Browsing by Author "Kurt, H."

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Open Access
Broadband one way propagation via dielectric waveguides with unequal effective index
(IEEE, 2014) Öner, B. B.; Üstün, K.; Kurt, H.; Okyay, Ali Kemal; Turhan-Sayan, G.
We present an efficient approach for broad band one way propagation of light by parallel and unequal dielectric waveguides leading different effective phase shifts. Three dimensional numerical simulations show that 30% operating bandwidth is achieved.
Open Access
Crossed chiral band approximation for wide-band self-collimation of light
(Institute of Physics Publishing, 2020) Gümüş, M. A.; Akçaalan, Önder; Kurt, H.
We propose a perspective to the evaluation of the wide bandwidth phenomenon for low-symmetric photonic structures, by introducing the band tailoring and chiral band approximation on the self-collimation effect. In the case of the crossing of the bands, we claim the excitation of the lower mode can provide the utilization of the entire bandwidth by suppressing the intersection regions where the frequencies tend to mix. Thereby, we design broadband self-collimation capable, defect-free photonic structures and examine their performances. A fractional bandwidth of 0.35 (a/λ = 0.429 − 0.607) and 0.37 (a/λ = 0.481 − 0.701) are achieved for radii of r = 0.25a and r = 0.23a, respectively. We explore the full-range collimation by using transmission and E-field intensity analyses in addition to band diagrams and group velocity dispersions. Moreover, we indicate all-angle collimation validity even for highly tilted sources up to an angle of 80◦ .
Open Access
Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides
(Elsevier, 2012-02-28) Akosman, Ahmet E.; Mutlu, Mehmet; Kurt, H.; Özbay, Ekmel
A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.
Open Access
Dynamic beam splitter employing an all-dielectric metasurface based on an elastic substrate
(OSA - The Optical Society, 2020) Koçer, Hasan; Durna, Y.; Kurt, H.; Özbay, Ekmel
Beam splitters are an indispensable part of optical measurements and applications. We propose a dynamic beam splitter incorporating all-dielectric metasurface in an elastic substrate under external mechanical stimulus of stretching. The optical behavior at 720 nm wavelength shows that it can be changed from a pure optical-diode-like behavior to a dynamic beam splitter. Although the structure is designed running at 720 nm, the design approach with appropriate materials can be used at any wavelength. Various cases, including wavelength and polarization dependencies, are thoroughly investigated to demonstrate the principles of operating conditions of two different regimes of the designed metasurface.
Open Access
Enhanced superprism effect in symmetry reduced photonic crystals
(American Institute of Physics, 2018) Gumus, M.; Giden, I. H.; Akcaalan, O.; Turduev, M.; Kurt, H.
We propose compact S-vector superprism providing broadband wavelength sensitivity within a/λ = 0.610-0.635, where "a" is the lattice constant, λ is the incident wavelength, and S denotes the Poynting vector. The reported configuration overcomes strong beam divergence and complex beam generation due to the self-collimation ability of the low symmetric primitive photonic crystal (PhC) cells. Analytical calculations of equi-frequency contours, photonic band structures, and group velocity dispersions are performed by solving Maxwell's equations and using the plane wave expansion method. Besides, finite-difference time-domain analyses are also conducted. The designed superprism induces large refracted angle variation for different frequencies when the incident angle is fixed: 4% change of incident frequencies results in approximately 40° deflected angle difference with a maximum 68.9° deflection angle inside the PhC. Meanwhile, for a fixed incident wavelength, a large output variation occurs if the incident angle is altered. Microwave experimental results are found to be in good agreement with the numerical analyses.
Open Access
The focusing effect of graded index photonic crystals
(AIP Publishing LLC, 2008) Kurt, H.; Colak, E.; Cakmak, O.; Caglayan, H.; Özbay, Ekmel
We describe an approach to implement graded index (GRIN) structures using two-dimensional photonic crystals (PCs). The lattice spacing along the transverse direction to propagation is altered and we show, both theoretically and experimentally, that such a spatial perturbation is an effective way to obtain GRIN PC. The response of the structure to spatially wide incident beams is investigated and strong focusing behavior is observed. The large spot size conversion ratio can be attainable and is mainly limited by the finite size of the structure. The designed GRIN PC shows promise for use in optical systems that require compact and powerful focusing elements compared to the traditional bulky lenses.
Open Access
High efficiency of graded index photonic crystal as an input coupler
(AIP Publishing LLC, 2009) Cakmak, A. O.; Colak, E.; Caglayan, H.; Kurt, H.; Özbay, Ekmel
A graded index photonic crystal (GRIN PC) configuration was placed at the input side of a photonic crystal waveguide (PCW) in order to efficiently couple the light waves into the waveguide. We compared the transmission efficiencies of light in the absence and presence of the GRIN PC structure. We report a significant improvement in coupling when the GRIN PC is incorporated with the PCW. The intensity profiles were obtained by carrying out the experiments at microwave frequencies. Finite difference time domain based simulations were found to be in good agreement with our experimental results.
Open Access
Multiple slow waves and relevant transverse transmission and confinement in chirped photonic crystals
(OSA Publishing, 2014-09-02) Serebryannikov, A. E.; Cakmak, A. O.; Colak, E.; Caglayan, H.; Kurt, H.; Özbay, Ekmel
The dispersion properties of rod-type chirped photonic crystals (PhCs) and non-channeled transmission in the direction of the variation of structural parameters from one cell of such a PhC to another are studied. Two types of configurations that enable multiple slow waves but differ in the utilized chirping scheme are compared. It is demonstrated that the multiple, nearly flat bands with a group index of refraction exceeding 180 can be obtained. For these bands, transmission is characterized by multiple narrow peaks of perfect transmission, strong field enhancement inside the slab, and large values of the Q-factor. Among the bands, there are some that show negative phase velocity. Symmetry with respect to the slab mid-plane must be kept in order to obtain constructive interferences that are necessary for reflection-free transmission. It is shown that 15 and more slow wave bands can be obtained in one configuration. The corresponding transmission peaks are well separated from each other, being the only significant feature of the transmission spectrum, while the Q-factor can exceed 105. The observed features are preserved in a wide range of the incidence angle variation. They can be used for tuning the locations and spectral widths of the transmission peaks. Some comparisons with the chirped multilayer structures have been carried out. ©2014 Optical Society of America.
Open Access
Nanosphere concentrated photovoltaics with shape control
(Wiley, 2020-12) Esmaeilzad, N. S.; Demir, Ahmet Kemal; Hajivand, J.; Çiftpınar, H.; Turan, R.; Kurt, H.; Bek, A.
Dielectric colloidal nanospheres (NSs) are promising candidates for light management in photonic devices such as solar cells (SCs). NS arrays can direct the broad incident solar radiation into a set of tighter foci, at which light intensity becomes considerably concentrated, enabling higher photovoltaic conversion efficiency. Furthermore, the NS arrays acting as an effective medium on the SC surface can reduce reflectance and facilitate improved forward scattering. Therefore, uniform arrays of NSs located on top of the SC can behave as antireflection coatings or as microlenses, which can be regarded as a surface distributed light concentrator within the framework of concentrated photovoltaics. Fabrication of NS‐based light‐trapping structures is low‐cost and less complicated than common alternatives such as vacuum evaporated multilayer antireflection coatings. In this work, experimental demonstration and computational confirmation on the shape adjustment of such NS structures for improved light harvesting and efficiency enhancement in Si SCs are studied. The light conversion efficiency of Si solar cells is shown to improve by more than 27% with shape adjustment of NS arrays.
Open Access
Submicron size all-semiconductor vertical cavities with high Q
(Institute of Electrical and Electronics Engineers Inc., 2019) Demir, Abdullah; Apaydın, D.; Kurt, H.
The miniaturization of lasers promises on-chip optical communications and data processing speeds that are beyond the capability of electronics and today's high-speed lasers. Lasers with low-power consumption are one of the most important parts in creating a photonics integrated architecture. This requirement was the motivating force behind the development of small laser and nanolasers. Here, we propose a new method that could be utilized to fabricate such a laser. Oxide-VCSELs require strict control of the oxidation process with significantly reduced reliability for small size, and micropillars have degraded Q with fabrication artifacts for submicron diameter pillars. We propose to use a phase-shifting current-blocking (PSCB) layer serving dual function for a nanocavity device (Fig. 1a) providing both optical- and electrical-confinement via lithographically defined and selectively-biased buried structures. Phase-shifting leads to optical-confinement tuning by layer thickness control and current-blocking provides electrical-confinement. By modifying the dimensions of these layers, the confinement can be tuned by lithographic means. We studied the electromagnetic wave propagation and analyzed the quality factor (Q) of these cavities based on 3D finite difference time domain (FDTD) calculations.
Open Access
Theoretical and experimental investigations of asymmetric light transport in graded index photonic crystal waveguides
(AIP Publishing, 2014) Giden, I. H.; Yilmaz, D.; Turduev, M.; Kurt, H.; Colak, E.; Özbay, Ekmel
To provide asymmetric propagation of light, we propose a graded index photonic crystal (GRIN PC) based waveguide configuration that is formed by introducing line and point defects as well as intentional perturbations inside the structure. The designed system utilizes isotropic materials and is purely reciprocal, linear, and time-independent, since neither magneto-optical materials are used nor time-reversal symmetry is broken. The numerical results show that the proposed scheme based on the spatial-inversion symmetry breaking has different forward (with a peak value of 49.8%) and backward transmissions (4.11% at most) as well as relatively small round-trip transmission (at most 7.11%) in a large operational bandwidth of 52.6 nm. The signal contrast ratio of the designed configuration is above 0.80 in the telecom wavelengths of 1523.5-1576.1 nm. An experimental measurement is also conducted in the microwave regime: A strong asymmetric propagation characteristic is observed within the frequency interval of 12.8 GHz-13.3 GHz. The numerical and experimental results confirm the asymmetric transmission behavior of the proposed GRIN PC waveguide. (C) 2014 AIP Publishing LLC.
Open Access
Two-dimensional complex parity-time-symmetric photonic structures
(American Physical Society, 2015-02) Turduev, M.; Botey, M.; Giden, I.; Herrero, R.; Kurt, H.; Özbay, Ekmel; Staliunas, K.
We propose a simple realistic two-dimensional complex parity-time-symmetric photonic structure that is described by a non-Hermitian potential but possesses real-valued eigenvalues. The concept is developed from basic physical considerations to provide asymmetric coupling between harmonic wave components of the electromagnetic field. The structure results in a nonreciprocal chirality and asymmetric transmission between in- and out-coupling channels into the structure. The analytical results are supported by a numerical study of the Bloch-like mode formations and calculations of a realistic planar semiconductor structure. © 2015 American Physical Society.
Open Access
Uncovering the non-radiative thermal characteristics of a passive radiative cooler under real operating conditions
(Institute of Physics Publishing Ltd., 2022-12-12) Koçer, Hasan; Durna, Yılmaz; Işık, Halil; Soydan, Mahmut Can; Khalichi, Bahram; Ghobadi, Amir; Kurt, H.; Özbay, Ekmel
Passive radiative cooling (PasRadCool), which emits thermal energy from objects to deep cold space through atmospheric transparency, offers complementary and alternative green energy solutions for passive cooling of buildings, clothing, and renewable energy harvesting. Depending on the spectral emissive/absorptive properties of the unit under test (UUT), radiative heat exchanges occur between the UUT, atmosphere, and sun, while at the same time non-radiative heat exchange occurs. The performance of the PasRadCool is determined by the combined thermal and thermodynamic effects of both exchange mechanisms. Although the non-radiative heat exchange, which consists of conductive and convective processes to the outer surfaces of the UUT and the surrounding air fluid, is very sensitive to environmental changes, the actual performance is not fully determined since this feature is considered statically in many studies. Herein, we propose a method that reveals the non-radiative thermal characteristics of the PasRadCool under real operating conditions. With a photonic radiative cooler structure, which we manufacture as a proof of concept, we perform nighttime field test measurements in varying non-radiative thermal conditions. The proposed method extracts the time-dependent non-radiative heat transfer coefficient of the UUT as accurately as possible. We also confirm that our experimental result shows good agreement with both numerical and analytical methods. The proposed approach, which highlights the realistic thermal management of PasRadCool, is not specific to the circumstances of our study and can be applied to all PasRadCool situations with different geometry, material, and environmental conditions.
Open Access
Unveiling the optical parameters of vanadium dioxide in the phase transition region: a hybrid modeling approach
(Royal Society of Chemistry, 2020) Çakır, Mehmet Cihan; Koçer, Hasan; Durna, Y.; Yıldırım, Deniz Umut; Ghobadi, Amir; Hajian, Hodjat; Aydın, K.; Kurt, H.; Sağlam, N.; Özbay, Ekmel
The phase change behavior of vanadium dioxide (VO2) has been widely explored in a variety of optical and photonic applications. Commonly, its optical parameters have been studied in two extreme regimes: hot (metallic) and cold (insulating) states. However, in the transition temperatures, VO2 acts like an inherent metamaterial with mixed metallic-insulating character. In this range, the portions of metallic and insulating inclusions are tuned by temperature, and therefore a gradual change of optical parameters can be achieved. In this paper, a universal hybrid modeling approach is developed to model VO2 in the intermediate region. For this aim, the measured reflectivity data, is analyzed and matched through the transfer matrix method (TMM) simulations where an effective medium theory (EMT) is employed. Based on the findings of this approach, not only the relative portions of inclusions are tailored but also their grain shapes are significantly altered in the transition range. Finally, the modeling approach is testified by experimental findings through dynamic device applications operating at short and mid infrared wavelengths. In addition, the hysteretic behaviors on electrical, optical, and structural parameters of the VO2 film along the heating and cooling cycles are demonstrated by the experiments and scrutinized by the simulations.