Faculty Publications
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Item A method to generate a single sub-50-as pulse by employing a 5-fs laser pulse with intensity above the saturation intensity, was proposed. The neon target medium of 0.5 mm length and 5 Torr pressure, was placed 10 mm beyond the laser focus. Harmonic radiation from neon atoms, was obtained by solving the time-dependent Schrödinger equation coupled with Maxwell equations. The results show that the positive chirp contained in the broad continuum radiation can be compensated by an x-ray filter with a negative group delay dispersion in the spectral region of interest.(American Physical Society, Single sub-50-attosecond pulse generation from chirp-compensated harmonic radiation using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004Item We utilize the dispersion property of an X-ray filter material for the generation of a single sub-50-as pulse from high-order harmonics. The attosecond pulse, formed by selecting the spectral range of high-order harmonic radiation, contains an intrinsic chirp corresponding to the quadratic phase variation during a half cycle of a laser pulse. We show that this chirp can be compensated by using the negative group-delay dispersion of a thin X-ray filter, compressing the attosecond pulse down to sub-50-as.(Compression of harmonie pulses by using material dispersion) Kim, K.T.; Kim, C.M.; Baik, M.G.; Umesh, G.; Nam, C.H.2004Item Self-compression of attosecond high-order harmonic pulses(2007) Kim, K.T.; Kang, K.S.; Park, M.N.; Imran, T.; Umesh, G.; Nam, C.H.Self-compression of attosecond high-order harmonic pulses in the harmonic generation medium itself has been demonstrated. The attosecond pulses were generated in an argon-filled gas cell and compressed by exploiting the dispersion characteristics of argon. Since the harmonic generation medium itself was used as the compression medium, continuous chirp control was easily achieved by adjusting the gas pressure. The optimized attosecond pulse was also the most intense, and its duration of 206 as was very close to the transform-limited value of 200 as. © 2007 The American Physical Society.Item Total internal reflection quasiphase matching-based broadband second harmonic generation in a plane-parallel uniaxial crystal of lithium niobate(SPIE spie@spie.org, 2018) Barma, M.D.; Deb, S.; Saha, A.The present work analytically investigates the generation of broadband second harmonic (SH) using total internal reflection (TIR) quasiphase matching technique in a plane-parallel uniaxial crystal having optic axis nonparallel with respect to the horizontal base. A computer-aided simulation has been performed using lithium niobate as the nonlinear material to determine the possibility of generating broadband SH intensity when broadband fundamental laser radiation is allowed to undergo TIR inside the crystal. The simulated results indicate a peak conversion efficiency of 5.22% with a spectral bandwidth (BW) of 116 nm, centered at 2.258 ?m in a 10-mm-long slab. The effect of variation in operating temperature, crystal length, crystal thickness, angle of deviation of optic axis, and angle of incidence of fundamental beam at the air-prism interface on efficiency and BW of resulting SH output has been studied. © 2018 Society of Photo-Optical Instrumentation Engineers (SPIE).Item Effect of Erioglaucine dye dopant on the structural, optical, mechanical, electrical and nonlinear properties of ammonium dihydrogen phosphate single crystal(Elsevier B.V., 2020) Fernandes, J.M.; Mahendra, K.; Udayashankar, N.K.The structural, optical, mechanical, electrical and nonlinear properties of standard Ammonium Dihydrogen Phosphate (NH4H2PO4, ADP) single crystals incorporating organic Erioglaucine dye dopant are presented. The effect of Erioglaucine dye dopant at varied concentration on these properties has been investigated through measurement of powder X-Ray Diffraction (XRD), UV–Vis and photoluminescence spectroscopy, Vickers microhardness, light dependent I–V measurements and Second Harmonic Generation studies. These measurements reveal that doping with increasing dye concentration of Erioglaucine leads to change in properties of the ADP single crystals, making them suitable for optoelectronic applications. The Erioglaucine doped ADP single crystals were grown using solvent evaporation technique at room temperature. Optical properties like absorbance and emission of these crystals are determined using UV–vis and photoluminescence spectroscopy, respectively. Optical bandgap and photoluminescence of the crystals are found to increase with dye doping, indicating their suitability in photonic applications. The mechanical properties of the crystals are determined using Vickers microhardness measurement technique. Light dependent I–V measurements exhibit negative photoconductivity behavior of the ADP crystals. However, the current through the crystals is observed to increase with increase in doping concentration of the Erioglaucine dye. Second Harmonic Generation studies show enhancement in nonlinearity for doped crystals. Our experiments indicate gradual variance in the crystallinity, emission, hardness, conductivity and nonlinearity of the sample with change in dye concentration. © 2020 Elsevier B.V.Item Investigation of structural and physico-chemical properties of rice starch with varied amylose content: A combined microscopy, spectroscopy, and thermal study(Elsevier B.V., 2022) Govindaraju, I.; Zhuo, G.-Y.; Chakraborty, I.; Melanthota, S.K.; Mal, S.; Sarmah, B.; Baruah, V.J.; Mahato, K.K.; Mazumder, N.Starch from a given botanical source can vary considerably in terms of physicochemical properties in its native and hydrolyzed forms. The current study investigated the structural and functional characteristics of starch from ten indigenous rice varieties endemic to Northeast India. In vitro enzymatic hydrolysis was used to reveal the dextrose equivalent profile of each type of starch. Gezep Sali and Betguti Sali respectively exhibited the highest and lowest starch hydrolysis. Among the ten rice varieties, amylose content varied between 7.50 and 28.58%. Optical and scanning electron microscopy (SEM) revealed the polyhedral shape of the native starch granules and deformation of the shape upon enzymatic hydrolysis. Second harmonic generation (SHG) microscopy and X-ray diffraction (XRD) analysis confirmed the presence of and variations in starch crystallinity. XRD revealed spectral peaks characteristic of A-type starch crystals in the native form. The elevated intensity of XRD peaks in hydrolyzed starch granules confirmed the occurrence of amylose hydrolysis rather than hydrolysis in amylopectin regions. Fourier transform infrared (FTIR) spectra revealed the common stretching and bending of bonds in all native starches; however, changes were observed in the fingerprint region (1080, 1000, 926 cm−1) of hydrolyzed starch granules, which indicates the amylolysis of the amylose region and disturbances in the ordered arrangement in the crystalline part. Differential scanning calorimeter (DSC) endotherms revealed the highest and lowest gelatinization peak temperatures in Harfoni (78 °C) and Tulosi Sali (41 °C) rice cultivars, respectively. The findings in this study can help to optimize the usage of rice starch in food and non-food industries. Furthermore, understanding the control points of starch digestion and genetically tailoring rice grains with different digestibility could be beneficial for nutraceutical applications. © 2021 Elsevier LtdItem Internal damage growth in quasi-brittle fibre-reinforced cementitious materials under cyclic compressive loading(Structural Engineering Research Centre, 2023) Vidya Sagar, R.; Basu, D.J.; Suhas Reddy, K.V.; Prathap, Y.; Bhuvaneswari, G.; Sai Keerthi, P.This article reports a comparison between the internal damage growth in cementitious materials without fibres and with fibres subjected to elevated amplitude cyclic compressive loading. The damage progression was assessed using ultrasonic testing method and Acoustic Emission (AE) testing. The intricate fracture mechanism in the test specimens causes rise to a higher harmonic generation, which was used as an indicator to the internal damage. The decrement in wave peak amplitude with higher harmonic generation may be regarded as a ‘internal damge growth’ in the deformable solid. The complexity in the fracture mechanism in fibrous cementitious matrix influenced the heterogeneity of the specimen, which is reflected by the steep decrement in the slope of the line plotted using normalized higher harmonic ratio and load. It was observed that the ‘magnitude of the total damage’ developed in plain concrete at the last loading phase was relatively lower than brass coated steel fibre reinforced concrete. This was supported by the damage parameter based on generated AE, where final failure of the specimens preceded an AE avalanche. Therefore, the utilization of a combination of nondestructive testing techniques such as AE and nonlinear ultrasonic testing can offer a more comprehensive understanding of the progression of damage in quasi-brittle cementitious materials. © 2023, Structural Engineering Research Centre. All rights reserved.