Researcher:
Aksun, M. İrşadi

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Faculty Member

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M. İrşadi

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Aksun

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Aksun, M. İrşadi
Aksun, Muhammet İrşadi

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2002 - 2009172010 - 202010

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Researcher Of Publications: search.filters.isAuthorOfPublication.44f6c585-9298-4374-9a7d-b214b96fb022

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Now showing 1 - 10 of 27
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    Enhancement of resolution and propagation length by sources with temporal decay in plasmonic devices
    (Springer, 2020) Department of Electrical and Electronics Engineering; Department of Electrical and Electronics Engineering; Tetikol, Hüseyin Serhat; Aksun, M. İrşadi; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; College of Engineering; N/A; 28358
    Highly lossy nature of metals has severely limited the scope of practical applications of plasmonics. The conventional approach to circumvent this limitation has been to search for new materials with more favorable dielectric properties (e.g., reduced loss), or to incorporate gain media to overcome the inherent loss. In this study, however, we turn our attention to the source and show that the wealth of new SPP modes with simultaneous complex frequencies and complex wave vectors that are otherwise unreachable can be excited by imposing temporal decay on the excitation. Therefore, to understand the possible implications of these new modes and how to be able to tune them for specific applications, we propose a framework of pseudo-monochromatic modes that are generated by introducing exponential decays into otherwise monochromatic sources. Within this framework, the dispersion relation of complex SPPs is re-evaluated and cast to be a surface rather than a curve, depicting all possible omega-kpairs (both complex in general) that are supported by the given geometry. To demonstrate the potentials of the complex modes and the use of the framework to study them selectively, we have chosen two important, and somewhat limiting, features of SPPs to investigate; resolution in plasmonic lenses and propagation length in SPP waveguides. While the former is mainly used to validate the proposed method and the framework on the recent improvement of resolution in plasmonic superlenses, the latter provides a novel approach to extend the propagation length of the SPP modes in planar waveguides significantly. Since the improvement in propagation length due to the introduction of temporal decay to the excitation is rather counter-intuitive, the dispersion-based theoretical predictions (the proposed approach) have been validated via the FDTD simulations of Maxwell's equations in the same geometry without any a priori assumptions on the frequency or the wave vector.
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    Characterization of finite photonic crystals with defects
    (Institute of Electrical and Electronics Engineers (IEEE), 2011) N/A; Department of Electrical and Electronics Engineering; Karabulut, Emine Pınar; Aksun, M. İrşadi; Reseacher; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; College of Engineering; N/A; 28358
    A simple computational approach is proposed to obtain the dispersion characteristics that could be observed outside of general finite-extent photonic crystals with defects. Since introducing and tailoring defects in photonic crystals are crucial for designing practical devices, the proposed method may play an important role in characterization and optimization of such defects. The method uses reflection data, due to an incident plane wave at a given frequency, collected at the front interface of a photonic crystal. It is simple and applicable for general photonic crystals, that is, photonic crystals with any periodicity, 1D, 2D, and 3D, and even with any kind of defects. The validity of the method was tested and verified on 1D and 2D finite photonic crystals, for which the reflection coefficient data at the front interface can be easily obtained by analytical means and numerical simulations, respectively. In addition, different types of defects, like random and periodic defects, were studied and it has been shown that the method is capable of providing information pertinent to the outside world on the defect modes.
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    Closed-form representations of field components of fluorescent emitters in layered media
    (Optical Soc Amer, 2009) Doğan, Mehmet; Swan, Anna K.; Goldberg, Bennett B.; Ünlü, M. Selim; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358
    Dipole radiation in and near planar stratified dielectric media is studied theoretically within the context of fluorescence microscopy, as fluorescent emitters are generally modeled by electric dipoles. Although the main emphasis of this study is placed on the closed-form representations of the field components of fluorescent emitters in layered environments in near- and far-field regions, the underlying motive is to understand the limits of spectral self-interference fluorescence microscopy in studying the dipole orientation of fluorophores. Since accurate calculations of the field components of arbitrarily polarized electric dipoles in layered environments are computationally very time-consuming, a method for finding their closed-form representations is proposed using the closed-form potential Green's functions previously developed for microwave applications. The method is verified on typical geometries used in spectral self-interference microscopy experiments, where a dipole emitter is positioned over a slab of SiO2 on top of a Si substrate. In addition to facilitating efficient calculation of near and intermediate fields of fluorescent emitters, closed-form Green's functions for fields would also play a crucial role in developing efficient and rigorous computational analysis and design tools for optical passive devices such as optical antennas by significantly improving the computational cost of the numerical solution of the integral equation.
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    Clarification of issues on the closed-form Green's functions in stratified media
    (Institute of Electrical and Electronics Engineers (IEEE), 2005) Dural, G.; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358
    The closed-form Green's functions (CFGF), derived for the vector and scalar potentials in planar multilayer media, have been revisited to clarify some issues and misunderstandings on the derivation of these Green's functions. In addition, the range of validity of these Green's functions is assessed with and without explicit evaluation of the surface wave contributions. As it is well-known, the derivation of the CFGF begins with the approximation of the spectral-domain Green's functions by complex exponentials, and continues with applying the Sommerfeld identity to cast these approximated spectral-domain Green's functions into the space domain in closed forms. Questions and misunderstandings of this derivation, which have mainly originated from the approximation process of the spectral-domain Green's functions in terms of complex exponentials, can be categorized and discussed under the topics of: 1) branch-point contributions; 2) surface wave pole contributions; and 3) the accuracy of the obtained CFGF. When these issues are clarified, the region of validity of the CFGF so obtained may be defined better. Therefore, in this paper, these issues will be addressed first, and then their origins and possible remedies will be provided with solid analysis and numerical demonstrations.
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    A novel approach for the efficient and accurate computation of sommerfeld integral tails
    (Institute of Electrical and Electronics Engineers (IEEE), 2015) Karabulut, Emine Pınar; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358
    This paper presents a general computational method to evaluate slowly converging infinite integrals efficiently and accurately. The method applies a subspace algorithm to the set of partial integrals and approximates them interms of complex exponentials. The residue of the exponential term with zero pole directly corresponds to the result of the integration. The method is applied to Sommerfeld integral tails, which have an oscillating and slowly converging nature. The performance of the method is then compared to the generalized weighted averages algorithm which is one of the most efficient extrapolation methods for the convergence acceleration of the sequences obtained for the calculation of Sommerfeld integral tails.
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    Analysis of multiple vertical strips in planar geometries via DCIM-MoM
    (Springer-Verlag Berlin, 2006) Kınayman, Noyan; N/A; Department of Electrical and Electronics Engineering; Önal, Tayyar; Aksun, M. İrşadi; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 28358
    Vertical metallizations in planar geometry, like via holes in MMICs, shorting strips, and probe feeds in microstrip antennas, have become the integral parts of high-frequency circuits and/or multifunction antennas. Therefore, an efficient full-wave electromagnetic simulation algorithm needs to be developed for the analysis of planar geometries with multiple vertical metallizations. In this study, it is demonstrated that using the method of moments (MoM) in conjunction with the discrete complex image method (DCIM) for the analysis of printed structures with multiple vertical strips results in a robust and efficient full-wave analysis tool. The use of DOM together with MoM has already proved to be very efficient for printed geometries, where efficiency implies the overall computational performance. However, the approach proposed here is not only efficient in this sense but also extremely efficient to handle multiple vertical metallization.
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    Critical study of DCIM, and development of efficient simulation tool for 3D printed structures in multilayer media
    (The Electromagnetics Academy, 2006) Department of Electrical and Electronics Engineering; N/A; Aksun, M. İrşadi; Önal, Tayyar; Faculty Member; Master Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; 28358; N/A
    Since the discrete complex image method (DCIM) has been widely used in conjunction with the Method of Moments (MoM) to efficiently analyze printed structures, some lingering issues related to the implementation of DCIM and their brief clarifications are first reviewed. Then, an efficient and rigorous electromagnetic simulation algorithm, based on the combination of MoM and DCIM, is proposed and developed for the solution of mixed-potential integral equation (MPIE) for printed structures with multiple vertical strips in multilayer media. The algorithm is possibly the most efficient approach to handle multiple vertical conductors, even spanning more than one layer, in printed circuits.
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    Characterization of finite photonic crystals
    (IEEE, 2008) N/A; N/A; Department of Electrical and Electronics Engineering; Karabulut, Emine Pınar; Aksun, M. İrşadi; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 28358
    A new approach is proposed to obtain the dispersion characteristics of finite-extent photonic crystals. The method provides (w - k) diagram and information on the effective constitutive parameters of the structure and is applicable for general photonic crystals, that is, 1D, 2D and 3D photonic crystals with any kind of defects. The method utilizes the reflection data due to an incident plane wave at a given frequency, collected at the front interface of the photonic crystal for different numbers of unit cells. The reflection data can be obtained either analytically or by means of simulations or measurements. The method is verified on 1D and 2D perfect photonic crystals and 1D photonic crystal with defect.
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    Enhancing the robustness of the discrete complex image method for planar multilayered media
    (Institute of Electrical and Electronics Engineers (IEEE), 2007) Michalski, Krzysztof A.; Mosig, Juan R.; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358
    A simple method is described that enhances the robustness of the discrete complex image method (DCIM) for Green functions in planar multilayered media. The standard DCIM can fail in the large- ρ range, where ρ is the horizontal distance between the source and field-points-even if the guided waves are extracted prior to the complex image approximation. This can occur, in particular, when a significant lateral wave component is present. In the improved method, the spectral kernels are sampled close to the branch point, thus better capturing the lateral wave contribution to the spectrum. As a result, the applicability of the DCIM is extended to larger ρ values.
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    Discrete complex image method for planar multilayers with uniaxial anisotropy
    (Institute of Electrical and Electronics Engineers (IEEE), 2007) Michalski K.A.; Department of Electrical and Electronics Engineering; Aksun, M. İrşadi; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 28358
    The discrete complex image method (DCIM) is applied to compute the complete set of the mixed-potential integral equation (MPIE) kernels for planar multilayers with uniaxial anisotropy. It is assumed that the layered medium is of infinite lateral extent and the optic axis is normal to the stratification. Sample results are presented for a structure comprising positive- and negative-uniaxial layers and are shown to be in excellent agreement with reference data.