RESUMEN

Imaging with resolutions much below the wavelength λ - now common in the visible spectrum - remains challenging at lower frequencies, where exponentially decaying evanescent waves are generally measured using a tip or antenna close to an object. Such approaches are often problematic because probes can perturb the near-field itself. Here we show that information encoded in evanescent waves can be probed further than previously thought, by reconstructing truthful images of the near-field through selective amplification of evanescent waves, akin to a virtual superlens that images the near field without perturbing it. We quantify trade-offs between noise and measurement distance, experimentally demonstrating reconstruction of complex images with subwavelength features down to a resolution of λ/7 and amplitude signal-to-noise ratios < 25dB between 0.18-1.5 THz. Our procedure can be implemented with any near-field probe, greatly relaxes experimental requirements for subwavelength imaging at sub-optical frequencies and opens the door to non-invasive near-field scanning.

RESUMEN

Terahertz band is envisaged to provide substantially higher capacity and much lower latency for wireless communications in contrast to microwave frequencies. Moving to higher frequencies comes with its own unique challenges to be addressed, such as poor coupling efficiency from free space into and out of planar air-core waveguides. Here, we propose a framework for rapid design and low-cost fabrication of terahertz horn couplers. The horn couplers are first designed by maximizing the field overlap integral on apex and aperture interfaces, then fabricated exploiting 3D printing technique, and finally sputtered with a thin layer of gold. A 28~µm standard deviation of the surface roughness height of the 3D printed horn couplers is calculated. Experimental demonstrations show that the proposed horn coupler improves the transmittance of a hybrid photonic crystal waveguide by 20 dB in comparison with the previous pinhole-based coupling configuration. This work provides a fast, convenient and economical approach for design and fabrication of customized couplers for any waveguide size, with a cost of only 5% of commercially available counterparts, and could be integrated in 3D-printed terahertz devices during fabrication.

RESUMEN

We describe the fabrication of metamaterial magnifying hyperlenses with subwavelength wire array structures for operation in the mid-infrared (around 3 µm). The metadevices are composed of approximately 500 tin wires embedded in soda-lime glass, where the metallic wires vary in diameter from 500 nm to 1.2 µm along the tapered structure. The modeling of the hyperlenses indicates that the expected overall losses for the high spatial frequency modes in such metadevices are between 20 dB to 45 dB, depending on the structural parameters selected, being promising candidates for far-field subdiffraction imaging in the mid-infrared. Initial far-field subdiffraction imaging attempts are described, and the problems encountered discussed.

RESUMEN

Combining the advantages of photonics (low loss) and plasmonics (high field confinement), while mitigating their respective drawbacks, can greatly enhance the light-matter interaction efficiency. Here we make use of a time-dependent coupled mode theory in order to theoretically model the excitation of a plasmonic nanoantenna via a photonic resonator. We show that including the light source-nanoantenna direct coupling substantially enhances the overall coupling efficiency, maintaining the same input power.

RESUMEN

We design a hollow-core terahertz (THz) waveguide guiding a single linearly polarized mode. This is achieved using a hybrid cladding, where we introduce a ring of subwavelength structures, including metal wires and air-holes. The wire-based cladding is extremely anisotropic, reflecting only transverse magnetic (TM) modes. The polarization of TM modes is further manipulated by replacing some wires with air-holes. Numerical simulations confirm the guidance of only an x-polarized TM2 mode over 0.36-0.46 THz in a wavelength-scale core (diameter of 1 mm). The propagation losses are of the order 0.25 dB/cm, with low bend losses <0.3 dB/cm at 0.4 THz for a bend radius of 5 cm.

RESUMEN

Hyperlenses and hyperbolic media endoscopes can overcome the diffraction limit by supporting propagating high spatial frequency extraordinary waves. While hyperlenses can resolve subwavelength details far below the diffraction limit, images obtained from them are not perfect: resonant high spatial frequency slab modes as well as diffracting ordinary waves cause image distortion and artefacts. In order to use hyperlenses as broad-band subwavelength imaging devices, it is thus necessary to avoid or correct such unwanted artefacts. Here we introduce three methods, namely convolution, field averaging, and power averaging, to remove imaging artefacts over wide frequency bands, and numerically demonstrate their effectiveness based on simulations of a wire medium endoscope. We also define a projection in spatial Fourier space to effectively filter out all ordinary waves, leading to considerable reduction in image distortion. These methods are outlined and demonstrated for simple and complex apertures.

RESUMEN

Recent advances in metamaterials have revealed the possibility of overcoming the diffraction limit, opening the door for high-density-integration photonic devices including waveguides and cavities. Here we investigate the condition required to have air cavities within a uniaxial metamaterial clad waveguide. Our work reveals that air-cavity sizes much smaller than the operating wavelength (D2h/λ3=1/(352×100)) are achievable under specific cladding material conditions, which could have a great impact on the miniaturization of electromagnetic devices. Harnessing metamaterials enables engineering of the required condition at a desired wavelength, unlike plasmonic cavities where the condition is reached at a specific wavelength.

RESUMEN

We study full vector imaging of two dimensional source fields through finite slabs of media with extreme anisotropy, such as hyperbolic media. For this, we adapt the exact transfer matrix method for uniaxial media to calculate the two dimensional transfer functions and point spread functions for arbitrary vector fields described in Cartesian coordinates. This is more convenient for imaging simulations than the use of the natural, propagation direction-dependent TE/TM basis, and clarifies which field components contribute to sub-diffraction imaging. We study the effect of ordinary waves on image quality, which previous one-dimensional approaches could not consider. Perfect sub-diffraction imaging can be achieved if longitudinal fields are measured, but in the more common case where field intensities or transverse fields are measured, ordinary waves cause artefacts. These become more prevalent when attempting to image large objects with high resolution. We discuss implications for curved hyperbolic imaging geometries such as hyperlenses.

RESUMEN

We analyze the dependence of the electromagnetic properties of wire array metamaterial media on the choice of metal, and identify promising material combinations for use in the near and mid infrared. We propose a figure of merit for the metal optical quality and consider it as a function of several parameters, such as material loss, wavelength of operation and wire diameter. Accordingly, we select promising material combinations, based on optical quality and fabrication compatibility, and simulate the loss of the quasi-TEM mode, for different wavelengths between 1 and 10 µm. We conclude that wire arrays are unlikely to deliver on their many promises at 1 µm, but should prove useful beyond 3 µm.

RESUMEN

We report on the fabrication and characterization of an elliptical hollow fiber inner coated with a silver layer and a dielectric layer for polarization maintaining and low loss transmission of terahertz (THz) radiation. The primary purpose of adding the dielectric layer is to prevent the silver layer from oxidation. The thickness of the dielectric layer is non-uniform owing to the surface tension of the coating, which was initially applied as a liquid. Transmission loss and polarization maintenance are experimentally characterized. Effects of the dielectric layer on transmission properties are analyzed by comparing the fiber to Ag-only fiber. Results show that a dielectric layer with thickness less than λ/10 can effectively decreases the power distributed on the metal surface and thus can practically reduce loss resulting from roughness of the silver layer. Bending effects on transmission loss and polarization maintenance are also investigated.

RESUMEN

Efficient nonlinear frequency conversion requires a phase matching condition to be satisfied. We analyze the dispersion of the modes of hyperbolic wire metamaterials and demonstrate that phase matching at infrared wavelengths can be achieved with a variety of constituent materials, such as GaAs, in which phase matching cannot easily be achieved by conventional means. Our finding promises access to many materials with attractive nonlinear properties.

RESUMEN

We propose, numerically analyze and experimentally demonstrate a novel refractive index sensor specialized for low index sensing. The device is based on a directional coupler architecture implemented in a single microstructured polymer optical fiber incorporating two waveguides within it: a single-mode core and a satellite waveguide consisting of a hollow high-index ring. This hollow channel is filled with fluid and the refractive index of the fluid is detected through changes to the wavelength at which resonant coupling occurs between the two waveguides. The sensor design was optimized for both higher sensitivity and lower detection limit, with simulations and experiments demonstrating a sensitivity exceeding 1.4 × 10(3) nm per refractive index unit. Simulations indicate a detection limit of ~2 × 10(-6) refractive index units is achievable. We also numerically investigate the performance for refractive index changes localized at the surface of the holes, a case of particular importance for biosensing.

RESUMEN

Metamaterials with extreme anisotropy overcome the diffraction limit by supporting the propagation of otherwise evanescent waves. Recent experiments in slabs of wire media have shown that images deteriorate away from the longitudinal Fabry-Perot resonances of the slab. Existing theoretical models explain this using nonlocality, surface waves, and additional boundary conditions. We show that image aberrations can be understood as originating from cavity resonances of uniaxial media with large local axial permittivity. We apply a simple cavity resonator model and a transfer matrix approach to replicate salient experimental features of wire media hyperlenses. These results offer avenues to reduce observed imaging artefacts, and are applicable to all uniaxial media with large magnitude of the axial permittivity, e.g., wire media and layered media.

RESUMEN

Using conventional materials, the resolution of focusing and imaging devices is limited by diffraction to about half the wavelength of light, as high spatial frequencies do not propagate in isotropic materials. Wire array metamaterials, because of their extreme anisotropy, can beat this limit; however, focusing with these has only been demonstrated up to microwave frequencies and using propagation over a few wavelengths only. Here we show that the principle can be scaled to frequencies orders of magnitudes higher and to considerably longer propagation lengths. We demonstrate imaging through straight and tapered wire arrays operating in the terahertz spectrum, with unprecedented propagation of near field information over hundreds of wavelengths and focusing down to 1/28 of the wavelength with a net increase in power density. Applications could include in vivo terahertz-endoscopes with resolution compatible with imaging individual cells.

RESUMEN

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate (1). They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves (2). This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks (3), negative refractive index materials (4), and lenses that resolve objects below the diffraction limit (5). However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size (6-7). Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response (8). We combine the stack-and-draw technique used to produce microstructured polymer optical fiber (9) with the Taylor-wire process (10), using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 °C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Asunto(s)

RESUMEN

We characterize spatial dispersion in longitudinally invariant drawn metamaterials with a magnetic response at terahertz frequencies, whereby a change in the angle of the incident field produces a shift in the resonant frequency. We present a simple analytical model to predict this shift. We also demonstrate that the spatial dispersion is eliminated by breaking the longitudinal invariance using laser ablation. The experimental results are in agreement with numerical simulations.

Asunto(s)

RESUMEN

We present a novel method for producing drawn metamaterials containing slotted metallic cylinder resonators, possessing strong magnetic resonances in the terahertz range. The resulting structures are either spooled to produce a 2-dimensional metamaterial monolayer, or stacked to produce three-dimensional multi-layered metamaterials. We experimentally investigate the effects of the resonator size and number of metamaterial layers on transmittance, observing magnetic resonances between 0.1 and 0.4 THz, in good agreement with simulations. Such fibers promise future applications in mass-produced stacked or woven metamaterials.

RESUMEN

We experimentally demonstrate solid-core photonic crystal fibers that guide via the inhibited coupling mechanism. We measure an overall transmission window of more than an octave, as well as an uninterrupted width of almost one octave. The fiber is fabricated in polymer, with high-index ring-shaped inclusions. This type of fiber was conceived based on a simple model which shows that the cutoffs of the modes of a thin ring cluster around the cutoffs of planar waveguide modes. The model shows that such ring based fibers are closely related to kagome and square lattice hollow core fibers, and have transmission bandwidths that could in principle reach 1.6 octaves. Measured transmission properties are in good agreement with rigorous modelling.

Asunto(s)

RESUMEN

We present the design of an invisible metamaterial fibre operating at optical frequencies, which could be fabricated by adapting existing fibre drawing techniques. The invisibility is realised by matching the refractive index of the metamaterial fibre with the surroundings. We present a general recipe for the fabrication of such fibres, and numerically characterise a specific example using hexagonally arranged silver nanowires in a silica background. We find that invisibility is highly sensitive to details of the metamaterial boundary, a problem that is likely to affect most invisibility and cloaking schemes.

Asunto(s)

RESUMEN

We investigate higher order core-modes of solid-core photonic bandgap fibers experimentally and theoretically. We observe that for some wavelengths ranges the second mode is guided while the fundamental mode is not. We interpret this behavior in terms of the band diagrams and full numerical simulations, in good agreements with experiments. The sole guidance of the second, ring shaped modes observed at the edges of bandgaps could be of use for generation of vortex beams.