V. P. Mahadevan Pillai

University of Kerala, Tiruvananantapuram, Kerala, India

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Publications (110)94.87 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Zinc oxide is a wide, direct band gap II-VI oxide semiconductor. Pure and Eu-doped ZnO films are prepared by RF Magnetron sputtering at different doping concentrations (0.5, 1, 3 and 5 wt %). The films are annealed at 500 0C in air for two hours. The structural, morphological and optical properties of the films are characterized using XRD, micro-Raman, AFM, UV-Visible and photoluminescence spectroscopy. The thickness of the films is measured using stylus profilometer. XRD analysis shows that all the films are highly c-axis oriented exhibiting a single peak corresponding to (002) lattice reflection plane of hexagonal wurtzite crystal phase of ZnO. The micro-Raman spectra analysis reveals the presence of E2 high mode in all the samples which is the intrinsic characteristic of hexagonal wurtzite structure of ZnO. The appearance of LO modes indicates the formation of defects such as oxygen vacancies in the films. AFM micrographs show uniform distribution of densely packed grains of size with well defined grain boundaries. All the films exhibit very high transmittance (above 80%) in the visible region with a sharp fundamental absorption edge around 380 nm corresponding to the intrinsic band edge of ZnO. All the films show PL emission in the UV and visible region.
    IOP Conference Series Materials Science and Engineering 08/2014; 64(1):012027.
  • K Sathesh Kumar, P Srinivasan, V P Mahadevan Pillai
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    ABSTRACT: Single crystals of para amino hippuric acid (PAHA) were grown by slow evaporation technique. The spectral and its structural properties of the crystals were studied by FT-IR, micro-Raman and factor group analysis. The optical transparency in the UV-Visible regions was found to be good for non-linear optics (NLO) applications. Thermogravimetric analysis (TGA) and Differential Thermal Analysis (DTA) showed that the compound decomposes beyond 300°C. The dielectric behavior of the compound predicts low dielectric loss at high frequency applied whereas in the case of mechanical behavior of the specimen hardness increases with increasing applied load. After certain weight increase, hardness gets saturated in the region of ⩾110. Relative second harmonic efficiency of the compound is found to be 1.8 times greater than that of potassium di-phosphate reference.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 05/2014; 132C:263-270. · 1.98 Impact Factor
  • 01/2014; 82(2).
  • K. Sathesh Kumar, P. Srinivasan, V.P. Mahadevan Pillai
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 01/2014; 132:263–270. · 1.98 Impact Factor
  • Sensors and Actuators B Chemical 10/2013; 187:611-621. · 3.54 Impact Factor
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    ABSTRACT: ZnO thin films are grown on quartz substrates at various substrate temperatures (ranging from 573 to 973K) under an oxygen ambience of 0.02mbar by using pulsed laser ablation. Influence of substrate temperature on the structural, morphological, optical and electrical properties of the ZnO thin films are investigated. The XRD and micro-Raman spectra reveal the presence of hexagonal wurtzite structure of ZnO with preferred orientation (002). The particle size is calculated using Debye-Scherer equation and the average size of the crystallites are found to be in the range 17-29nm. The AFM study reveals that the surface morphology of the film depends strongly on the substrate temperature. UV-Visible transmittance spectra show highly transparent nature of the films in visible region. The calculated optical band gap energy is found to be decrease with increase in substrate temperatures. The complex dielectric constant, the loss factor and the distribution of the volume and surface energy loss of the ZnO thin films prepared at different substrate temperatures are calculated. All the films are found to be highly porous in nature. The PL spectra show very strong emission in the blue region for all the films. The dc electrical resistivity of the film decreases with increase in substrate temperature. The temperature dependent electrical measurements done on the film prepared at substrate temperature 573K reveals that the electric conduction is thermally activated and the activation energy is found to be 0.03911eV which is less than the reported values for ZnO films.
    Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 09/2013; 118C:724-732. · 1.98 Impact Factor
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    ABSTRACT: A novel ZnO incorporated MoO3 nanostructured thin film system exhibiting high sensitivity and selectivity to ethanol has been developed. The MoO3:ZnO nanostructures exhibit enhanced ethanol sensing performance in non-humid and humid (75% r.H. at 21 °C) atmospheres compared to the pure MoO3 layer; with increase in ZnO concentrations, the sensitivity and stability increased, and the response/recovery time decreased. The response (Gethanol/Gair) of the 25% MoO3:ZnO sensor at an operating temperature of 300 °C against 500 ppm ethanol is up to 171 under non-humid and 117 under humid (75% r.H.) conditions. By comparing the response of the 25% ZnO added MoO3 sensor toward various gases (H2, CO, C3H6, CH4 and C2H5OH), distinctive selectivity to ethanol is observed. The ethanol sensitivity action over MoO3 nanostructures can be ascribed to the catalytic oxidation of ethanol to acetaldehyde, and the enhancement of gas sensing response of the MoO3:ZnO system can be attributed to more active centers that are obtained from the enhanced oxygen vacancy defects induced by ZnO. The presence of a humid atmosphere has a dramatic influence on the sensor performance towards ethanol; the sensitivity diminishes drastically due to the partial site precluding nature of the adsorbed hydroxyl groups to the analyte. The ZnO incorporated MoO3 nanostructure based sensing layers in the present work show significantly superior ethanol sensing performance to the works previously reported for various metal oxide systems.
    J. Mater. Chem. C. 06/2013; 1(25):3976-3984.
  • S. Karpagam, S.K. Sudheer, V.P.M. Pillai
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    ABSTRACT: A new method for Discrete Gyrator Transform (DGT) based image encryption and decryption using Double Random Phase Mask (RPM) is proposed. In the present work encryption and decryption has been done using Gyrator Transform (GT) a kind of discrete algorithm using convolution operation. The proposed method uses DGT to reduce the computational load. The robustness of the proposed algorithm to blind decryption in terms of different key values has been calculated. Numerical simulations and some analysis for security have been presented to verify its validity and efficiency.
    Fiber Optics in Access Network (FOAN), 2013 4th International Workshop on; 01/2013
  • N. Anand, S.K. Sudheer, V.P. Mahadevan Pillai
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    ABSTRACT: Design and analysis of a Nonlinear Photonic Crystal Fiber having low confinement loss, low birefringence and low effective mode area has been presented using COMSOL Multiphysics software. The suggested design has a central hole filled with Nematic Liquid Crystal (NLC) surrounded by air holes and a Perfectly Matched Layer (PML). Results show that by using circular holes of small diameter and materials with considerable refractive index difference, the nonlinearity and confinement increases, but the effective area and birefringence properties of the fiber decreases.
    Fiber Optics in Access Network (FOAN), 2013 4th International Workshop on; 01/2013
  • B. Soumya, S.K. Sudheer, V.P. Mahadevan Pillai
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    ABSTRACT: Here, we designed a highly nonlinear and low loss Photonic Crystal Fiber (PCF) structure with square lattice using COMSOL multiphysics. Propagation properties of proposed PCF has been analysed. The analysis has been carried out using finite element method. The proposed fiber obtained low confinement loss, minimum dispersion for wide range of wavelength. Results shows that the proposed PCF gives large negative dispersion for a wide range of wavelength and this makes it suitable for dispersion compensating applications and also it gives high nonlinear coefficient of 117 W-1Km-1. The square lattice PCF structure with elliptical holes is also designed and its birefringence property is compared with PCF with circular holes. Results shows that PCF with elliptical holes shows high birefringence as compared to other and such high birefringent PCF can be used for polarization maintaining applications.
    Fiber Optics in Access Network (FOAN), 2013 4th International Workshop on; 01/2013
  • B. Soumya, S.K. Sudheer, V.P. Mahadevan Pillai
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    ABSTRACT: Photonic Crystal Fibers (PCFs) are a new class of optical waveguides which offer exceptional light guiding mechanism and deliver significantly improved performance compared to conventional optical fibers. In the present investigation, we propose an solid core PCF for three different values of air filling fraction. The proposed PCF is simulated by using COMSOL Multiphysics. The nonlinearity of proposed PCF is found to be varying with respect to its air filling fraction and a nonlinear coefficient of 148 W-1Km-1 is obtained for PCF with air filling fraction of 0.7. The variation of other parameters like effective refractive index, confinement loss and effective area with respect to wavelength and air filling fraction is also investigated. The supercontinuum spectrum is generated using the designed highly nonlinear fiber and the effect of pump power on the generated output spectrum is studied and results shows that the bandwidth of output spectrum increases with pump power provided its pulse width remains constant.
    Fiber Optics in Access Network (FOAN), 2013 4th International Workshop on; 01/2013
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    J. R. Rani, V. P. Mahadevan Pillai, G. Anoop, M. Gopal, V. Ganeshan
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    ABSTRACT: Tin oxide (SnO2)-doped Si nanorings of diameter in the range of 100 nm to 170 nm with an average width of 25 nm are synthesized by off-axis laser ablation (PLD) and are characterized by different techniques. The AFM observations show that the surface morphological features of films depend on the tin oxide concentration. The bandgap energies of undoped quantum dots are found to be 2.29 eV, while it decreases to 2.15 eV and 1.5 eV for 3 wt.% and 0.1 wt.% SnO2-doped samples, respectively. The increase in the value of bandgap energy can be attributed to size reduction of particles. The Raman spectra of SnO2-doped films are characterized by a broad Raman band with intensity maximum around 478 cm-1. Raman spectrum shows frequency shift which may be due to changes in the Si–O bond length or Si–O–Si bond angle. The activation energy at higher temperature is found to be 16.99 meV for 3SnSi209, 21 meV for 0.1SnSi209 and 18.1 meV for undoped silicon which shows that defect levels are present in all the samples, the conduction is due to the presence of holes. The synthesized films exhibit PL peak in the visible region. The PL emission peak and PL intensity depend on dopants and it is concluded that luminescence does not originate from localized states in gap but from extended states. The size and shape of nanostructures depend on the SnO2 concentration and the doping effects can be used as a significant guideline for tuning the electronic and optical properties of Si.
    Nano brief reports and reviews 01/2013; · 1.17 Impact Factor
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    ABSTRACT: Silver incorporated tungsten oxide thin films are prepared by RF magnetron sputtering technique. The effect of silver incorporation in micro structure evolution, phase enhancement, band gap tuning and other optical properties are investigated using techniques such as x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and UV-Visible spectroscopy. Effect of silver addition in phase formation and band gap tuning of tungsten oxide thin films are investigated. It is found that the texturing and phase formation improves with enhancement in silver content. It is also found that as the silver incorporation enhances the thickness of the films increases at the same time the strain in the film decreases. Even without annealing the desired phase can be achieved by doping with silver. A broad band centered at the wavelength 437 nm is observed in the absorption spectra of tungsten oxide films of higher silver incorporation and this can be attributed to surface plasmon resonance of silver atoms present in the tungsten oxide matrix. The transmittance of the films is decreased with increase in silver content which can be due to increase in film thickness, enhancement of scattering, and absorption of light caused by the increase of grain size, surface roughness and porosity of films and enhanced absorption due to surface plasmon resonance of silver. It is found that silver can act as the seed for the growth of tungsten oxide grains and found that the grain size increases with silver content which in turn decreases the band gap of tungsten oxide from 3.14 eV to 2.70 eV.
    Journal of Applied Physics 12/2012; 112(11). · 2.21 Impact Factor
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    Dataset: 05676474
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    ABSTRACT: In this paper we have demonstrated a simple and secure method to encrypt and decrypt a color image. In the proposed method, the color image is initially encrypted with jigsaw transform and followed by double random phase encryption. The jigsaw transform is employed at the input plane and random phase masks (RPMs) are placed at the Fourier planes. The jigsaw transformed image is Fourier transformed and multiplied with the RPM1. Inverse transform of this image is performed and multiplied with the RPM2 gives the encrypted image. The jigsaw transform indices of the transformed image and the random phase code of the encrypted image form the keys for the successful retrieval of the data. Encrypting with this technique makes it almost impossible to retrieve the image without using both the right keys. Results of the computer simulation have been presented in support of the proposed idea. Mean square error (MSE) between the decrypted and the original image has also been calculated in support of the technique.
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    ABSTRACT: Multilayer ensembles of alumina capped, widely dispersed silicon nanoparticles (Si-nps) with mean diameter in the range of about 1–4 nm were grown using pulsed laser deposition. With photo-excitation at ~3.82 eV, photoluminescence (PL) was found to emanate from these Si-nps mainly in the UV spectral region centered at about 3.37 eV due to the Γ25–Γ15 transitions. It was found that while the bandgap measured from photoabsorption spectra showed blueshift with decreasing mean size of the Si-nps, spectral position of the PL peaks remained almost insensitive to variation in the mean size. The PL peak at about 3.37 eV was observed to vanish at temperatures higher than 70 K and another one at about 3.31 eV attributed to the TO phonon replica disappeared above 100 K. In general FWHM of both these PL peaks was found to increase monotonically from about 6 to 19 meV and the peak positions were found to undergo redshift with increase in the sample temperature from 10 to 100 K. These observations could be explained by applying Bose statistics for a 6 meV confined phonon mode broadening of the Si-nps and using the Varshni equation, respectively. Si-nps grown in oxygen ambient at 600°C showed significant enhancement in the PL intensity with increasing pressure. These findings elicit that light emission from Si-nps is either due to the nanoparticles of about 1 nm and smaller size primarily driven by the quantum confinement or due to an interface state pumped by these Si-nps.
    International Journal of Nanoscience 04/2012; 10(01n02).
  • S. R. Chalana, R. Vinodkumar, I. Navas, V. Ganesan, V. P. Mahadevan Pillai
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    ABSTRACT: Nanostructured zinc suplhide thin films are successfully deposited on quartz substrates using pulsed laser deposition (PLD) under different argon pressures (0, 5, 10, 15 and 20Pa). The influence of argon ambience on the microstructural, optical and luminescence properties of zinc sulphide (ZnS) thin films is systematically investigated. The GIXRD data suggests rhombohedral structure for ZnS films prepared under different argon ambience. Self assembly of grains into well-defined patterns along the y direction is observed in the AFM image of the film deposited under argon pressure 20Pa. All the films show a blue shift in optical band gap. This can be due to the quantum confinement effect and less widening of conduction and valence band for the films with less thickness and smaller grain size. The PL spectra of the different films are recorded at excitation wavelengths 250nm and 325nm and the spectra are interpreted. The PL spectra of the films recorded at excitation wavelength 325nm show intense yellow emission. The film deposited under an argon pressure of 15Pa shows the highest PL intensity for excitation wavelength 325nm. For the PL spectra (excitation at 250nm), the highest PL intensity is observed for the film prepared under argon free ambience. In our study, 15Pa is the optimum argon pressure for better crystallinity and intense yellow emission when excited at 325nm.
    Journal of Luminescence 04/2012; · 2.14 Impact Factor
  • Journal of Nanoscience Letters. 01/2012;
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    ABSTRACT: Fabry-Perot filters based on fiber Bragg gratings are widely used as optical narrowband filters and as sensors. Here we propose and demonstrate all optical clock recovery (10Gbit/s, 40Gbit/s & 100Gbit/s) with Fabry-Perot filters made up of fiber Bragg gratings.
    Fiber Optics and Photonics (PHOTONICS), 2012 International Conference on; 01/2012
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    ABSTRACT: We report all optical Integrator based on FBGs and FP_FBGs which has been analyzed to determine the impulse response for various impulse pulse excitations.
    Fiber Optics and Photonics (PHOTONICS), 2012 International Conference on; 01/2012