Hosseinifar M.,Islamic Azad University at Tehran |
Ahmadi V.,Tarbiat Modares University |
Abaeiani G.,Laser and Optics Research School
Journal of Lightwave Technology | Year: 2011
A novel structure of unitraveling carrier photodiodes (UTC-PDs) with high-speed, high-efficiency, and wavelength selective characteristics is presented. Including the essential features of resonant cavity enhanced photodiodes and waveguide photodiodes, we propose microring-based UTC-PDs (MR-UTC-PDs) which can achieve excellent high-speed and high-quantum efficiency characteristics simultaneously. The photoresponse of MR-UTC-PDs is based on a drift-diffusion model. Photoresponse characteristics of MR-UTC-PDs, which depend on device parameters and coupling conditions, are investigated and it is shown that the overcoupled structures are suitable for high-speed photodetection. The important features of the device, such as efficiency enhancement and wavelength selectivity are discussed and the trade-off between 3 dB bandwidth and efficiency is solved for nanoscaled absorption layer. Additionally, the bandwidth-efficiency product in the order of several hundreds gigahertz can be obtained even with low photoabsorption layers. © 2006 IEEE.
Shiri H.M.,Payame Noor University |
Aghazadeh M.,Laser and Optics Research School
Journal of the Electrochemical Society | Year: 2012
Capsule-like NiO nanoparticles were prepared by heat-treatment of the cathodically electrodeposited nickel hydroxide at 300C for three hours. The hydroxide precursor was deposited galvanostatically onto the stainless steel substrate from aqueous Ni(NO 3) 2 solution (0.01 M). Morphological, structural and chemical characteristics of the synthesized NiO were examined by means of scanning and transmission electron microscopy (SEM and TEM) as well as X-ray diffraction (XRD) and FT-IR spectroscopy. The XRD and TEM results revealed that the oxide products have well-crystallized cubic structure with capsule-like morphology of various length and diameters in the order of 20-70 nm and 10-30 nm, respectively. Capacitive performance of NiO nanocapsules were evaluated by cyclic voltammetry and galvanostatic chargedischarge measurements in a potential window between -0.2 and +0.5 V vs. AgAgCl. Electrochemical measurements revealed that the NiO nanocapsules have a specific faradaic pseudocapacitance as high as 307.2 F g -1 in 1 M KOH solution and good cycle life (approximately displayed 86.4 of its maximum capacitance after 3000 cycle discharging). These results showed that the prepared NiO nanostructure can be recognized as a promising candidate for the supercapacitor applications. © 2012 The Electrochemical Society.
Adelkhani H.,Material Research School |
Ghaemi M.,Golestan University |
Ruzbehani M.,Laser and Optics Research School
International Journal of Electrochemical Science | Year: 2011
Manganese dioxide samples were prepared by pulse current electrodeposition. The effects of the pulse current parameters on the type of hysteresis loop of the adsorption isotherms, the surface area, and nano-structure morphology of the samples were investigated in details by using the Barrett-Joyner-Halenda (BJH) method and the Scanning Electron Microscopy (SEM) technique. Results indicate that the applied duty cycles play key roles in different characteristics of the samples. Samples obtained at duty cycles up to 30% show a decreasing trend of the surface area as well as increasing grain size and represent a type H4 hysteresis as duty cycles decrease. At duty cycle of 30%, the surface area has been increased, whereas at higher duty cycle of 70%, a decrease in surface area and the grain size of samples has been observed. Even though type H2 hysteresis prevailed at duty cycles higher than 70%, an overlapping of both type H4 and H2 hysteresis loops were obtained at duty cycles lower than 70%. These behaviors were explained and discussed on the basis of the formation and dissolution mechanism of the passive layers during on- and off-times. © 2011 by ESG.
Ruzbehani M.,Laser and Optics Research School
Asian Journal of Control | Year: 2010
A new tracking controller for discrete-time Single Input Single Output (SISO) non-minimum phase (NMP) systems is presented. In the proposed method, after cancellation of poles and cancelable zeros of the system, the controller adds some NMP zeros to compensate the effect of NMP zero (zeros) of the system. As a result, the phase of the overall transfer function will be almost linear and its magnitude approaches unity for all frequencies. The method can be applied even to the systems with complex conjugate NMP zeros. As well, it is applicable to the systems for which the conventional methods cannot properly be used. Furthermore, a generalization of method to continuous-time systems is another given result. Several examples are provided to illustrate the effectiveness of the method. © 2009 John Wiley and Sons Asia Pte Ltd and Chinese Automatic Control Society.
Karimi M.,Laser and Optics Research School |
Farahbod A.H.,Laser and Optics Research School
Optics Communications | Year: 2014
Shooting method is a simple and powerful method to solve boundary value problems. By using shooting method and appropriate definition of comparative function with a proper guess for the initial value of pump and signal the computation process has short time and quick converge to correct answers. In this paper by definition of the answer ranges and proper comparative function, the requirement of proper initial guesses for forward signal and backward pump to solve rate equation is resolved. By putting initial and end of answer ranges as two set guesses, the signal and pump initial values were achieved fast and easily using the second method. In other words using answer ranges, the shooting algorithm was equipped with a process of response screening. This computational procedure, without requirement of experimental lookout can be used for fiber lasers and amplifiers. We also show that the former optimum length definition in strongly pumped power lasers cannot be used in lasers with conventional pump power and must be replaced with suitable one. Forward, backward and bi-directional pump scheme have been investigated using the proposed calculation procedure. An effect of background loss on optimum fiber laser length is also verified and the effects of input and output mirrors reflectivity, dopant concentration and fiber length in laser output power is investigated. © 2014 Elsevier B.V.
Hassanpour A.,K. N. Toosi University of Technology |
Bananej A.,Laser and Optics Research School
Optik | Year: 2013
The effect of time-temperature gradient annealing on the TiO 2 single layers depositing in the same conditions by electron gun were discussed. Investigating the optical properties of the layers which were calculated by envelope method revealed that not only the extinction coefficient but also porous ratio of the layer may affect the laser resistance of the thin films. The experimental results are in a good agreement with theoretical analysis which is based on the well known Yoshida's model. It can be seen the positive impact of higher porous ratio on the laser induced damage threshold values of the TiO 2 thin films. In addition, according to the XRD patterns of samples, it can be seen that our novel annealing procedure have a significant influence on the grain size of the TiO 2 material. Consequently, comparing the laser induced damage threshold showed that different annealing procedures may modify laser resistance of TiO 2 thin films by influencing their optical or structural properties. © 2011 Elsevier GmbH. All rights reserved.
Rashidian Vaziri M.R.,Laser and Optics Research School
Optics Communications | Year: 2015
In an influential paper Jamshidi-Ghaleh and Mansour  (Opt. Commun. 234 (2004) 419), have reported on a new method for measuring the nonlinear refractive index of materials using the rotational moiré deflectometry technique. In the cited work, the authors apply the ray matrix theory for finding the beam deflection angle on the plane of the first grating in the used geometry. To this end, using the parabolic approximation, the exponential term in the beam irradiance is expanded and retaining the first two resultant terms, the nonlinear sample is treated as a thin lens with a position dependent focal length. In this comment, the effective focal length of the nonlinear sample has been rederived in detail using the Gaussian beam theory and it is shown that it must contain a correction factor. The relative error introduced by ignoring this factor can be as large as 73.5-84.4% in determining the nonlinear refractive index of thin samples. © 2014 Elsevier B.V. All rights reserved.
Bananej A.,Laser and Optics Research School |
Hassanpour A.,K. N. Toosi University of Technology
Applied Surface Science | Year: 2012
As a result of using time-temperature gradient annealing, the structural and optical properties and hence, thermal properties of ZrO 2 thin films deposited by electron gun, enhanced considerably compared with rapid thermal annealing. Also, it can be seen time- temperature gradient annealing caused to decrease the total stress and number of thermal barrier along the film. Therefore, according to our experimental results, which is agree with theoretical analysis, the laser induced damage threshold of ZrO 2 films can be enhanced significantly compared with rapid annealing procedure. © 2011 Elsevier B.V. All rights reserved.
Vaziri M.R.R.,Laser and Optics Research School
Applied Optics | Year: 2013
In this paper, the Z-scan theory for nonlocal nonlinear media has been further developed when nonlinear absorption and nonlinear refraction appear simultaneously. To this end, the nonlinear photoinduced phase shift between the impinging and outgoing Gaussian beams from a nonlocal nonlinear sample has been generalized. It is shown that this kind of phase shift will reduce correctly to its known counterpart for the case of pure refractive nonlinearity. Using this generalized form of phase shift, the basic formulas for closed- and open-aperture beam transmittances in the far field have been provided, and a simple procedure for interpreting the Z-scan results has been proposed. In this procedure, by separately performing open- and closed-aperture Z-scan experiments and using the represented relations for the farfield transmittances, one can measure the nonlinear absorption coefficient and nonlinear index of refraction as well as the order of nonlocality. Theoretically, it is shown that when the absorptive nonlinearity is present in addition to the refractive nonlinearity, the sample nonlocal response can noticeably suppress the peak and enhance the valley of the Z-scan closed-aperture transmittance curves, which is due to the nonlocal action's ability to change the beam transverse dimensions. © 2013 Optical Society of America.
Pazokian H.,Laser and Optics Research School
Optical and Quantum Electronics | Year: 2016
A thick rim (~200 µm) formed around the ablated area following irradiation of a polyethersulfone film with ultra-short laser pulses, produced by a KrF laser. The possible mechanisms for generation of the heat affected zone around the ablated area were discussed and the reason for the formation of such a thick rim was found. © 2015, Springer Science+Business Media New York.