reduce island growth substrate cooling and wetting layers are used. Our aim is to find deposition conditions when influence of thermal expansion mismatch on smoothness of deposited layers can be diminished. Quality of surfaces is assessed using standard deviation of average roughness measured with atomic force microscope for films deposited at different rates and different temperatures.
Propagation of light through layered metamaterials consisting of a metal-dielectric stack may be described as linear spatial filtering. We present the modelling and optimization strategy for engineering such metamaterials, as well as the measurement results of spatial filters consisting of titanium oxide and silver layers evaporated with PVD. Depending on the point spread function, the metamaterial can be applied for subdiffraction spatial filtering or for classical spatial filtering operations. We optimize the metamaterial with respect to the shape of the complex amplitude transfer function, the average transmission coefficient and to average reflections. The shape of the point spread function can only be tailored in a limited degree, due to the limited number of the degrees of freedom contained in the structure, and only in one, planarly or radially oriented dimension. The metamaterial optimised for high-pass filtering consists of several substructures, each of which is an individual cavity, and is optimized by tuning the resonance order of these cavities. In this way we obtain a high transmission for a broad range of spatial frequencies. This metamaterial can be applied to modify the contrast of the object or to introduce a phase-contrast. It may be used for far-field imaging. As an example, we propose to apply it as a novel phase-step visualization photonic element.
Interest in plasmonic lenses dates back to the seminal paper of Pendry [Phys. Rev. Lett. 85, 3966 (2000)] who has shown that superresolution is possible due to imaging through a negative-refractive-index material. Experimental verifications of near-field to near-field imaging properties of a single Ag nanolayer have proven that a resolution reaching one-sixth of the illumination wavelength is possible. The images have been recorded in a photoresist spin-coated onto an Ag layer. In this paper, images are recorded using a scanning near-field optical microscope (SNOM) working in the transmission mode with tapered-fibre metal-coated probes and aperture diameters <_ 100 nm. This recording method allows for separate recording of monochromatic images from the same lens, here we report on samples illuminated using the 404 nm mercury line. Moreover, with SNOM recording several uses of a single lens are possible. We consider dependence of the resolution on the roughness of the outer surface in the following multilayers: Ag/Ge/sapphire, Cr/sapphire, and Ag/SiO<sub>2</sub>/Cr/sapphire. Further research on reduction of chromium layer roughness is necessary.
In the recent decade metamaterials with magnetic permeability different than unity and unusual response to the magnetic
field of incident light have been intensively explored. Existence of magnetic artificial materials created an interest in a
scanning near-field magnetic microscope for studies of magnetic responses of subwavelength elementary cells of those
metamaterials. We present a method of measuring magnetic responses of such elementary cells within a wide range of
optical frequencies with single probes of two types. The first type probe is made of a tapered silica fiber with radial
metal stripes separated by equidistant slits of constant angular width. The second type probe is similar to metal coated,
corrugated, tapered fiber apertured SNOM probe, but in this case corrugations are radially oriented. Both types of probes
have internal illumination with azimuthally polarized light. In the near-field they concentrate into a subwavelength spot
the longitudinal magnetic field component which is much stronger than the perpendicular electric one.
Aperture probes of scanning near-field optical microscopes (SNOM) offer resolution which is limited by a sum
of the aperture diameter at the tip of a tapered waveguide probe and twice the skin depth in metal used for
coating. An increase of resolution requires a decrease of the aperture diameter. However, due to low energy
throughput of such probes aperture diameters usually are larger than 50 nm. A groove structure at fiber
core-metal coating interface for photon-to-plasmon conversion enhances the energy throughput 5-fold for Al
coated probes and 30-fold for Au coated probes due to lower losses in the metal. However, gold coated probes
have lower resolution, first due to light coupling from the core to plasmons at the outside of the metal coating,
and second due to the skin depth being larger than for Al. Here we report on the impact of a metal bilayer
of constant thickness for coating aperture SNOM probes. The purpose of the bilayer of two metals of which
the outer one is aluminum and the inner is a noble metal is to assure low losses, hence larger transmission.
Using body-of-revolution finite-difference time-domain simulations we analyze properties of probes without
corrugations to measure the impact of using a metal bilayer and choose an optimum bi-metal configuration.
Additionally we investigate how this type of metalization works in the case of grooved probes.
We consider two kinds of plasmonic nanolenses which focus radially polarized Laguerre-Gauss beam into
subwavelength spot. The first one is free-standing opaque metal layer with concentric grooves on both sides [Phys. Rev.
Lett. 102, 183902 (2009)]. The second has slits instead of grooves thus concentric rings have to be integrated with
dielectric matrix. Constructive interference of far-field radiation of SPPs scattered on the back side of the lenses gives
subwavelength size foci approaching the Rayleigh resolution limit. We investigate transmission and focusing properties
of considered metal structures. Choice of appropriate metal such as silver, gold, copper or aluminum strongly affects
transmission. Parameters of surface structure determine efficient photon-plasmon coupling and plasmon scattering
phenomenon thus influence both transmission and focusing effect. Finally, the choice of dielectric function of
surrounding medium gives another degree of freedom to fulfill momentum matching condition for resonant photonplasmon
interaction. In this paper, taking into account the above parameters, we show an optimization procedure, which
leads to high transmission, tight focal spot and large focal length of the considered plasmonic nanolenses.
Rapid development of novel, functional metamaterials made of purely dielectric, plasmonic, or composite
structures which exhibit tunable optical frequency magnetic responses creates a need for new measurement
techniques. We propose a method of actively measuring magnetic responses, i.e. magnetic dispersion, of such
metamaterials within a wide range of optical frequencies with a single probe by exciting individual elementary
cells within a larger matrix. The probe is made of a tapered optical fiber with a radially corrugated metal
coating. It concentrates azimuthally polarized light in the near-field below the apex into a subwavelength
size focus of the longitudinal magnetic field component. An incident azimuthally polarized beam propagates
in the core until it reaches the metal stripes of constant angular width running parallel to the axis. For a
broad frequency range light-to-plasmon coupling is assured as the lattice constant changes with the radius
due to constant angular width. Bound plasmonic modes in slits between the metal stripes propagate toward
the apex where circular currents in stripes and displacement currents in slits generate a strong longitudinal
magnetic field. The energy density of the longitudinal magnetic component in the vicinity of the axis is
much stronger than that of all the other components combined, what allows for pure magnetic excitation of
magnetic resonances rather than by the electric field. The scattered signal is then measured in the far-field
We present a method of fabricating aperture tapered-fiber metal-coated SNOM probes with a corrugated core surface
which assures efficient photon-to-plasmon conversion and thus high energy throughput. High energy throughput allows
for a small apex aperture and high resolution. The procedure consists of recording of Bragg grating in the to-be-tapered
part of a Ge-doped silica fiber and chemical etching with the Turner method. Bragg gratings are recorded with UV light
through nearly sinusoidal phase masks of chosen lattice constants. The refractive index contrast in the Bragg grating
differentiates the etch rate of the Ge-doped hydrogenated fiber core in exposed and unexposed parts by about 100
nm/min at room temperature.
Recently, 3D silver nanolenses with concentric slits with an on-axis stop and with concentric corrugations on both
surfaces and no hole on the optical axis were proposed. The nanolenses illuminated with a radially polarized visible
range Laguerre-Gauss beam focus light into subwavelength spots and act as high numerical aperture refractive optical
systems. Focal lengths range from one to a few wavelengths. Due to constructive interference of far-field radiation of
SPPs generated on the back side the lens focuses without contribution of the evanescent field. In this technical note we
investigate transmission and focusing properties of lenses of both kinds made of different metals: silver, gold, copper,
Resolution of scanning near-field optical microscopes is limited by a sum of the aperture diameter at the tip of a
tapered waveguide probe and twice the skin depth in metal used for coating. To increase the resolution we need
to decrease the aperture diameter, to this end increase of energy throughput is necessary. Recently, we proposed
that the interface between the fiber core and metal coating is structured into parallel grooves of different profiles
curved inward the core. The role of grooves is to facilitate conversion of photons to plasmons at the core -
cladding interface. In this paper we prove that a singe groove is enough to increase energy throughput by 500%
in the case of aluminum cladding and 3000% for gold cladding over probes without corrugations. Moreover, one
groove assures better transmission than a set of 10 grooves. Our investigations of aluminum, copper and gold
coated probes are carried out using finite-difference time-domain simulations within the optical range 400-700
nm, a computational volume equal 30μm<sup>3</sup>, and discretisation step 0.5 nm.
In a numerical experiment, we optimise performance of plasmonic lenses with different structures of single metal
nanolayers. The nanolenses, either with double sided grooved or with slits act like classical, high-numerical aperture,
refractive objectives. Their focal regions are well defined and different from those of diffractive optical elements. The
narrowest rotationally symmetric foci are achieved for a Laguerre-Gauss intensity profile with radial polarization. The
highest transmission reaching 80% is achieved for high slit width-to-lattice constant ratios when light is waveguided in
annular slits. In grooved and continuous metal lenses transmission reaches 30% due to resonant tunnelling of plasmons.
Location of slit/groove edges, which act as sources of spherical waves, and light intensity at them decides on interference
of radial and longitudinal electric field components in focal region. Proper choice of lattice constants and surface
structure allows for focal length several times larger than the free space light wavelength. All simulations are made using
body-of-revolution finite difference time domain method and Drude model parameters of silver. In simulations we accept
parameters of the nanolenses which are possible to fabricate with technical equipment available to us.
We characterise two geometries of silver-dielectric layered or single layer patterned lenses for subwavelength
imaging in the visible spectral range. The first consists of a periodic multilayer operating for the TM polarisation
in a planar geometry, and the other is a grooved structure with rotational symmetry operating for the radial
polarisation. For the multilayer superlens, diffraction-free propagation is conditioned on the phase flatness of the
transfer function. Low-loss, diffraction-free transmission is demonstrated at micrometer distances and compared
to diffractive propagation involving evanescent waves. The silver single layer lens, in turn, has double-sided
grooves and no on-axis aperture. In another version the single layer lens has slits and no on-axis aperture, all
rings and a stop are integrated with a fiber. Both lenses focus a far-field source into a far-field spot. They
perform like a high numerical aperture optical objective and obey the diffraction limit.
Superfocusing of light, far better than the diffraction limit, is of crucial importance for scanning near-field optical
microscopy (SNOM), optical chemical sensing, and nanolithography. For SNOM applications there are two typical
geometries. The first are tapered-fiber metal-coated aperture probes, which are being constantly improved. The other are
tapered metal or metal-coated apertureless tips, which are continuously brought to perfection. We propose a modification
of a metal-coated fiber tip, which has an additional, thin, dielectric coating with refractive index greater than that of air,
what leads to higher field enhancement at the tip. The excitation signal is an internal, radially polarized Laguerre-Gauss
beam. There is no sound theoretical model to describe nanofocusing of plasmons and we limit the scope of investigations
to body-of-revolution finite-difference time-domain (BOR FDTD) simulations using in-house code. We find that with an
increase of the refractive index of nanocladdings the maximum enhancement occurs for increasingly longer wavelengths.
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 0.15λ<sup>2</sup>. 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.
Development of nanotechnologies demands optical characterization and measurement techniques that yield information with resolutions well below the diffraction limit. This requires an increase of the resolution of scanning near-field optical microscopes (SNOMs) from 50-70 nm commercially available nowadays in the visible range, to beneficial 30 nm, where λ is the wavelength of light in free space. High resolution SNOM probes would be crucial in measurements of point spread functions of superlenses based on negative refraction and characterization of plasmonic circuitry.
The resolution of SNOMs is ▵r = d + 2a, where d is the diameter of a radiating aperture of a tapered-fiber metal-coated probe and a is a skin depth, that is the distance the electromagnetic field penetrates the metal coating. The size of the radiated field does not exceed the diameter ▵r when the aperture-sample distance h is kept constant by the shear-force tuning fork method. One of the resolution parameters, the skin depth a, depends on the metal that coats the dielectric probe and the shape of the metal rim. For Ag and Al, the values of a are on the level of 10nm, when measured on a flat metal surface illuminated with a plane wave. Thus, the other resolution parameter which we intend to decrease is a probe diameter d. The probe should radiate enough energy to be detected in a reasonable scanning measurement time. Recently, we proved that probe emission depends on the charge density induced on the probe rim. To increase this density we propose enhancement of the photon-plasmon coupling on the interface between the dielectric core and the metal coating. To this end we corrugate the interface. In this paper we analyze the role of parameters of the corrugations and report on attempts to fabricate them.