Somnath Ghosh

Bio: Somnath Ghosh is an academic researcher from Indian Institute of Technology, Jodhpur. The author has contributed to research in topics: Microstructured optical fiber & Optical fiber. The author has an hindex of 28, co-authored 287 publications receiving 3318 citations. Previous affiliations of Somnath Ghosh include University of Calcutta & Indian Institute of Science.

Effect of Si/Al Ratio on Performance of Fly Ash Geopolymers at Elevated Temperature

Abstract: This paper presents the results of an experimental investigation on performance of fly ash-based geopolymer pastes at elevated temperature exposure. Geopolymer paste specimens having Si/Al in the range 1.7–2.2, manufactured by activating low calcium fly ash with a mixture of sodium hydroxide and sodium silicate solution were subjected to temperatures up to 900 °C. The effect of Si/Al ratio was studied on the basis of physical appearance, weight losses, residual strength, volumetric shrinkage and water sorptivity at different temperatures. Scanning electron microscopy along with energy dispersive X-ray and X-ray diffraction analysis were also conducted to examine changes in microstructure and mineralogy during the thermal exposure. Specimens gradually changed in colour from grey to light red accompanied by the appearance of small cracks as the temperature was increased to 900 °C. Loss of weight and volumetric strain due to elevated temperature exposure were higher in specimens manufactured with lesser Si/Al ratios. Geopolymer paste specimen containing maximum Si/Al of 2.2 performed best in terms of residual compressive strength after exposure to elevated temperatures.

Effect of silica fume additions on porosity of fly ash geopolymers

Abstract: This paper presents the results of an experimental study performed to investigate effect of incorporating silica fume in the fly ash geopolymer on its porosity and compressive strength. Geopolymer specimens were prepared by activating fly ash incorporated with additional silica fume in the range of 2.5% to 5% with a mixture of sodium hydroxide and sodium silicate having Na2O content of 8%. The characterization of the geopolymer specimens was done with ESEM/EDAX and MIP tests. Addition of silica fume up to 5% enhanced compressive strength of geopolymer mortars. However, further increase of silica fume caused a decrease in compressive strength. SEM micrographs for specimens incorporated with silica fume showed better microstructure and exhibited lesser porosity. MIP results of paste specimens indicate higher pore volume in the specimen prepared with additional silica fume while for mortar specimens; the pore volume was seen lesser in specimens with additional silica fume. Silica fume may be used as an additional material to improve or modify some properties of the resulting geopolymer.

A spin-adapted linear response theory in a coupled-cluster framework for direct calculation of spin-allowed and spin-forbidden transition energies

Abstract: In this paper, we have spin-adapted our recently formulated linear response theory in a coupled-cluster framework. This allows us to calculate directly both the spin-allowed and the spin-forbidden transition energies from a single methodology. We have introduced rank-zero and rank-one spin operators to construct excitation operators for singlet-singlet and singlet-triplet transitions respectively and utilised the graphical methods of spin algebra to integrate the spin variables. It has been shown how a suitable parameterisation of the reduced Hugenholtz matrix elements of the excitation operator in terms of Goldstone matrix elements makes the resulting system of equations simple, compact and suitable for computer implementation. A pilot calculation has been performed to test the applicability of the theory.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Exceptional points in optics and photonics.

Abstract: BACKGROUND Singularities are critical points for which the behavior of a mathematical model governing a physical system is of a fundamentally different nature compared to the neighboring points. Exceptional points are spectral singularities in the parameter space of a system in which two or more eigenvalues, and their corresponding eigenvectors, simultaneously coalesce. Such degeneracies are peculiar features of nonconservative systems that exchange energy with their surrounding environment. In the past two decades, there has been a growing interest in investigating such nonconservative systems, particularly in connection with the quantum mechanics notions of parity-time symmetry, after the realization that some non-Hermitian Hamiltonians exhibit entirely real spectra. Lately, non-Hermitian systems have raised considerable attention in photonics, given that optical gain and loss can be integrated as nonconservative ingredients to create artificial materials and structures with altogether new optical properties. ADVANCES As we introduce gain and loss in a nanophotonic system, the emergence of exceptional point singularities dramatically alters the overall response, leading to a range of exotic functionalities associated with abrupt phase transitions in the eigenvalue spectrum. Even though such a peculiar effect has been known theoretically for several years, its controllable realization has not been made possible until recently and with advances in exploiting gain and loss in guided-wave photonic systems. As shown in a range of recent theoretical and experimental works, this property creates opportunities for ultrasensitive measurements and for manipulating the modal content of multimode lasers. In addition, adiabatic parametric evolution around exceptional points provides interesting schemes for topological energy transfer and designing mode and polarization converters in photonics. Lately, non-Hermitian degeneracies have also been exploited for the design of laser systems, new nonlinear optics phenomena, and exotic scattering features in open systems. OUTLOOK Thus far, non-Hermitian systems have been largely disregarded owing to the dominance of the Hermitian theories in most areas of physics. Recent advances in the theory of non-Hermitian systems in connection with exceptional point singularities has revolutionized our understanding of such complex systems. In the context of optics and photonics, in particular, this topic is highly important because of the ubiquity of nonconservative elements of gain and loss. In this regard, the theoretical developments in the field of non-Hermitian physics have allowed us to revisit some of the well-established platforms with a new angle of utilizing gain and loss as new degrees of freedom, in stark contrast with the traditional approach of avoiding these elements. On the experimental front, progress in fabrication technologies has allowed for harnessing gain and loss in chip-scale photonic systems. These theoretical and experimental developments have put forward new schemes for controlling the functionality of micro- and nanophotonic devices. This is mainly based on the anomalous parameter dependence in the response of non-Hermitian systems when operating around exceptional point singularities. Such effects can have important ramifications in controlling light in new nanophotonic device designs, which are fundamentally based on engineering the interplay of coupling and dissipation and amplification mechanisms in multimode systems. Potential applications of such designs reside in coupled-cavity laser sources with better coherence properties, coupled nonlinear resonators with engineered dispersion, compact polarization and spatial mode converters, and highly efficient reconfigurable diffraction surfaces. In addition, the notion of the exceptional point provides opportunities to take advantage of the inevitable dissipation in environments such as plasmonic and semiconductor materials, which play a key role in optoelectronics. Finally, emerging platforms such as optomechanical cavities provide opportunities to investigate exceptional points and their associated phenomena in multiphysics systems.