Tevhit Karacali

Aging of porous silicon and the origin of blue shift

Aging effects of porous silicon (PS) and the origin of blue shift are investigated. Photoluminescence (PL) measurements of the PS prepared with HF-EtOH solution showed a 210 meV blue shift after 1.5 months. It is found from deconvolution of the PL spectra that this shift is not fully related to the quantum confinement (QC) effect. For stable PS formation, a HFEtOH-H2O2 solution is used. A stable luminescence at 2.01 eV with a Gaussian distribution is obtained when the samples are kept in H2O2 for 45 min after the anodization.

Novel Design of Porous Silicon Based Sensor for Reliable and Feasible Chemical Gas Vapor Detection

In this study, an innovative design for porous silicon (PSi) based sensors is proposed to eliminate some problems of conventional PSi sensors such as undesired reflections from imprecise positioning of the fiber optic cable and the PSi structure. Our design has three stages as hole milling for fiber socket, fabrication of filter structure, and integration of the fiber optic cable to the bulk Si. We have carried out reflectivity measurements of alcohols with close refractive index values by the proposed and conventional sensors. From the measurements, we note that both sensors have equal sensitivity in identifying alcohols, that repeatability of our proposed sensor is relatively higher, and that our sensor allows easy positioning and flexibility in sensing applications. Nonetheless, our proposed sensor necessitates a thorough fabrication process and a methodological preparation.

The effect of silicon loss and fabrication tolerance on spectral properties of porous silicon Fabry-Perot cavities in sensing applications

In this paper, we investigate the effect of non-uniformities (enlargement of current passage, non-equal surface current densities, etc.) in axial as well as transverse directions of a porous silicon Fabry-Perot (FP) cavity as well as loss nature of bulk silicon on spectral properties of this cavity, even that cavity is created with an anisotropic etching process. Without correct and comprehensive characterization of such cavities by incorporating these non-uniformities and inherent lossy nature of a cavity, detection and identification of biological and chemical molecules by that cavity may yield unpredictable and misleading results. From our simulations, we note the following two key points. First, effects of the refractive index and the thickness of microcavity region of a lossless or lossy FP cavity on resonance wavelength is more prevailing than those of first and last layers. Second, the effect of some small loss inside the FP cavity is not detectable by the measurement of resonance wavelength whereas the same influence is noticeable by the measurement of reflectivity. We carried out some measurements from two different regions on the fabricated cavities to validate our simulation results. From a practical point of view in correct detection and/or identification of lossy biological or chemical vapor by FP cavities, we conclude that not only the measurement of resonance wavelength as well as its shift but also the reflectivity value at the resonance wavelength or some specific wavelengths should be utilized.

Single and Double Fabry–PÉrot Structure Based on Porous Silicon for Chemical Sensors

This paper presents sensing of chemicals using porous silicon as optical sensor fabricated by periodically modulating the porosity of silicon to produce multilayered structures. Single and double Fabry-Perot (FP) structure is designed by using electrochemical anodical etching technique. The operation of chemical sensor is based on the change of effective refractive index of the porous silicon medium, induced by condensation of solvent vapors around the pillars. Resonant wavelengths of single and double multilayer with a microcavity have presented a red shift when exposed to vapor of solvents. On the other hand, resonant wavelength of double FP structure sandwiched with a diffusion layer has presented different optical response when exposed to vapor of solvents. This structure actually has two stop bands with two resonant wavelengths. While of the beginning first and second stop band and resonant wavelength shift together to the infrared region continuously, after awhile second stop band stopped but the first stop band continued to shifting to the infrared region. Time dependence of optical response of proposed structure can be used for identification chemicals.

Study of structural and optical properties of zinc oxide rods grown on glasses by chemical spray pyrolysis

We have investigated morphological and optical properties of zinc oxide rods. Highly structured ZnO layers comprising with well-shaped hexagonal rods were prepared by spray pyrolysis deposition of zinc chloride aqueous solutions at ∼550°C. The rods were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, photoluminescence, and ultraviolet and visible absorption spectroscopy measurements. The deposition of the 0.1mol/L solution at ∼550°C resulted in crystals with a diameter of 400-1000nm and length of 500-2000nm. Sharp near-band edge emission peaks, centered at 3844 and 3680A, dominated the PL spectra of ZnO at 300K and 6.2K, respectively. In addition to this, absorption coefficient was determined by absorption measurement. X-ray diffraction, scanning electron microscopy and atomic force microscopy, results suggest that ZnO rods, prepared by spray pyrolysis, have high crystalline quality. This is desirable in nanotechnology applications.

Electrical transport properties of p-GaTe grown by directional freezing method

p-type GaTe single crystals have been grown using the directional freezing method with different growth rates. Temperature-dependent Hall effect and resistivity measurements were carried out in the 80–325 K temperature range and at 1.6 T magnetic field. The free carrier concentration in the exhaust region was in the range of 9 × 1015–8 × 1016 cm−3 for growth rates of 3.3–0.39 µm s−1 and the hole mobility at 300 K ranged from 4.8 to 21 cm2 V−1 s−1. A systematic dependence of the hole carrier concentration and room temperature hole mobility on the growth rate was not observed. From temperature-dependent Hall measurements, we have found that the compensation ratio ND/NA was in the range of 0.39–0.69 regardless of the growth rate. The highest mobility of 342 cm2 V−1 s−1 at 100 K was achieved after annealing at 200 °C for 30 min. The fitting of the temperature-dependent free carrier concentration, using the single acceptor–single donor model, was used to determine the compensation ratio. Finding Nmax(m*)3/2 = 3.15 indicates high effective mass for holes or the possibility of a few close maxima of valence band edges with different effective masses.

Reference-Plane-Invariant Effective Thickness and Electromagnetic Property Determination of Isotropic Metamaterials Involving Boundary Effects

It is well recognized that near-field effects become dominant when the metamaterial (MM) is in resonance. In addition, any inaccurate information of the location of reference planes, and the effective length can seriously affect the accuracy of retrieved electromagnetic properties of MMs. By considering all these issues, in this research paper, we propose a retrieval method for reference-plane invariant and thickness-independent determination of electromagnetic parameters of MM slabs involving boundary effects. Our method first accomplishes determination of effective length of MMs and calibration-plane factors using scattering parameter measurements, aside the resonance region, of two identical MMs with different lengths. Our method then incorporates near-field effects in accurate retrieval of electromagnetic properties of MMs. The method is verified by scattering parameters simulated for a homogeneous conventional material and a weakly or negligibly coupled inhomogeneous MM slab made by two metallic concentric split-ring-resonators. Consequences of an inaccurate information of reference-plane transformation factors and the value of effective lengths and of noninclusion of near field effects on the retrieved electromagnetic properties are thoroughly discussed by way of few examples to substantiate the accuracy of the proposed method.

Reference-plane-invariant and thickness- and branch-index-independent retrieval of effective parameters of bi-anisotropic metamaterials

We propose a retrieval method for reference-plane-invariant electromagnetic parameter measurements of bi-anisotropic metamaterial slabs without resorting to accurate information of the slab thickness and the branch index. To extract reference-plane distances, the slab thickness, and the branch index, we first approximate wave impedances and refractive index away from the slab resonance frequency and then use scattering parameters to calculate the refractive index and the branch index. Once these quantities are determined, they are used as inputs for the retrieval of electromagnetic properties of slabs over the whole band. Different approximations for refractive index and wave impedances are applied to demonstrate the applicability and accuracy of our proposed method. We tested our method for electromagnetic parameter extraction of bi-anisotropic split-ring-resonator and Omega-shaped MM slabs with different number of unit cells. From our analysis, we note that inaccurate information of reference-plane distances, the slab length, and the branch index not only changes the amplitude but also shifts the response of the electromagnetic properties. We show that the presented method can be applied for accurate electromagnetic parameter extraction of bi-anisotropic MM slabs.

Characterization of Porous Silicon Fabry–Pérot Optical Sensors for Reflectivity and Transmittivity Measurements

We investigate the effect of fabrication parameters (nonequal surface current densities, impurities inside the structure, etc.) and loss factor on reflectivity and transmittivity measurements from porous silicon Fabry-Pérot cavities with finite-size substrate thicknesses. We apply the formalism based on wave cascade matrix method for obtaining dependencies of reflectivity and transmittivity. From our analysis, we note the following results. First, resonance behavior of reflectivity and transmittivity changes only when optical/physical properties of middle layers of the cavity alter. Second, for lossless cavities, while reflectivity and transmittivity considerably change with surface characteristics (optical/physical properties of first layers), transmittivity is significantly modified by a change of optical/physical properties of middle layers (microcavity region). Third, loss inside a FP cavity makes the transmittivity more immune to variations in optical/physical properties of middle layers. Finally and most importantly, transmittivity values at resonance wavelength as well as the resonance wavelength shift can be utilized for the identification of unknown chemical/biological molecules by lossless FP cavities. For validation of these results, we carried out reflectivity and transmittivity measurements from some arbitrarily chosen positions but around the center of two fabricated FP cavities resonating at 1456 nm and at 542 nm.

Determination of constitutive parameters of homogeneous metamaterial slabs by a novel calibration-independent method

A calibration-independent line-line method for broadband and simultaneous constitutive parameters determination of homogeneous metamaterial (MM) slabs is proposed. It is shown that the sufficient condition for parameters retrieval by the proposed method is to measure uncalibrated (raw) complex scattering parameters of measurement cells (different air regions in free-space) which are completely and partially loaded by the two identical metamaterial slabs. The stability of derived equations for different measurement uncertainty cases is analyzed. We have validated the proposed method by using simulated scattering parameters of a MM slab with split-ring-resonators and then by comparing the extracted electromagnetic parameters with those of a general method used in the literature in the cases with and without a small offset in reference-plane positions (as well as other measurement errors). From this comparison, we note that while the general method does not eliminate those errors, the proposed method not onl...