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Ghai D.P.,Laser Science and Technology Center
Applied Optics

Generation of optical vortices using a new design of adaptive helical mirror (AHM) is reported. The new AHM is a reflective device that can generate an optical vortex of any desired topological charge, both positive and negative, within its breakdown limits. The most fascinating feature of the AHM is that the topological charge of the optical vortex generated with it can be changed in real time by varying the excitation voltage. Generation of optical vortices up to topological charge 4 has been demonstrated. The presence of a vortex in the optical field generated with the AHM is confirmed by producing both fork and spiral fringes in an interferometric setup. Various design improvements to further enhance the performance of the reported AHM are discussed. Some of the important applications of AHM are also listed. © 2011 Optical Society of America. Source

Kumar V.,Indian Institute of Technology Delhi | Singhal G.,Laser Science and Technology Center | Subbarao P.M.V.,Indian Institute of Technology Delhi
Applied Thermal Engineering

The constant rate of momentum change (CRMC) is a new approach towards design of supersonic ejectors. CRMC methodology was first proposed by Eames [1] in a study which was primarily based on isentropic flow inside the diffusing region of a supersonic ejector. The prime benefit that accrues from employing a CRMC ejector is that it can effectively eliminate the irreversibility associated with occurrence of thermodynamic shock process. The present study examines the supersonic flow in a CRMC ejector from the perspective of an adiabatic flow with frictional effects inside the variable cross-section of supersonic ejector, which is apparently more realistic. An analytical model has been discussed for the prediction of flow parameter variation in a space marching formulation taking into account change in localized frictional coefficient due to corresponding changes at each step. The analytical results have been validated by conducting a computational study based on 2-D axi-symmetric viscous compressible flow formulation with turbulence in FLUENT. The results are in good agreement at on-design conditions. The predictions especially for the recovered pressure made through the analytical formulation incorporating friction are found to be in significantly better agreement than the isentropic approach. The experimental validation for the approach has also been presented with the results being in close agreement with analytically predicted values. © 2013 Elsevier Ltd. All rights reserved. Source

Verma A.,Indian Institute of Technology Delhi | Malhan N.,Laser Science and Technology Center | Ganguli A.K.,Indian Institute of Technology Delhi
Materials Letters

We have studied the synthesis of Nd doped Y 3Ga 5O 12 (Nd:YGG) nano-powder and optimized the conditions for obtaining pure phases by the coprecipitation route using NH 4HCO 3 as the precipitant. The precursor obtained was calcined at 900 °C for 2 h to transform it into pure Nd:YGG. Rietveld refinement of X-ray diffraction data confirms the cubic structure of YGG. The particle size of Nd:YGG was found to be ∼ 10 nm (a much smaller size than most of the earlier reports on YGG). High-resolution transmission electron microscopic (HRTEM) images showed the highly crystalline nature of as-synthesized YGG nanopowder. Photoluminescence spectroscopy confirms that the doping of Nd 3+ ions substitutes the Y 3+ ions in the YGG crystalline lattice which enables Nd:YGG nanopowders to be a suitable candidate for YGG based transparent ceramics for laser applications. © 2012 Elsevier B.V. All rights reserved. Source

Joshi D.,Indian Institute of Technology Delhi | Joshi D.,Laser Science and Technology Center | Soni R.K.,Indian Institute of Technology Delhi
Applied Physics A: Materials Science and Processing

Partially oxidized spherical silver nanoparticles (AgNPs) of different size are prepared by pulsed laser ablation in water and directly conjugated to protein S-ovalbumin for the first time and characterized by various optical techniques. UV-Visible spectrum of AgNPs showed localized surface plasmon resonance (LSPR) peak at 396 nm which red shift after protein addition. Further the increased concentration of AgNPs resulted a decrease in intensity and broadening of S-ovalbumin peak (278 nm), which can be related to the formation of protein NPs complex caused by the partial adsorption of S-ovalbumin on the surface of AgNPs. The red shift in LSPR peak of AgNPs after mixing with S-ovalbumin and decrease in protein-characteristic peak with increased silver loading confirmed the formation of protein-AgNPs bioconjugates. The effect of laser fluence on the size of AgNPs and nanoparticle-protein conjugation in the size range 5-38 nm is systematically studied. Raman spectra reveal broken disulphide bonds in the conjugated protein and formation of Ag-S bonds on the nanoparticle surface. Fluorescence spectroscopy showed quenching in fluorescence emission intensity of tryptophan residue of S-ovalbumin due to energy transfer from tryptophan moieties of albumin to AgNPs. Besides this, small blue shift in emission peak is also noticed in presence of AgNPs, which might be due to complex formation between protein and nanoparticles. The binding constant (K) and the number of binding sites (n) between AgNPs and S-ovalbumin have been found to be 0.006 M-1 and 7.11, respectively. © 2014 Springer-Verlag Berlin Heidelberg. Source

Kumaresan V.,Anna University | Chandrasekaran P.,SRM University | Nanda M.,Laser Science and Technology Center | Maini A.K.,Laser Science and Technology Center | Velraj R.,Anna University
International Journal of Refrigeration

This present study presents the solidification behavior of water based nanofluid phase change material encapsulated in a spherical container. The nanofluid phase change material (NFPCM) was prepared by dispersing the multi wall carbon nanotubes (MWCNT) with volume fractions of 0.15%, 0.3%, 0.45%, and 0.6% in de-ionized (DI) water as the base phase change material. The solidification experiments were conducted with DI water and the NFPCM and maximum reductions of 14% and 20.1% were observed in the solidification time with the NFPCMs at surrounding bath temperature of -9 C and -12 C respectively. The presence of MWCNT also acted as nucleating agent that caused appreciable reduction in the subcooling. The enhanced thermal transport properties of the NFPCM are very useful to operate the cool thermal energy storage (CTES) system at higher operating temperature of the secondary refrigerant. It is predicted that there is a possible energy saving potential of approximately 6-9% in the CTES using the NFPCMs. © 2013 Elsevier Ltd and IIR. All rights reserved. Source

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