5941 research outputs found
Sort by
Remembering Sir CV Raman
No full textAn article on Sir CV Raman's 125th birth anniversary , the man who taught Raman the Russian language recollects the experience
Towards an anomaly-free quantum dynamics for a weak coupling limit of Euclidean gravity: Diffeomorphism covariance
No full textOpen Access.The GNewton→0 limit of Euclidean gravity introduced by Smolin is described by a generally covariant U(1)3 gauge theory. In an earlier paper, Tomlin and Varadarajan constructed the quantum Hamiltonian constraint of density weight 4/3 for this U(1)3 theory so as to produce a nontrivial anomaly-free loop quantum gravity type representation of the Poisson bracket between a pair of Hamiltonian constraints. These constructions involved a choice of regulating coordinate patches. The use of these coordinate patches is in apparent conflict with spatial diffeomorphism covariance. In this work we show how an appropriate choice of coordinate patches together with suitable modifications of these constructions results in the diffeomorphism covariance of the continuum limit action of the Hamiltonian constraint operator, while preserving the anomaly-free property of the continuum limit action of its commutator
Temperature measurement of laser-cooled atoms using vacuum Rabi splitting
No full textOpen Access.We laser cool rubidium atoms to form a magneto-optical trap, within a Fabry-Perot cavity, and demonstrate strong coupling of the cold atoms to the cavity. The coupling strength is measured using the vacuum Rabi frequency splitting (VRS) of transmitted, cavity-coupled light on atomic resonance. The VRS is measured for two- and three-level atomic systems and the atom-cavity-coupling strength for each system is determined. By allowing the laser-cooled atoms to expand for different times and measuring the corresponding VRS, we determine the number of atoms overlapped with the cavity mode as a function of time. This time-of-flight of atoms from the cavity permits the measurement of the temperature of the laser-cooled atoms. Finally, the need for this technique and its utility is discussed
Size-dependent optical properties of Au nanorods
No full textRestricted Access.In the fast evolving field of nanoscience and nanotechnology, where size and shape are crucial in deciding the optoelectronic properties of nanomaterials, the understanding of size and shape dependent behavior is of direct relevance to device applications. Present study reports the synthesis of Au nanorods with well controlled aspect ratios, and the influence of the aspect ratio on the surface enhanced Raman scattering (SERS) activity using crystal violet (CV) as the probe molecule. The influence of pH and the concentrations of reducing agent and Ag ions in controlling the aspect ratio of gold nanorods are also investigated. The structural and optical properties of the synthesized samples have been characterized by transmission electron microscopy (TEM) and UV–visible absorption spectroscopy. The nonlinear optical (NLO) transmission of the Au nanorods investigated using the open aperture Z-scan technique revealed the absorption saturation followed by an optical limiting behavior, which may find potential applications in optoelectronic nanodevices
Photo and electrically switchable B7 mesophase exhibiting asymmetric bent-core liquid crystals
No full textRestricted Access.The first bent-core liquid crystal materials with a photo-active azo linkage exhibiting the B7 mesophase are reported. The lower homologues of the bent-core compounds display the B1 phase and higher homologues show the B7 mesophase. The mesomorphic properties were characterized using polarized optical microscopy, differential scanning calorimetry and X-ray diffraction. The B7 phase of these materials establishes ferroelectric switching characteristics with anticlinic organization (SmCAPF) of the molecules in the layers. The bent-core molecules with an azo linkage are important for photoisomerization studies, which indicate trans to cis isomerization at 30 s, whereas the reverse processes take place over 40 s in chlorofor
Synthesis of Symmetrical and Unsymmetrical Triphenylene Discotic Liquid Crystals Using Antimony(V)Chloride Under Scholl Oxidation
No full textOpen AccessTriphenylene-based discotic liquid crystals, useful in studying the energy and charge migration in self-organized systems, are the most widely synthesized and studied discotic liquid crystals. In this paper, we report an efficient synthetic procedure for the preparation of symmetrical and unsymmetrical triphenylene discotic liquid crystals using antimony pentachloride as a novel reagent. Scholl oxidative trimarization of 1,2-dialkoxybenzenes with SbCl5 yields hexaalkoxytriphenylenes in good yield, while the oxidative coupling of a 3,3’,4,4’-tetraalkoxybiphenyl with a 1,2,3-trialkoxybenzene affords an unsymmetrically substituted heptaalkoxy-triphenylene derivative. The potential of this new reagent was compared with the other known reagents for the synthesis of alkoxytriphenylene
SARAS: a precision system for measurement of the cosmic radio background and signatures from the epoch of reionization
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)SARAS is a correlation spectrometer purpose designed for precision measurements of the cosmic radio background and faint features in the sky spectrum at long wavelengths that arise from redshifted 21-cm from gas in the reionization epoch. SARAS operates in the octave band 87.5–175 MHz. We present herein the system design arguing for a complex correlation spectrometer concept. The SARAS design concept provides a differential measurement between the antenna temperature and that of an internal reference termination, with measurements in switched system states allowing for cancellation of additive contaminants from a large part of the signal flow path including the digital spectrometer. A switched noise injection scheme provides absolute spectral calibration. Additionally, we argue for an electrically small frequency-independent antenna over an absorber ground. Various critical design features that aid in avoidance of systematics and in providing calibration products for the parametrization of other unavoidable systematics are described and the rationale discussed. The signal flow and processing is analyzed and the response to noise temperatures of the antenna, reference termination and amplifiers is computed. Multi-path propagation arising from internal reflections are considered in the analysis, which includes a harmonic series of internal reflections. We opine that the SARAS design concept is advantageous for precision measurement of the absolute cosmic radio background spectrum; therefore, the design features and analysis methods presented here are expected to serve as a basis for implementations tailored to measurements of a multiplicity of features in the background sky at long wavelengths, which may arise from events in the dark ages and subsequent reionization era