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Tuning of random lasers by means of external magnetic fields based on the Voigt effect
, Article Optics and Laser Technology ; Volume 47 , April , 2013 , Pages 121-126 ; 00303992 (ISSN) ; Mahdavi, S. M ; Bahrampour, A. R ; Sharif University of Technology
2013
Abstract
It has been proposed that emission spectrum of random lasers with magnetically active semiconductor constituents can be made tunable by external magnetic fields. By employing the FDTD method, the spectral intensity and spatial distribution of electric field are calculated in the presence of an external magnetic field. It is numerically shown that due to the magneto-optical Voigt effect, the emission spectrum of a semiconductor-based random laser can be made tunable by adjusting the external magnetic field. The effect of magnetic field on the localization length of the laser modes is investigated. It is also shown that the spatial distribution of electric field exhibited remarkable...
Tunability of terahertz random lasers with temperature based on superconducting materials
, Article Journal of Applied Physics ; Volume 112, Issue 4 , 2012 ; 00218979 (ISSN) ; Bahrampour, A. R ; Mahdavi, S. M ; Hosseini, M ; Sharif University of Technology
2012
Abstract
We theoretically demonstrate the tunabiltiy of terahertz random lasers composed of high temperature superconductor YBCO and ruby layers as active medium. The considered system is a one-dimensional disordered medium made of ruby grain and YBCO. Finite-difference time domain method is used to calculate the emission spectrum and spatial distribution of electric field at different temperatures. Our numerical results reveal that the superconductor based random lasers exhibit large temperature tunability in the terahertz domain. The emission spectrum is significantly temperature dependent, the number of lasing modes and their intensities increase with decreasing temperature. Also, we make some...
Temperature tunable random laser using superconducting materials
, Article Optics Communications ; Volume 285, Issue 7 , 2012 , Pages 1900-1904 ; 00304018 (ISSN) ; Hosseini, M ; Bahrampour, A. R ; Mahdavi, S. M ; Sharif University of Technology
2012
Abstract
We propose that spectral intensity of superconductor based random lasers can be made tunable by changing temperature. The two fluid model and wavelength dependent dispersion formula have been employed to describe the optical response of the superconducting materials. Random laser characteristics have been calculated using the one dimensional FDTD method. Our simulation results reveal that the emission spectrum can be manipulated through the ambient temperature of the system. It is observed that transition from metal phase to pure superconducting phase leads to the enhancement of the laser emission. Furthermore, spatial distribution of the fields in one dimensional disordered media is very...
Numerical study of random lasing in three dimensional amplifying disordered media
, Article Optics Communications ; Volume 285, Issue 6 , 2012 , Pages 1314-1322 ; 00304018 (ISSN) ; Bahrampour, A. R ; Mahdavi, S. M ; Golshani Gharyeh Ali, M ; Sharif University of Technology
2012
Abstract
In this paper, the lasing action in three-dimensional active random systems has been numerically investigated. Here, random systems of spherical dielectric particles imbedded in an active medium are considered. The quasi steady state approximation for the population inversion of the active medium is applied to solve three dimensional governing equations. Results show that when the density of particles increases to an upper limit, the intensity of lasing modes is enhanced. Also, the effects of pumping rate and particle size on the number of lasing modes and their intensity are studied. Lasing threshold of laser modes in different disordered systems is calculated and it is shown that by an...
Mode analysis of fiber Bragg grating random lasers in the presence of mode competition
, Article Optics and Laser Technology ; Volume 44, Issue 4 , 2012 , Pages 969-975 ; 00303992 (ISSN) ; Golshani Gharyeh Ali, M ; Mahdavi, S. M ; Bahrampour, A. R ; Sharif University of Technology
2012
Abstract
In the presence of self and cross saturation effects, the laser governing equations are nonlinear. To solve the governing equations of a fiber Bragg grating random laser, on the basis of the transfer matrix method, a simple and fast iterative method is introduced. The method is employed to obtain the frequency, threshold pumping rate and spatial distribution of different laser modes. Numerical calculations show that by increasing the number of fiber Bragg grating, the number of lasing modes increases, while the threshold pumping rate of the first mode decreases. Our theoretical results are in agreement with the experimental observations reported by previous authors
Ultraviolet photodetectors based on ZnO sheets: The effect of sheet size on photoresponse properties
, Article Applied Surface Science ; Volume 258, Issue 14 , 2012 , Pages 5405-5411 ; 01694332 (ISSN) ; Pazoki, M ; Mahdavi, S. M ; Bahrampour, A. R ; Taghavinia, N ; Sharif University of Technology
2012
Abstract
In this work, ultraviolet photodetectors based on electrodeposited ZnO sheet thin films were fabricated on a glass substrate. Before electrodeposition, a thin buffer layer of ZnO was deposited on the glass by pulsed laser deposition method. This layer not only acted as a nucleation site for ZnO sheet growth, but also made it possible to use cheap glass substrate instead of conventional fluorine-doped tin oxide (FTO) substrate. Our results showed that photoresponse properties of the photodetectors strongly depend on the sheet sizes. The smaller sheets exhibited enhanced photosensitivity, shortened fall times and decreased gain compared to larger ones. We showed that photodetectors based on...
Numerical Simulation, Tunability and Experimental Investigation of Random Lasers and Using ZnO Nanosheets in these Lasers and UV Photodetectors
, Ph.D. Dissertation Sharif University of Technology ; Mahdavi, Mohammad (Supervisor) ; Bahrampour, Ali Reza (Supervisor) ; Taghavinia, Nima (Co-Advisor)
Abstract
In this thesis, first, random lasing in a three dimensional disordered medium has been simulated using FDTD method and some properties of random lasers which were observed by others in the previous experiments have theoretically been shown. Our simulation results show that when the density of particles increases to an upper limit, the intensity of lasing modes is enhanced. Also, the effects of pumping rate and particle size on the number of lasing modes and their intensity are studied. Lasing threshold of laser modes in different disordered systems is calculated and it is shown that by an appropriate selection of the central frequency of gain line-shape, the output power intensity of random...
Hybrid multiwalled carbon nanotubes and trioxide tungsten nanoparticles for hydrogen gas sensing
, Article Journal of Physics D: Applied Physics ; Volume 42, Issue 16 , 2009 ; 00223727 (ISSN) ; Iraji zad, A ; Sharif University of Technology
2009
Abstract
The WO3/multiwalled carbon nanotubes (MWNTs) hybrid films were fabricated by spin-coating on an alumina substrate. The structure, morphology, chemical composition and quality of the as-prepared WO3 nanoparticles, functionalized MWNTs and WO3/MWNTs hybrid films were studied by SEM, TEM, XRD, XPS, DLS and Raman spectroscopy methods. Results show that WO3 nanoparticles were nucleated on oxygenated functionalized MWNTs in hybrid suspension. After coating and annealing the films at 350 °C, the hybrid WO3/MWNTs films present rather linear gas sensitivity as a function of hydrogen concentrations at 300 °C. Results are discussed based on surface increase as well as development of two types of...
The moment method for fiber raman amplifier gain ripple minimization
, Article Optics Communications ; Volume 281, Issue 14 , 2008 , Pages 3673-3680 ; 00304018 (ISSN) ; Farman, F ; Ghasempour, A ; Sharif University of Technology
2008
Abstract
We aim to propose a novel fiber Raman amplifier modeling based on the moment method, which is previously introduced for modeling the inhomogeneous Erbium doped fiber amplifiers and recently employed to analyze the fiber Raman amplifier with continuous pump spectrum. In this model, the number of governing equations is independent of the number of signals and according to the degree of accuracy it is proportional to the number of pumps. This method is employed to analyze the Raman fiber amplifiers with an arbitrary input signal line shape and to minimize the gain ripple of the fiber Raman amplifier with respect to the pump powers and pump frequencies. © 2008 Elsevier B.V. All rights reserved
The effect of the Cr and Mo on the physical properties of electrodeposited Ni-Fe alloy films
, Article Journal of Alloys and Compounds ; Volume 386, Issue 1-2 , 2005 , Pages 43-46 ; 09258388 (ISSN) ; Iraji zad, A ; Dolati, A ; Ghasempour, R ; Sharif University of Technology
2005
Abstract
Ni-Fe and the Ni-Fe-Cr-Mo alloy films were electrodeposited from a chloride solution containing complexing compounds on ITO/glass substrates. AFM studies revealed a spherical structure for the alloy films with roughnesses from 10 up to 50 nm. The Ni-Fe-Cr-Mo alloy films were found to have a fine-grain structure. The quaternary alloy compounds containing up to 4% Cr and 2% Mo showed higher resistivity and little decrease in magnetoresistance (MR) relative to the Ni80Fe20 alloy compound. The MR of the quaternary alloy compounds was decreased for higher Cr and Mo contents. Moreover, the anisotropy (AMR) was also decreased by the addition of Cr and Mo to Permalloy films. Auger studies indicated...
Visible light photoinactivation of bacteria by tungsten oxide nanostructures formed on a tungsten foil
, Article Applied Surface Science ; Volume 338 , May , 2015 , Pages 55-60 ; 01694332 (ISSN) ; Azimirad, R ; Amini, A ; Akhavan, O ; Sharif University of Technology
Elsevier
2015
Abstract
Antibacterial activity of tungsten oxide nanorods/microrods were studied against Escherichia coli bacteria under visible light irradiation and in dark. A two-step annealing process at temperatures up to 390 °C and 400-800 °C was applied to synthesize the tungsten oxide nanorods/microrods on tungsten foils using KOH as a catalyst. Annealing the foils at 400 °C in the presence of catalyst resulted in formation of tungsten oxide nanorods (with diameters of 50-90 nm and crystalline phase of WO3) on surface of tungsten foils. By increasing the annealing temperature up to 800 °C, tungsten oxide microrods with K2W6O19 crystalline phase were formed on the foils. The WO3 nanorods showed a strong...
Characterization of Ni-P-SiO2-Al2O3 nano-composite coatings on aluminum substrate
, Article Materials Chemistry and Physics ; Volume 189 , 2017 , Pages 207-214 ; 02540584 (ISSN) ; Afshar, A ; Sadreddini, S ; Ghanbari, A. A ; Sharif University of Technology
Elsevier Ltd
2017
Abstract
In the present work, nano-composites of Ni-P-SiO2-Al2O3 were coated on a 6061 aluminum substrate. The surface morphology of the nano-composite coating was studied by field emission scanning electron microscopy (FESEM). The amount of SiO2 in the coating was determined by Energy Dispersive Analysis of X-Ray (EDX) and the crystalline structure of the coating was examined by X-ray diffractometer (XRD). All the experiments concerning the corrosion behavior of the coating carried out in 3.5%wt NaCl solution and evaluated by electrochemical impedance spectroscopy (EIS) and polarization technique. The results showed that an incorporation of SiO2 and Al2O3 in Ni-P coating at the SiO2 concentration of...
Analysis of the forces acting on apex seal of a wankel engine
, Article Australian Journal of Basic and Applied Sciences ; Volume 4, Issue 9 , 2010 , Pages 4205-4212 ; 19918178 (ISSN) ; Shadloo, M. S ; Ghasempour, M ; Kimiaeifar, A ; Sharif University of Technology
2010
Abstract
The acceleration acting on apex seals, and engine speed fluctuations together with the leading and trailing cells gas pressure forces are analyzed in the present work using the second Newton law. The research shows that cell gas pressure effects are important and inclusion of these effects will provide results which are more compatible with the reality and lead to a much more accurate analysis of the system. Theoretical calculations reveal that gas cell pressure forces represent about 78% of the overall calculated force and ignoring these pressure forces lead to the same magnitude of inaccuracy in predictions
Time-dependent theory for random lasers in the presence of an inhomogeneous broadened gain medium such as PbSe quantum dots
, Article Applied Optics ; Volume 52, Issue 6 , 2013 , Pages 1317-1324 ; 1559128X (ISSN) ; Mahdavi, S. M ; Bahrampour, A. R ; Sharif University of Technology
2013
Abstract
Time-dependent model is presented to simulate random lasers in the presence of an inhomogeneous gain medium. PbSe quantum dots (QDs) with an arbitrary size distribution are treated as an inhomogeneous gain medium. By introducing inhomogeneity of the PbSe QDs in polarization, rate, and Maxwell's equations, our model is constructed for a one-dimensional disordered system. By employing the finite difference time-domain method, the governing equations are numerically solved and lasing spectra and spatial distribution of the electric field are calculated. The effect of increasing the pumping rate on the laser characteristics is investigated. The results show that the number of lasing modes and...
A Nested Logit analysis of the influence of distraction on types of vehicle crashes
, Article European Transport Research Review ; Volume 10, Issue 2 , 2018 ; 18670717 (ISSN) ; Mahmoudzadeh, A ; Kermanshah, M ; Sharif University of Technology
Springer Verlag
2018
Abstract
Purpose: This work aims to study factors, such as driver characteristics, environmental conditions, and vehicle characteristics, that affect different crash types with a special focus on distraction parameters. For this purpose, distraction factors are divided into five groups: cellphone usage, cognitive distractions, passengers distracting the driver, outside events attracting the driver’s attention, and in-vehicle activities. Methods: Taking the crashes that occurred in the USA into account, the crash types are divided into two main groups, single-vehicle crashes and two-vehicle crashes. Since there were different crash types (alternatives) in the dataset and the probable correlation in...
What factors results in having a severe crash? a closer look on distraction-related factors
, Article Cogent Engineering ; Volume 6, Issue 1 , 2019 ; 23311916 (ISSN) ; Mahmoudzadeh, A ; Kermanshah, M ; Sharif University of Technology
Cogent OA
2019
Abstract
This study provides a comprehensive literature review to summarize all contributing factors and the logit-based models that were used to predict the severity of crashes. Using the General Estimates Systems (GES) dataset, as a subset and a branch of the National Automotive Sampling System in the US, a Generalized Ordered Logit (GLM) model is developed to predict the crash severity. The developed severity model detects the most important parameters based on characteristics of the driver, the environment, the vehicle, the road, and the type of crash. This study aims to take a more in-depth look into the distraction-related factors as one of the most important groups of contributing factors to...
Design and fabrication of sensitive carbon nanotubes/PMMA film for acetone vapour detection
, Article International Journal of Nanomanufacturing ; Volume 5, Issue 3-4 , 2010 , Pages 268-277 ; 17469392 (ISSN) ; Iraji Zad, A ; Hormozi Nezhad, M. R ; Sharif University of Technology
2010
Abstract
We present gas sensing property of carbon nanotubes (CNTs) polymethylmethacrylate (PMMA) composite as the active element for acetone vapour detection at room temperature. The polymeric films were formed by spin coating method on SiO2 substrates. Then they were activated by dropping the CNTs suspension in acetone on the PMMA films. The CNT/PMMA films were characterised by SEM, TEM and Raman spectroscopy. Variation of film's electrical resistance after exposure polar and non-polar gases is utilised as the principle of gas sensing. The experimental results showed that the samples present chemical selectivity and reversibility toward polar gases especially acetone vapour
Pd doped WO3 films prepared by sol-gel process for hydrogen sensing
, Article International Journal of Hydrogen Energy ; Volume 35, Issue 2 , 2010 , Pages 854-860 ; 03603199 (ISSN) ; Iraji zad, A ; Rahimi, F ; Ghasempour, R ; Sharif University of Technology
2010
Abstract
The sol gel method was employed to prepare peroxopolytungstic acid (P-PTA). Palladium chloride salt was dissolved in the sol with different Pd:W molar ratios and coated on Al2O3 substrates by spin coating method. XRD and XPS techniques were used to analyze the crystal structure and chemical composition of the films before and after heat treatment at 500 °C. We observed that Pd can modify the growth kinetic of tungsten trioxide nanoparticles by reducing the crystallite size and as a result can improve hydrogen sensitivity. Resistance-sensing measurements indicated sensitivity of about 2.5 × 104 at room temperature in hydrogen concentration of 0.1% in air. Considering all sensing parameters,...
A performance and functional assertion-based verification methodology at transaction-level
, Article 19th International Conference on Microelectronics, ICM, Cairo, 29 December 2007 through 31 December 2007 ; 2007 , Pages 133-136 ; 9781424418473 (ISBN) ; Gharehbaghi, A. M ; Hessabi, S ; Sharif University of Technology
2007
Abstract
In this paper, we present an assertion-based verification methodology for system-level design. Transactionlevel concepts are integrated with an assertion language to introduce a useful, effective and familiar assertion description language. Our assertion verification language is capable of specifying system-level assertions for validating performance as well as functional properties. Proper-ties can be verified using offline simulation trace analysis. C++ trace checkers are automatically generated to validate particular simulation runs or to analyze their performance characteristic(s). Using a JPEG decoder as a case study, we demonstrate that the assertion-based verification is highly useful...
System-level assertion-based performance verification for embedded systems
, Article 13th International Computer Society of Iran Computer Conference on Advances in Computer Science and Engineering, CSICC 2008, Kish Island, 9 March 2008 through 11 March 2008 ; Volume 6 CCIS , 2008 , Pages 243-250 ; 18650929 (ISSN); 3540899847 (ISBN); 9783540899846 (ISBN) ; Gharehbaghi, A. M ; Hessabi, S ; Sharif University of Technology
2008
Abstract
As contemporary digital systems specifically embedded systems become more and more complex, taking advantage of system-level design is being more widespread. Many embedded systems must operate under strict timing constraints. One of the best methods for examining timing constraints in an embedded system can be done via the performance verification. In this paper an assertion-based verification methodology has been proposed for verifying system-level timing constraints in an embedded system. Performance assertions are specified by an assertion language at the transaction-level of abstraction. A Turing machine and a structure named performance evaluator have been coupled to provide a...