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Article Citation - WoS: 21Citation - Scopus: 20Nonlinear Optical Absorption Characteristics of Pbmoo4 Single Crystal for Optical Limiter Applications(Elsevier, 2022) Pepe, Yasemin; Isik, Mehmet; Karatay, Ahmet; Gasanly, Nizami; Elmali, AyhanMolybdate materials take great interest due to their photocatalytic and optoelectronic applications. In this report, PbMoO4 single crystal, one of the member of molybdate materials, is grown by Czochralski technique and the change of nonlinear absorption characteristic depending on the input intensity was reported. Linear absorption analysis revealed the band gap energy and Urbach energy as to be 3.12 and 0.52 eV, respectively. Nonlinear absorption characteristics of the PbMoO4 single crystal was examined with the open aperture (OA) Z-scan experiments at 532 nm excitation wavelength under various input intensities. Fitting results of the OA Z-scan experiments indicated that PbMoO4 single crystal has nonlinear absorption (NA) behavior, and NA coefficient (beta(eff)) increased from 7.11 x 10(-8) to 1.96 x 10(-7) m/W with increasing input intensity. This observation was associated with the increase of the contribution of the free carrier absorption to the NA with the generation of more excited electrons with increasing input intensity. At the 532 nm excitation wavelength (2.32 eV), the dominant mechanisms were revealed as one photon and free carrier absorptions. The optical limiting threshold of the PbMoO4 single crystal was obtained to be 4.91 mJ/cm(2). The reported results indicated that PbMoO4 single crystal can be a good optical limiter in the visible wavelength region due to its effective NA behavior.Article Citation - WoS: 28Citation - Scopus: 30Composition-tuned band gap energy and refractive index in GaSxSe1-x layered mixed crystals(Elsevier Science Sa, 2017) Isik, Mehmet; Gasanly, NizamiTransmission and reflection measurements on GaSxSe1-x mixed crystals (0 <= x <= 1) were carried out in the 400-1000 nm spectral range. Band gap energies of the studied crystals were obtained using the derivative spectra of transmittance and reflectance. The compositional dependence of band gap energy revealed that as sulfur (selenium) composition is increased (decreased) in the mixed crystals, band gap energy increases quadratically from 1.99 eV (GaSe) to 2.55 eV (GaS). Spectral dependencies of refractive indices of the mixed crystals were plotted using the reflectance spectra. It was observed that refractive index decreases nearly in a linear behavior with increasing band gap energy for GaSxSe1-x mixed crystals. Moreover, the composition ratio of the mixed crystals was obtained from the energy dispersive spectroscopy measurements. The atomic compositions of the studied crystals are well-matched with composition x increasing from 0 to 1 by intervals of 0.25. (C) 2016 Elsevier B.V. All rights reserved.Article Citation - WoS: 8Citation - Scopus: 9Experimental and Theoretical Investigation of the Mechanical Characteristics of Sillenite Compound: Bi12geo20<(Elsevier Science Sa, 2021) Surucu, Gokhan; Isik, Mehmet; Gencer, Aysenur; Gasanly, NizamiThe present study reports the mechanical and elastic characteristics of Bi12GeO20 (BGO) compound by experimental nanoindentation measurements and density functional theory (DFT) calculations. X-ray diffraction pattern of BGO was plotted and revealed diffraction peaks were associated with Miller indices of cubic crystalline structure with lattice constant of a = 10.304 angstrom. Two- and three-dimensional representations of Young's modulus, linear compressibility, shear modulus and Poisson's ratio were presented according to DFT calculations. The calculated elastic constants pointed out the mechanically stable and anisotropic behavior of the BGO. The hardness and Young's modulus ranges of the BGO calculated from DFT studies were found as 3.7-6.3 GPa and 61.7-98.9 GPa, respectively. Hardness and Young's modulus of BGO single crystal were also obtained by analyzing force-dependent nanoindentation experimental data. It was observed that hardness and Young's modulus decrease with increase of load in the low applied loads and then reaches saturation in the high applied loads. This behavior is known as indentation size effect. True hardness value was determined from proportional specimen resistance model as 4.1 GPa. The force independent region presented the Young's modulus as 114 GPa. (C) 2021 Elsevier B.V. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Analysis of Optical Constants and Temperature-Dependent Absorption Edge of Gas0.75se0.25< Layered Crystals(Pergamon-elsevier Science Ltd, 2017) Isik, Mehmet; Gasanly, NizamiGaS0.75Se0.25 single crystals were optically characterized through transmission and reflection measurements in the wavelength range of 450-1000 nm. Derivative spectrophotometry analyses on temperature dependent transmittance spectra showed that band gap energies of the crystal increase from 239 eV (T=300 K) to 2.53 eV (T=10 K). Band gap at zero temperature, average phonon energy, electron phonon coupling parameter and rates of change of band gap energy with temperature were found from the temperature dependences of band gap energies under the light of different models reported in literature. Furthermore, the dispersion of room temperature refractive index was discussed in terms of single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, zero-frequency refractive index, were determined as a result of analyses. (C) 2017 Elsevier Ltd. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Temperature-Tuned Band Gap Energy and Oscillator Parameters of Gas0.5se0.5< Single Crystals(Elsevier Gmbh, Urban & Fischer verlag, 2016) Isik, Mehmet; Tugay, Evrin; Gasanly, NizamiTemperature-dependent transmission and room temperature reflection measurements were carried out on GaS0.5Se0.5 single crystal in the wavelength range of 380-1000 nm to investigate its optical parameters. The analysis of the temperature-dependent absorption data showed that direct and indirect band gap energies increase from 2.36 to 2.50 eV and 2.27 to 2.40 eV, respectively, as temperature is decreased from 300 to 10 K. The rates of change of the direct and indirect band gap energies with temperature was found around -7.4 x 10(-4) eV/K from the analysis of experimental data under the light of theoretical relation giving the band gap energy as a function of temperature. The absolute zero value of the band gap energies were also found from the same analysis as 2.50 eV (for direct) and 2.40 eV (for indirect). Wemple-DiDomenico single effective oscillator model, Sellmeier oscillator model and Spitzer-Fan model were used for the room temperature reflection data to find optical parameters of the crystal. (C) 2016 Elsevier GmbH. All rights reserved.Article Citation - WoS: 21Citation - Scopus: 21The Role of Defects on the Transition From Saturable Absorption To Nonlinear Absorption of Bi12geo20< Single Crystal Under Increasing Laser Excitation(Elsevier, 2022) Pepe, Yasemin; Isik, Mehmet; Karatay, Ahmet; Yildiz, Elif Akhuseyin; Gasanly, Nizami; Elmali, AyhanThis work reports defect and input intensity dependent nonlinear optical behaviors of Bi12GeO20 (BGO) single crystal. Open aperture (OA) Z-scan experiments were performed with 532 nm excitation wavelength under 4 ns and 100 fs pulsed laser irradiation. Obtained data were fitted with a theoretical model considering one-photon, two-photon and free carrier absorption contributions to nonlinear absorption due to longer lifetime of localized defect states than that of used laser pulse durations. At low input intensities, the BGO single crystal showed saturable absorption (SA) behavior and transition to nonlinear absorption (NA) behavior observed with further increase of the input intensities both of pulse durations. At low input intensity, the OPA mechanism is dominant and results in SA by filling of the defect states due to defect state at around one photon energy (2.32 eV). At higher input intensity, multi-photon, two-photon and free carrier absorption become dominant mechanisms, and nonlinear absorption behavior was observed. The lowest saturation threshold was found as 1.36 x 1010 W/cm2 with nanosecond pulses. We have revealed the mechanisms contributing both SA and NA, and determined saturation intensity threshold and effective nonlinear absorption coefficients. Our findings indicate that the tails of defect states overlap in the energy band gaps especially in sufficiently disordered crystal. With this way, the spectral range for saturable absorption and nonlinear absorption can be broadened.Article Citation - WoS: 22Citation - Scopus: 22Wavelength Dependence of the Nonlinear Absorption Performance and Optical Limiting in Bi12tio20 Single Crystal(Elsevier, 2023) Pepe, Yasemin; Isik, Mehmet; Karatay, Ahmet; Gasanly, Nizami; Elmali, AyhanIn this study, the influence of excitation wavelength and input intensity on the nonlinear absorption (NA) mechanism and optical limiting behavior of the Bi12TiO20 (BTO) single crystal were reported. The energy band gap of the BTO single crystal was obtained to be 2.38 eV. Urbach energy revealed that the single crystal has a highly defective structure. Open aperture (OA) Z-scan experiments were conducted at 532 and 1064 nm exci-tation wavelengths at various input intensities. Obtained experimental data were analyzed with a theoretical model considering one photon, two-photon and free carrier absorption contributions to NA. The obtained results revealed that the BTO single crystal possesses NA. The NA coefficient increased with increasing input intensity at 532 nm excitation wavelength, while it decreased with increasing input intensity at 1064 nm excitation wave-length. Due to the intense localized defect states distribution at the energy of 532 nm excitation wavelength within the band gap, increasing contribution to NA came from one photon absorption (OPA), sequential two -photon absorption (TPA) and free carrier absorption (FCA) with increasing input intensity. The filling of the defect states at 1064 excitation wavelength caused a reduction in NA due to increasing saturable absorption with increasing input intensity. TPA coefficients were also found from the fitting ignoring the defect states. As ex-pected, the values of the nonlinear absorption coefficient beta eff are higher than that of the TPA coefficients for both excitation wavelengths. The optical limiting threshold of the BTO single crystal was obtained to be 6.62 mJ/cm2. The results of the present works indicated that BTO single crystal can be used as a potential optical limiter.Article Citation - WoS: 12Citation - Scopus: 14Excitation Wavelength Dependent Nonlinear Absorption Mechanisms and Optical Limiting Properties of Bi12sio20 Single Crystal(Elsevier, 2023) Dogan, Anil; Karatay, Ahmet; Isik, Mehmet; Pepe, Yasemin; Gasanly, Nizami; Elmali, AyhanNonlinear absorption mechanisms (NA), excitation wavelength dependence, and defect states of Bi12SiO20 (BSO) single crystal were investigated. The band gap and Urbach energies were found to be 2.51 and 0.4 eV from the absorption spectra. To evaluate the effect of excitation energy on the NA mechanism of the BSO single crystal, open aperture Z-scan experiment with 4 ns laser pulse at 532 and 1064 nm wavelengths with different intensities was performed. Obtained data were analyzed with a theoretical model considering the contributions of one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption (FCA) to NA. The results indicated that the NA behavior decreased with increasing of the pump intensity as the defect states at around 2.32 eV by OPA at 532 nm, and TPA at 1064 nm excitations. The dominant NA mechanisms are OPA and sequential TPA at 532 nm as compared to the 1064 nm. A higher NA coefficient was obtained at 532 nm as compared to 1064 nm excitation. This observation was attributed to higher contribution of OPA at 532 nm even at lower input intensities compared to TPA contribution at 1064 nm. Onset optical limiting thresholds were found as 0.34 and 0.68 mJ/cm2 for 532 and 1064 nm input beams, respectively. In the light of the results, the BSO single crystal may be used as a saturable absorber or an optical limiter at convenient input intensity by effectively adjusting defect states and excitation wavelength.Article Citation - WoS: 7Citation - Scopus: 7Optical Properties of Gas Crystals: Combined Study of Temperature-Dependent Band Gap Energy and Oscillator Parameters(Natl inst Science Communication-niscair, 2017) Isik, Mehmet; Tugay, Evrin; Gasanly, Nizami; Department of Electrical & Electronics EngineeringOptical parameters of gallium sulfide (GaS) layered single crystals have been found through temperature-dependent transmission and room temperature reflection experiments in the wavelength range of 400-1100 nm. Experimental data demonstrates the coexistence of both optical indirect and direct transitions and the shift of the absorption edges toward lower energies by increasing temperature in the range of 10-300 K. Band gap at zero temperature, average phonon energy and electron phonon coupling parameter for indirect and direct band gap energies have been obtained from the analyses of temperature dependences of band gap energies. At high temperatures kT>> (E-ph), rates of band gap energy change have been found as 0.56 and 0.67 me V/K for E-gi and E-gd, respectively. Furthermore, the dispersion of refractive index has been discussed in terms of the Wemple-DiDomenico single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, oscillator strength and zero-frequency refractive index, have been found to be 4.48 eV, 24.8 eV, 6.99x10(13) m(-2) and 2.56, respectively. The results of the present work will provide an important contribution to the research areas related to the characterization and optoelectronic device fabrication using GaS layered crystals.Article Citation - WoS: 7Citation - Scopus: 6Tunable Nonlinear Absorption and Optical Limiting Behavior of Nabi(moxw1-x< Single Crystals With Ratio of Molybdenum/Tungsten(Iop Publishing Ltd, 2023) Pepe, Yasemin; Yildiz, Elif Akhuseyin; Isik, Mehmet; Karatay, Ahmet; Gasanly, Nizami; Elmali, AyhanThe compositional effect of Mo/W ratio on linear, nonlinear absorption and optical limiting behavior of the NaBi(MoxW1-xO4)(2) single crystals grown by Czochralski technique was investigated. X-ray diffraction patterns of the studied crystals presented well-defined peaks associated with the tetragonal crystalline structure. The nonlinear absorption performance and optical limiting threshold were determined using an open-aperture Z-scan technique. A theoretical model including one photon absorption (OPA), two photon absorption (TPA) and free carrier absorption was used to determine the nonlinear absorption parameters. All of the results showed that defect states, which strongly affect nonlinear absorption (NA) and optical limiting behaviors, can be tuned with the Mo/W ratio, enabling NaBi(MoxW1-xO4)(2) single crystals to be used in desired optoelectronic applications. Linear optical absorption analysis revealed that bandgap energy and defect states can be tuned by changing the Mo/W ratio in the crystal structure. The obtained results showed that all the studied crystals had NA behavior and the nonlinear absorption coefficient decreased with increasing Mo/W ratio. Sequential TPA is the main NA mechanism for these crystals due to the fact that the incident light energy is lower than the bandgap energies and the existence of the real intermediate state around 2.32 eV.
