Papers by mojtaba shahraki
Ultrathin nano-ring metasurface absorber in visible regime based on circuit model
The European Physical Journal Plus
Absorption improvement of organic solar cells using grating structures in active layer and antireflection coating layer
Journal of Nanophotonics
Frontiers of Optoelectronics, 2022
This paper investigates how the dimensions and arrangements of stadium silicon nanowires (NWs) af... more This paper investigates how the dimensions and arrangements of stadium silicon nanowires (NWs) affect their absorption properties. Compared to other NWs, the structure proposed here has a simple geometry, while its absorption rate is comparable to that of very complex structures. It is shown that changing the cross-section of NW from circular (or rectangular) to a stadium shape leads to change in the position and the number of absorption modes of the NW. In a special case, these modes result in the maximum absorption inside NWs. Another method used in this paper to attain broadband absorption is utilization of multiple NWs which have different geometries. However, the maximum enhancement is achieved using non-close packed NW. These structures can support more cavity modes, while NW scattering leads to broadening of the absorption spectra. All the structures are optimized using particle swarm optimizations. Using these optimized structures, it is viable to enhance the absorption by s...
Optimization of light absorption in ultrathin elliptical silicon nanowire arrays for solar cell applications
Journal of Modern Optics, 2022
International Journal of Optics and Photonics, 2021
In this paper, the optical properties of laterally oriented core-shell nanowire silicon solar cel... more In this paper, the optical properties of laterally oriented core-shell nanowire silicon solar cells (NWSCs) are optimized. The optimum structure consists of an array with non-uniform hexagonal nanowires (NWs). Each NW is constructed from an amorphous silicon layer sandwiched between two crystalline silicon layers. In order to improve the light absorption and short circuit current density (Jsc) of NWSC, a particle swarm optimization (PSO) algorithm is used to optimize the geometrical parameters of NWs. It is shown that the optimized structure has advantageous performance in terms of light absorption and Jsc. Finally, a multiple structure composed of two NWs with different morphologies and the optimized dimensions is proposed to utilize NWSCs better.
Absorption improvement of a-Si/c-Si rectangular nanowire arrays in ultrathin solar cells
Journal of Photonics for Energy, 2021
Abstract. Optical absorption of rectangular a-Si/c-Si solar cells is investigated. Each nanowire ... more Abstract. Optical absorption of rectangular a-Si/c-Si solar cells is investigated. Each nanowire consists of a thin layer of a-Si, which surrounded by c-Si layers. The proposed structure has a very simple geometry compared with other nanowire solar cells, while its absorption rate is comparable to very complex structures. Periodic, semiperiodic, and multiple rectangular nanowires are used to achieve broadband absorption. In order to maximize the short-circuit current density of the proposed nanowire solar cells, particle swarm optimization algorithm has been performed. Irregularities have been observed to be one of the ways of absorbing broadband.
In this paper , we propose an all-optical switch based on nonlinear metal-insulator -metal (MIM) ... more In this paper , we propose an all-optical switch based on nonlinear metal-insulator -metal (MIM) nanoplasmonic structures. The nonlinear Ker r effect and the intr insic nonlinear proper ties of gold are used to implement the plasmonic switch. Increasing the intensity of the input light causes significant nonlinear effects. By examining the transmission spectrum, the proposed structure has a minimum at 860 nm. When the light intensity increases, the resonance wavelength changes. The result showed that the proposed switch has a threshold power equal to 0.07 mW/μm and the transmission ratio of about 0.8.
In this paper, the optical properties of laterally oriented core-shell nanowire silicon solar cel... more In this paper, the optical properties of laterally oriented core-shell nanowire silicon solar cells (NWSCs) are optimized. The optimum structure consists of an array with non-uniform hexagonal nanowires (NWs). Each NW is constructed from an amorphous silicon layer sandwiched between two crystalline silicon layers. In order to improve the light absorption and short circuit current density (Jsc) of NWSC, particle swarm optimization (PSO) algorithm is used to optimize the geometrical parameters of NWs. It is shown that the optimized structure has advantageous performance in terms of light absorption and Jsc.
In this paper, we proposed an all-optical plasmonic switch based on metal-insulator-metal (MIM) s... more In this paper, we proposed an all-optical plasmonic switch based on metal-insulator-metal (MIM) structures. We used the intrinsic nonlinear properties of gold to implement the switch. The proposed switch consists of a bus waveguide side coupled with a pair of asymmetric vertical cavities. We obtained the transmission spectrum of the structure for low input intensities. The results showed that a sharp dip occurs at the wavelength of 860 nm. Due to the nonlinear properties of gold and the nonlinear Kerr effects, the proposed switch has a high transmission ratio of about 0.8 and a low threshold power of 0.07 mW/µm2. The threshold power of the structure with and without using the gold nanostructure shows a reduction of 50%. The result showed that the proposed switch has the potentiality to be applied in the plasmonic integration circuits.
Current Applied Physics, 2017
In this paper, an analytical approach based on transmission line equivalent circuit (TLEC) is use... more In this paper, an analytical approach based on transmission line equivalent circuit (TLEC) is used to model planar solar cells. The proposed method has been applied to the all-dielectric planar solar cells with multilayer anti-reflection coating (ARC) and non-periodic distributed Bragg reflector (DBR) nanolayers. This analytical approach is very efficient and easy to be implemented. Consequently, it is straightforward to employ several optimization procedures to determine the optimum thickness for different layers. In this paper, optimization process according to the Fuzzy adaptive modified particle swarm optimization (FAMPSO) algorithm is utilized to maximize the absorption enhancement of these types of solar cells. Results demonstrate the possibility of wideband absorption in all-dielectric planar solar cells and comparable to the conventional light trapping structures.
IEEE Transactions on Nanotechnology, 2017
In this paper, a circuit modeling is used to investigate the effects of waveguide grating nanostr... more In this paper, a circuit modeling is used to investigate the effects of waveguide grating nanostructures on the performance of ultrathin solar cells. The proposed method is based on transmission line equivalent circuit (TLEC) and it is identified for both of the transverse electric (TE) and transverse magnetic (TM) polarizations. Under assumption of weakcoupling condition, this method can effectively describe the absorption inside ultrathin solar cells with the one-dimensional grating nanostructure etched into the anti-reflection coating layers. The proposed approach consists of two different circuits accounting for the background and scattering fields. The final result of TLEC method comprises of the superposition of the background and scattering TLEC circuits.
Omnidirectional broadband absorption enhancement in laterally assembled quadrangle silicon nanowire solar cells
Journal of Modern Optics, 2016
Abstract In this paper, effects of geometry and spatial placement of quadrangle silicon nanowires... more Abstract In this paper, effects of geometry and spatial placement of quadrangle silicon nanowires (NWs) on the performance of laterally assembled NW solar cells are investigated. Two different approaches are proposed to reach the broadband absorption enhancement in these types of solar cells. In the first method, a semi-periodic array is used to have a better utilization of the optical antenna effect. The current density can be enhanced as a result of a compromise between the diffraction and optical antenna effects. In the second approach, multiple NWs with different geometries are employed to use the cavity modes of the various NWs. The best current density is achieved for the combination of both of these methods. The different parameters in each structure are selected using the optimization algorithm. Finally, a multilayer structure with the optimized dimensions is proposed to obtain the maximum achievable current density in ultrathin solar cells. The broadband absorption enhancement in the proposed structure is preserved for a wide range of incident angles. Using the multilayer NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.
Polarization effects on modulation instability of silicon on insulator waveguides
Journal of Nanophotonics, 2016
Abstract. Considering the effects of linear polarization, modulation instability (MI) in silicon ... more Abstract. Considering the effects of linear polarization, modulation instability (MI) in silicon on insulator (SOI) waveguides is studied. Impacts of differential group delay, group velocity dispersion, third-order dispersion, and linear loss on gain and frequency of MI, in linearly polarized radiations, are presented theoretically. Fully numerical simulations are performed to verify calculations as well. The results demonstrate strong MI in the normal dispersion regime of SOI waveguides compared to silica fibers with much higher frequencies.
Optimal design of laterally assembled hexagonal silicon nanowires for broadband absorption enhancement in ultrathin solar cells
Optical Engineering, 2015
Abstract. Design approaches to carry out broadband absorption in laterally assembled hexagonal si... more Abstract. Design approaches to carry out broadband absorption in laterally assembled hexagonal silicon nanowire (NW) solar cells are investigated. Two different methods are proposed to improve the current density of silicon NW solar cells. It is observed that the key to the broadband absorption is disorder and irregularity. The first approach to reach the broadband absorption is using multiple NWs with different geometries. Nevertheless, the maximum enhancement is obtained by introducing irregular NWs. They can support more cavity modes, while scattering by NWs leads to broadening of the absorption spectra. An array of optimized irregular NWs also has preferable features compared to other broadband structures. Using irregular NW arrays, it is possible to improve the absorption enhancement of solar cells without introducing more absorbing material.
Frequency Tuning and Bandwidth Enhancement of Raman Amplification in SiG-OI Waveguides
Recent Patents on Signal Processinge, 2012
ABSTRACT In this paper, some patents on silicon Raman amplifiers are investigated. These patents ... more ABSTRACT In this paper, some patents on silicon Raman amplifiers are investigated. These patents emphasize on the length reduction of amplifiers compared with the silica ones. However, the bandwidth of the amplifier in these patents is very low. In addition, while the pump wavelength is constant, the central frequency remains unchanged. The germanium doping leads to tuning of central frequency and bandwidth enhancement of Raman gain spectrum. Thus, the effect of germanium doping on the characteristics of major patents on silicon Raman amplifiers is investigated in this paper. Amplifier gain depends on various parameters such as the Raman gain coefficient. The effects of germanium doping on the impressive parameters of the silicon Raman amplifiers are also presented. By usage of the variable germanium concentration waveguides, the bandwidth of amplifiers is enhanced which has a great effect on communication systems. Furthermore, the characteristics of waveguide are estimated in order to achieve a flat gain amplification in silicon germanium on insulator (SiGe-OI) waveguides.
Reformation of an SiGe HBT structure for reducing the effect of parasitic barriers
2010 Second Pacific-Asia Conference on Circuits, Communications and System, 2010
In this paper, an n-p-n SiGe heterojunction bipolar transistor (HBT) is investigated. The emitter... more In this paper, an n-p-n SiGe heterojunction bipolar transistor (HBT) is investigated. The emitter and collector of this transistor are made up of silicon, and germanium dopant is linearly diffused in the base region. Adding germanium dopant causes the formation of parasitic barriers in base-emitter junction of transistor. The barrier effects are reduced by this structure. In this model, choosing
Australian Journal of Basic and …, 2011
Abstract: In this paper, the effect of Ge profile on the characteristics of an SiGe heterojunctio... more Abstract: In this paper, the effect of Ge profile on the characteristics of an SiGe heterojunction bipolar transistor (HBT) is investigated. The serious problem of the HBT structure is the formation of parasitic barriers, due to the addition of germanium dopants to the base. ...
The effects of polarization on soliton interactions inside optical fibers
Optical and Quantum Electronics, 2022
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Papers by mojtaba shahraki