A review of Terahertz technology and Metamaterial based electromagnetic absorber at Terahertz band

 Terahertz (THz) technology, a new step towards the wireless communication is a recent topic of research. THz radiation has unique properties which make it better than microwaves, infrared, X-rays, etc. These radiations have wide applications in various fields like imaging, spectroscopy, security, biomedicine, sub-millimeter astronomy, in communication, etc. THz radiation sources and detectors play a vital role in the THz communication system.  The sources and detectors for THz radiation are still in a developing stage. As in naturally occurring materials, there is lack of good terahertz characteristic, so researchers are moving towards artificial one, i.e., the metamaterial based design. Metamaterial based design of generation and detection of THz radiation is the recent demands in the field of THz communication. Hence, we are focussing on the metamaterial based terahertz transmitter and receiver. So in this paper, a brief study of the THz technology based on metamaterial has been carried out and presented here

Design and Analysis of Metamaterial Electromagnetic Wave Absorbers at THz Frequency

The subject of THz research and technology has advanced dramatically over the previous two decades. In the THz regime, a metamaterial-based absorber is in high demand. Metamaterials (MMs) can be employed as an effective medium by changing the shape to influence the electromagnetic characteristics. All incident radiations at a specific operational frequency are absorbed by a unity absorber, while transmissivity, reflectivity, scattering, and all other light propagation pathways are inhibited. The main focus of this study is on metamaterial-based perfect absorbers (MMPA) in the THz regime. This paper presents an MMPA with two metallic layers and a dielectric. The suggested MMPA is investigated utilising three different materials: gold, silver, and copper, with a comparison between them shown her

Shadow of Kottler black hole in the presence of plasma for a co-moving observer

We consider a spherically symmetric black hole metric in (3+1)-dimensions in presence of a positive cosmological constant $\Lambda$. We use a general approach as proposed in \cite{1} to transform the metric in co-moving coordinates. Then for a general metric we have evaluated the angular shadow size both in absence and presence of plasma. By using the aberration relation, we then find the corresponding shadow radius as viewed by a co-moving observer for Schwarzschild de-Sitter or Kottler spacetime. The effect of cosmological constant $\Lambda$ and plasma parameter $k$ on the angular size of black hole shadow has been observed in detail. Finally, by using the observed angular size of M87$^*$ black hole, we constrain the value of the plasma parameter $k$ with a specific observational value of the cosmological constant $\Lambda$ (or the Hubble constant $H_0$).Comment: 21 pages, 12 figures, comments are welcom

Virtual transitions in an atom-mirror system in the presence of two scalar photons

We examine the virtual transition of an atom-mirror system with the simultaneous emission of two scalar photons, where the atom and the mirror admit a relative acceleration between them. For the single photon emission, literature (Phys. Rev. Lett. 121 (2018) 071301) dictates that the transition probabilities of two individual systems, such as an atom accelerating with respect to the mirror and its reverse, turn out to be equivalent under the exchange of the frequencies of atom and the field. Addressing the observational merit of such excitation process, a detectable probability ($P \sim 10^{-2}$) is also reported in the above literature. In the present manuscript our finding dictates that the simultaneous emission of dual photon instead of one, destroys the equivalence between the transition probabilities as reported in the above literature.Comment: 9 pages, 3 figure

Stability, quasinormal modes in a charged black hole in perfect fluid dark matter

In this work, we study time-like and null geodesics in a charged black hole background immersed in perfect fluid dark matter (PFDM). Using the condition for circular geodesics, we evaluate the energy ($E$) and angular momentum ($L$) in terms of the radius ($r_c$) of the circular orbits. The existence and finite-ness of $E$ and $L$ constrain the possible range of PFDM parameter ($\chi$) and the radius of the circular orbit ($r_c$). We then use the Lyapunov exponent ($\lambda$) to study the stability of the geodesics. Then we analyze the critical exponent ($\gamma$) useful for determining the possibility of detection of gravitational wave signals. After that, we study the perturbation due to a massless scalar field in such a background and calculate the quasinrmal mode (QNM) frequencies and their dependence on PFDM parameter $\chi$ and black hole charge $Q$. Also, we compare the obtained QNM frequencies both in the exact case and in the eikonal limit. We also calculate the quality factor of the oscillating system and study its dependence on $\chi$ and $Q$. Finally, we evaluate the black hole shadow radius $R_s$ and graphically observe the effect of $\chi$ and $Q$ on it.Comment: 29 pages, 18 Figures; Comments are welcom

Bandgap Engineering of ZnO Nanostructures through Hydrothermal Growth

he optical, electronic and magnetic properties changes with the reduction of particle sizes, this is because the nano sized structures exhibits certain unique properties different from their bulk counterpart. Bandgap engineering for nano structures is one of the prominent area of research now. All though the bandgap increases with the reduction in the particle size, however there are several other ways by which the band gap can be changed. Fast crystallization method has been reported here and is compared with the conventional method. Absorption spectra are obtained using UV-vis optical spectrometer and from the absorption spectra tauc plot is drawn to find out the band gap theoretically. The band gap of nano particles and nano rods grown with the same nano particles as seeds are found to be different. The work shows a comparative study on the resultant bandgap for ZnO nano particles and ZnO nano rods grown with hydrothermal method. It is observed that the fast crystallization method results in decreases of band gap by around .8 eV compared to conventional method.Keywords:ZnO, nano particle, nano rod,bandga

Improvement of photocatalytic activity of Zinc Oxide nanoparticles using Zinc Sulphide Shell

Core/shell nanoparticles having distinct attributes of both core and shell have, of late, become attractive candidates for applications involving surface engineering of nanoparticles. In this work, the synthesis of ZnO/ZnS core/shell nanoparticles by wet chemical method is reported, formation of which is confirmed by HRTEM analysis. Characterization of the core/shell nanoparticles was done using absorption spectroscopy and HRTEM. ZnO/ZnS core/shell nanoparticles exhibited higher photocatalytic activity than the already proved good photocatalyst ZnO. This was supported by photocatalytic degradation experiments carried out on an organic dye, methylene blue, under direct sunlight irradiation. The enhancement in the photocatalytic activity is attributed to the increase of the surface reactivity of the as synthesized material, which is confirmed through the blue shift in the UV Visible absorption spectra. This material can be a promising photocatalyst in the field of water purification.Keywords: visible light, photocatalysis, ZnO, ZnO/ZnS Core/Shell, methelyne blu

STUDY ON THE HYDROTHERMAL GROWTH OF ZnO NANORODS FOR PIEZOTRONIC APPLICATIONS

The capability of a certain material to generate an electric charge in response to applied mechanical stress is called as piezoelectric Effect. Metal oxidesemiconductors having high piezoelectric coefficient can be cost effectively manufactured by a simple hydrothermal methods at low temperature. These nanostructures are capable of transforming mechanical deformation into electrical power. The nanostructure morphologies and dimensions can be controlled by controlling the growth conditions. When subjected to mechanical deformations, these nanostructures are capable of transforming mechanical deformation into electrical power. Due to the structural noncentralsymmetry,ZnO nanostructures exhibit anisotropic piezoelectric properties. High aspect ratio ZnO nanostructures can be merely designed using hydrothermal methods and these nanowires or nanorods show piezoelectric properties. When subjected to mechanical deformations, these nanostructures undergo a charge separation due to inherent structural asymmetry. Tapping of the separated charges and subsequent accumulation can give a manifestation of mechanical to electrical energy transformation and lead to energy harvesting

Cranial osteology of Hypoptophis (Aparallactinae : Atractaspididae: Caenophidia), with a discussion on the evolution of its fossorial adaptations

Fossoriality evolved early in snakes, and has left its signature on the cranial morphology of many extinct Mesozoic and early Caenozoic forms. Knowledge of the cranial osteology of extant snakes is indispensable for associating the crania of extinct lineages with a particular mode of life; this applies to fossorial taxa as well. In the present work, we provide a detailed description of the cranium of Hypoptophis wilsonii, a member of the subfamily Aparallactinae, using micro-computed tomography (CT). This is also the first thorough micro-CT-based description of any snake assigned to this African subfamily of predominantly mildly venomous, fossorial, and elusive snakes. The cranium of Hypoptophis is adapted for a fossorial lifestyle, with increased consolidation of skull bones. Aparallactines show a tendency toward reduction of maxillary length by bringing the rear fangs forward. This development attains its pinnacle in the sister subfamily Atractaspidinae, in which the rear fang has become the "front fang" by a loss of the part of the maxilla lying ahead of the fang. These dentitional changes likely reflect adaptation to subdue prey in snug burrows. An endocast of the inner ear of Hypoptophis shows that this genus has the inner ear typical of fossorial snakes, with a large, globular sacculus. A phylogenetic analysis based on morphology recovers Hypoptophis as a sister taxon to Aparallactus. We also discuss the implications of our observations on the burrowing origin hypothesis of snakes.Peer reviewe