Yasa Ekşioğlu

Optical Response of Plasmonic Nanohole Arrays: Comparison of Square and Hexagonal Lattices

Nanohole arrays in metal films allow extraordinary optical transmission (EOT); the phenomenon is highly advantageous for biosensing applications. In this article, we theoretically investigate the performance of refractive index sensors, utilizing square and hexagonal arrays of nanoholes, that can monitor the spectral position of EOT signals. We present near- and far-field characteristics of the aperture arrays and investigate the influence of geometrical device parameters in detail. We numerically compare the refractive index sensitivities of the two lattice geometries and show that the hexagonal array supports larger figure-of-merit values due to its sharper EOT response. Furthermore, the presence of a thin dielectric film that covers the gold surface and mimics a biomolecular layer causes larger spectral shifts within the EOT resonance for the hexagonal array. We also investigate the dependence of the transmission responses on hole radius and demonstrate that hexagonal lattice is highly promising for applications demanding strong light transmission.

Matter-wave interferometry in periodic and quasi-periodic arrays

Abstract We calculate within a Bose–Hubbard tight-binding model the matter-wave flow driven by a constant force through a Bose–Einstein condensate of 87 Rb atoms in various types of quasi-onedimensional arrays of potential wells. Interference patterns are obtained when beam splitting is induced by creating energy minigaps either through period doubling or through quasi-periodicity governed by the Fibonacci series. The generation of such condensate modulations by means of optical-laser structures is also discussed.

Dynamical analysis of a weakly coupled nonlinear dielectric waveguide: Surface-plasmon model as another type of Josephson junction

We propose that a weakly-coupled nonlinear dielectric waveguide – surface-plasmon system can be formulated as a new type of Josephson junction. Such a system can be realized along a metal dielectric interface where the dielectric medium hosts a nonlinear waveguide (e.g. fiber) for soliton propagation. We demonstrate that the system is in close analogy to the bosonic Josephson-Junction (BJJ) of atomic condensates at very low temperatures, yet exhibits different dynamical features. In particular, the inherently dynamic coupling parameter between soliton and surfaceplasmon generates self-trapped oscillatory states at nonzero fractional populations with zero and π time averaged phase difference. The salient features of the dynamics are presented in the phase space. PACS numbers: 05.45.-a, 42.65.-k, 03.75.Lm

Dissipative Josephson junction of an optical soliton and a surface plasmon

We examine the dynamics of a dissipative photonic Josephson junction formed by the weak coupling of an optical soliton in a nonlinear dielectric waveguide and a co-propagating surface plasmon along a parallel metal surface with a linear dielectric spacer. We employ a heuristic model with a coupling function that depends on the soliton amplitude, and consider two phenomenological dissipation mechanisms separately: angular velocity dissipation and population imbalance dissipation. In the former dissipation mechanism, the system exhibits phase-slip phenomenon where the odd-π phase modes decay into even-π phase modes. The latter damping mechanism sculptures the phase-space significantly by introducing complex features, among which Hopf type bifurcations are notable. We show that some of the bifurcation points expand to stable limit cycles for certain regimes of the model parameters.

Extraordinary Transmission Characteristics of Subwavelength Nanoholes with Rectangular Lattice

We investigate the extraordinary optical transmission (EOT) properties of nanohole arrays with a rectangular lattice for label-free refractive index sensing applications. We show that the deviation within the periodicities along the two axes at the nanohole plane leads to more advantageous spectral quality of EOT signal compared to the conventional square lattice geometries. We introduce a way to further improve the sensitivity of the aperture system by carefully choosing the periodicities. We introduce nanohole arrays with a rectangular lattice supporting EOT signals with larger figure-of-merit values as well as enabling much stronger light transmission. We also model a nanohole system covered with a thin dielectric layer, mimicking biomolecules captured on the gold surface, in order to show its biosensing capability. We also show that certain deviation amounts between periodicities create spectral splitting within the EOT signal leading to larger spectral shifts in the presence of a thin dielectric film.

Self-pulsing and chaos in Kerr-nonlinear coupled ring resonators

We investigate the dynamics of nonlinear resonant structures consisting of two coupled ring resonators. The time evolution of the system is modeled by difference-differential equations that take into account non-instantaneous Kerr response and the effect of loss. We demonstrate transitions from stable states to periodic (self-pulsing) and chaotic states. We identify parameters that can significantly affect onset and period of self-pulsing solutions.

Multi-Band Plasmonic Platform Utilizing UT-Shaped Graphene Antenna Arrays

In this work, we introduce a plasmonic platform based on UT-shaped graphene antenna arrays. The proposed multi-resonant platform shows three different resonances, which can be independently tuned. The physical origin of these modes is shown with finite-difference time-domain (FDTD) nearfield distribution analyses, which are used to statically tune each resonance wavelength via the geometrical parameters, corresponding to different nearfield localization. We achieve statistical tuning of multiple resonances also by changing the number of graphene layers. Another static tuning of the optical response of the UT-shaped graphene antenna is achieved via the chemical potential and the relaxation time.

Investigation of plasmonic transmission in UT shaped graphene arrays

In this work, we studied the multi-band plasmonic UT graphene antenna arrays. The proposed model shows three different resonance frequencies. We show nearfield distributions of corresponding resonance frequencies and investigate the effect of the geometrical parameters, chemical potential, relaxation time, thickness of the substrate and different refractive index of the material on the spectral position of the UT-shaped graphene antenna.

Self-pulsing in nonlinear Kerr-type coupled ring resonators with effect of loss

We focus on dynamical analysis of nonlinear structures consisting of coupled ring resonators. We formulate a system of difference-differential equations that take into account non-instantaneous Kerr response and the effect of loss. The system is applied to investigation of a double-ring structure in the all-pass filter configuration. We observe rich dynamics, transitions from steady state to Hopf bifurcations and chaos. The system is highly sensitive for the values of detuning from resonance and input power. The influence of loss on oscillatory states is also presented.

Physics and advanced simulations of photonic and plasmonic structures

In this contribution, we present the main results of our joint scientific theoretical project with the Czech Science Foundation (2010-2013) Physics and advanced simulations of photonic and plasmonic structures, arisen from the cooperation of three laboratories of Czech Technical University in Prague, Institute of Photonics and Electronics of the Academy of Sciences of the Czech Republic, and Brno University of Technology. First, we present the basics of our in-house methods and numerical tools for the analysis of such structures, developed independently within the scope of the project, together with their mutual comparison. Three linear frequency-domain modal three-dimensional (3D) numerical methods developed and adapted for modelling photonic / plasmonic guiding and resonant subwavelength (SW) structures, will be mentioned, namely, aperiodic rigorous coupled-wave analysis (aRCWA) method, bi-directional mode expansion propagation method (BEP) based on the Fourier series (BEX), as well as the finite difference (FD) / finite element (FE)-BEP technique, connecting the eigensolvers with advanced BEP-based scattering matrix algorithm. Subsequently, a special original method suitable for treating nonlinear structures with Kerr nonlinearities, based on the eigenmode expansion (EME), has been developed (NL-EME) and applied, too. These methods, together with several approximate methods, have formed a solid portfolio for subsequent analysis of various photonic and plasmonic subwavelength structures of interest. The project generated several novel and interesting results, introducing novel structure designs in the following areas: novel magnetooptic (MO) guiding structures with non-reciprocal properties, advanced plasmonic structures based on hybrid dielectric plasmonic slot waveguides, nonlinear plasmonic couplers, SW grating structured waveguides, 3D resonant high-Q nanostructures, gain-loss guiding structures as photonic analogues of quantum structures with parity-time (PT)-symmetry breaking. Selected results of modelling of these promising SW structure designs will be presented and discussed, together with a new result based on our recent investigation of the plasmon-soliton interaction.