International Journal of

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Larger width of P-cladding layer in p-i-n waveguide of traveling wave electroabsorption modulator (TWEAM) results in lower resistance and microwave propagation loss which provides an enhanced high speed electro-optical response. In this paper, a fullvectorial finite-difference-based optical mode solver is presented to analyze mushroom-type TWEAM for the first time. In this analysis, the discontinuities of the normal components of the electric field across abrupt dielectric interfaces which are known as the limitations of scalar and semivectorial approximation methods are considered. The optical field distributions in mushroom-type TWEAM and conventional ridge-type TWEAM of the same active region for 1.55 μm operation are presented. The important parameters in the high-frequency TWEAM design such as optical effective index which defines optical velocity and transverse mode confinement factor are calculated. The modulation response of mushroom-type TWEAM is calculated by considering interaction of microwave and optical fields in waveguide and compared to that of conventional ridge-type TWEAM. The calculated 3dB bandwidths for ridge-type and mushroom-type TWEAM are about 139 GHz and 166 GHz for 200 μm and 114 GHz and 126 GHz for 300 μm waveguide length, respectively.

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We have studied light-induced birefringence (LIB) and surface relief grating (SRG) formation in the series of methylacrylate polymers. The effect of material structure such as length of photochromic side chain, glass transition temperature and molecular structure of azo units on LIB and SRG are studied. The optical formation of self-induced SRG on films of these materials is also presented.

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A modulated holding beam shining a VCSEL above threshold can improve modulation bandwidth by adjusting the semiconductor laser’s relaxation oscillation frequency. The improved modulation characteristics are accompanied by reduced damping rate. By choosing correct parameters, dynamical behavior of system and cavity solitons is changed.

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In this paper, we used the time-of-flight (TOF) of a charge packet, that injected by a voltage pulse to calculate the drift velocity and mobility of holes in organic semiconducting polymers. The technique consists in applying a voltage to the anode and calculating the time delay in the appearance of the injected carriers at the other contact. The method is a simple way to determine the charge transport properties of organic semiconductors. The effect of charge trapping mechanism on the carrier mobility in the organic layer is investigated and it was shown that at higher applied voltage, 100 V, hole mobility increases rapidly in comparison with lower applied voltage (50 V).

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Photonic crystal design procedure for negative refraction has so far been based on trial and error. In this paper, for the first time, a novel and systematic design procedure based on physical and mathematical properties of photonic crystals is proposed to design crystal equi-frequency contours (EFCs) to produce negative refraction. The EFC design is performed by the help of rectangular stair-case (RSC) photonic crystals. The RSC crystal is then converted to more common structures like pillar crystals by matching Fourier coefficients of periodic electric permittivity. Methods to design common crystals which have approximately equal Fourier components to the RSC crystal are also discussed. The proposed procedure can be used to design metamaterials without the difficulties of large trial and error. The devised procedure can also be applied in designing other structures involving photonic crystals.

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The nonlinear effects of the second harmonic generation have been investigated for the propagation of light along the axis of fibers of wagon wheel cross sectional shape. Nodal finite element formulation is utilized to obtain discretized Helmholtz equations under appropriate boundary conditions. The hierarchical p-version nodal elements are used for meshing the cross section of wagon wheel fiber. The fiber material has been chosen to be LiTaO₃ to provide proper second harmonic generation. Propagation of generated second harmonics for two incident field amplitudes are studied in this work.

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In this paper, we have proposed and demonstrated a new method of atomic population transfer. The transition dynamic of a two-level system is studied in a full quantum description of the Jaynes-Cummings model. Solving the time-dependent Schrödinger equation, we have investigated the transition probabilities numerically and analytically by using a sudden boost of the laser frequency. The results show that complete population transfer can be achieved by adjusting the time of the frequency boost.

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Topological Insulators are systems where the broken time reversal symmetry gives rise to protected edge modes that support backscatter-free and one-way propagation of electromagnetic waves by opening non-trivial bandgaps. In this study we investigate a one-way topologically protected waveguide in the frequency range of f=6.0 to 8.0 GHz. The time reversal symmetry is broken by an applied magnetic field in the z direction. We show that the waveguide propagates the light in only one direction that can be controlled by the applied magnetic field and no backscattering is present in the waveguide which results in a near 100% transmission of light to the output. Furthermore, we investigate effect of the applied magnetic field on the topological properties of the system by considering the material dispersion of the rods. Our results show that 3 different frequency ranges will be supported by the edge modes at each given magnetic field. By increasing the magnitude of the applied magnetic field, a blue shift in the non-trivial bandgap is seen, where it can be used to tailor the modes for the waveguide.

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In this paper, a thin film silicon solar cell with anti-reflection coatings on front surface and the combination of periodic grating and photonic crystal on its back surface has been considered. The thickness and number of anti-reflection coatings, as well as the geometric and physical parameters of photonic crystal and grating are optimized to increase the optical absorption of solar cell. The simulations have been performed using the finite difference time domain method with Lumercial software. The results show that the optical absorption of solar cell has been increased significantly by utilizing the anti-reflection coatings, photonic crystal and grating.

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Two-dimensional nanomaterials have attracted increasing attention for enhancing surface plasmon resonance (SPR) biosensors application. In this work, we use the graphene layer to improve the sensitivity of the SPR biosensors based on the conventional Kretschmann configuration. We employ Tungsten disulfide (WS₂) and Molybdenum disulfide (MoS₂) Two-dimensional materials as an interlayer to enhance the sensitivity of Au/Graphene biosensor in angle interrogation method. The transfer matrix method (TMM) is used to analyze the characteristics of the device. Results show that using WS₂ in Au/Graphene structure increases sensitivity by about 12.64%, which is higher than MoS₂. Combining graphene based SPR and ellipsometry as a highly sensitive, label-free, real-time, and versatile method can be used to measure a very small concentration of biomolecules, which leads to 170-fold enhancement compared to angle interrogation method and improves the detection accuracy and quality factor.

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Photobiomodulation therapy (PBMT) or Low level light Therapy (LLLT), is the stimulatory effect of light on the cell behavior. It has been considered as a potential therapeutic intervention. Glioblastoma is a malignant primary brain tumor without any effective treatment. This in vitro study investigated the effect of PBMT on proliferation rate and vital activity of human glioblastoma U87 cell line. Three different wavelengths were considered: 632 nm (red light, 2.1 mW/cm²), 534 nm (green light, 1.2 mW/cm²), and 457 nm (blue light, 6.5 mW/cm²). The cell behavior was studied during a period of four hours up to 60 hours after irradiation. The irradiated cells were inspected by different assays for cell count, cell viability, cell death, and free radical production rate and were compared with the control non-irradiated ones. The results show a reduction in cell viability for all the three wavelengths. However, the effect is more pronounced for blue light. Cell death assessments, staining and flow cytometry, and NBT assay shows that blue light is not lethal, but that it reduces the free radical production rate. Temporal analysis shows that the maximum effect on cell proliferation will be observed around 48 hours after irradiation. It could be concluded that light, particularly shorter wavelengths, has an inhibitory effect on the in vitro proliferation rate of U87 cell line by affecting the energetics of the cell. The effect is stimulatory and persistent for periods comparable to cell doubling time.

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Ti-doped tungsten disulfide (WS2) nanosheet-semiconductor is studied for thermo- optical and electronic properties. Thermal diffusivity (D) thermal conductivity (κ) and absorbance were determined as a function of Ti- dopant (0, 7, 14, and 28%). The research focused on the effect of different Ti-dopant concentrations, and we tried to evaluate the thermal parameters using photohermal lens technique as a simple, non-contacting method. The results show an increase in the values of D by 5 times with an increment of Ti-dopant from 0% to 28%. The addition of Ti did not produce any additional phase in the material, although, the separation of the crystallographic planes reduced, indicating the presence of the Ti atoms in the crystal structure.

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This article examines the impact of graded-index reflectors (GIRs) constructed from cholesteric liquid crystals (CLCs) on the modification of escape-cone loss and the improvement of luminescent solar concentrator (LSC) efficiency. GIRs are comprised of a structure in which the refractive index changes gradually with a spatial function. In this study, the LSC is a planar optical waveguide made of Poly (methyl methacrylate) (PMMA) with dimensions of 5×5×0.5 cm³ and contains CH₃NH₃PbBr₃ perovskite material as a fluorophore. Two types of GIRs are employed at the bottom of the LSC: periodic and quasi-periodic GIRs. Periodic reflectors (PRs) have a refractive index matrix that gradually changes with a periodic sinusoidal function, while the refractive index matrix of the quasi-periodic reflectors (QPRs) gradually changes with a chirped sinusoidal function. To analyze the models of the study, Monte-Carlo and Finite-Difference Time-Domain (FDTD) methods were utilized in conjunction with experimental results. The results indicated that the reflection band of a PR exhibited the maximum overlap (lowest escape cone) with the dye emission spectrum when the mirror's reflection band underwent a redshift of 20 nm compared to the dye emission spectrum. On the other hand, the QPR generated a broader reflection band, resulting in complete overlap and higher efficiency. Moreover, GIRs enhanced sunlight absorption in the LSC by reflecting transmitted solar photons through it. The optical efficiency (OE) of the LSC increased by 12% (33%) once a periodic (quasi-periodic) reflector is utilized. Furthermore, GIRs reduced escape cone loss, thereby increasing reabsorption, and subsequently, the system selected a lower optimal concentration to minimize reabsorption losses.

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In this research, palladium nanoparticles (Pd NPs) were first synthesized using laser ablation in the deionized (DI) water environment. Also, metal-organic framework (MOF) was produced using the solvothermal method at a temperature of 150°C. To accumulate Pd NPs on the synthesized MOF, ultrasonic and magnetic stirring methods were used. Different analytical methods were used to investigate the structure and morphology of the synthesized nanocomposite. Also, the sensitivity of the synthesized nanocomposite to ethanol and methanol organic vapors was investigated. The results showed an increase in the response of the MOF in the presence of nanoparticles.