Sub‐wavelength interference pattern from volume plasmon polaritons in a hyperbolic medium

Inside of a hyperbolic medium, the principal components of the permittivity tensor have opposite signs causing the medium to exhibit a ‘metallicbr’ type of response to light wave sin one direction, and a ‘dielectric’ response in the other. Our study shows that inside hyperbolic media, volume plasmon polaritons (VPPs) propagate along the characteristic planes, forming distinct, directionally dependent optical responses. This is similar to the propagation of conventional surface plasmon polaritons (SPPs) along the planar interfaces separating the isotropic dielectrics and metallic slabs. Interestingly, the plasmon polariton propagates along the resonance cone in a volume of hyperbolic metamaterial crossing the interfaces of the constitutive materials. The Young's double‐slit scheme is used to study the spatially‐confined diffraction in a hyperbolic slab, made of many thin planar layers of a metal and dielectric, to obtain the sub‐wavelength interference pattern at the output interface. Proof‐of‐concept systems for producing such patterns applicable to nanolithography and subwavelength probes are demonstrated.     

Optical resonances and angular filtering functionality of subwavelength hyperbolic etalons

We show that a slab of subwavelength thickness can exhibit etalon resonances and can provide angular filtering functionality at optical wavelengths if it is filled by an anisotropic medium whose principal permittivities have different signs (hyperbolic medium) and have amplitudes smaller than one. This is possible since extraordinary plane waves hyperbolic dispersion allows the vacuum radiation to couple with medium plane waves whose longitudinal wavenumbers are sufficiently large to allow the settlement of standing waves within the nanometric slab thickness. We consider a mixture of metal nanoparticles dispersed within a liquid crystal matrix and we show that it can be designed to exhibit the considered unusual optical response.     

Lasing action assisted by long‐range surface plasmons

It is shown that lasing action at subwavelength scales can be achieved in realistic plasmonic systems supporting long‐range surface plasmons (LRSPPs). To this end, a general numerical framework has been developed that is able to accurately account for the full spatio‐temporal lasing dynamics and the vastly different length‐ and time‐scales featured by this class of systems. Starting from a loss compensation regime for propagating LRSPPs, it is shown how the introduction of an optical feedback mechanism induces the formation of a self‐sustained laser oscillation at moderate pump intensities. The simplicity of the proposed subwavelength scale laser offers significant potential as a novel class of planar light sources in complex plasmonic circuits.     

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.     

Enhancement of spatial coherence by surface plasmons

We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Young's interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. The visibility of the interference pattern can be manipulated by tuning the wavelength of one of the optical sources.     

Acoustic resonators for far-field control of sound on a subwavelength scale

We prove experimentally that broadband sounds can be controlled and focused at will on a subwavelength scale by using acoustic resonators. We demonstrate our approach in the audible range with soda cans, that is, Helmholtz resonators, and commercial computer speakers. We show that diffraction-limited sound fields convert efficiently into subdiffraction modes in the collection of cans that can be controlled coherently in order to obtain focal spots as thin as 1/25 of a wavelength in air. We establish that subwavelength acoustic pressure spots are responsible for a strong enhancement of the acoustic displacement at focus, which permits us to conclude with a visual experiment exemplifying the interest of our concept for subwavelength sensors and actuators.     

THz metamaterials made of phonon-polariton materials

In this paper, we demonstrate numerically various phenomena and possibilities that can be realized in THz metamaterials made of phonon-polariton materials. Such phenomena include hyperbolic dispersion relation, subwavelength imaging using backward propagation and backward radiation, total transmission and subwavelength guiding exploiting Mie-resonant scattering in permittivity near zero host, and toroidal dipolar response. The systems that we use to demonstrate most of these phenomena are two-dimensional periodic systems of μm-scale rods in a host, where both rods and host are made of polaritonic alkali-halide materials.     

Topological transition in coated wire medium

Employing nonlocal homogenization approach, we investigate the properties of a metamaterial consisting of parallel metallic wires with dielectric coating. We demonstrate that manipulation of dielectric contrast between wire dielectric shell and host material at fixed frequency results in dynamic switching of metamaterial dispersion regime from elliptic to the hyperbolic one, i.e. the topological transition takes place. It is proved that such transition can be induced by the variation of the metamaterial temperature. Our findings thus pave a way to the implementation of a tunable ‘elliptic‐hyperbolic’ metamaterial.     

Particle sorting using a subwavelength optical fiber

Size‐based particle sorting using a subwavelength optical fiber was demonstrated with 600‐nm and 1‐μm sizes of polystyrene particles. Optical forces acting on the particles were calculated based on three‐dimensional finite‐difference time‐domain simulations at wavelengths of 808, 1047, and 1310 nm propagating in a subwavelength optical fiber with diameter of 800 nm. Calculations indicate that by launching two counterpropagating laser beams at different wavelengths into the fiber, the directions of the resultant optical scattering forces acting on the two particle sizes can be opposite along the fiber, which leads to a countertransport of the particles. To verify the theoretical prediction, experiments were performed using the 800‐nm fiber to sort the two particle sizes. The results show that with two counterpropagating beams at 808 and 1310 nm, a continuous particle sorting was achieved. Measured particle velocities were in agreement with the theoretical calculations.     

Near-field Moiré effect mediated by surface plasmon polariton excitation

We have demonstrated a surface plasmon polariton mediated optical Moiré effect by inserting a silver slab between two subwavelength gratings. Enhancement of the evanescent fields by the surface plasmon excitations on the silver slab leads to a remarkable contrast improvement in the Moiré fringes from two subwavelength gratings. Numerical calculations, which agree very well with the experimental observation of evanescent-wave Moiré fringes, elucidate the crucial role of the surface plasmon polaritons. The near-field Moiré effect has potential applications to extend the existing Moiré techniques to subwavelength characterization of nanostructures.     

Dipole radiation near hyperbolic metamaterials: applicability of effective-medium approximation

We investigate the radiation rate of a dipole in close proximity to a hyperbolic metamaterial and confirm that both the radiation rate and its fraction directed into the metamaterial are greatly increased compared to bulk dielectric or metal. However, we find that the homogenized effective-medium approach greatly overestimates the Purcell factor compared to metal-dielectric subwavelength multilayers with previously reported layer thicknesses.     

Focusing microwaves into subwavelength dimensions with a half-cylindrical hyperlens based on split ring resonators

Hyperlenses based on metamaterials can be applied to subwavelength imaging in the lightwave band.In this letter,we demonstrate both through simulations and experimentally verified results that our proposed halfcylindrical shaped hyperlens can be used for super-resolution microwave focusing in a TE mode.Based on split ring resonators,the hyperlens satisfies a hyperbolic dispersion relationship.Simulations demonstrate that the focused spot size and position are insensitive to the rotation angle of the hyperlens around its geometric center.Experimental results show that a focused spot size 1/3 of the vacuum wavelength is achieved in the microwave band.     

Optical cavity resonance with magnetized plasma

Indefinite media with mixed signs of dielectric tensor elements possess unbounded equifrequency surfaces that have been utilized for diverse applications such as superimaging, enhanced spontaneous emission, and thermal radiation. One particularly interesting application of indefinite media is an optical cavity supporting anomalous scaling laws. In this Letter, we show that by replacing an indefinite medium with magnetized plasma one can construct a tunable indefinite cavity. The magnetized plasma model is based on realistic semiconductor material properties at terahertz frequencies that show hyperbolic dispersion in a certain frequency regime. The hyperbolic dispersion features are utilized for the design of optical cavities. Dramatically different sizes of cavities can support the same resonance mode at the same frequency. For a cavity of fixed size, the anomalous scaling law between the resonance frequency and mode number is confirmed. The resonance frequency can be strongly modulated by changing the strength of the applied magnetic field. The proposed model provides active controllability of terahertz resonances on the deep subwavelength scale with realistic semiconductor materials.     

Near-field observation of subwavelength confinement of photoluminescence by a photonic crystal microcavity

We present a direct, room-temperature near-field optical study of light confinement by a subwavelength defect microcavity in a photonic crystal slab containing quantum-well sources. The observations are compared with three-dimensional finite-difference time-domain calculations, and excellent agreement is found. Moreover, we use a subwavelength cavity to study the influence of a near-field probe on the imaging of localized optical modes.     

Sub-diffraction-limited far-field imaging in infrared

We investigated a far-field superlens operating at mid-infrared wavelength that allows resolving subwavelength features in the far-field. By utilizing evanescent enhancement provided by surface plasmon excitation of silver nanorods and Moiré effect, we numerically demonstrated that subwavelength information of an object can be converted to propagating information. This information can then be captured by conventional optical components. A simple image reconstruction algorithm can restore the subwavelength object. A sub-diffraction-limited resolution of 2.5 μm at 6-μm wavelength is demonstrated.     

Quantum imaging with incoherent photons

We propose a technique to obtain subwavelength resolution in quantum imaging with potentially 100% contrast using incoherent light. Our method requires neither path-entangled number states nor multiphoton absorption. The scheme makes use of N photons spontaneously emitted by N atoms and registered by N detectors. It is shown that for coincident detection at particular detector positions a resolution of lambda/N can be achieved.     

Lessons from nature: biomimetic subwavelength structures for high‐performance optics

In nature, optical structures in the subwavelength range have been evolved over millions of years. For example, in the form of ‘moth‐eye’ structures they show a strong anti‐reflective effect on the compound eyes of night‐active insects and therefore offer a successful protection over predators. In this contribution the advantages and challenges to transfer this natural concept of subwavelength structured optical interfaces to high‐end optical systems are discussed. Here, in comparison to alternative conventional multilayer systems, the bioinspired anti‐reflective structures offer a wide wavelength range and a broad angle dependency. Additionally, adhesion problems are reduced drastically. Simultaneously to the theoretical consideration of the best profile form of the subwavelength structures, appropriate realization technologies have been developed in recent years, where both top‐down and bottom‐up approaches have been investigated. Depending on the choice of the structuring technique, anti‐reflective subwavelength structures are applicable to a wide spectrum of optical elements ranging from micro‐optical components to aspheres for applications in imaging and also illumination setups of high‐end optical instruments.     

A Photonic Nanojet as Tunable and Polarization‐Sensitive Optical Tweezers

The ability to manipulate small objects with focused laser beams has opened a venue for investigating dynamical phenomena relevant to both fundamental and applied sciences. However, manipulating nano‐sized objects requires subwavelength field localization, provided by auxiliary nano‐ and microstructures. Particularly, dielectric microparticles can be used to confine light to an intense beam with a subwavelength waist, called a photonic nanojet (PNJ), which can provide sufficient field gradients for trapping nano‐objects. Herein, the scheme for wavelength‐tunable and nanoscale‐precise optical trapping is elaborated, and the possibility of lateral nanoparticle movement using the PNJ's side lobes is shown for the first time. In addition, the possibility of subwavelength positioning using polarization switching is shown. The estimated stability with respect to Brownian motion is higher compared to conventional optical trapping schemes.     

Controlling the phase delay of light transmitted through double-layer metallic subwavelength slit arrays

We demonstrate that the phase of light transmitted through double-layer subwavelength metallic slit arrays can be controlled through lateral shift of the two layers. Our samples consist of two aluminum layers, each of which contains an array of subwavelength slits. The two layers are placed in sufficient proximity to allow coupling of the evanescent fields at resonance. By changing the lateral shift between the layers from zero to half the period, the phase of the transmitted electromagnetic field is increased by pi, while the transmitted intensity remains high. Such a controllable phase delay could open new capabilities for nanophotonic devices that cannot be achieved with single-layer structures.