Publication Date: 2022
Scientific Reports (20452322)12(1)
A novel boomerang-like alumina based antireflective coating with ultra-low reflectance has been produced for light incidence angles form 0 up to 45°. Boomerang-like alumina nanostructures have been fabricated on the BK7 glass substrates by dip-coating and surface modification via hot water treatment. To achieve the lowest residual reflectance, the effect of dip-coating rate and hot-water temperature in the treatment process has been investigated and optimized. To further investigate the boomerang-like alumina nanostructure and extract its graded refractive index profile by fitting the measured reflectance spectrum with the simulated one, a simulation based on the finite-difference time-domain (FDTD) method has been performed. The average reflectance measured at normal incidence for double-sided coated BK7 glass substrates is only 0.3% in the visible spectral region. Considering both sides, the average reflectance of the substrate decreased in the spectral range of 400–700 nm down to 0.4% at incidence angles of 45° by applying the boomerang-like alumina antireflection coatings. The optimized single layer boomerang-like alumina coating on the curved aspheric lens exhibited a low average reflectance of less than 0.14% and an average transmittance of above 99.3% at normal incidence. The presented process is a simple and cost-effective route towards broadband and omnidirectional antireflection coatings, which have promising potential to be applied on substrates having large scales with complex geometric shapes. © 2022, The Author(s).
Publication Date: 2021
Advanced Theory and Simulations (25130390)4(5)
Infrared neural stimulation techniques have potential applications in the diagnosis and treatment of numerous neurological and psychiatric disorders. There has been little progress in the computational modeling of these techniques and further improvement is needed in this area. In this paper, a comprehensive computational model is presented for simulating the complete mechanism of direct and plasmonic nanoparticle-mediated infrared neural stimulation techniques in schematic samples of experimental setups. The simulation process involves three phases: 1) Simulating the light transmission and absorption in setups containing pure water or a gold nanorod solution using developed 3D, time-independent, and time-dependent Monte Carlo models, 2) calculating the spatiotemporal evolutions of temperature within the setup using the finite difference method and a presented novel method, and 3) simulating the thermally induced responses of lipid membranes using an improved method compared to existing theoretical models. The model is validated by comparing the computational results with existing experimental data. The effect of the laser pulse characteristics, nanofluid properties, and some other related parameters on the thermally induced membrane responses is investigated. The computational results help to optimize the parameters selection and maximize the overall efficiency of the infrared neural stimulation techniques. © 2021 Wiley-VCH GmbH
Publication Date: 2020
Proceedings of SPIE - The International Society for Optical Engineering (1996756X)11460
In recent decades, research in the field of optoelectronics has attracted great interest. Modulators are one of the most usable optical components in the optical communication systems. The most important parameters for a modulator are high modulation speed, small footprint, and large optical bandwidth. In this paper we design and fabricate an all-optical intensity modulator based on optical microfiber in which graphene oxide replaces the clad of microfiber. We chemically etched the clad of multimode optical fiber. Then we coated aqueous solution of graphene oxide with concentration 5 mg/ml on the microfiber that has 90° curvature. The graphene oxide was irradiated directly by the pump laser (power=500 mW) at wavelength 405 nm from outside the microfiber. We were able to achieve a maximum modulation depth of 27.3% by graphene oxide. The modulator has been built has many advantages such as compatibility with optical communications, easy fabrication, and low cost. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
Publication Date: 2020
Applied Physics B: Lasers and Optics (09462171)126(5)
To achieve a high-sensitivity surface plasmon resonance sensor, a sensor based on Ag-MgF2 grating was designed and fabricated. A suitable-thickness MgF2 was suggested to prevent the oxidation of silver while avoiding reducing its plasmonic properties. The combination of an interference lithography approach, the material used for the fabrication of grating, and angular interrogation method led to a less costly sensor. The sensitivity and figure of merit of the proposed sensor approached 85.61 deg/RIU and 51 RIU−1, respectively, which is higher than the experimental values reported so far for grating-based sensors. It was shown that by optimization of the silver-based structure, it has great potential for use in sensor applications. It was observed that based on the made grating pattern, the numerical results were closer to experimental results by considering the grating pattern in a sine form. The effect of temperature on sensor performance was experimentally investigated. It was demonstrated that the change in the resonance angle with the temperature in this structure was equal to 0.02 deg/°C and it was also experimentally shown that temperature changes in the analyte refractive index had the most effect on the variations of the SPR response with temperature. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
Publication Date: 2019
Journal of the Optical Society of America B: Optical Physics (07403224)36(7)pp. 1748-1757
Light propagation in an asymmetric multilayered structure composed of a linear defective photonic crystal and nonlinear graphene layer is theoretically investigated. A BA4∕graphene∕AB4AABB4 structure with a slight perturbation in the AABB4 part can provide non-reciprocal transmission to achieve an optical diode. This structure was optimized in order to have a low-operation input intensity, high-contrast, small-size all-optical diode without an additional pump signal. In this work, four different passive all-optical diodes are presented. The best contrast ratio is 6.3, and the minimum operation input intensity for diode action is 2.7 × 10−3 MW cm2 . This structure gives an all-optical diode with a smaller size and higher efficiency in comparison to previous works. © 2019 Optical Society of America.
Publication Date: 2019
Applied Optics (21553165)58(33)pp. 9039-9043
In this paper, an easy and cost-efficient method to create a single anti-reflective layer on a silicon substrate is presented. In this experiment, by using the silver nanoparticles on a silicon substrate at room temperature, a porous nanostructure is created. This nanostructure significantly reduces the reflection and increases the transmission in a wide spectral range. By using this method, a porous layer on one side of the silicon substrate is fabricated. The results show that the reflection reduces to 31% and the transmission increases to 68% in the 3500–5000 nm spectral range, and these values are close to the reflection and transmission of a sample with ideal one-sided anti-reflection. © 2019 Optical Society of America.
Publication Date: 2019
Materials Research Express (20531591)6(5)
The effect of second phase size (200, 400, 600 and 800 nm) and volume fraction (10, 20 and 30%) on light transmission behavior of ZnS/diamond composite was simulated using Mie theory and FDTD method. It is expected that the results of FDTD method to be close to experimental results. To validate this approach, simulation results of Mie theory and FDTD method were compared with experimental light transmission (T) of 88.7-11.3 vol% ZnS/diamond composite, reported previously by Xue and et al, and it is shown that results of FDTD method matched well with these experimental results. Because of considering particles interaction and the higher accuracy of the FDTD method in calculating the scattering cross section than the Mie theory, the difference between simulation curves of ZnS/diamond nanocomposite based on Mie theory and FDTD method became more significant with increasing second phase size particularly at greater volume fractions and at short wavelengths. Simulated light transmission curves obtained by FDTD method showed that the optimized second phase size should be smaller than 800 nm for volume fraction of 10% and smaller than 200 nm for volume fractions of 20 and 30%. On the other words, within these sizes, the reduction of light transmission of ZnS/diamond composite compared to monolithic ZnS is lower than 10%. © 2019 IOP Publishing Ltd.
Publication Date: 2018
Journal of Physical Chemistry B (15205207)122(29)pp. 7319-7331
Neural stimulation has widespread applications in investigating brain functions, restoring impaired neural functions, and treating numerous neurological/psychiatric diseases. Use of infrared pulses to stimulate neurons (infrared neural stimulation) offers a direct and non-invasive technique. Recent research has demonstrated that transient heating associated with the absorption of infrared light by the local aqueous medium around the cell membrane can stimulate nerves. One mechanism for this stimulation is due to a thermally induced increase in the membrane electrical capacitance, which causes cell depolarization as well as action potential production under certain physiological conditions. A theoretical and computational model helps better understand the mechanism of thermally induced electrical capacitance changes and optimize the stimulus parameters. In this article, we develop the existing theoretical models for membrane electrical capacitance and its thermally induced changes. We improve the formulation of Gouy-Chapman-Stern theory by Genet et al. and Shapiro et al. with the addition of a diffuse layer to the electrical double layer and by modifying the relation of Stern layer capacitance, to calculate the membrane capacitive charge and capacitive current. We also present a new method to calculate the membrane electrical capacitance and the rate of its thermally induced changes. In our calculations, two new factors are considered including the temperature dependence of the surface charge density and the hydrophobic core dielectric constant of the lipid bilayer. Our developed model predicts rates of 0.3 and 0.26%/°C for the thermally induced capacitance changes of the artificial lipid bilayer under two different sets of conditions previously reported by Shapiro et al. and Carvalho-de-Souza et al., respectively. Our model is in very good agreement with the corresponding experimental values given by these groups. The presented model is also able to calculate the membrane capacitive currents and investigate the voltage dependence of this current. © 2018 American Chemical Society.
Publication Date: 2017
Optics Communications (00304018)383pp. 159-164
The all-optical switch is realized based on nonlinear transmission changes in Fano resonance of 2D photonic crystals (PhC) which enhances the light intensity on the graphene in PhC; and in this study, the graphene layer is used as the nonlinear material. The refractive index change of graphene layer leads to a shift in the Fano resonance frequency due to the input light intensity through the Kerr nonlinear effect. Through finite-difference time-domain simulation, it is found that the high performance of all-optical switching can be achieved by the designed structure with a threshold pump intensity as low as MW/cm2. This structure is featured by optical bistability. The obtained results are applicable in micro optical integrated circuits for modulators, switches and logic elements for optical computation. © 2016
Publication Date: 2017
Optics Communications (00304018)395pp. 195-200
An electro-optical modulator is demonstrated based on Fano-resonance effect in an out-of-plane illumination of one-dimensional slab photonic crystal composed of two graphene layers. It has been shown that high sensitivity of the Fano-resonance and electro-refractive tuning of graphene layers provides a suitable condition to obtain an electro-optical modulator with low energy consumption (8 pJ) with contrast of ~0.4. © 2016 Elsevier B.V.
Publication Date: 2016
Journal of Nanophotonics (19342608)10(3)
We report a simple way of etching lithium niobate (LN) to build ridge waveguides. Argon plasma is used in an RF-sputtering chamber to etch the LN. The height of waveguide walls reaches 2.5 μm and a titanium self-alignment in-diffusion process is used to make the waveguide. Several diffusion times and different waveguides width are used to compare the mode properties and proportion of light that is confined in the ridge section of the waveguide. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).
Publication Date: 2015
Nanoscale (20403372)7(26)pp. 11379-11385
We design a compact, all-optical THz wave generator based on self-modulation in a 1-D slab photonic crystal (PhC) waveguide with a single sub-nanometer graphene layer by using enhanced nonlinearity of graphene. It has been shown that at the bandgap edge of higher bands of a 1-D slab PhC, through only one sub-nanometer graphene layer we can obtain a compact, high modulation factor (about 0.98 percent), self-intensity modulator at a high frequency (about 0.6 THz) and low threshold intensity (about 15 MW per square centimeter), and further a compact, all-optical THz wave generator by integrating the self-modulator with a THz photodiode or photonic mixer. Such a THz source is expected to have a relatively high efficiency compared with conventional sources based on optical methods. The proposed THz source can find wide applications in THz science and technology, e.g., in THz imaging, THz sensors and detectors, THz communication systems, and THz optical integrated logic circuits. © The Royal Society of Chemistry.
Publication Date: 2015
Laser Physics (1054660X)25(3)
The mobile light detection and ranging DIAL system of Malek Ashtar University of Technology has been developed for the detection of chemical warfare agents whose absorption wavelengths are in the range of 9.2-10.8 μm tunable CO2 lasers of the system. In this paper, this system is first described and then ammonia detection is analyzed experimentally. Also, experimental results of detecting a sarin agent simulant, dimethyl-methyl phosphonate (DMMP), are presented. The power levels received from different ranges to detect specific concentrations of NH3 and DMMP have been measured and debated. The primary test results with a 150 ns clipped pulse width by passive pinhole plasma shutter indicate that the system is capable of monitoring several species of pollutants in the range of about 1 km, with a 20 m spatial and 2 min temporal resolution. © 2015 Astro Ltd.
Publication Date: 2015
EPJ Applied Physics (12860042)70(2)
In this article, a theoretical model is presented to calculate the laser clipped pulse temporal width by the pinhole plasma shutter, and then the model results are compared with the experimental results of CO2 laser clipped pulses by aluminum and copper pinhole plasma shutters. In this model, it is assumed that the laser clipped pulse width is approximately equal to the sum of the plasma formation time and the plasma propagation time in order to reach from pinhole edges to the pinhole center. Furthermore, we assume that the plasma formation time is approximately equal to the time for the surface temperature of pinhole metal plate to reach the boiling point by absorbing the laser pulse energy. Heat conduction equation is used to calculate the time of plasma formation, and Taylor-Sedov's model is used to calculate the plasma propagation time to reach the pinhole center. By these assumptions, a relationship has been established between the laser clipped pulse width on the one hand, and thermo-dynamical and optical parameters of plasma shutter and the involved laser optical parameters on the other. Results of this model are in good agreement with experimental results. © EDP Sciences, 2015.
Publication Date: 2014
IEEE Sensors Journal (1530437X)14(11)pp. 4055-4058
We present the first experimental study for enhancement of PtSi Schottky detectors using photonic crystal (PC) structures. PCs can be used for simultaneous reduction of reflection and increase of absorption. The 2-D PCs are fabricated by interference lithography and reactive ion etching. In PC with 4.2-μ m depth, the average responsivity is enhanced by a factor of ∼ 7) with respect to regular detectors. We show that the light absorption enhancement is not sufficient to explain efficiency enhancement. The extra enhancement may be due to nanoscale roughness on the PC walls that affect the carrier collection efficiency and cutoff wavelength. © 2014 IEEE.
Publication Date: 2013
Applied Surface Science (01694332)264pp. 1-6
A nanoporous tapered silicon (Si) photonic crystal (PC) is realized. The PCs with this structure, which may be called hybrid PC-porous can significantly reduce the surface reflection over the broad wavelength range of 400-2000 nm. Moreover, the absorption enhances in this structure significantly. The PCs are fabricated by interference lithography and then nanoporous structure is applied on it using metal assisted chemical etching. The measured reflectance and absorption across a spectral range of 400-2000 nm are, approximately 3% and 96%, respectively. The improvement on the reflectance and absorption are about 90% and 70% compared to bare Si respectively; which is promising in the utilization of this structure for various applications. © 2012 Elsevier B.V. All rights reserved. All rights reserved.
Publication Date: 2012
Journal Of The European Optical Society-Rapid Publications (19902573)7
Broadband antireflection layers have been fabricated by two dimensional (2D) photonic crystals (PCs) with tapered pillars on the Si substrate. These PCs have been produced by interference lithography and reactive ion etching (RIE) techniques. The effect of depth and the filling factor (FF) of the PCs on the reflectance magnitude and bandwidth has been investigated. The obtained reflectance was less than 1% in the broad spectral range from 400 to 2100 nm. Our numerical simulation shows the PC pillars slope has an essential effect in the reduction of the reflection. However, our results show that the existence of RIE grasses in the PCs, which are created in the RIE process, does not influence the performance of the antireflection layer. This leads to a simpler fabrication process.
Publication Date: 2012
Microelectronic Engineering (01679317)97pp. 201-203
Using interference lithography and deposition technique we have fabricated large scale quasi one dimensional polymer-dielectric photonic crystal that provides sharp and deep Fano resonance in the transmission spectrum of the PC at normal incidence. Due to high sensitivity of the polymer refractive index to the temperature, the obtained Fano resonance provides suitable condition to be used as a low threshold and high contrast thermo-optical switch. We show that by increasing 125 °C in the PC temperature, the transmittance increases five times near the deep of the Fano resonance. © 2012 Elsevier B.V. All rights reserved.
Publication Date: 2011
Optics Communications (00304018)284(8)pp. 2230-2235
We investigate the potential of plasmonic resonance in metal nanocomposite materials for the design of photonic crystal all optical switches by numerical methods. We study the absorption effect of the plasmonic resonance on the Fano resonances of one dimensional photonic crystal slabs covered by a metal nanocomposite layer. It is shown that the absorption reduces the contrast of the Fano resonances. However, for adequate metal nanoparticle concentrations it is possible to achieve both sufficiently sharp Fano resonance and strong Kerr nonlinearity, which provides a suitable condition for the design of high contrast and low threshold switches. © 2011 Elsevier B.V. All rights reserved.
Publication Date: 2011
IEEE Photonics Technology Letters (10411135)23(20)pp. 1436-1438
In this study, we fabricate a nanocomposite photonic crystal (PC) by doping low concentration gold nanoparticles in a one-dimensional slab polymer PC. By coating a ZnS layer, we get a fairly sharp Fano resonance dip with quality factor about 30 in the transmission spectrum of the PC near the surface plasmonic resonance of gold nanoparticles. We show theoretically that in spite of absorption by the gold nanoparticles this nanocomposite PC can be used as a good switch with low pump threshold intensity, high contrast, and high efficiency. © 2011 IEEE.
