Alessandro Tuniz

Metamaterial fibres for subdiffraction imaging and focusing at terahertz frequencies over optically long distances

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.

Drawn metamaterials with plasmonic response at terahertz frequencies

Electromagnetic metamaterials attract much attention since they can be engineered to exhibit optical properties not found in nature. Their fabrication, however, is challenging, especially in volume. We introduce drawing as a means of fabricating metamaterials, thus demonstrating a terahertz metamaterial. We codraw polymethyl-methacrylate and indium, producing several meters of metamaterial with wire diameters down to ∼10u2002μm, and lattice constants of ∼100u2002μm. We experimentally characterize the transmission properties of different samples, observing high-pass filtering between 0.3–0.4 THz, in good agreement with simulations.

Stacked-and-drawn metamaterials with magnetic resonances in the terahertz range

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.

Weaving the invisible thread: design of an optically invisible metamaterial fibre.

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.

Fiber-drawn double split ring resonators in the terahertz range

We present a novel method for producing metamaterials based on double split ring resonators with a magnetic resonance at terahertz (THz) frequencies. The resonators were made by fiber drawing, a scalable method capable of producing large volumes of metamaterials, demonstrating that this technique can be extended to complex meta-atoms. The observed resonances occur at larger wavelengths relative to the resonator size, compared to single split ring resonators, and are in good agreement with simulations.

Fiber metamaterials with negative magnetic permeability in the terahertz

We present a novel method for producing metamaterials with a terahertz magnetic response via fiber drawing, which can be inexpensively scaled up to mass production. We draw a centimeter preform to fiber, spool it, and partially sputter it with metal to produce extended slotted resonators. We characterize metamaterial fiber arrays with different orientations via terahertz time domain spectroscopy, observing distinct magnetic resonances between 0.3 and 0.4 THz, in excellent agreement with simulations. Numerical parameters retrieval techniques confirm that such metamaterials possess negative magnetic permeability. Combined with fiber-based negative permittivity materials, this will enable the development of the first woven negative index materials, as well as the fabrication of magnetic surface plasmon waveguides and subwavelength waveguides.

Imaging performance of finite uniaxial metamaterials with large anisotropy

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.

Spatial dispersion in three-dimensional drawn magnetic metamaterials

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.

Fabricating Metamaterials Using the Fiber Drawing Method

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.

Two-dimensional imaging in hyperbolic media-the role of field components and ordinary waves.

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.