Sankalpa Ghosh

Electron optics with magnetic vector potential barriers in graphene

An analysis of electron transport in graphene in the presence of various arrangements of delta-function like magnetic barriers is presented. The motion through one such barrier gives an unusual non-specular refraction leading to asymmetric transmission. The symmetry is restored by putting two such barriers in opposite directions side by side. Periodic arrangements of such barriers can be used as Bragg reflectors whose reflectivity has been calculated using a transfer matrix formalism. Such Bragg reflectors can be used to make resonant cavities. We also analyze the associated band structure for the case of infinite periodic structures.

Electron transport and Goos-Hänchen shift in graphene with electric and magnetic barriers: optical analogy and band structure.

Transport of massless Dirac fermions in graphene monolayers is analysed in the presence of a combination of singular magnetic barriers and applied electrostatic potential. Extending a recently proposed (Ghosh and Sharma 2009 J. Phys.: Condens. Matter 21 292204) analogy between the transmission of light through a medium with modulated refractive index and electron transmission in graphene through singular magnetic barriers to the present case, we find the addition of a scalar potential profoundly changes the transmission. We calculate the quantum version of the Goos-Hänchen shift that the electron wave suffers upon being totally reflected by such barriers. The combined electric and magnetic barriers substantially modify the band structure near the Dirac point. This affects transport near the Dirac point significantly and has important consequences for graphene-based electronics.

Generation of polarization-entangled photons using type-II doubly periodically poled lithium niobate waveguides

In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.

Phase stabilization by rapid thermal annealing in amorphous hydrogenated silicon nitride film

We have studied the effect of rapid thermal annealing (RTA) in the context of phase evolution and stabilization in hydrogenated amorphous silicon nitride (a-SiN(x):H) thin films having different stoichiometries, deposited by an Hg-sensitized photo-CVD (chemical vapor deposition) technique. RTA-treated films showed substantial densification and increase in refractive index. Our studies indicate that a mere increase in flow of silicon (Si)-containing gas would not result in silicon-rich a-SiN(x):H films. We found that out-diffusion of hydrogen, upon RTA treatment, plays a vital role in the overall structural evolution of the host matrix. It is speculated that less incorporation of hydrogen in as-deposited films with moderate Si content helps in the stabilization of the silicon nitride (Si(3)N(4)) phase and may also enable unreacted Si atoms to cluster after RTA. These studies are of great interest in silicon photonics where the post-treatment of silicon-rich devices is essential.

ELECTRON OPTICS WITH DIRAC FERMIONS: ELECTRON TRANSPORT IN MONOLAYER AND BILAYER GRAPHENE THROUGH MAGNETIC BARRIER AND THEIR SUPERLATTICES

In this review article we discuss the recent progress in studying ballistic transport for charge carriers in graphene through highly inhomogeneous magnetic field known as magnetic barrier in combination with gate voltage induced electrostatic potential. Starting with cases for a single or double magnetic barrier we also review the progress in understanding electron transport through the superlattices created out of such electromagnetic potential barriers and discuss the possibility of experimental realization of such systems. The emphasis is particularly on the analogy of such transport with propagation of light wave through medium with alternating dielectric constant. In that direction we discuss electron analogue of optical phenomena like Fabry–Perot resonances, negative refraction, Goos–Hanchen effect, beam collimation in such systems and explain how such analogy is going to be useful for device generation. The resulting modification of band structure of Dirac fermions, the emergence of additional Dira...

Numerical study of one-dimensional and interacting Bose–Einstein condensates in a random potential

We present a detailed numerical study of the effect of a disordered potential on a confined one-dimensional Bose‐Einstein condensate, in the framework of a mean-field description, using a highly efficient and fast converging numerical scheme. For repulsive interactions, we consider the Thomas‐Fermi and Gaussian limits and for attractive interactions the behaviour of soliton solutions. We find that the average spatial extension of the stationary density profile decreases with an increasing disorder strength both for repulsive and attractive interactions among bosons. In the Thomas‐Fermi limit, a strong localization of the bosons is obtained in momentum space around the state k = 0. The time-dependent density differs considerably in the cases we have considered. For attractive and disordered Bose‐Einstein condensates, we show evidence of a bright soliton with an overall unchanged shape, but a disorder-dependent width. For weak disorder, the soliton is delocalized and for stronger disorder, it bounces back and forth between high potential barriers. (Some figures in this article are in colour only in the electronic version)

Reversal of Klein reflection by magnetic barriers in bilayer graphene.

Abstract Whereas massless Dirac fermions in monolayer graphene exhibit Klein tunneling when passing through a potential barrier upon normal incidence, such a barrier totally reflects massive Dirac fermions in bilayer graphene due to difference in chirality. We show that, in the presence of magnetic barriers, such massive Dirac fermions can have transmission through even at normal incidence. The general consequence of this behaviour for multilayer graphene consisting of massless and massive modes are mentioned. We also briefly discuss the effect of a bias voltage on such magnetotransport.Massless Dirac fermions in monolayer graphene exhibit total transmission when normally incident on a scalar potential barrier, a consequence of the Klein paradox originally predicted by O Klein for relativistic electrons obeying the 3 + 1 dimensional Dirac equation. For bilayer graphene, charge carriers are massive Dirac fermions and, due to different chiralities, electron and hole states are not coupled to each other. Therefore, the wavefunction of an incident particle decays inside a barrier as for the non-relativistic Schrödinger equation. This leads to exponentially small transmission upon normal incidence. We show that, in the presence of magnetic barriers, such massive Dirac fermions can have transmission even at normal incidence. The general consequences of this behavior for multilayer graphene consisting of massless and massive modes are mentioned. We also briefly discuss the effect of a bias voltage on such magnetotransport.

Generation of modal- and path-entangled photons using a domain-engineered integrated optical waveguide device

Integrated optical devices are expected to play a promising role in the field of quantum information science and technology. In this paper we propose a scheme for the generation of nondegenerate, copolarized, modal, and path-entangled photons using a directional coupler and an asymmetric Y-coupler geometry in type-0 phase-matched domain-engineered lithium niobate (LN) waveguide. The nonlinearity in LN is tailored in such a way that quasi-phase-matching conditions for two different spontaneous parametric down-conversion processes are obeyed simultaneously, leading to a modal and path-entangled state at the output. Assuming typical values of various parameters, we show, through numerical simulations, that an almost maximally entangled state is achievable over a wide range of waveguide parameters. For the degenerate case, the proposed scheme gives a NOON state for N = 2. The generated entangled photon pairs should have potential applications in quantum information schemes and also in quantum metrology. By appropriate domain engineering and component designing, the idea can be further extended to generate hyperentangled and two-photon multipath-entangled states, which may have further applications in quantum computation protocols.

Spinor Dipolar Bose-Einstein Condensates: Classical Spin Approach

Bose-Einstein condensates which are dominated by magnetic dipole-dipole interaction are discussed under spinful situations. We treat the spin degrees of freedom as a classical spin vector, approaching from the large spin limit to obtain an effective minimal Hamiltonian. This is a version extended from a nonlinear sigma model. By solving the Gross-Pitaevskii equation, we find several novel spin textures where the mass density and spin density are strongly coupled, depending upon trap geometries due to the long-range and anisotropic natures of the dipole-dipole interaction.

Cavity Optomechanics with Synthetic Landau Levels of Ultracold Fermi Gas

Ultracold fermionic atoms placed in a synthetic magnetic field arrange themselves in Landau levels. We theoretically study the optomechanical interaction between the light field and collective excitations of such fermionic atoms in synthetic magnetic field by placing them inside a Fabry-Perot cavity. We derive the effective Hamiltonian for particle hole excitations from a filled Landau level using a bosonization technique and obtain an expression for the cavity transmission spectrum. Using this we show that the cavity transmission spectrum demonstrates cold atom analog of Shubnikov-de Haas oscillation in electronic condensed matter systems. We discuss the experimental consequences for this oscillation for such a system and the related optical bistability.