Haluk Şafak - Academia.edu (original) (raw)

Papers by Haluk Şafak

The Broad Band Optical Absorber Designed By Multilayered Hyperbolic Metamaterial

Selçuk Üniversitesi Fen Fakültesi Fen Dergisi

In this study, a multilayered hyperbolic metamaterial has been designed that exhibits a broad ban... more In this study, a multilayered hyperbolic metamaterial has been designed that exhibits a broad band optical absorption, especially in optical region. In that proposed structure, three different metals (Ag, Au, and Al) and a semiconductor material (InGaAs) have been employed. Optical calculations are performed by using the Transfer Matrix Method. Optical absorption and reflection spectra are determined for different incident angle values. Hyperbolic dispersion regions of the metal-dielectric multilayered structures have been determined, also contour graphs of the structures, depend on the incident angles are obtained. Consequently, the best optical absorption response is found for the structure in which Ag is used as metal component.

Bu calismada ince fimleriri optikseI karakterizasyonunda sikca kullanilan polarimetrik bir yontem... more Bu calismada ince fimleriri optikseI karakterizasyonunda sikca kullanilan polarimetrik bir yontem olan Elipsometri teknigi incelenmistir. Yapilan calismada bir elektromagnetik dalganin elipsometre duzenegi icersindeki davranisi ayrintili olarak ele alinmistir. Daha sonra elipsometrik parametreler ve denklemler tanimlanmis ve her hangi bir ince film icin kirilma indisi ve sogurma katsayisinin elipsometrik yontemle nasil hesaplandigi aciklanmistir.

Bazı Tek ve Çift Eksenli Nonlineer Kristallerin Optik Özellikleri

Bu calismada, Optiksel Parametrik Salinim (OPO) sistemlerinde kullanilan bazi tek ve cift eksenli... more Bu calismada, Optiksel Parametrik Salinim (OPO) sistemlerinde kullanilan bazi tek ve cift eksenli kristallerin temel fiziksel ve optik ozellikleri incelenmistir. Yuksek donusum kazancina sahip koherent isik elde etmek icin gereken kosullar tartisilmistir. Ayrica, faz uyum kosullarinin kristallerin cesitli parametreleriyle degisimleri incelenmistir. Numerik hesaplamalar KDP ve KTP kristalleri icin gerceklestirilmistir.

Anizotropik Ortamda Işığın Davranışı: Stokes- Mueller Matris Hesaplaması

Anizotropik bir ortamin cift kiricilik ve cift renklilik ozellikleri Mueller matris formalizmi ce... more Anizotropik bir ortamin cift kiricilik ve cift renklilik ozellikleri Mueller matris formalizmi cercevesinde analitik olarak incelenmis ve farkli olasi durumlar icin lineer ve dairesel kutuplanma derecelerinin isigin ortam icerisinde aldigi yol ile degisimi arastirilmistir. Bu degisimlerin incelenmesi sonucu, cift kiricilik ozelliginin cift renklilikten daha baskin oldugu durumlarda, anizotropik bir ortamda ilerleyen isigin kutuplanma derecesinin bir salinim davranisi gosterdigi, diger taraftan cift renklilik ozelliginin cift kiriciliktan daha buyuk oldugu durumlarda ise kutuplanma derecesinin hiperbolik bir degisim gosterdigi sonucuna varilmistir.

FARKLI ORTAMLAR İÇİN JONES-MUELLER MATRiS HESAPLAMASI

Istanbul University - DergiPark, Dec 1, 2005

l~tgtn Kutuplann1a tcn1sili <;e$itli yonten1lcrle belirlenebilir. I~tk izotropik bir ortan1da ... more l~tgtn Kutuplann1a tcn1sili <;e$itli yonten1lcrle belirlenebilir. I~tk izotropik bir ortan1da ilerlerken kutuplann1a durumunda l1erhangi bir degi~tne soz konusu degildir. Bu durun1da, t ~tgtn sogurultnastndan dolayt sadece ~iddetinde bir azalma soz ~onusudur. Fakat eger t~tk anizotropik bir ortan1 boyunca ilerlerse kutuplanma durun1lan kayda deger bir bi<;imde degi ~ir. l3u <;al1~n1ada Ani7otropik ortan1 boyunca ilerleyen t~tgn1 kutuplanrna ternsili 2x2'1ik Jones tnatrisi ile ter11sil edildi [ l ]. Ancak ktstnen k.utuplann11~ l ~tgtn kutuplann1a gosterin1i 4x4 lUk Mueller rnatrisi ile temsil edilnli$tir. Mueller n1atrisi 1erhangi b ir ani zotropi k ortatnda i lerleyen ~~ 1gu1 temel ozelliklerin bel irler.

Journal of Alloys and Compounds, 2015

The optical behaviors of molybdenum oxide thin films are highly important due to their widespread... more The optical behaviors of molybdenum oxide thin films are highly important due to their widespread applications. In the present paper, the effect of thickness on the structure, morphology and optical properties of molybdenum oxide (MoO x) thin films prepared on Corning glass substrates using thermal evaporation technique was studied. The structure and morphology of films were characterized using Xray diffraction (XRD) and scanning electron microscopy (SEM), respectively, while their optical properties were investigated by UV-VIS-NIR spectrophotometry in the spectral range from 300 to 2500 nm. It was observed that whole films have amorphous structure and also they showed rather high transmittance values reached nearly up to 90%. Absorption analysis showed two types of electronic transitions; both direct and indirect interband transition energy values of films decrease from 4.47 to 3.45 eV and from 3.00 to 2.75 eV, respectively, with increasing the film thickness, while the width of the localized states tail increases with thickness. This decrease in the band gap value can be attributed to the rising oxygenion vacancy densities with the thickness. The refractive indices of films were calculated from Sellmeier coefficients determined by nonlinear curve fitting method based on the measured transmittance spectral data. The dispersion of the refractive index was discussed in terms of the Wemple-DiDomenico singleoscillator model. The dispersion parameters such as average oscillator energy, E o , the dispersion energy, E d , and static refractive index n o were evaluated and they found to vary significantly with the film thickness.

PHOTOIONIZATION CROSS-SECTION AND OSCILLATOR STRENGTH OF HYDROGENIC IMPURITIES IN ZnS/SiO 2 QUANTUM DOTS

International Journal of Modern Physics B, 2009

In this study, the photoionization cross-section and oscillator strength for the intersubband ele... more In this study, the photoionization cross-section and oscillator strength for the intersubband electronic transitions associated with an on-center impurity in the ZnS / SiO 2 spherical quantum dot have been calculated. The effects of dot radius, the normalized photon energy and the potential barrier height on the cross-section have been investigated. In the calculations, both the infinite and finite confinement cases have been considered.

$Research paper thumbnail of Photoionization cross section and refractive-index change of hydrogenic impurities in a CdS-SiO2 spherical quantum dot$

Central European Journal of Physics, 2010

In this study, we calculate the photoionization cross section and refractive-index change of an o... more In this study, we calculate the photoionization cross section and refractive-index change of an on-center hydrogenic impurity in a CdS-SiO2 spherical quantum dot. In numerical calculations, both the finite- and infinite-confinement cases are considered and a variational scheme is adopted to determine the energy eigenvalues for the impurity. The variations of the photoionization cross section with the dot radius, the refractive-index change, and the normalized photon energy are investigated, and the effect of the potential-barrier height on the cross section is discussed. The results obtained show that the photoionization cross section and the refractive-index change in CdS-SiO2 spherical quantum dots are sensitively dependent on the incident optical intensity and on the dot sizes.

The double Gaussian distribution of inhomogeneous barrier heights in the organic-on-inorganic Schottky devices

physica status solidi (a), 2012

ABSTRACT We have fabricated an Au/perylene–monoimide (PMI)/n-Si organic-on-inorganic Schottky dev... more ABSTRACT We have fabricated an Au/perylene–monoimide (PMI)/n-Si organic-on-inorganic Schottky device by spin coating of PMI solution on an n-Si semiconductor wafer. Current–voltage (I–V) measurements on the device in the temperature range of 75–300 K were carried out. An abnormal decrease in the experimental barrier height ΦB and an increase in the ideality factor n with a decrease in temperature have been observed. This behaviour has been explained on the basis of thermionic emission theory with a double Gaussian distribution of the barrier heights due to the barrier height inhomogeneities.

The normal reflectance of single crystal Si was measured at 0.5 - 5.6 eV range by unpolarized lig... more The normal reflectance of single crystal Si was measured at 0.5 - 5.6 eV range by unpolarized light. Optical parameters of Si were determined from this reflectance data by using two different methods: oscillator fit procedure and Kramers-Kronig analysis. The refractive index, extinction coefficient and the real and imaginary parts of complex dielectric constant obtained by these two methods were compared. A good agreement was found between two methods. Furthermore, the optical parameters so obtained seem to be in a satisfactory agreement with the literature.

Analysis of I–V measurements on Ag/p-SnS and Ag/p-SnSe Schottky barriers

Solid-State Electronics, 2002

In this study, we have performed current–voltage (I–V) measurements on single crystals p-SnS and ... more In this study, we have performed current–voltage (I–V) measurements on single crystals p-SnS and p-SnSe at different temperatures in the vicinity of room temperature. These compound semiconductors belong to IV–VI layered material class. Hence, they show strong anisotropy for all properties. We realized all measurements on the easy cleavage plane perpendicular c-axis. From I–V characteristics, we have tried to determine

Physical Review B, 2005

Zhang and Xie ͓Phys. Rev. B 68, 235315 ͑2003͔͒ presented their results for the second harmonic ge... more Zhang and Xie ͓Phys. Rev. B 68, 235315 ͑2003͔͒ presented their results for the second harmonic generation ͑SHG͒ susceptibility in electric-field-biased parabolic and semiparabolic quantum wells ͑QW's͒. In this Comment, we demonstrate that the results presented for the SHG coefficient in a parabolic QW with an applied electric field are not physically sound and they contradict with well-established properties of the Hermite polynomials which just happen to be the wave functions describing such a system. It is shown that in a parabolic QW with an applied electric field intersubband transitions contribute to neither second nor any higher order harmonic generation.

physica status solidi (b), 2007

In this study, a theoretical investigation of intersubband nonlinear optical rectification in Al ... more In this study, a theoretical investigation of intersubband nonlinear optical rectification in Al x l Ga 1-x l As/ GaAs/Al x r Ga 1-x r As asymmetric rectangular quantum wells is presented. The electronic states in the asymmetric rectangular quantum well are described within the framework of the envelope function approach including the effects of band nonparabolicity and the effective mass mismatch. The nonlinear optical rectification is calculated using the density matrix formalism. It is found that the nonlinear optical rectification in the asymmetric rectangular quantum well depends sensitively on the parameters such as the width and the asymmetry of the potential well. The adjustable parameters allow for tuning of the asymmetric rectangular quantum well system to the desired wavelength while retaining a large optical rectification coefficient. This gives a new degree of freedom in various device applications based on nonlinear optical properties. Band nonparabolicity is found to significantly influence both electronic states and nonlinear optical rectification. Moreover the resulting optical rectification coefficient is much larger than the ones for bulk GaAs and some other theoretical studies in literature.

Optical Constants of CuInSe2Thin Films Prepared by Two-Stage Process

Physica Scripta, 2005

Page 1. Optical Constants of CuInSe2 Thin Films Prepared by Two-Stage Process This article has be... more Page 1. Optical Constants of CuInSe2 Thin Films Prepared by Two-Stage Process This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2005 Phys. Scr. 71 221 (http://iopscience.iop.org/1402-4896/71/2/019) ...

Second harmonic generation in an asymmetric rectangular quantum well under hydrostatic pressure

Physica B: Condensed Matter, 2007

The effects of structure parameters and hydrostatic pressure on the electronic states and the sec... more The effects of structure parameters and hydrostatic pressure on the electronic states and the second harmonic generation (SHG) susceptibility of asymmetric rectangular quantum well (ARQW) are studied. The asymmetry of the potential can be controlled by changing the structural parameters and this adjustable asymmetry is important for optimizing the SHG susceptibility. We have calculated analytically the electronic states in ARQW

Nonlinear optical rectification in semiparabolic quantum wells with an applied electric field

Physica B: Condensed Matter, 2005

Microelectronic Engineering, 2005

This paper summarizes the first results of characteristics parameters obtained from current-volta... more This paper summarizes the first results of characteristics parameters obtained from current-voltage (I-V) measurements for Ag/p-SnS and Ag/p-SnSe structure. The reverse and forward bias current-voltage characteristics of Ag Schottky contacts on a Bridgman-Stockbarger grown p-SnS and p-SnSe layered semiconducting material have been measured at various temperatures. We have tried to determine contact properties such as apparent barrier heights U B0 , ideality factor n and series resistance R s. The apparent barrier height and ideality factor calculated by using thermionic emission theory were found to be strongly temperature dependent. Evaluating forward I-V data reveals a decrease at the apparent barrier height, but an increase at the ideality factor with decrease in temperature. It is shown that the values of R s estimated from CheungÕs method were strongly temperature dependent and decreased with increasing temperature. It has been found that both contacts are of Schottky type.

Frequency dependent interface state properties of a Schottky device based on perylene-monoimide deposited on n-type silicon by spin coating technique

Materials Science in Semiconductor Processing, 2013

ABSTRACT We have reported a detailed investigation of frequency dependent properties of the Au/pe... more ABSTRACT We have reported a detailed investigation of frequency dependent properties of the Au/perylene-monoimide (PMI)/n-Si Schottky diodes in this study. Schottky diodes based on PMI have been fabricated by spin coating method. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics have been measured in the frequency range from 30 kHz to 1 MHz at room temperature. The values of measured capacitance CmCm and conductance GmGm under both reverse and forward bias have been corrected for the effect of series resistance to obtain the real diode capacitance and the conductance values. The density of interface states (DitDit) distribution profiles as a function of frequency has been extracted from the corrected C–V and G–V measurements. Interface trap states of the PMI/n-Si Schottky device have decreased by increasing the applied frequency and were found to be 8.13×1011 and 1.75×1011 eV−1 cm−2 for 30 kHz and 1 MHz, respectively.

Excitonic effects on the nonlinear optical properties of small quantum dots

Journal of Physics D: Applied Physics, 2008

The excitonic effects on the nonlinear optical properties of small quantum dots with a semiparabo... more The excitonic effects on the nonlinear optical properties of small quantum dots with a semiparabolic confining potential are studied under the density matrix formalism. First, within the framework of the strong confinement approximation, we present the excitonic states and then calculate the excitonic effects on the nonlinear optical properties, such as second harmonic generation, third harmonic generation, nonlinear absorption coefficient

Intersubband resonant enhancement of the nonlinear optical properties in compositionally asymmetric and interdiffused quantum wells

Journal of Applied Physics, 2008

ABSTRACT We report the resonant enhancement of the second- and third-order optical nonlinearities... more ABSTRACT We report the resonant enhancement of the second- and third-order optical nonlinearities in compositionally asymmetric quantum wells with finite confining potential and interdiffused quantum wells. The energy levels and the envelope wave functions in quantum wells are obtained by solving numerically the Schrödinger equation. The optical rectification, second and third harmonic generation coefficients are calculated within the framework of the density matrix formalism. The effect of the structure parameters such as the well width and the barrier height on the nonlinear optical properties is investigated in detail. The resulting nonlinear susceptibilities obtained in both quantum wells are considerably larger than those of bulk GaAs.