Towards subdiffraction imaging with wire array metamaterial hyperlenses at MIR frequencies.

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.

Gain investigation of Perylene-Red-doped PMMA for stimulated luminescent solar concentrators.

Luminescent solar concentrators (LSCs) utilizing stimulated emission by a seed laser are a promising approach to overcome the limitations of conventional LSCs, with a significant reduction of the photovoltaic material. In our previous work, we demonstrated the principle of a stimulated LSC (s-LSC) and correspondingly developed a model for quantifying the output power of such a system, taking into account different important physical parameters. The model suggested Perylene Red (PR) dye as a potential candidate for s-LSCs. Here, we experimentally investigate the gain of PR-doped polymethyl methacrylate (PMMA) required for s-LSCs using a single pump wavelength (instead of the solar spectrum) as a proof of principle. The results found from the experiment are well matched with the previously developed numerical model except for gain saturation, which occurs at a comparatively small seed laser signal power. To investigate the gain saturation, two approaches were taken: investigating (i) spectral hole burning and (ii) triplet state absorption. Experimental investigation of spectral hole burning with PR dyes showed a small effect on the gain saturation. We developed a general state model considering triplet state absorption of the PR dyes for the second approach. The state model suggests that the PR dyes suffer from significant triplet state absorption loss, which obstructs the normal operation of the PR-based s-LSC system.

Removing image artefacts in wire array metamaterials.

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.

Luminescent solar concentrators utilizing stimulated emission.

Luminescent solar concentrators (LSCs) are an emerging technology that aims primarily to reduce the cost of solar energy, with great potential for building integrated photovoltaic (PV) structures. However, realizing LSCs with commercially viable efficiency is currently hindered by reabsorption losses. Here, we introduce an approach to reducing reabsorption as well as improving directional emission in LSCs by using stimulated emission. Light from a seed laser (potentially an inexpensive laser diode) passes through the entire area of the LSC panel, modifying the emission spectrum of excited dye molecules such that it is spectrally narrower, at wavelengths that minimize reabsorption to allow net gain in the system, and directed towards a small PV cell. A mathematical model, taking into account thermodynamic considerations, of such a system is presented which identifies key parameters and allows evaluation in terms of net effective output power.

Optical gain characterization of Perylene Red-doped PMMA for different pump configurations.

The optical gain is measured in Perylene Red (PR)-doped polymethyl methacrylate (PMMA) slabs for copropagating and transverse pumping configurations based on a single-pass pump-probe method where a small signal is used as a probe beam. The gain is characterized in terms of the stimulated gain coefficient (g(S)) for both pump configurations. This material property determines the strength of pump absorption and coupling to the probe signal beam through stimulated emission. For copropagating pumping, g(S) was found to be (3.05±0.17)×10(-3) m/W for ∼0.05 mM PR-doped PMMA using a 633 nm probe laser, pumping with a 532 nm CW laser. For transverse pumping, g(S) was found to be (3.28±0.09)×10(-3) m/W for a ∼0.15 mM sample. The small difference in the gain coefficient is attributed to the difference in concentration. The stimulated gain coefficient, a material property of the gain medium independent of the pump configuration and experimental setup, offers a useful and convenient way to characterize the optical gain for solid-state lasers or amplifiers.

All-plastic fiber-based pressure sensor.

We present a feasibility study and a prototype of an all-plastic fiber-based pressure sensor. The sensor is based on long period gratings inscribed for the first time to the best of our knowledge by a CO2 laser in polymethyl methacrylate (PMMA) microstructured fibers and coupled to a pod-like transducer that converts pressure to strain. The sensor prototype was characterized for pressures up to 150 mbars, and various parameters related to its construction were also characterized in order to enhance sensitivity. We consider this sensor in the context of future applications in endoscopic pressure sensors.

Modeling of stimulated emission based luminescent solar concentrators.

The efficiency improvement of luminescent solar concentrators (LSCs) necessary for practical realization is currently hindered by one major loss mechanism: reabsorption of emitted photons by the luminophores. Recently, we explored a promising technique for reducing reabsorption and also improving directional emission in LSCs utilizing stimulated emission, rather than only spontaneous emission, with an inexpensive seed laser. In this work, a model is developed to quantify the gain (i.e. the amount of amplification of a low power seed laser propagating through the solar-pumped concentrator) of stimulated-LSCs (s-LSCs) considering the effects of different important physical parameters. The net optical output power, available for a small PV cell, from the concentrator can also be determined from the model, which indicates the performance of s-LSCs. Finally, the performance of different existing material systems is investigated using literature values of the parameters required for the model, and a set of optimal parameters is suggested for practical realization of such a device.

Metal selection for wire array metamaterials for infrared frequencies.

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.

Holey fiber mode-selective couplers.

Directional mode coupling in an asymmetric holey fiber coupler is demonstrated both numerically and experimentally for the first time. The holey fiber mode couplers have interesting spectral characteristics and are also found to exhibit increased dimensional tolerances. Following a design based on numerical investigations, a dual-core polymer holey fiber coupler for LP(01) and LP(11) mode multiplexing was fabricated via a drilling and drawing technique. The measurements are compared with the simulation results.

Elliptical metallic hollow fiber inner-coated with non-uniform dielectric layer.

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.

Multimodal optogenetic neural interfacing device fabricated by scalable optical fiber drawing technique.

We present a novel approach to the design and manufacture of optrodes for use in the biomedical research field of optogenetic neural interfacing. Using recently developed optical fiber drawing techniques that involve co-drawing metal/polymer composite fiber, we have assembled and characterized a novel optrode with promising optical and electrical functionality. The fabrication technique is flexible, scalable, and amenable to extension to implantable optrodes with high-density arrays of multiple electrodes, waveguides, and drug delivery channels.

Refractive index sensor based on a polymer fiber directional coupler for low index sensing.

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.

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.

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.

Hybrid hollow core fibers with embedded wires as THz waveguides.

We experimentally demonstrate broadband terahertz (THz) pulse propagation through hollow core fibers with two or four embedded Indium wires in a THz time-domain spectroscopy (THz-TDS) setup. The hybrid mode is guided in the air core region with power attenuation coefficients of 0.3 cm(-1) and 0.5 cm(-1) for the two-wire and four-wire configurations, respectively.

A complementary study to "Hybrid hollow core fibers with embedded wires as THz waveguides" and "Two-wire terahertz fibers with porous dielectric support:" comment.

In a recent paper, Anthony et al. [Opt. Express 21, 2903 (2013)] demonstrated broadband terahertz pulse propagation through the hollow core fibers with two embedded Indium wires. In another paper by A. Markov et al. [Opt. Express 21, 12728 (2013)], we proposed a plasmonic THz fiber featuring two metallic wires held in place by the porous dielectric cladding functioning as a mechanical support. Although the cross sections of the two waveguides look very similar, we were surprised to find that the guidance mechanisms for these two waveguides are quite different. In fact, waveguide considered by A. Markov et al. was guiding a plasmonic mode, while the waveguide presented by Anthony et al. was guiding a dielectric waveguide-like mode. Finally, we have realized that by reducing the waveguide dimensions by a factor of ~10-20 one can transition from the dielectric waveguide guidance as it is demonstrated by Anthony et al. to plasmonic guidance as reported in A. Markov et al. Therefore, we conclude that both waveguide are essentially identical, while their guidance mechanism changes as a function of the waveguide overall size.

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.

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.

Spectroscopy of 3D-trapped particles inside a hollow-core microstructured optical fiber.

We report on the demonstration of three-dimensional optical trapping inside the core of a hollow-core microstructured optical fiber specifically designed and fabricated for this purpose. Optical trapping was achieved by means of an external tweezers beam incident transversely on the fiber and focused through the fiber cladding. Trapping was achieved for a range of particle sizes from 1 to 5 µm, and manipulation of the particles in three-dimensions through the entire cross-section of the fiber core was demonstrated. Spectroscopy was also performed on single fluorescent particles, with the fluorescence captured and guided in the fiber core. Video tracking methods allowed the optical traps to be characterized and photobleaching of single particles was also observed and characterized.

Multicore composite single-mode polymer fiber.

We study, fabricate and characterize an all-solid polymer composite waveguide consisting of a multicore fiber for single-mode operation down to the visible. The individual cores of the multicore structure are arranged such that they strongly interact, to form the composite core. The behavior and parameters of the multicore geometry are analyzed in order to achieve true single-mode operation. The composite core fiber is fabricated with off-the-shelf poly-methyl-methacrylate (PMMA) and Zeonex 480R polymers and characterized.