arXiv subjects

S. Ramakrishna

Publications and source records attributed to S. Ramakrishna.

7 recordsLinked to original sources

arXiv2026

Unambiguous Vector Magnetometry with Structured Light in Atomic Vapor

Absorption profiles of vector light upon interaction with atomic vapor carries distinct signatures of external magnetic field vector. However, this signature becomes ambiguous for anti parallel magnetic field vectors of equal magnitude, which makes their absorption profiles visually indistinguishable. To resolve this ambiguity, we present theoretical analysis of the interaction of vector light with optically polarized atoms immersed in reference and test magnetic fields. Furthermore, we demonstrate the complete characterization of the arbitrarily oriented test magnetic field via Fourier analysis of the absorption profile. This analysis reveals a one to one correspondence between the magnetic field properties and the profiles contrast and rotational angle. Our findings open an avenue to design an optical vector atomic magnetometer based on structured light fields.

physics.atom-ph↗

arXiv2025

Interaction of a Poincar\'e beam with optically polarized atoms in the presence of constant magnetic field

Recent studies have highlighted the frequent applications of structured light modes in optically pumped atomic magnetometers. In this work, we theoretically explore how a Poincar\'e beam probes an optically polarized atomic medium. Specifically, we consider atoms polarized by a plane wave with linear polarization, immersed in a constant external magnetic field. We analyze how the polarization of the pump and probe light fields, along with the external magnetic field, impact the absorption profile. To this end, we employ a density matrix approach based on the Liouville-von Neumann equation. Our results reveal that the absorption profile exhibits an asymmetric pattern that depends on the magnetic field strength and the mutual orientation of the pump and probe light propagation directions relative to the quantization axis. For illustration, we assume the incoming radiation drives an electric dipole transition, $5s \, ^2S_{1/2}$ ($F=1$) $\rightarrow$ $5p \, ^2P_{3/2}$ ($F=0$), in rubidium atoms subjected to a magnetic field. These findings may aid in designing future experiments on optically pumped atomic magnetometers utilizing structured light modes.

physics.atom-ph↗

arXiv2024

Atomic photoexcitation as a tool for probing purity of twisted light modes

The twisted light modes used in modern atomic physics experiments can be contaminated by small admixtures of plane wave radiation. Although these admixtures hardly reveal themselves in the beam intensity profile, they may seriously affect the outcome of high precision spectroscopy measurements. In the present study we propose a method for diagnosing such a plane wave contamination, which is based on the analysis of the magnetic sublevel population of atoms or ions interacting with the "twisted + plane wave" radiation. In order to theoretically investigate the sublevel populations, we solve the Liouville-von Neumann equation for the time evolution of atomic density matrix. The proposed method is illustrated for the electric dipole $5s \, {}^{2}\mathrm{S}_{1/2} \, - \, 5p \, {}^{2}\mathrm{P}_{3/2}$ transition in Rb induced by (linearly, radially, or azimuthally polarized) vortex light with just a small contamination. We find that even tiny admixtures of plane wave radiation can lead to remarkable variations in the populations of the ground-state magnetic sublevels. This opens up new opportunities for diagnostics of twisted light in atomic spectroscopy experiments.

physics.atom-ph↗

arXiv2022

Polarization effects in the total rate of biharmonic $ω+ 3ω$ ionization of atoms

The total ionization rate of biharmonic ($ω+ 3ω$) ionization is studied within the independent particle approximation and the third order perturbation theory. Particular attention is paid to how the polarization of the biharmonic light field affects the total rate. The ratios of the biharmonic ionization rates for linearly and circularly polarized beams as well as for corotating and counterrotating elliptically polarized beams are analyzed, and how they depend on the beam parameters, such as photon frequency or phase between $ω$ and $3ω$ light beams. We show that the interference of the biharmonic ionization amplitudes determines the dominance of a particular beam polarization over another and that it can be controlled by an appropriate choice of beam parameters. Furthermore, we demonstrate our findings for the ionization of neon $L$ shell electrons.

physics.atom-ph↗

arXiv2022

Photoexcitation of atoms by cylindrically polarized Laguerre-Gaussian beams

We analyze the photo-excitation of atoms with a single valence electron by cylindrically polarized Laguerre-Gaussian beams. Theoretical analysis is performed within the framework of first-order perturbation theory and by expanding the vector potential of the Laguerre-Gaussian beam in terms of its multipole components. For cylindrically polarized Laguerre-Gaussian beams, we show that the (magnetic) sub-components of electric-quadrupole field vary significantly in the beam cross-section with beam waist and radial distance from the beam axis. We discuss the influence of varying magnetic multipole component in the beam cross-section on the sub-level population of a localized atomic target. In addition, we calculate the total excitation rate of electric quadrupole transition ($4s \;^{2}S_{1/2} \rightarrow 3d \;^{2}D_{5/2}$) in a mesoscopic target of Ca$^{+}$ ion. These calculations shows that the total rate of excitation is sensitive to the beam waist and the distance between center of the target and the beam axis. However, the excitation by cylindrically polarized Laguerre-Gaussian beam is found more efficient in driving electric quadrupole transition in the mesoscopic atomic target than the circularly polarized beams.

physics.atom-ph↗

arXiv2017

Emergence of Landauer Transport from Quantum Dynamics: A Model Hamiltonian Approach

The Landauer expression for computing current-voltage characteristics in nanoscale devices is efficient and widely applicable but not suited to transient phenomena and time dependent currents because it assumes that the charge carrier population attains a time independent dynamic equilibrium as soon as the external voltage is turned on. In this article, we construct a very general expression for a time dependent current in an electrode-molecule-electrode arrangement. Utilizing a model Hamiltonian, we propagate the Schrodinger wave function equation to numerically compute the time dependent population in the individual sub-systems. The current in each electrode (defined in terms of the rate of change of the corresponding population) has two components, one due to the charges originating from the same electrode and the other due to the charges initially residing at the other electrode. We derive an analytical expression for the first component and illustrate that it agrees reasonably with the numerical counterpart at early times. The structural form reveals that the initial occupancy can be factored out of the time dependent segment of the expression. We take this cue to construct a Landauer style formula and demonstrate that the current obtained from this simplified formula overlaps with our most general numerical current only after the charge flow settles into a steady state. Thus, we illustrate the emergence of Landauer transport from a true first-principles quantum dynamics calculation without any prior assumptions. Subsequently, we investigate the ingredients in our model that regulate the onset time scale of this Landauer regime. We compare the performance of our general current expression with the Landauer current for time dependent electronic coupling. Finally, we comment on the applicability of the Landauer formulas to compute hot-electron current arising upon plasmon decoherence.

cond-mat.mes-hall↗

arXiv2017

Friction of rubber with surfaces patterned with rigid spherical asperities

This paper reports on the frictional properties of smooth rubber substrates sliding against rigid surfaces covered with various densities of colloidal nano-particles (average diameter 77 nm). Friction experiments were carried out using a transparent Poly(dimethyl siloxane) (PDMS) rubber contacting a silica lens with silica nano-particles sintered onto its surface. Using a previously described methodology (Nguyen \textit{et al.}, \textit{J. of Adhesion} \textbf{87} (2011) 235-250 ), surface shear stress and contact-pressure distribution within the contact were determined from a measurement of the displacement field at the surface of the PDMS elastomer. Addition of silica nano-particles results in a strong, pressure-independent enhancement of the frictional shear stress as compared to the smooth lens. The contribution of viscoelastic losses to these increased frictional properties is analyzed in the light of a numerical model that solves the contact problem between the rubber and the rough surface. An order-of-magnitude agreement is obtained between experimental and theoretical results, the latter showing that the calculation of viscoelastic dissipation within the contact is very sensitive to the details of the topography of the rigid asperities.

cond-mat.soft↗