Subwavelength Region Research Articles

Highly tapered pentagonal bipyramidal Au microcrystals with high index faceted corrugation: Synthesis and optical properties

Focusing light at sub-wavelength region opens up interesting applications in optical sensing and imaging beyond the diffraction limit. In the past, tapered Au wires with carved gratings have been employed to achieve nanofocusing. The fabrication process however, is expensive and the obtained wires are polycrystalline with high surface roughness. A chemical synthetic method overcoming these hurdles should be an attractive alternative. Here, we report a method to chemically synthesize Au microcrystals (~10 μm) bearing pentagonal bipyramidal morphology with surface corrugations assignable to high index planes. The method is a single step solid state synthesis at a temperature amenable to common substrates. The microcrystals are tapered at both ends forming sharp tips (~55 nm). Individual microcrystals have been used as pick and probe SERS substrates for a dye embedded in a polymer matrix. The unique geometry of the microcrystal also enables light propagation across its length.

Exact nonparaxial transmission of subwavelength detail using superoscillations

Any object, described by a target function that need not be band-limited, can be sampled at any chosen set of points and then propagated without evanescent waves, using a function which is band-limited, so as to be imaged exactly (i.e. nonparaxially) at multiples of a given repetition distance. If the samples span a sub-wavelength region, the repeated images are superoscillatory. The number N of samples is equal to the repetition distance measured in wavelengths and also to the number of plane waves in the propagating field. If N ≫ 1, the waves form a quasi-continuum, and asymptotics enables an almost-explicit description of the superoscillations. But the matrix involved is ill-conditioned (many of its eigenvalues are very small), so this method of sub-wavelength imaging would be pathologically sensitive to noise, and the depth of focus is exponentially small.

Analytical Expressions for the Electromagnetic Dyadic Green’s Function in Graphene and Thin Layers

An analytical general analysis of the electromagnetic dyadic Green's function for 2-D sheet (or a very thin film) is presented, with an emphasis on the case of graphene. A modified steepest descent treatment of the fields from a point dipole given in the form of Sommerfeld integrals is performed. We sequentially derive the expressions for both out-of-plane and in-plane fields of both polarizations. It is shown that the analytical approximation provided is very precise in a wide range of distances from a point source down to a deep subwavelength region (1/100 of wavelength). We separate the contribution from the pole, the branch point, and discuss their interference. The asymptotic expressions for the fields are composed of the plasmon, Norton wave, and the components corresponding to free space.

On the coupling of resonance and Bragg scattering effects in three-dimensional locally resonant sonic materials

Three-dimensional (3D) locally resonant sonic materials (LRSMs) are studied theoretically for purpose of optimising their sub-wavelength performance by coupling resonance and Bragg scattering effects together. Through the study of effective sound speeds of LRSMs, we find that the starting frequency of Bragg scattering can be shifted to sub-wavelength region by softening coats of resonators when the matrix is a low shear-velocity medium. A similar result can be achieved by compressing the lattice constant. By using a layer-multiple-scattering method, we investigate the complex band structure and the transmission spectrum of an LRSM whose Bragg gap is already close to the resonance gap in frequency. The wave fields of the composite simulated by COMSOL are further analysed at several typical frequencies. The result shows that the approaching of two kinds of gaps not only broadens the bandwidth of the resonance gap, but also increases the depth of the Bragg gap since the interaction between resonant modes and scattering waves are enhanced. By varying the shear velocity of coats, we obtain a coupled gap, which exhibits a broad transmission gap in the sub-wavelength region. When the loss of coats is considered, the coupled gap can not only maintain a good sound blocking performance, but also perform an efficient absorption in the low frequency region.

Microwave magnetoelectric fields and their role in the matter-field interaction

We show that in a source-free subwavelength region of microwave fields, there can exist field structures with a local coupling between the time-varying electric and magnetic fields differing from the electric-magnetic coupling in regular-propagating free-space electromagnetic waves. To distinguish such field structures from regular electromagnetic (EM) field structures, we term them as magnetoelectric (ME) fields. We study a structure and conservation laws of microwave ME near fields. We show that there exist sources of microwave ME near fields-the ME particles. These particles are represented by small quasi-two-dimensional ferrite disks with magnetic-dipolar-oscillation spectra. The near fields originating from such particles are characterized by topologically distinctive power-flow vortices, nonzero helicity, and a torsion degree of freedom. The paper consists of two main parts. In the first one, we give a theoretical background of properties of the electric and magnetic fields inside and outside of a ferrite particle with magnetic-dipolar-oscillation spectra resulting in the appearance of microwave ME near fields. In the second main part, we represent numerical and experimental studies of the microwave ME near fields and their interactions with matter. Based on the obtained properties of the ME near fields, we discuss possibilities for effective microwave sensing of natural and artificial chiral structures.

Metamaterial-based microfluidic sensor for dielectric characterization

A microfluidic sensor is implemented from a single split-ring resonator (SRR), a fundamental building block of electromagnetic metamaterials. At resonance, an SRR establishes an intense electric field confined within a deeply subwavelength region. Liquid flowing in a micro-channel laid on this region can alter the local field distribution and hence affect the SRR resonance behavior. Specifically, the resonance frequency and bandwidth are influenced by the complex dielectric permittivity of the liquid sample. The empirical relation between the sensor resonance and the sample permittivity can be established, and from this relation, the complex permittivity of liquid samples can be estimated. The technique is capable of sensing liquid flowing in the channel with a cross-sectional area as small as (0.001λ0)2, where λ0 denotes the free-space wavelength of the wave excitation. This work motivates the use of SRR-based microfluidic sensors for identification, classification, and characterization of chemical and biochemical analytes.

Transmission enhancement of acoustic waves through a thin hard plate embedded with elastic inclusions

We study the transmission response of acoustic waves through a water-immersed thin hard plate embedded with elastic inclusions. The transmission spectra show a striking peak at the subwavelength region, whose position is almost irrelevant with the incident angle. Our study states that the transmission enhancement stems from the resonant excitation of the localized bending mode in the inclusion. Different from the well-known Fabry-Perot effect, here the resonant frequency reduces rapidly as the sample is thinned down. The theory is further validated by experiment. Potential applications of the abnormal transmission effect produced by such thin plate structures can be anticipated.

Generation of surface optical vortices by evanescent Bessel beams

We present that a set of high order Bessel beams when internally reflected at a dielectric/vacuum interface can create distinct evanescent light modes in each of which the intensity distribution is restricted to a sub-wavelength region near the interface outside the dielectric. We believe that this could really facilitate lateral optical manipulation of nano-particles and neutral atoms along the interface. Similarly, the set-up could lead to the formation of two-dimensional surface optical vortices that possibly will give rise to attractive phenomena, including pattern rotation liken to a surface optical Ferris wheel. Applications are envisaged to be in atom lithography, optical surface tweezers, and spanners. Key words: Bessel beam, optical surface tweezers, optical surface spanners.

Guided-mode resonance excitation of waveguide grating at oblique incidence in the non-subwavelength region

Guided-mode resonance (GMR) excitation of a waveguide grating at oblique incidence in the non-subwavelength region is investigated using the equivalent homogeneous thin-film model. The nondegenerate resonance locations are obtained and the spectral properties are demonstrated by using the rigorous coupled-wave analysis (RCWA) method based on the calculated parameters. It is shown that the dispersion equation of the slab waveguide can provide a reliable approximation in estimating the GMR locations even in the non-subwavelength region. The nondegenerate resonance in the subwavelength region at oblique incidence can be used to tailor an optical filter with good filtering features. Counter-propagating higher-order evanescent waves in the waveguide layer are possible at oblique incidence in the non-subwavelength region. The energy of the electric-field in the waveguide tends to be concentrated as the resonance wavelength decreases, and the lateral enhancement of the standing-wave electric-field (SWEF) can be available due to Bragg reflection in the vicinity of the resonance wavelength.

Formation of optical fields of stimulated Raman scattering with a resolution beyond the Abbe diffraction limit by spherical microlens cavities with whispering gallery modes: Near-field approximation

We consider a significant lowering of the threshold of stimulated Raman scattering in solid fused silica spherical microlenses cavities that is caused by an increase in the integral overlap factor of whispering gallery modes. The structure of focal regions of a microlens is shown to have the shape of honeycombs, forming a photonic crystal or a photonic nanojet. We show that, at comparatively small numerical apertures NA = 0.7–0.8, which correspond to hemispherical microlenses, a spherical microlens cavity exhibits the possibility of focusing laser radiation beyond the Abbe diffraction limit. This enables the possibility of wide practical applications of microspheres as a focusing element the resolving power of which exceeds the Abbe diffraction limit in the near field. The whispering-gallery-mode spherical microlens cavity makes it possible to perform laser generation with a duration of a coherent pulse in the subfemtosecond range and to form a subwavelength focal region of the near field. This ensures the possibility of detecting single molecules of a substance in the subwavelength range in the near field and can be used to increase the sensitivity of intracavity spectroscopy methods and as microlasers for excitation of molecules in metal molecular nanoswitches and semiconductor heterostructures. From an array of microlens cavities, metamaterials with a negative refractive index can be formed.

Single-atom localization via resonance-fluorescence photon statistics

We present a scheme for subwavelength single-atom localization that is based on the photon statistics measurement of the resonance fluorescence from the two-level atomic system. The photon number distribution of the resonance fluorescence depends on the position of the atom inside the standing-wave driving field via the position-dependent Rabi frequency. We show that this position-dependent Rabi frequency can be used to localize the atom within the subwavelength region.

Tapered plasmonic waveguides with efficient and broadband field transmission

In this paper we suggest, numerically simulate and discuss a tapered plasmonic nanostructure which allows efficient transmission of the wave beam energy into a weakly subwavelength (λ/4…λ/2) spatial region within a broad frequency range. The frequency stable location of this region becomes possible due to unusual plasmonic modes we have theoretically revealed and used. Our structure is promising for prospective transducers between conventional optical waveguides and subwavelength nanoguides or nanocircuits.

Effective model and investigation of the near-field enhancement and subwavelength imaging properties of multilayer arrays of plasmonic nanospheres

When a small radiating or scattering object is placed near a multilayer array of plasmonic nanospheres, on the other side the optical near field is enhanced due to the excitation of resonant modes in the layers. For some particular frequencies, the field behind the array is concentrated in a subwavelength region, creating a super resolution effect. Resonating layers are able to reproduce (transport) part of the evanescent spectrum to the other side of these layers which otherwise would decay rapidly. We explore the mechanism of evanescent field transport and subwavelength field concentration on the other side of the layered material and show the relationship between near-field enhancement, field concentration, and modal dispersion characteristics. A detailed investigation of these phenomena is carried out by using an effective numerical model based on the array scanning method (ASM) combined with the Ewald method to accelerate the convergence of the dyadic Green function calculation. The subwavelength-sized spheres forming the arrays are represented as single dipole radiators, and the model of their interactions takes into account all the radiative and reactive field components.

Growth and characterization of sub-wavelength-sized GaInP ridge structures on GaAs substrates

GaInP ridge structures with ridge-top facet widths in the sub-wavelength region (0.5–0.6 μm) and aligned along the [1 1 0] direction were successfully fabricated by selective-area metalorganic vapor phase epitaxy on (0 0 1) GaAs substrates using SiO 2 stripes as a mask. An X-ray diffraction study confirmed that the lattice mismatch of GaInP grown on both the (0 0 1) flat and (1 1 1)B sidewall facets of the ridges can be controlled to within about 0.3%, making possible the fabrication of high-efficiency optical devices using the grown structures. Spontaneous atomic ordering similar to that appearing in GaInP layers grown on unpatterned (0 0 1)GaAs substrates was observed on the (0 0 1) flat facet of the ridge structure, while the (1 1 1)B facet was completely disordered.

Optical manipulation at planar dielectric surfaces using evanescent Hermite–Gaussian light

We show that a set of high order Hermite–Gaussian light beams when internally reflected at a dielectric/vacuum interface can generate well-defined evanescent light modes in each of which the intensity distribution is confined to a sub-wavelength region near the interface outside the dielectric. We suggest that this could greatly facilitate lateral optical manipulation of nano-particles and neutral atoms along the interface. Equally significantly, the scenario could lead to the formation of two-dimensional optical potential arrays and surface optical lattices that could form a suitable architecture for the implementation of quantum computing using neutral atoms.

A reflectometer for the combined measurement of refractive index, microroughness, macroroughness and gloss of low-extinction surfaces

Roughness, porosity and gloss are important properties of many surfaces including coated and printed paper, paints and mineral-filled plastics. A visible light reflectometer is described for essentially simultaneous measurement of refractive index, gloss, microroughness (rms amplitude in the sub-wavelength region) and the two-dimensional forward scattering pattern (yielding the surface slope distribution and associated statistics) of glossy or semi-glossy low-extinction surfaces. The principal novelty is the ability to obtain this useful combination of data in a single measurement. In combination with an x–y stage, the reflectometer can readily produce surface maps of the various measurement parameters and it is expected that the data should offer improved insights into cause and effect in paper and other surfaces, for example to identify causes of gloss and print mottle. Illustrative experimental data are provided and some comparisons made between data obtained by reflectometry and by alternative techniques such as spectroscopic ellipsometry and atomic force microscopy.

Photodetector Using Surface-Plasmon Antenna for Optical Interconnect

AbstractWe used a surface-plasmon antenna to obtain small photodetectors for LSI on-chip optical interconnection by using near-field light generated by the antenna. Such near-field devices are not constrained by the diffraction limit and they offer an approach to integrated nanoscale photonic devices. A small semiconductor structure is located near the antenna to absorb the near-field light. This structure can be made as small as the Schottky depletion layer, so the separation between electrodes can be reduced to almost the size of the near-field region. We have demonstrated a “Si nano-photodiode” or plasmon photodiode that uses the near-field localized in a subwavelength region, which is usually relatively large in size because of the long absorption length for Si (˜10 μm at a wavelength of ˜800 nm). The Si nano-photodiode has a fast impulse response with a full-width at half-maximum of ˜20 ps even when the bias voltage is small (˜1 V or less). We demonstrated an on-chip optical interconnect chip to operate circuitry in an LSI chip by using waveguide-coupled Si nano-photodiodes.

Slow electromagnetic wave guided in subwavelength region along one-dimensional periodically structured metal surface

A perfect electric conductor surface with one-dimensional periodic rectangle holes is proposed as a surface-plasmon-like waveguide, where designed surface plasmon modes with very low group velocity are confined in a subwavelength region. It is shown that the half maximum of electric field intensity of the mode can be confined in a 0.20λ×0.10λ subwavelength region on the transversal plane and the group velocity approaches to zeros, where λ is the wavelength in vacuum.

Enhanced Fluorescence from Arrays of Nanoholes in a Gold Film

Arrays of sub-wavelength holes (nanoholes) in gold films were used as a substrate for enhanced fluorescence spectroscopy. Seven arrays of nanoholes with distinct periodicities (distances between the holes) were fabricated. The arrays were then spin-coated with polystyrene films containing different concentrations of the fluorescent dye oxazine 720. The dye was excited via resonant extraordinary transmission of the laser source through the nanoholes. Enhanced fluorescence was observed when the geometric characteristics of the arrays allowed for an enhancement in the transmitted excitation. This enhancement occurred via surface plasmon excitation by the laser and a consequential increase in the local electromagnetic field in a sub-wavelength region at the metal-film interface. It was demonstrated that the sensitivity of the fluorescence measurement (change in signal vs change in dye concentration in the polymer film) is significantly larger at the surface plasmon resonance conditions than that obtained from equivalent films on glass substrates. Enhancement factors for the fluorescence emission were calculated for each array, with a maximum enhancement of close to 2 orders of magnitude as compared to the emission of films on glass. The results presented here indicate that arrays of nanoholes are interesting substrates for the development of fluorescence sensors based on surface plasmon resonance, as they provide a platform that allows both spatial confinement and enhancement of excitation light. Moreover, the collinear characteristics of the present optical setup, due to the resonant extraordinary transmission through the nanohole arrays, are more conducive to miniaturization and chip integration than more traditional experimental geometries.

Observation of Faraday Rotation and Magnetic Circular Dichroism in an Optical Near-Field Probe Coated with Fe

We demonstrate a novel fiber device exhibiting magnetic circular dichroism (MCD) and Faraday rotation in sharpened optical fibers coated with Fe. The degree of MCD was 0.68 in a magnetic field of 0.35 T and the Faraday rotation angle was as great as 110 degrees. Such great magneto-optical effect is due to optical near-field interactions in the sub-wavelength region, i.e., in the tip of the near-field fiber probe. These effects can be attributed to the large magnitude of the magneto optical coefficient of Fe.