A. Rostami

All-optical filter design: Electromagnetically induced transparency and ring resonator

In this paper, we present a novel method based on Electromagnetically Induced Transparency (EIT) and ring resonator for description of all-optical controllable filters that is so necessary for development of all-optical networks and systems. Impurities including 4-level atoms or nanocrystals added to ring resonator for control of the absorption and the index of refraction. Parameters of the control electromagnetic field or fields are used to manipulate the absorption and the index of refraction of the ring resonator. Also, optical filtering behavior of the ring resonator is studied. It is shown that using the control field, optical filtering properties of the proposed structure can be controlled.

All-optical pole and zero blocks for implementation of optical engineering systems

Basic units for realization of all-pole and zero transfer functions in all-optical engineering systems using the ring resonators are introduced. Using our proposed basic blocks for generation of pole and zero, we can implement any type and any degree of optical engineering systems (e.g. filters and optical processing systems). In this work, z-transform is used as mathematical tools. In this case, the LP Butterworth optical filter design using our proposal is presented as an example.

Broadband negative refractive index in the visible spectrum

In this paper, a composite medium based metamaterial with random distribution of nanoparticles in vacuum host to achieve negative effective refractive index in the visible wavelength range is suggested for invisibility purposes. Our calculations show that structures including single metal (dielectric) spheres and core-shell structures with metallic core and dielectric shell, which consists two-layer particles with uniform sizes cannot support negative effective refractive index. For this purpose the structures consist of two-layer nanospheres with different sizes and fill fraction has been proposed. Since, band width of negative refractive index is narrow, the three layer nanospheres has been studied and investigated. We show that in this case with increasing the refractive index of middle and outer layers, negative value of effective refractive index can be increased. Also, we show that using different sizes of nanomaterials in host medium, band width is increased. Finally, superposition of three layer spherical nanoparticles with different outer radius and applied single doped semiconductor spheres, has been proposed. We show that Band width with negative permittivity and permeability can be optimized.

Performance Analysis of Ultra-Thin Silicon Based Tunnel Junctions for Tandem Solar Cell Applications

Nowadays silicon solar cells have an efficiency of up to 20 % and in order to increase the efficiency of them, fabrication of multi-junction thin film solar cells with different band gaps is one of the most promising approaches. The silicon based tandem solar cells are third generation new style solar cells with ultra-high efficiency. The sub-cells in a tandem solar cell have different energy band gaps. In order to match the currents between sub-cells, tunnel junctions are used to connect the sub-cells. This work will concentrate on simulating the tunnel junction for application as part of multi-junction solar cell. In this way dopant concentration is changed and the tunnel junction current-voltage characteristics and their Energy band diagram in different dopant levels under equilibrium condition for moderate and usual doping have been calculated. An n++-Si/p++-Si tunnel junction is selected to simulate the overall characteristics of cell by numerical finite element method. We have simulated a symmetric silicon tunnel junction with thickness of 25 nm for n-type and 25 nm for p-type silicon by changing doping value from 1 × 10e20 cm−3 to 2 × 10e20 cm−3. The simulation results show that the doping concentration of 2 × 10e20 cm−3 is suitable for both sides.

Effect of Dopant Concentrations on Conversion Efficiency of SiC-Based Intermediate Band Solar Cells

It was recognized that the introducing of a narrow metallic band states in the crystal structure of semiconductors make materials that they can be used as intermediate band materials for improving the power conversion efficiency of high band gap single junction solar cells. In these structures intermediate bands would serve as a “stepping stone” for photons with different energies to excite electrons from the valence to the conduction bands. Low-energy photons can be captured by this method that would pass through a conventional solar cell. An optimal IBSC (intermediate band solar cells) has a total band gap of about 1.95 eV and 3C-SiC has the closest band gap to this value (band gap of 2.2 eV). Excellent electronic properties of 3C-SiC such as high electron mobility and saturated electron drift velocity and its suitable band gap makes it an important alternative material for light harvesting technologies instead of conventional semiconductors like silicon. In this condition detailed balance analysis predicts a limiting efficiency of more than 55 % for an optimized, single junction intermediate band solar cell that it is higher than efficiency of an optimized two junction tandem solar cell. In this study we have analyzed Fe doped 3C-SiC by ab initio calculations for Fe concentration of 1.05, 1.85, 3.22, and 5.55 %. The results show conversion efficiency for designed solar cell change with altering in Fe contents. The maximum efficiency has been obtained for crystals with 3 % Fe3+ as dopant in 3C-SiC structure.

Full-optical spectrum analyzer design using EIT based Fabry-Perot interferometer

We present the full-optical realization of spectrum analyzer system based on electromagnetic induced transparency (EIT) and Fabry-Perot interferometer. In this system, the index of refraction variation due to variation of the coupling field intensity in the EIT process can be used for detection of the incident light components in the frequency domain. For extracting the frequency component, the transmission coefficient of the Fabry-Perot interferometer is used. Our proposed system can monitor the incident spectrum content at least for 5-THz bandwidth.

Optical Properties of Coated Nanospheres in Visible Wavelength Range

In this paper we have studied polymeric structures which possess coated spherical nanoparticles in visible wavelength range. Medium with metallic cores and silicon (Si) shells and structures composed by Si cores and metallic shells have been studied. Since, the size of particles is very small related to the incident wavelength, semi-static approximation and Clausius-Mossotti formula have been used in order to calculate the effective permittivity. Resonance wavelength of the structure depends on size and filling fraction of the guest nanoparticles. Resonance wavelength has been obtained by the mathematical relations and simulation results. Studied structures are applicable in invisibility.

Spatial mode conversion by non-degenerate four wave mixing

We investigate coupling and power transfer between two transverse modes in a single quantum-well traveling wave semiconductor optical amplifier (SOAs) by non-degenerate four wave mixing. By this approach the mode purity achieve 99.99% at the end of active region of SOA and the mode conversion can be controlled by the adjusting pump and probe power.

A novel multiband filter design based on ring resonators and DSP approach

This paper proposes a novel ring resonator based optical filter that has an outstanding multi narrow band response due to adopting quasi structures such as Thue-Morse sequence as the radius-pattern. This capability introduces this design approach as an effective method for the design of filters for emerging dense wavelength division multiplexing networks. The design process incorporates analysing through the transfer matrix method and the powerful discrete-time signal processing techniques. Giving an adequate overview of analysing basic optical building blocks in the Z-domain, the procedure develops to analysing any optical structure imposed by mathematical sequences. The proposal is discussed employing pole-zero diagrams, discrete-time signal processing approach including apodization techniques. The point of the discrete-time signal processing approach is that the effect of dominant optical parameters over operation is clarified through the pole-zero position. Features like number of poles, bandwidth, and position of stop-bands can be controlled using ring diameter ratio. Finally, apodization of coupling coefficients attains a filter with an FWHM of 0.3 nm.

Temperature effect on Intermediate Band Solar Cells (IBSCs)

Temperature has profound effect on the performance of solar cells. Most of the electrical processes in the semiconductor devices depend on the temperature and revealed in dramatically variations in their characteristics such as open circuit voltage, short circuit current, power conversion efficiency and the band gap of semiconductor. The aim of this paper is investigation of temperature effects on the Intermediate Band Solar Cells (IBSCs). The theoretical results indicate that performance of this type of the solar cells is low at high temperatures. Increasing in temperature from 300K to 600K decreases the efficiency of solar cell from 63% to 59 % and this decrement continues with temperature increment. This is while temperature can decrease the open circuit voltage and increase the short circuit current.