Ozan Aktas

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.

Spontaneous High Piezoelectricity in Poly(vinylidene fluoride) Nanoribbons Produced by Iterative Thermal Size Reduction Technique

We produced kilometer-long, endlessly parallel, spontaneously piezoelectric and thermally stable poly(vinylidene fluoride) (PVDF) micro- and nanoribbons using iterative size reduction technique based on thermal fiber drawing. Because of high stress and temperature used in thermal drawing process, we obtained spontaneously polar γ phase PVDF micro- and nanoribbons without electrical poling process. On the basis of X-ray diffraction (XRD) analysis, we observed that PVDF micro- and nanoribbons are thermally stable and conserve the polar γ phase even after being exposed to heat treatment above the melting point of PVDF. Phase transition mechanism is investigated and explained using ab initio calculations. We measured an average effective piezoelectric constant as -58.5 pm/V from a single PVDF nanoribbon using a piezo evaluation system along with an atomic force microscope. PVDF nanoribbons are promising structures for constructing devices such as highly efficient energy generators, large area pressure sensors, artificial muscle and skin, due to the unique geometry and extended lengths, high polar phase content, high thermal stability and high piezoelectric coefficient. We demonstrated two proof of principle devices for energy harvesting and sensing applications with a 60 V open circuit peak voltage and 10 μA peak short-circuit current output.

Macroscopic assembly of indefinitely long and parallel nanowires into large area photodetection circuitry.

Integration of nanowires into functional devices with high yields and good reliability turned out to be a lot more challenging and proved to be a critical issue obstructing the wide application of nanowire-based devices and exploitation of their technical promises. Here we demonstrate a relatively easy macrofabrication of a nanowire-based imaging circuitry using a recently developed nanofabrication technique. Extremely long and polymer encapsulated semiconducting nanowire arrays, mass-produced using the iterative thermal drawing, facilitate the integration process; we manually aligned the fibers containing selenium nanowires over a lithographically defined circuitry. Controlled etching of the encapsulating polymer revealed a monolayer of nanowires aligned over an area of 1 cm(2) containing a 10 × 10 pixel array. Each light-sensitive pixel is formed by the contacting hundreds of parallel photoconductive nanowires between two electrodes. Using the pixel array, alphabetic characters were identified by the circuitry to demonstrate its imaging capacity. This new approach makes it possible to devise extremely large nanowire devices on planar, flexible, or curved substrates with diverse functionalities such as thermal sensors, phase change memory, and artificial skin.

Nanoscale charging hysteresis measurement by multifrequency electrostatic force spectroscopy

We report a scanning probe technique that can be used to measure charging of localized states on conducting or partially insulating substrates at room temperature under ambient conditions. Electrostatic interactions in the presence of a charged particle between the tip and the sample is monitored by the second order flexural mode, while the fundamental mode is used for stabilizing the tip-sample separation. Cycling the bias voltage between two limits, it is possible to observe hysteresis of the second order mode amplitude due to charging. Results are presented on silicon nitride films containing silicon nanocrystals.

Label-Free Biosensing with High Selectivity in Complex Media using Microtoroidal Optical Resonators

Although label-free biosensors comprised of optical microcavities inherently possess the capability of resolving molecular interactions at individual level, this extreme sensitivity restricts their convenience for large scale applications by inducing vulnerability towards non-specific interactions that readily occur within complex media. Therefore, the use of optical microresonators for biosensing is mostly limited within strictly defined laboratory conditions, instead of field applications as early detection of cancer markers in blood, or identification of contamination in food. Here, we propose a novel surface modification strategy suitable for but not limited to optical microresonator based biosensors, enabling highly selective biosensing with considerable sensitivity as well. Using a robust, silane-based surface coating which is simultaneously protein resistant and bioconjugable, we demonstrate that it becomes possible to perform biosensing within complex media, without compromising the sensitivity or reliability of the measurement. Functionalized microtoroids are successfully shown to resist nonspecific interactions, while simultaneously being used as sensitive biological sensors. This strategy could pave the way for important applications in terms of extending the use of state-of-the-art biosensors for solving problems similar to the aforementioned.

Parametrically coupled multiharmonic force imaging

We report use of nonlinear tip-sample interactions to parametrically convert the frequency components of periodic tip-sample interaction forces to frequencies where they can be resonantly detected. One flexural mode of a cantilever is used for tapping-mode imaging and another flexural mode is used for detection of forces converted in presence of an externally injected mechanical oscillation at the difference frequency of the detecting mode and a harmonic of the tapping mode. Material contrast in attractive and repulsive regimes are demonstrated on samples with polymethyl methacrylate patterns and with deoxyribonucleic acid strands on silicon.

Tapered nanoscale chalcogenide fibers directly drawn from bulk glasses as optical couplers for high-index resonators

We report production of air-clad tapered chalcogenide fibers by directly drawing bulk glasses between cleaved tips of tapered silica fibers. Exploiting these tapered fibers with nanoscale waists as evanescent optical couplers, we demonstrate phase-matched coupling of light into on-chip whispering gallery mode chalcogenide microresonators with coupling efficiencies as high as 95%. To the best of our knowledge, this is the first-time demonstration of critical coupling into high-index microresonators by using high-index tapered fibers. The tapered chalcogenide fibers can also be utilized as optical couplers for microresonators made of various high-index materials, as well as for nonlinear optical applications.

Wavelength Conversion and Supercontinuum Generation in Silicon Optical Fibers

This paper describes the state of the art in wavelength conversion and supercontinuum generation using glass-clad silicon core optical fibers. Such semiconductor fibers have enjoyed considerable attention due to their intrinsically high third-order nonlinearities, which are markedly higher than in conventional infrared glasses. Results to date from small core silicon fibers fabricated using both the high-pressure chemical vapor deposition technique and the molten core drawing method are presented. Also discussed are directions for continued study and development, including engineering the dispersion and nonlinear properties as well as improved interconnection.

Chalcogenide microresonators tailored to distinct morphologies by the shaping of glasses on silica tapers

Production of chalcogenide (As₂Se₃) microresonators in sphere, loop, and bottle morphologies by the shaping of glasses at appropriate temperatures between cleaved silica tapers is reported. The quality factors exceed Q_S=6.2×10⁵, Q_B=6.7×10⁵, and Q_L=1.6×10⁴ for the sphere, bottle, and loop microresonators, respectively. All-optical thermally assisted tuning with a rate of 0.61 nm/mW is demonstrated for a bottle microcavity pumped via a silica taper at a wavelength of 670 nm. This technique enables practical and robust in situ production of chalcogenide microresonators thermally spliced to silica fibers in several morphologies with a wide tuning range of size.

Tapered silicon core fibers with nano-spikes for optical coupling via spliced silica fibers

Reported here is the fabrication of tapered silicon core fibers possessing a nano-spike input that facilitates their seamless splicing to conventional single mode fibers. A proof-of-concept 30 µm cladding diameter fiber-based device is demonstrated with nano-spike coupling and propagation losses below 4 dB and 2 dB/cm, respectively. Finite-element-method-based simulations show that the nano-spike coupling losses could be reduced to below 1 dB by decreasing the cladding diameters down to 10 µm. Such efficient and robust integration of the silicon core fibers with standard fiber devices will help to overcome significant barriers for all-fiber nonlinear photonics and optoelectronics.