The resonance conditions of localized surface plasmon resonances (LSPRs) can be perturbed in any number ways making plasmon nanoresonators viable tools in detection of e.g. phase changes, pH, gasses, and single molecules. Precise measurement via LSPR of molecular concentrations hinge on the ability to confidently count the number of molecules attached to a metal resonator and ideally to track binding and unbinding events in real-time. These two requirements make it necessary to rigorously quantify relations between the number of bound molecules and response of plasmonic sensors. This endeavor is hindered on the one hand by a spatially varying response of a given plasmonic nanosensor. On the other hand movement of molecules is determined by stochastic effects (Brownian motion) as well as deterministic flow, if present, in microfluidic channels. The combination of molecular dynamics and the electromagnetic response of the LSPR yield an uncertainty which is little understood and whose effect is often disregarded in quantitative sensing experiments. Using a combination of electromagnetic finite-difference time-domain (FDTD) calculations of the plasmon resonance peak shift of various metal nanosensors (disk, cone, rod, dimer) and stochastic diffusion-reaction simulations of biomolecular interactions on a sensor surface we clarify the interplay between position dependent binding probability and inhomogeneous sensitivity distribution. We show, how the statistical characteristics of the total signal upon molecular binding are determined. The proposed methodology is, in general, applicable to any sensor and any transduction mechanism, although the specifics of implementation will vary depending on circumstances. In this work we focus on elucidating how the interplay between electromagnetic and stochastic effects impacts the feasibility of employing particular shapes of plasmonic sensors for real-time monitoring of individual binding reactions or sensing low concentrations – which characteristics make a given sensor optimal for a given task. We also address the issue of how particular illumination conditions affect the level of uncertainty of the measured signal upon molecular binding.
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.
Interaction of light with metals in the form of surface plasmons is used in a wide range of applications in which the scattering decay channel is important. The absorption channel is usually thought of as unwanted and detrimental to the efficiency of the device. This is true in many applications, however, recent studies have shown that maximization of the decay channel of surface plasmons has potentially significant uses. One of these is the creation of electron-hole pairs or hot electrons which can be used for e.g. catalysis. Here, we study the optical properties of hetero-metallic nanostructures that enhance light interaction with the catalytic elements of the nanostructures. A hybridized LSPR that matches the spectral characteristic of the light source is excited. This LSPR through coupling between the plasmonic elements maximizes light absorption in the catalytic part of the nanostructure. Numerically calculated visible light absorption in the catalytic nanoparticles is enhanced 12-fold for large catalytic disks and by more 30 for small nanoparticles on the order of 5 nm. In experiments we measure a sizable increase in the absorption cross section when small palladium nanoparticles are coupled to a large silver resonator. These observations suggest that heterometallic nanostructures can enhance catalytic reaction rates.
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-C61-butyric acid methyl ester (PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). 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 the optical properties of plasmonic glasses which are metal-dielectric composites composed of metallic inclusions in a host dielectric medium. The investigated structures are of quasi-random nature, described by the pair correlation function, featuring a minimum center-to-center distance between metallic inclusions and long range randomness. Plasmonic glasses exhibiting short-range order only may be fabricated using bottom-up, self-assembly methods and have been utilized in a number of applications such as plasmonic sensing or plasmon-enhanced solar harvesting, and may be also employed for certain non-linear applications. It is therefore important to quantify their properties. Using theoretical methods we investigate optical of 1D, 2D, and 3D structures composed of amorphous distributions of metallic spheres. It is shown, that the response of the constituent element, i.e. the single sphere localized surface plasmon resonance, is modified by the scattered fields of the other spheres in such a way that its peak position, peak amplitude, and full-width at half-maximum exhibit damped oscillations. The oscillation amplitude is set by the particle density and for the peak position may vary by up to 0.3 eV in the optical regime. Using a modified coupled dipole approach we calculate the effective (average) polarizability of plasmonic glasses and discuss their spectra as a function of the dimensionality, angle of incidence and polarization, and the minimum center-to-center distance. The analytical model is complemented and validated by T-Matrix calculations of the optical cross-sections of amorphous arrays of metallic spheres obtained using a modification of the Random Sequential Adsorption algorithm for lines, surfaces, and volumes.
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 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.
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
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 30μm3, and discretisation step 0.5 nm.
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,
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.
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.
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.
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.
Interest in optical devices that image with superresolution and inherent optical parallelism continues. Recently, the
concept of superresolution is pursued along the lines of negative refraction and transparent multilayer, metallo-dielectric
photonic band gap structures. Flat superlenses image from the near-field to the near- or far-field with resolution beyond
the diffraction limit. There is a need for characterization methods which allow measurement of the point spread function
of such devices. Scanning near-field microscopes (SNOMs) measure the field intensity in the vicinity of objects as close
as 5 nm due to shear-force technique. Improvement of transversal resolution up to λ/20 may be possible due to
considerable improvement of energy throughput of SNOM probes. To this aim we propose to corrugate the dielectric
core-metal coating interface of SNOM probes. The corrugations facilitate the excitation of surface plasmons, which
enhance the transport of energy to the probe aperture.
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.
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.