Mehmet Bayindir

Transmission properties of composite metamaterials in free space

We propose and demonstrate a type of composite metamaterial which is constructed by combining thin copper wires and split ring resonators (SRRs) on the same board. The transmission measurements performed in free space exhibit a passband within the stop bands of SRRs and thin wire structures. The experimental results are in good agreement with the predictions of the transfer matrix method simulations.

Photonic-crystal-based beam splitters

We proposed and demonstrated two different methods to split electromagnetic waves in three-dimensional photonic crystals. By measuring transmission spectra, it was shown that the guided mode in a coupled-cavity waveguide can be splitted into the coupled-cavity or planar waveguide channels without radiation losses. The flow of electromagnetic waves through output waveguide ports can also be controlled by introducing extra defects into the crystals. Our results may have an important role in the design of efficient power splitters in a photonic circuit.

Photonic crystal-based resonant antenna with a very high directivity

We investigate the radiation properties of an antenna that was formed by a hybrid combination of a monopole radiation source and a cavity built around a dielectric layer-by-layer three-dimensional photonic crystal. We measured a maximum directivity of 310, and a power enhancement of 180 at the resonant frequency of the cavity. We observed that the antenna has a narrow bandwidth determined by the cavity, where the resonant frequency can be tuned within the band gap of the photonic crystal. The measured radiation patterns agree well with our theoretical results.

Highly Transparent, Flexible, and Thermally Stable Superhydrophobic ORMOSIL Aerogel Thin Films

We report preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica (ORMOSIL) aerogel thin films from colloidal dispersions at ambient conditions. The prepared dispersions are suitable for large area processing with ease of coating and being directly applicable without requiring any pre- or post-treatment on a variety of surfaces including glass, wood, and plastics. ORMOSIL films exhibit and retain superhydrophobic behavior up to 500 °C and even on bent flexible substrates. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of <5°) by calcination at high temperatures. The wettability of the coatings can be changed by tuning the calcination temperature and duration. The prepared films also exhibit low refractive index and high porosity making them suitable as multifunctional coatings for many application fields including solar cells, flexible electronics, and lab on papers.

Solid-State Emissive BODIPY Dyes with Bulky Substituents As Spacers

Bright fluorescence of the BODIPY dyes, just like most other fluorophores, is quenched in the solid state due to reabsorption and self-quenching. However, introduction of bulky tert-butyl substituents on the meso-phenyl groups result in more spaced packing in the solid state, resulting in highly luminescent powders and films.

Metal-insulator-semiconductor optoelectronic fibres

The combination of conductors, semiconductors and insulators with well-defined geometries and at prescribed length scales, while forming intimate interfaces, is essential in most functional electronic and optoelectronic devices. These are typically produced using a variety of elaborate wafer-based processes, which allow for small features, but are restricted to planar geometries and limited coverage area. In contrast, the technique of fibre drawing from a preformed reel or tube is simpler and yields extended lengths of highly uniform fibres with well-controlled geometries and good optical transport characteristics. So far, this technique has been restricted to particular materials and larger features. Here we report on the design, fabrication and characterization of fibres made of conducting, semiconducting and insulating materials in intimate contact and in a variety of geometries. We demonstrate that this approach can be used to construct a tunable fibre photodetector comprising an amorphous semiconductor core contacted by metallic microwires, and surrounded by a cylindrical-shell resonant optical cavity. Such a fibre is sensitive to illumination along its entire length (tens of meters), thus forming a photodetecting element of dimensionality one. We also construct a grid of such fibres that can identify the location of an illumination point. The advantage of this type of photodetector array is that it needs a number of elements of only order N, in contrast to the conventional order N2 for detector arrays made of photodetecting elements of dimensionality zero.

Transmission and reflection properties of composite double negative metamaterials in free space

We report free space transmission and the first reflection measurements of a composite double negative (DNG) metamaterial, also known as a left-handed material (LHM). The metamaterial composes of the split-ring-resonators and discontinuous thin wires. Very high transmission values of the metamaterial are observed within a frequency range for which both effective permeability and permittivity are expected to be negative.

Arrays of indefinitely long uniform nanowires and nanotubes

Nanowires are arguably the most studied nanomaterial model to make functional devices and arrays. Although there is remarkable maturity in the chemical synthesis of complex nanowire structures, their integration and interfacing to macro systems with high yields and repeatability still require elaborate aligning, positioning and interfacing and post-synthesis techniques. Top-down fabrication methods for nanowire production, such as lithography and electrospinning, have not enjoyed comparable growth. Here we report a new thermal size-reduction process to produce well-ordered, globally oriented, indefinitely long nanowire and nanotube arrays with different materials. The new technique involves iterative co-drawing of hermetically sealed multimaterials in compatible polymer matrices similar to fibre drawing. Globally oriented, endlessly parallel, axially and radially uniform semiconducting and piezoelectric nanowire and nanotube arrays hundreds of metres long, with nanowire diameters less than 15 nm, are obtained. The resulting nanostructures are sealed inside a flexible substrate, facilitating the handling of and electrical contacting to the nanowires. Inexpensive, high-throughput, multimaterial nanowire arrays pave the way for applications including nanowire-based large-area flexible sensor platforms, phase-changememory, nanostructure-enhanced photovoltaics, semiconductor nanophotonics, dielectric metamaterials,linear and nonlinear photonics and nanowire-enabled high-performance composites.

Turn-on Fluorescent Dopamine Sensing Based on in Situ Formation of Visible Light Emitting Polydopamine Nanoparticles

Dopamine is the principle biomarker for diseases such as schizophrenia, Huntingtons, and Parkinsons, and the need is urgent for rapid and sensitive detection methods for diagnosis and monitoring of such diseases. In this Article, we report a turn-on fluorescent method for rapid dopamine sensing which is based on monitoring the intrinsic fluorescence of in situ synthesized polydopamine nanoparticles. The assay uses only a common base and an acid, NaOH and HCl to initiate and stop the polymerization reaction, respectively, which makes the assay extremely simple and low cost. First, we studied the in situ optical properties of polydopamine nanoparticles, for the first time, which formed under different alkaline conditions in order to determine optimum experimental parameters. Then, under optimized conditions we demonstrated high sensitivity (40 nM) and excellent selectivity of the assay. With its good analytical figures of merit, the described method is very promising for detection of dopamine related diseases.

Hollow multilayer photonic bandgap fibers for NIR applications

Here we report the fabrication of hollow-core cylindrical photonic bandgap fibers with fundamental photonic bandgaps at near-infrared wavelengths, from 0.85 to 2.28 microm. In these fibers the photonic bandgaps are created by an all-solid multilayer composite meso-structure having a photonic crystal lattice period as small as 260 nm, individual layers below 75 nm and as many as 35 periods. These represent, to the best of our knowledge, the smallest period lengths and highest period counts reported to date for hollow PBG fibers. The fibers are drawn from a multilayer preform into extended lengths of fiber. Light is guided in the fibers through a large hollow core that is lined with an interior omnidirectional dielectric mirror. We extend the range of materials that can be used in these fibers to include poly(ether imide) (PEI) in addition to the arsenic triselenide (As(2)Se(3)) glass and poly(ether sulfone) (PES) that have been used previously. Further, we characterize the refractive indices of these materials over a broad wavelength range (0.25 - 15 microm) and incorporated the measured optical properties into calculations of the fiber photonic band structure and a preliminary loss analysis.