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Label-free super-resolution (LFSR) imaging relies on light-scattering processes in nanoscale objects without a need for fluorescent (FL) staining required in super-resolved FL microscopy. The objectives of this Roadmap are to present a comprehensive vision of the developments, the state-of-the-art in this field, and to discuss the resolution boundaries and hurdles which need to be overcome to break the classical diffraction limit of the LFSR imaging. The scope of this Roadmap spans from the advanced interference detection techniques, where the diffraction-limited lateral resolution is combined with unsurpassed axial and temporal resolution, to techniques with true lateral super-resolution capability which are based on understanding resolution as an information science problem, on using novel structured illumination, near-field scanning, and nonlinear optics approaches, and on designing superlenses based on nanoplasmonics, metamaterials, transformation optics, and microsphere-assisted approaches. To this end, this Roadmap brings under the same umbrella researchers from the physics and biomedical optics communities in which such studies have often been developing separately. The ultimate intent of this paper is to create a vision for the current and future developments of LFSR imaging based on its physical mechanisms and to create a great opening for the series of articles in this field.

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotubes (MWCNTs) coated on fullerene electron transport layers (ETLs): C 60 and C 70 . Such electric double-layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning of the electronic concentration in MWCNTs and the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to the decrease of the series and increase of the shunt resistances of OPVs and allows use of thick (up to 200 nm) ETLs, increasing the durability of OPVs. Two stages of OPV enhancement are described upon the increase of gating bias V g : at small (or even zero) V g , the extended interface of ILs and porous transparent MWCNTs is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped J - V curve via improving the electron collection by an n-doped MWCNT cathode with an ohmic interfacial contact. The J - V curves further improve at higher gating bias V g due to the increase of the Fermi level and decrease of the MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNTs while ions of ILs penetrate into the fullerene. At this step, the internal built-in field is created within OPV, which helps in exciton dissociation and charge separation/transport, increasing further the J sc and the fill factor. The ionic gating concept demonstrated here for most simple classical planar small-molecule OPV cells can be potentially applied to more complex highly efficient hybrid devices, such as perovskite photovoltaic with an ETL or a hole transport layer, providing a new way to tune their properties via controllable and reversible interfacial doping of charge collectors and transport layers.

In this Letter, we experimentally show that a submicron layer of a transparent conducting oxide that may serve a top electrode of a photovoltaic cell based on amorphous silicon when properly patterned by notches becomes an efficient light-trapping structure. This is so for amorphous silicon thin-film solar cells with properly chosen thicknesses of the active layers (p-i-n structure with optimal thicknesses of intrinsic and doped layers). The nanopatterned layer of transparent conducting oxide reduces both the light reflectance from the photovoltaic cell and transmittance through the photovoltaic layers for normal incidence and for all incidence angles. We explain the physical mechanism of our light-trapping effect, prove that this mechanism is realized in our structure, and show that the nanopatterning is achievable in a rather easy and affordable way that makes our method of solar cell enhancement attractive for industrial adaptations.

The Purcell effect is defined as a modification of the spontaneous emission rate of a quantum emitter at the presence of a resonant cavity. However, a change of the emission rate of an emitter caused by an environment has a classical counterpart. Any small antenna tuned to a resonance can be described as an oscillator with radiative losses, and the effect of the environment on its radiation can be modeled and measured in terms of the antenna radiation resistance, similar to a quantum emitter. We exploit this analogue behavior to develop a general approach for calculating the Purcell factors of different systems and various frequency ranges including both electric and magnetic Purcell factors. Our approach is illustrated by a general equivalent scheme, and it allows resenting the Purcell factor through the continuous radiation of a small antenna at the presence of an electromagnetic environment.

We perform complex investigation of the distribution of electromagnetic fields in the vicinity of an array of silver nanoantennas, which can operate as an efficient light trapping structure in the visible spectral range. In theory, this array should support unusual collective modes that possess an advantageous distribution of local electric fields, ensuring both strong field localization beneath nanoantennas and a low level of optical losses inside the metal. Using an aperture-type near-field scanning optical microscope (NSOM), we obtain near-field patterns that show excellent agreement with the NSOM signal, directly reconstructed from rigorous numerical simulations using an approach based on the electromagnetic reciprocity theorem. The agreement between theory and experiment allows us to claim the first-time experimental verification of the existence of collective modes with such properties in an array of silver nanoantennas. The confirmation of this physical phenomenon opens the door to a new class of light-trapping structures for photovoltaics.

We introduce the novel concept of superdirective nanoantennas based on the excitation of higher-order magnetic multipole moments in subwavelength dielectric nanoparticles. Our superdirective nanoantenna is a small Si nanosphere containing a notch, and is excited by a dipole located within the notch. In addition to extraordinary directivity, this nanoantenna demonstrates efficient radiation steering at the nanoscale, resulting from the subwavelength sensitivity of the beam radiation direction to variation of the source position inside the notch. We compare our dielectric nanoantenna with a plasmonic nanoantenna of similar geometry, and reveal that the nanoantenna's high directivity in the regime of transmission is not associated with strong localization of near fields in the regime of reception. Likewise, the absence of hot spots inside the nanoantenna leads to low dissipation in the radiation regime, so that our dielectric nanoantenna has significantly smaller losses and high radiation efficiency of up to 70%.

Understanding the impact of order and disorder is of fundamental importance to perceive and to appreciate the functionality of modern photonic metasurfaces. Metasurfaces with disordered and amorphous inner arrangements promise to mitigate problems that arise for their counterparts with strictly periodic lattices of elementary unit cells such as, e.g., spatial dispersion, and allows the use of fabrication techniques that are suitable for large scale and cheap fabrication of metasurfaces. In this study, we analytically, numerically and experimentally investigate metasurfaces with different lattice arrangements and uncover the influence of lattice disorder on their electromagnetic properties. The considered metasurfaces are composed of metal-dielectric-metal elements that sustain both electric and magnetic resonances. Emphasis is placed on understanding the effect of the transition of the lattice symmetry from a periodic to an amorphous state and on studying oblique illumination. For this scenario, we develop a powerful analytical model that yields, for the first time, an adequate description of the scattering properties of amorphous metasurfaces, paving the way for their integration into future applications.

Self-assembled plasmonic metamaterials are fabricated from silver nanoparticles covered with a silica shell. These metamaterials demonstrate topological darkness or selective suppression of reflection connected to global properties of the Fresnel coefficients. The optical properties of the studied structures are in good agreement with effective medium theory. The results suggest a practical way of achieving high phase sensitivity in plasmonic metamaterials., (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this work we theoretically reveal the huge local field enhancement in a so-called perfect plasmonic absorber. We study the power absorption of light in a planar grid modelled as an effective sheet with zero optical thickness. The key prerequisite of the total absorption is the simultaneous presence of both resonant electric and magnetic modes in the structure. We show that the needed level of the magnetic mode is achievable using the effect of substrate-induced bianisotropy. On the microscopic level this bianisotropy is a factor which results in the huge local field enhancement at the same wavelength where the maximal absorption holds.

The physics and applications of a broad class of artificial electromagnetic materials composed of lattices of aligned metal rods embedded in a dielectric matrix are reviewed. Such structures are here termed wire metamaterials. They appear in various settings and can operate from microwaves to THz and optical frequencies. An important group of these metamaterials is a wire medium possessing extreme optical anisotropy. The study of wire metamaterials has a long history, however, most of their important and useful properties have been revealed and understood only recently, especially in the THz and optical frequency ranges where the wire media correspond to the lattices of microwires and nanowires, respectively. Another group of wire metamaterials are arrays and lattices of nanorods of noble metals whose unusual properties are driven by plasmonic resonances., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Modified boundary conditions for composite material are suggested. The modified RT-retrieval procedure yields bulk values of effective impedance and refractive index, which are independent of system size and boundary realization, whereas the conductivities of the excess surface currents depend on the property of the interface. Simultaneous treatment of all the possible realizations of the system removes the dependence. The accuracy of the latter procedure is the same as the usage of static effective parameters, namely k(eff)d.

A structure comprising a coupled pair of two-dimensional arrays of oblate plasmonic nanoellipsoids in a dielectric host medium is proposed as a superlens in the optical domain for both horizontal and vertical polarizations. By means of simulations it is demonstrated that a structure formed by silver nanoellipsoids is capable of restoring subwavelength features of the object for both polarizations at distances larger than half wavelength. The bandwidth of subwavelength resolution is in all cases very large (above 13%).

It is demonstrated that the ultimate physical limit of resolution of novel imaging devices based on arrays of metallic rods is determined by the skin depth of the metal. Our theoretical and numerical results show that wire medium lenses may provide a unique solution for subwavelength imaging at frequencies up to the terahertz range and may enable image formation at a significant distance from the source plane.

A transversal mode with zero group velocity and nonzero phase velocity that can exist in chains of silver nanospheres in the optical frequency range is theoretically studied. It is shown that the external source radiating a narrow-band nonmonochromatic signal can excite in the chain a mixture of standing and slowly traveling waves. The standing-wave component (named the resonator mode) is strongly dominating. The physical reason for such a regime is a sign-varying distribution of power flux over the cross section of the chain. A possible application of the resonator mode for evanescent-wave enhancement and for subwavelength imaging in the visible is discussed.

The dispersion properties of rectangular metallic waveguides periodically loaded by uniaxial resonant scatterers are studied with help of an analytical theory based on the local field approach, the dipole approximation, and the method of images. The cases of both magnetic and electric uniaxial scatterers with both longitudinal and transverse orientations with respect to the waveguide axis are considered. It is shown that in all considered cases waveguides support propagating modes below the cutoff of the hollow waveguide within some frequency bands near the resonant frequency of the individual scatterers. The modes are forward ones except the case of transversely oriented magnetic scatterers when the mode turns out to be backward. The described effects can be applied for the miniaturization of the guiding structures.

Dispersion properties of electromagnetic crystals formed by small uniaxial resonant scatterers (magnetic or electric) are studied using the local field approach. The goal of the study is to determine the conditions under which the homogenization of such crystals is possible. Therefore the consideration is limited to the frequency region where the wavelength in the host medium is larger than the lattice periods. It is demonstrated that, together with the known restriction for the homogenization related to the large values of the material parameters, there is an additional restriction related to their small absolute values. On the other hand, the homogenization becomes allowed in both cases of large and small material parameters for special directions of propagation. Two unusual effects inherent to the crystals under consideration are revealed: a flat isofrequency contour that allows subwavelength imaging using the canalization regime and birefringence of the extraordinary modes which can be used for beam splitting.

In this work the analytical model explaining the main reason of transmission losses in the known two-phase lattice of Smith, Schultz, and Shelby representing a uniaxial variant of left-handed medium (LHM) at microwaves is presented. The role of electromagnetic interaction between split-ring resonators (SRRs) and straight wires leading to the dramatic increase of ohmic losses in SRRs within the band when the meta-material becomes a LHM is clarified. This paper explains why in this structure, rather high transmission losses are observed in the experimental data, whereas these losses in a separate lattice of SRRs and in a lattice of wires are negligible at these frequencies.

This work is dedicated to the theoretical analysis of wire media, i.e., lattices of perfectly conducting wires consisting of two or three doubly periodic arrays of parallel wires which are orthogonal to one another. An analytical method based on the local field approach is used. The explicit dispersion equations are presented and studied. The possibility of introducing a dielectric permittivity is discussed. The theory is validated by comparison with the numerical data available in the literature.

In this paper the electromagnetic properties of bianisotropic electromagnetic crystals are studied. The crystals are assumed to be rectangular lattices of perfectly conducting helicoidal spirals. The analytical theory of dispersion and plane-wave reflection refers to the case when the spiral step and the radius are small compared to the wavelengths in the host medium. The periods of the lattice can be arbitrary. Explicit closed-form expressions are derived for the effective material parameters of the medium in the low-frequency regime. The medium eigenmodes are elliptically polarized, and one of them propagates without interaction with the lattice. As to the other eigenmode, the lattice has strong spatial dispersion even at extremely low frequencies in the direction along the spiral axes. Numerical examples are given. An analogy between the spiral medium and the medium of loaded wires is indicated.

Two-dimensional electromagnetic crystals formed by rectangular lattices of thin ideally conducting cylinders periodically loaded by bulk reactive impedances are considered. An analytical theory of dispersion and reflection from this medium is presented. The consideration is based on the local field approach. The transcendental dispersion equation is obtained in the closed form and solved numerically. Different types of the loads such as inductive, capacitive, serial, and parallel LC circuits are considered. Typical dispersion curves and reflection coefficients are calculated and analyzed.