Showing papers by "Rajib Chakraborty published in 2015"

Design of tunable transmission filter using one-dimensional defective photonic crystal structure containing electro-optic material

Abstract: A narrowband tunable transmission filter suitable for wavelength division multiplexing is designed. The basic structure is a one-dimensional Fabry–Perot structure formed by layers of dielectric magnesium fluoride and electro-optic lithium niobate, which act as low and high refractive index material layers, respectively. A narrowband phase shifted transmission peak occurs within the stopband of the reflectance spectra of the structure by introducing the defect of a low-index material at a suitable position in the structure. The bandwidth of the peak depends on the number of bilayers and also on the operating wavelength. The phase shift of the transmission peak is linearly related to the wavelength under consideration. By adjusting the defect layer width, this shift of the transmission peak from the operating wavelength can be avoided. The device dimensions are so chosen that such a structure can be fabricated and used with presently available technology. A linear transmission peak tunability of 4 nm/10 V is achieved for this device by varying the refractive index of the electro-optic lithium niobate layer with externally applied voltage along its z axis. All the simulations have been carried out using the finite difference time domain method in a MATLAB® environment.

Zinc-indium-oxide sol-gel thin film: surface patterning, morphology and photocatalytic activity

Abstract: We report the fabrication of surface patterned zinc indium oxide thin films by pressure assisted capillary force lithography from the precursor solutions of varying indium content 55 maximum. The presence of various morphologies spherical, hexagonal plate, cotton ball, flower and islandlike of hexagonal ZnO together with the formation of light surface patterns was observed from electron microscopes field emission scanning electron microscope, TEM for the films deposited from the precursors of 45 indium. However, at 55 indium ZI55, the development of high fidelity surface patterns average periodicity 15m peak height 15nm was observed from atomic force microscopy (AFM) study. A gradual increase in photocatalytic activity PA of the films towards decomposition of rhodamine 6G dye under UV 254nm was found on increasing indium content. Moreover, an improvement of the PA over 13 times was noticed by nanoAu coating on ZI55. This fabrication strategy could be used for PA improvement of metal oxide semicond...

Further enhancement of light extraction efficiency from light emitting diode using triangular surface grating and thin interface layer

Abstract: Analysis has been done of improvement of forward directional light extraction efficiency of light emitting diodes (LEDs) by surface patterning of different types of one-dimensional profiles on indium-zinc-oxide films developed recently by our group using sol–gel technique. Finite-difference time-domain simulations by MEEP software have been used for this purpose. From the analysis, it is found that the patterned film is suitable for near-infrared LED. The optimized structure, which gives maximum improvement at around 1.040 μm wavelength, is determined and fabricated using soft lithography. Further enhancement of the light output of the LED with the fabricated gratings is possible by introducing an interlayer within the top contact layer. The mathematical formulation of the coupling of light in structured/multilayered surfaces is also discussed.

Analysis of electric field for inclined electrodes and use of such configuration for generating tunable differential polarization phase

Abstract: While analytical techniques to study electric field distribution for an electrode-pair placed on two parallel opposite faces of a crystal (or channel) exist, but for the configuration with inclined opposite crystal faces such analysis is not straightforward. Here Schwarz-Christoffel mapping method, a special form of Conformal mapping technique, is used to study electric field distribution for parallel and non-parallel electrode-pair respectively. Being an analytical technique, the simulations are fast compared to numerical methods. A scheme on how variable polarization phase can be introduced when the electrode-pair is placed on the non-parallel faces of a specially designed trapezoidal shaped electro-optic lithium niobate crystal is also given in this work. It is also shown that by using multiple strip electrodes, tunability of the polarization phase difference can be achieved. Random optimization based on genetic algorithm has been used to optimize the device structure parameters to obtain desired phase shift.

Design of multistack Fabry-Perot structure with defect as tunable transmission filter for CWDM using FDTD method

Abstract: The translational symmetry of the periodicity in a photonic crystal can be disturbed by introducing a controlled defect in its periodicity. The photon localization causes a pass band in the photonic bandgap. Based on this concept, we are proposing the design of a tunable narrow band filter for multiple wavelengths used for coarse wave length division multiplexing (CWDM) system. To achieve that, a multiple stack Fabry Perot structure with suitable stack materials and controllable defect is considered. The proposed Fabry Perot structure consists of periodic layers of electro optic material Lithium Niobate (nH) and Magnesium Fluoride (nL). The optical length of each such layer is their corresponding quarter wave length width at the design wavelength λ 0 (1.55 μm). The reflection band of the quarter wavelength multilayer structure is formed due to the periodic repetition of the (LH) N L, where L and H are the quarter wavelength width of the nL and nH material respectively and N is the number of bilayers. A quarter wave layer L is then inserted between the groups of (LH) N and (HL) N to form the Fabry Perot resonator structure which can be used for narrow band transmission filter. The simulation has been carried out in both FDTD and TMM method and compared. As the refractive index of the Lithium Niobate can be tuned electro-optically, the filter can also be made tunable.

Tunable differential polarization phase shifter using electro-optic property of trapezoidal lithium niobate crystal

Abstract: A mathematical model to study the phase difference introduced between the two orthogonal components of light wave due to electro-optic effect in a trapezoidal shaped lithium niobate single crystal has been made. Here, the electrodes are placed on the two opposite non-parallel faces such that the field is nearly perpendicular to the light beam direction. It is found that for the said geometry of the device structure, the maximum phase difference is a function of oblique angle between the two non-parallel faces, which has a critical value. Using random optimization technique based on genetic algorithm, the device parameters can be optimized for any required phase change between the two components of light wave. The phase difference between the two orthogonal components of light can be made tunable by using a multi-strip electrode structure. The advantage of this technique is that tunability can be obtained with a constant voltage source.

Generating multiple narrowband sources from a broadband LED using defects in its top 2D photonic crystal vertical stack

Abstract: Vertical 2D PhC having a T-type three port channel on top of InGaAs/GaAs broadband IR LED is considered. Top PhC is created by 2D periodic repetation of InAs rods in air. T-Channel is created by replacing some solid rods by air. Light from LED is focussed within the channel. Part of LED light having wavelength out side the PhC bandgap will pass normally and wavelength within PhC will be guided by the channel. By introducing two different defects in the two arms of ‘T’ channel tuning of two demultiplexed narrowband sources is obtained.

Experimental verification of MMI by singlemode-multimode-singlemode and multimode-singlemode structures

Abstract: Multimode Interference (MMI) based on self imaging phenomenon is investigated using matrix approach. Experimentally MMI is verified using singlemode-multimode-singlemode and multimodesinglemode structures of optical fiber. The results obtained are also verified by BPM technique.