Table of contents

Suddenly, at the end of the last century, classical optics and classical electrodynamics became fashionable again. Fields that several generations of researchers thought were comprehensively covered by the famous Born and Wolf textbook and were essentially dead as research subjects were generating new excitement. In accordance with Richard Feynman's famous quotation on nano-science, the optical community suddenly discovered that 'there is plenty of room at the bottom'—mixing light with small, meso- and nano-structures could generate new physics and new mind-blowing applications. This renaissance began when the concept of band structure was imported from electronics into the domain of optics and led to the development of what is now a massive research field dedicated to two- and three-dimensional photonic bandgap structures. The field was soon awash with bright new ideas and discoveries that consolidated the birth of the 'new optics'. A revision of some of the basic equations of electrodynamics led to the suspicion that we had overlooked the possibility that the triad of wave vector, electric field and magnetic field, characterizing propagating waves, do not necessarily form a right-handed set. This brought up the astonishing possibilities of sub-wavelength microscopy and telescopy where resolution is not limited by diffraction. The notion of meta-materials, i.e. artificial materials with properties not available in nature, originated in the microwave community but has been widely adopted in the domain of optical research, thanks to rapidly improving nanofabrication capabilities and the development of sub-wavelength scanning imaging techniques. Photonic meta-materials are expected to open a gateway to unprecedented electromagnetic properties and functionality unattainable from naturally occurring materials. The structural units of meta-materials can be tailored in shape and size; their composition and morphology can be artificially tuned, and inclusions can be designed and placed at desired locations to achieve new functionality. Among important developments in the new optics was the discovery that a metal film with arrays of small holes in it could be transparent to light beyond any intuitive expectations and that a properly designed metallic structure could be made completely 'invisible' at certain wavelengths. A strong technological drive towards device miniaturization (or, perhaps we should say 'nanoturization'?) has breathed new life into plasmonics—a field many thought had matured some time ago. Surface plasmon polarition waves have come to be seen as potential broadband information carriers for highly integrated photonic devices with research now concentrating on routing and controlling plasmon–polariton signals. Among other new topics in optical electrodynamics are frequency selective surfaces, optical effects of low-dimensional chirality and electrodynamics of toroidal structures.

This Special Issue of Journal of Optics A: Pure and Applied Optics on 'Nanostructured Optical Meta-Materials: Beyond Photonic Bandgap Effects' is a very representative cross-section of research in 'new optics', with papers covering essential issues in meta-materials research, surface plasmons, nanostructured surfaces, sub-wavelength imaging, nanostructured and random laser media and nonlinearities in nanostructured films.

As the Guest Editors of this Special Issue, we are deeply grateful to all contributing authors for their efforts and their willingness to share recent results within the framework of what promises to be a landmark collection of papers in the field of 'new optics'. We are especially proud that the authorship includes pioneers and newcomers to this intriguing and fertile field of research.

In this work, we review some of the key issues for designing dielectric and metallic arrays in the diffraction or refraction regimes with main emphasis on left-handed electromagnetism. We first discuss dispersion characteristics of periodic dielectric arrays which are structured on the wavelength scale (photonic crystals for optics and electromagnetic band gaps for microwaves) with special attention paid to propagation and refraction effects. Special attention was also paid to the isotropy properties in the Brillouin zone with the prospects of defining a negative refractive index. Then, we considered metallic structures which permit one to synthesize double-negative media with the goal of pushing their operation frequency into the infrared region. For both classes of microstructures and nanostructures, the technological challenges will be addressed by considering air hole arrays in a high refractive index semiconductor substrate and embedded C-shaped and wire metal arrays patterned on low index substrates.

Using numerical simulation techniques such as the transfer matrix method and the commercially available code Microwave Studio, we study the transmission properties of left-handed (LH) metamaterials and arrays of split-ring resonators (SRRs). We examine the dependence of the transmission through single- and double-ring SRRs on parameters of the system such as the size and shape of the SRRs, size of the unit cell, dielectric properties of the embedding medium where the SRRs reside, and SRR orientation relative to the incoming electromagnetic field. Moreover, we discuss the role of SRRs and wires on the electric cut-off frequency of the combined system of wires and SRRs, as well as the influence of the various system parameters on the LH transmission peak of a medium composed of SRRs and wires. Finally, demonstrating the disadvantages of the currently used SRR designs due to the lack of symmetry, we discuss more symmetric, multigap SRRs, which constitute very promising components for future two-dimensional and three-dimensional LH structures.

Electromagnetic properties of a new class of two-dimensional periodic nanostructured materials, sub-wavelength plasmonic crystals (SPCs), are investigated. An SPC is a periodic lattice of metallic inclusions with negative dielectric permittivity <0 imbedded in a dielectric host with _h>0, with the lattice period much smaller than the wavelength of light. It is found that two types of propagating electromagnetic waves are supported by SPCs: (a) scale-invariant modes whose dispersion relation is almost independent of the lattice period, and (b) scale-dependent narrow-band resonances whose dispersion strongly depends on the lattice period. The scale-invariant modes are accurately described using a frequency-dependent quasi-static dielectric permittivity _qs(ω) and a vacuum magnetic permittivity μ = 1. The scale-dependent resonances exist inside narrow frequency bands where they can have a modified magnetic permittivity . Magnetic properties originate from the non-vanishing magnetic moment produced by the currents inside any given plasmonic inclusion due to the close proximity of the adjacent inclusions. Applications of SPCs to the development of novel left-handed metamaterials in the optical range are discussed. A new paradigm of the SPC-based surface-enhanced Raman scattering is also introduced.

We compare the optical response of isolated nanowires, double-wire systems, and Π-structures, and show that their radiation is well described in terms of their electric and magnetic dipole moments. We also show that both dielectric permittivity and magnetic permeability can be negative at optical and near infrared frequencies, and demonstrate the connection between the geometry of the system and its resonance characteristics. We conclude that plasmonic nanowires can be employed for novel negative-index materials. Finally, we demonstrate that it is possible to construct a nanowire-based 'transparent nanoresonator' with dramatically enhanced intensity and metal concentration below 5%.

Frequency selective surfaces (FSSs) made up of periodic arrays of split ring resonators (SRRs) are analysed. This analysis includes complementary screens, or complementary SRR-FSSs (CSRR-FSSs). It is shown that these FSSs show a dual behaviour, with a stop/pass band behaviour at the frequency of resonance of the SRRs/CSRRs. Cross-polarization effects in the SRR and the CSRR are considered, and it is shown that they permit resonance to occur for normally incident plane wave excitation. This latter property of SRRs and CSRRs also implies that the FSSs considered may act as polarizers and polarization converters as well. An analytical theory, valid for perfectly conducting and infinitely thin screens, is proposed for the SRR-FSSs and CSRR-FSSs. These approximations are valid in the microwave and millimetre-wave range, and up to the terahertz range.

Planar periodic arrays of metallic elements printed on grounded dielectric substrates are presented to exhibit left-handed properties for surface wave propagation. The proposed structures dispense with the need for grounding vias and ease the implementation of uniplanar left-handed metamaterials at higher frequencies. A transmission line description is used for the initial design and interpretation of the left-handed property. A thorough study based on full wave simulations is carried out with regards to the effect of the element geometrical characteristics and the array periodicity to the properties of the artificial material. Dispersion curves are presented and studied. The distribution of the modal fields in the unit cell is also studied in order to provide an explanation of the material properties. The scalability of the proposed structures to infrared frequencies is demonstrated.

We consider propagation of electromagnetic waves in a circular fibre with a core of negative-refractive-index metamaterial. We study the fast and slow guided modes of a dispersive fibre and the mode properties depending on the fibre parameters. We show the peculiar mode properties of a fibre with simultaneously negative dielectric permittivity and magnetic permeability of the core: the perfect phase matching of the TE and TM slow modes, sign-varying energy flux, and the existence of TEM modes.

The evidence that double-negative media, with an effective negative permittivity and an effective negative permeability, can be manufactured to operate at frequencies ranging from microwave to optical is ushering in a new era of metamaterials. They are referred to here as 'left handed', even though a variety of names is evident from the literature. In anticipation of a demand for highly structured integrated practical waveguides, this paper addresses the impact of this type of medium upon waveguides that can be also nonlinear. After an interesting historical overview and an exposure of some straightforward concepts, a planar guide is investigated, in which the waveguide is a slab consisting of a left-handed medium sandwiched between a substrate and cladding that are simple dielectrics. The substrate and cladding display a Kerr-type nonlinear response. Because of the nonlinear properties of the Kerr media, the power flow direction can be controlled by the intensity of the electric field. A comprehensive finite-difference-time-domain (FDTD) analysis is presented that concentrates upon spatial soliton behaviour. An interesting soliton-lens arrangement is investigated that lends itself to a novel cancellation effect.

We predict that left-handed metamaterials with nonlinear response can support both TE- and TM-polarized self-trapped localized beams, or spatial electromagnetic solitons. Such solitons appear as single- and multi-hump beams, being either symmetric or antisymmetric, and they are modified dramatically by the hysteresis-type magnetic nonlinearity and the effective domains of negative magnetic permeability.

Since the concept of a surface collective excitation was first introduced by Ritchie, surface plasmons have played a significant role in a variety of areas of fundamental and applied research, from surface dynamics to surface-plasmon microscopy, surface-plasmon resonance technology, and a wide range of photonic applications. Here we review the basic concepts underlying the existence of surface plasmons in metallic structures, and introduce a new low-energy surface collective excitation that has been recently predicted to exist.

We outline a new concept for active plasmonics that exploits light-induced nanoscale structural transformations in the waveguide material. The concept is illustrated by numerical modelling and test experiments on a gallium–dielectric interface. We also discuss other possible implementations of the concept such as an electro-plasmon modulator, a plasmon detector and a switch that controls one plasmon wave with another.

After reviewing the present understanding of the transmission properties of single apertures and the enhanced transmission through hole arrays, experimental results are presented which show how localized surface plasmon (LSP) modes of individual apertures contribute to the transmission peaks associated with the periodic arrays. In particular, it is shown that the surface plasmon polaritons (SPPs) of the periodic structure dominate the spectral signature of the arrays. The results also indicate that the LSP of each hole can interact and that this can be controlled by appropriate arrangement of the apertures in the array.

In this paper we explore the existence of surface electromagnetic modes in corrugated surfaces of perfect conductors. We analyse two cases: one-dimensional arrays of grooves and two-dimensional arrays of holes. In both cases we find that these structures support surface bound states and that the dispersions of these modes have strong similarities with the dispersion of the surface plasmon polariton bands of real metals. Importantly, the dispersion relation of these surface states is mainly dictated by the geometry of the grooves or holes and these results open the possibility of tailoring the properties of these modes by just tuning the geometrical parameters of the surface.

Several configurations of aperture arrays have been investigated using a rigorous numerical–analytical technique. The characteristics of optical transmission through a periodic array of sub-wavelength holes in metal films have been simulated and the mechanisms of the transmittance enhancement are discussed. The effects of topological factors and array arrangement have been explored from the perspective of enhancing microwave transmission through arrays of sub-wavelength apertures. The features and properties of aperture arrays used in quasi-optical filters and wavelength selective structures are presented.

We summarize our work on polarization effects in arrays of low-symmetry L-shaped gold nanoparticles. A collection of experiments used to characterize both the linear and second-order optical responses of the arrays is discussed. The responses of the arrays are found to be exceptionally sensitive to polarization. This sensitivity is determined to arise from structural properties including particle size, shape, spacing, and orientation. Nonlinear polarization measurements are shown to yield unexpected and very interesting information concerning the symmetry of the nanoparticle arrays. The combined linear and nonlinear results confirm that the smallest details of arrays influence optical responses.

We demonstrate the generation of second-harmonic radiation in transmission through periodic and disordered arrays of sub-wavelength metallic apertures. For circular apertures in a square lattice, the second-harmonic signal peaks at incidence angles corresponding to enhanced transmission of the fundamental beam of 800 nm wavelength except at small incidence angles where the local symmetry minimizes the effective second-order nonlinear susceptibility of the apertures. Even though the linear transmission of the fundamental beam can be more than five times greater through the periodic array as compared to a disordered array, the strength of the second harmonic from the disordered array is greater at large incidence angles. By breaking the local symmetry through the use of apertures of non-centrosymmetric shape, the second-harmonic output occurs at fundamental transmission resonances at small incidence angles.

The paper presents a transmission line (TL) approach developed in the field of metamaterials for planar devices and circuits. Materials under consideration are dielectric substrates, containing or not nanoscaled ferromagnetic inclusions, that can be arranged in a periodic way with specific impedances and phase velocities. They form photonic or periodic bandgap (PBG) substrates, on which planar guiding structures are deposited. These PBG substrates show some similarities with integrated Bragg reflector topologies previously investigated in the optical range.

Using the TL concept, defect modes in PBG planar devices are studied. A theoretical explanation of defect mode operation, which is validated by measurement on a dielectric PBG structure, is proposed. It is then transposed to a magnetic PBG structure based on arrays of nanoscaled ferromagnetic nanowires. The effect on its frequency response of varying the defect length is discussed, in order to design planar sensors and filters.

Extension of the magnetic PBG structure for applications in the optical range is finally proposed.

As a step towards designing nanoscaled waveguides with desired properties we introduce a periodic arrangement of longitudinally directed resonant dipole particles over a conducting plane and study waveguiding properties of this system. Depending on the operational frequency range, the dipoles may be plasmon resonant particles (optics) or small loaded wire antennas (microwaves). The particles are positioned along a straight line and are close to each other, forming a coupled dipole array. An analytical solution for waves propagating along this 'metawaveguide' is presented and conditions for guided-wave solutions are established. The properties of this waveguiding structure can be tuned by varying the electromagnetic parameters of the particles. Slow and backward-wave ('left-handed') solutions are possible in particular cases.

A method is proposed for designing a two-dimensional randomly rough Dirichlet surface which, when illuminated at normal incidence, scatters a scalar plane wave with a specified angular distribution of its intensity. It is validated by computer simulation calculations.

Excitation of bound surface waves on textured metallic structures can lead to strong resonant absorption of incident radiation at frequencies determined by the surface profile. In the present study however, attention is turned to the role of the surface structure in the enhancement of transmission through a circular, subwavelength-diameter aperture. Undertaking the experiment at microwave wavelengths allows for a precision of manufacture and optimization of the surface structure that would be difficult to replicate at optical frequencies, and demonstrates that transmission enhancement may be achieved with near-perfect metals. Further, the use of a finite element method computational model to study the electromagnetic response of the sample allows for the fields associated with transmission enhancement to be examined, thereby obtaining a better understanding of the role of the surface profile in the enhancement mechanism.

Grating-coupling phenomena between surface plasmons and electromagnetic waves were studied in the microwave spectrum using metallic gratings. Transmission measurements were carried out to observe the transmitted radiation around the surface plasmon resonance frequencies. Grating structures with subwavelength apertures were designed for transmission experiments. Measurements were made in the microwave spectrum of 10–37.5 GHz, corresponding to a wavelength region of 8–30 mm. The Al samples had a grating periodicity of 16 mm. A 2 mm wide subwavelength slit was opened for transmission samples. Samples with one/double-sided gratings displayed remarkably enhanced transmission and directivity with respect to the reference sample without gratings. The experimental results agreed well with theoretical simulations. ∼50 % transmission at 20.7 mm, ∼25-fold enhancement, and ± 4° angular divergence were achieved with a ∼λ/10 aperture.

A new far-field optical microscopy technique capable of reaching nanometre-scale resolution has been developed recently using the in-plane image magnification by surface plasmon polaritons. This microscopy is based on the optical properties of a metal–dielectric interface that may, in principle, provide extremely large values of the effective refractive index n_eff up to 10²–10³ as 'seen' by the surface plasmons. Thus, the theoretical diffraction limit on resolution becomes λ/2n_eff, and falls into the nanometre-scale range. The experimental realization of the microscope has demonstrated optical resolution better than 50 nm for 502 nm illumination wavelength. However, the theory of such a surface plasmon-based far-field microscope presented so far gives an oversimplified picture of its operation. For example, the imaginary part of the metal's dielectric constant severely limits the surface plasmon propagation and the shortest attainable wavelength in most cases, which in turn limits the microscope magnification. Here I describe how this limitation has been overcome in an experiment, and analyse the practical limits on the surface plasmon microscope resolution. In addition, I present more experimental results, which strongly support the conclusion of extremely high spatial resolution of the surface plasmon microscope.

Near-field imaging through planar silver lenses has been demonstrated using a modified conformal-mask optical lithography arrangement. Dense feature resolution down to 250 nm (on a 500 nm period) has been achieved in 50 nm thick photoresist on silicon using broadband illumination from a mercury lamp. Finite difference time domain simulations have been performed to show the resolution improvements that can be expected for imaging through such silver lenses compared with near-field proximity imaging. The resolution enhancements that are predicted are in good agreement with the experimental results, and the conditions by which sub-diffraction-limited resolution may be achieved are given.

We present a terahertz near-field microscope which may serve as a contactless probe for identifying the dielectric properties of individual quantum systems. Terahertz images of organic and inorganic structures demonstrate the applicability on objects of submicron size. Spatial resolutions down to 150 nm have been achieved. The unexpectedly high image contrast results from a novel near-field imaging process.

We discuss the optical transport properties of complex photonic structures ranging from ordered photonic crystals to disordered strongly-scattering materials, with particular focus on the intermediate regime between complete order and disorder. We start by giving an overview of the field and explain the important analogies between the transport of optical waves in complex photonic materials and the transport of electrons in solids. We then discuss amplifying disordered materials that exhibit random laser action and show how liquid crystal infiltration can be used to control the scattering strength of random structures. Also we discuss the occurrence of narrow emission modes in random lasers. Liquid crystals are discussed as an example of a partially ordered system and particular attention is dedicated to quasi-crystalline materials. One-dimensional quasi-crystals can be realized by controlled etching of multi-layer structures in silicon. Transmission spectra of Fibonacci type quasi-crystals are reported and the (self-similar) light distributions of the transmission modes at the Fibonacci band edge are calculated and discussed.

We study the properties of super-radiant coherent emission from a two-level system of atoms embedded in a one-dimensional photonic band gap (1D-PBG) structure. A reduced system of equations is derived that enables us to explore the role of the 1D-PBG band edge in the emission characteristics of coherent pulses. The effect of the emitters' spatial location in the PBG structures is also studied in a finite 1D-PBG, where the super-radiant emission characteristics are also presented.

We report the fabrication of a two-dimensional periodic structure made from the conjugated, fluorescent polymer poly(p-phenylene vinylene), PPV, by direct-write near-field lithography of a photosensitive precursor of the conjugated polymer. The precursor film is illuminated with ultraviolet radiation using the apertured fibre of a home-built scanning near-field optical microscope (SNOM), and unexposed areas are subsequently dissolved in methanol. The fully conjugated polymer is obtained by thermal conversion under vacuum. The structure is approximately 10 µm × 10 µm, and consists of 30 nm tall pillars on a 330 nm periodicity lattice. We present a calculation of the photonic properties and propose that structures made with the method investigated here are promising candidates for photonic crystal lasers. We also report the results of preliminary investigations aimed at increasing the pillar height.

A ZnO infiltration technology was developed by chemical deposition from solution into a three-dimensional opal lattice; samples of the ZnO–opal composites were prepared with the predominating UV emission at room temperature. It is shown that the use of 'raw' opals and an incomplete filling of pores by semiconducting material increase the edge excitonic emission several-fold at room temperature. Angular dependences of the photoluminescence and reflectance spectra of the ZnO-infiltrated opal are studied. The suppression effect of the spontaneous emission in the stop band is observed. These results can be used to create effective laser light sources in the UV spectral range using the 'photonic crystal' effect.

We have found that R6G laser dye in a concentration of 0.1 g l⁻¹ mixed with a solution of aggregated silver nanoparticles exhibits a new emission band with a maximum at 612 nm. This band does not exist in pure dye of comparable concentration or in a mixture of dye with a solution of single silver nanoparticles. A qualitatively similar red-shifted emission band is observed in pure R6G dye at very high concentration (3.8 or 16.7 g l⁻¹). In both cases, no changes occur to the shapes of the absorption spectra of the dye.

We explain the observed spectral changes in terms of J-aggregates of R6G molecules whose formation is probable in the presence of Ag aggregates with a complicated surface structure and is much less likely in the case of adsorption of dye molecules on single Ag nanoparticles.

Alternatively, many features observed in the experiment can be explained by an enhancement of the rates of spontaneous radiative transitions in the proximity of metallic particles, which is due to a modification of the local density of electromagnetic modes in the vicinity of metal surfaces at energies resonant with surface plasmon resonances.

The mode coupling of organic photonic crystal lasers is enhanced by using a photonic crystal that consists of a thin layer of a high-index material. The high-index material increases the index contrast, the confinement in the waveguide and thus the mode coupling. Using such a photonic crystal gives rise to new design criteria. We investigate these criteria, and employ them in the design of an organic photonic crystal laser. Calculations of the coupling constant of the organic laser show that using high-index materials results in much higher coupling constants and thus smaller devices.

We present transient ellipsometry measurements made upon Au, Cu, Ag, Ni, Pd, Ti, Zr, and Hf thin films under identical experimental conditions. Using an elliptically polarized pump beam, we have simultaneously observed the specular inverse Faraday effect (SIFE) and specular optical Kerr effect (SOKE) contributions to the optical polarization response, in order to extract the real and imaginary parts of the non-vanishing components of the third-order optical susceptibility tensor. The signal magnitudes and the extracted tensor components show a systematic variation that reflects the underlying band structure of the different metals. A time delay observed between the SIFE and SOKE signals is interpreted in terms of lifetimes for the transient polarization that may be of comparable magnitude to the pulse width. This suggests that formulae derived in the continuous wave limit may not be applicable in the description of ultrafast nonlinearities in metallic samples. The implications of our results for different areas of applied optics are discussed.

Optical control of phase coexistence, previously used as a mechanism for achieving a nonlinearity at planar gallium/glass interfaces, can be exploited as a nonlinear response mechanism in a metallic nanoparticle film. Experimental measurements of the reflective and transmissive nonlinear response of a gallium nanoparticle film, manufactured on the tip of an optical fibre using the light-assisted self-assembly technique, are shown to be consistent with an effective medium theory for the optical properties of a layer of closely packed nanoshells comprising a core and surface layer in two different structural phases of the metal.

Photonic crystals can be viewed just as a subclass of a larger family of material systems called metamaterials in which the properties largely derive from the structure rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings for further templating other materials with photonic applications for instance. The tortuous route from materials to devices might perhaps find reward in the ease and low cost of fabrication of these materials. In this paper we present a review of recent work and work under way in our laboratory tending towards synthesis based on self-assembly to realize metamaterials in the optical range. This comprises the formation of the templates (opals) and subsequent synthesis of guest materials such as semiconductors, metals and insulators. The possibility of further processing allows additional two-dimensional and quasi-two-dimensional patterning for the design of new structures. In this paper we show how the raw matter can be checked for quality and learn how to use its optical properties to evaluate application potential. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined. We show some examples where opals are used to pattern the growth of other materials with photonic applications (such as metals and semiconductors) and developments leading to both vertical and lateral engineering are shown.