Kürşat Şendur

Broadband plasmonic nanoantenna with an adjustable spectral response

Six-particle and eight-particle common-gap plasmonic nanoantennas are utilized to obtain a broadband spectral response when illuminated with circular and elliptical polarization. Due to the insensitivity of dipole antennas to circular polarization, the resonant structures are brought together around the common-gap to expand the spectrum of the whole system. Their ability to focus light at different frequencies is demonstrated. The spectral response is manipulated by geometrical parameters and the strength of the spectral peaks is tailored through the ellipticity of the elliptically polarized light.

Circularly and elliptically polarized near-field radiation from nanoscale subwavelength apertures

With advances in nanotechnology, obtaining circularly and elliptically polarized optical spots beyond the diffraction limit is an emerging need for plasmonic applications. Two techniques are suggested to obtain circularly and elliptically polarized near-field radiation using subwavelength apertures. It is demonstrated that a square aperture can mediate diffraction limited circularly or elliptically polarized radiation into an optical spot with circular or elliptical polarization beyond the diffraction limit. Linearly polarized diffraction limited radiation is converted into a circularly or an elliptically polarized optical spot beyond the diffraction limit by creating asymmetry in the subwavelength aperture.

Patterned medium for heat assisted magnetic recording

Heat assisted magnetic recording (HAMR) is a potential solution to extend the limits of conventional magnetic recording. In HAMR, the heating of the recording medium is achieved with a near-field optical transducer. Although the literature suggests novel transducers, there is little consideration of the optical and thermal aspects of the magnetic medium. In this letter we suggest a recording medium that provides a significant enhancement in optical absorption and localized heating. The thermal profiles of the proposed medium and the conventional medium are compared using finite element method solutions of Maxwell’s and the heat transfer equations.

Unidirectional broadband radiation of honeycomb plasmonic antenna array with broken symmetry

Emerging plasmonic and photovoltaic applications benefit from effective interaction between optical antennas and unidirectional incident light over a wide spectrum. Here, we propose a honeycomb array of plasmonic nanoantennas with broken symmetry to obtain a unidirectional radiation pattern over a wide spectrum. The honeycomb nanoantenna array is based on a hexagonal grid with periodically arranged nanostructure building blocks. To analyze the far-field optical distribution and spectral behavior of the plasmonic antenna honeycomb, a two-dimensional Wigner-Seitz unit cell is used together with periodic boundary conditions. As a result of the vectoral superposition of the fields produced by the Wigner-Seitz unit cells, far-zone optical fields interfere constructively or destructively in different directions. The constructive interference along the arrays normal direction engenders unidirectional radiation. Due to the broken symmetry of the Wigner-Seitz cell, multiple resonances are supported by the plasmonic antenna honeycomb array over a broad spectrum.

Heat transfer enhancement with actuation of magnetic nanoparticles suspended in a base fluid

In this study, we have experimentally demonstrated that heat transfer can be substantially increased by actuating magnetic nanoparticles inside a nanofluid. In order to materialize this, we have utilized a miniature heat transfer enhancement system based on the actuation of magnetic nanoparticles dispersed in a base fluid (water). This compact system consists of a pool filled with a nanofluid containing ferromagnetic nanoparticles, a heater, and two magnetic stirrers. The ferromagnetic particles within the pool were actuated with the magnetic stirrers. Single-phase heat transfer characteristics of the system were investigated at various fixed heat fluxes and were compared to those of stationary nanofluid (without magnetic stirring). The heat transfer enhancement realized by the circulation of ferromagnetic nanoparticles dispersed in a nanofluid was studied using the experimental setup. The temperatures were recorded from the readings of thin thermocouples, which were integrated to the heater surface. The ...

Effect of fly height and refractive index on the transmission efficiency of near-field optical transducers

Heat-assisted magnetic recording is a potential remedy to extend the limits of magnetic recording. A high temperature with a steep gradient is used to reduce the local coercivity of the magnetic medium. To achieve such a thermal profile, an intense optical spot well below the diffraction limit is necessary. Transmission efficiency of a near-field optical transducer is affected by various factors at the head-medium interface. Effect of fly height and presence of high refractive index material at the head-medium interface is investigated. Favorable conditions are identified.

Boiling heat transfer enhancement of magnetically actuated nanofluids

Nanofluids offer a potential breakthrough as next-generation heat transfer fluids since they offer exciting new possibilities to enhance heat transfer performance compared to pure liquids. A major drawback for using nanofluids in practical applications is difficulty in maintaining their stability due to deposition on surfaces. In this study, we propose and experimentally investigate a magnetic actuation scheme to avoid this deposition. Two-phase heat transfer characteristics of the designed system have been experimentally investigated with magnetic actuation and compared to the results without magnetic actuation. Two phase average heat transfer enhancement observed with the suggested system was 17%. The average single phase enhancement is found as 29% with magnetic actuation. It was observed that magnetically actuated nanoparticles neither form any clusters nor precipitate after the experiments.

Absorption efficiency enhancement in inorganic and organic thin film solar cells via plasmonic honeycomb nanoantenna arrays

We demonstrate theoretically that by embedding plasmonic honeycomb nanoantenna arrays into the active layers of inorganic (c-Si) and organic (P3HT:PCBM/PEDOT:PSS) thin film solar cells, absorption efficiency can be improved. To obtain the solar cell absorption spectrum that conforms to the solar radiation, spectral broadening is achieved by breaking the symmetry within the Wigner-Seitz unit cell on a uniform hexagonal grid. For optimized honeycomb designs, absorption efficiency enhancements of 106.2% and 20.8% are achieved for c-Si and P3HT:PCBM/PEDOT:PSS thin film solar cells, respectively. We have demonstrated that the transverse modes are responsible for the enhancement in c-Si solar cells, whereas both the longitudinal and transverse modes, albeit weaker, are the main enhancement mechanisms for P3HT:PCBM/PEDOT:PSS solar cells. For both inorganic and organic solar cells, the absorption enhancement is independent of polarization.

Perpendicular oriented single-pole nano-optical transducer

Nano-optical transducers have been utilized in existing and emerging applications due to their ability to obtain small optical spots, large transmission efficiency, and narrow and adjustable spectral response. In emerging nano-optical applications, such as heat assisted magnetic recording (HAMR), these features are not sufficient. For example, in HAMR a transducer should also satisfy additional requirements, such as massproduction and integrability with other device components. In this study, the basic principles of Maxwells equations and image theory for good metals are utilized to design a perpendicular oriented single-pole nano-optical transducer, which can be integrated into the manufacturing technologies of current hard disk drive heads. The perpendicular oriented single-pole nano-optical transducer is investigated using 3-D finite element method. Gold transducers are investigated for both longitudinal and perpendicular orientations. The optical intensity profiles and spot sizes of longitudinal and perpendicular oriented transducers are compared for various fly heights. It is shown that a perpendicular ridge waveguide provides localized optical spots with intensities comparable to longitudinal transducers.

Increasing the stability of nanofluids with cavitating flows in micro orifices

One of the most critical challenges for nanofluids in practical applications is related to their stability and reusability since a gradual agglomeration of nanoparticles in nanofluids occurs with time and is accelerated by heating. In this study, we propose a technique to maintain the performance and stability of nanofluids with the use of cavitating flows through micro orifices to prevent agglomeration and sedimentation of nanoparticles, which will increase the durability of the nanofluids. γ-Al2O3 (gamma-alumina) nanoparticles with a mean diameter of 20 nm suspended in water were utilized. In the current approach, a flow restrictive element induces sudden pressure, which leads to cavitation bubbles downstream from the orifice. The emerging bubbles interact with the agglomerated structure of nanoparticles and decrease its size through hitting or shock waves generated by their collapse, thereby increasing the stability and reusability of nanofluids. The method does not involve any use of expensive surfact...