arXiv subjects

R. Arun

Publications and source records attributed to R. Arun.

47 records · Page 3Linked to original sources

arXiv2016

Superluminal light propagation via quantum interference in decay channels

We examine the propagation of a weak probe light through a coherently driven $Y$-type system. Under the condition that the excited atomic levels decay via same vacuum modes, the effects of quantum interference in decay channels are considered. It is found that the interference in decay channels results in a lossless anomalous dispersion between two gain peaks. We demonstrate that the probe pulse propagation can in principle be switched from subluminal to superluminal due to the decay-induced interference. We also show that the system exhibits a high index of refraction with negligible absorption for the driving fields. A dressed-state picture of the atom-light interaction is described to explain the numerical results.

quant-ph↗

arXiv2015

Effect of Transverse Magnetic Field on Dynamics of Current Driven Domain Wall Motion in the Presence of Spin-Hall Effect

Theoretically, we study the dynamics of a current induced domain wall in the bi-layer structure consists of a ferromagnetic layer and a non-magnetic metal layer with strong spin-orbit coupling in the presence of spin-Hall effect. The analytical expressions for the velocity and width of the domain wall interms of excitation angle are obtained by solving the Landau-Lifshitz-Gilbert equation with adiabatic, nonadiabatic and spin Hall effect-spin transfer torques using Schryers and Walker's method. Numerical results show that the occurance of polarity switching in the domain wall is observed only above the threshold current density. The presence of transverse magnetic field along with spin Hall effect-spin transfer torque enchances the value of the threshold current density, and the corresponding saturated velocity at the threshold current density is also increased.

cond-mat.mtrl-sci↗

arXiv2015

Dynamics of Current and Field Driven Domain Wall Motion under the Influence of Transverse Magnetic Field

The dynamics of transverse Neel domain wall in a ferromagnetic nanostrip in the presence of driving field, current and transverse magnetic field is investigated by the Landau-Lifshitz-Gilbert(LLG) equation with the adiabatic and non-adiabatic spin-transfer torques both analytically and numerically. The analytical expressions for the velocity, width, excitation angle and displacement for the domain wall are obtained by using small angle approximation along with Walkers trial function. The results show that the initial velocity of the domain wall can be controlled by the adiabatic spin-transfer torque and the saturated velocity can be controlled by the non-adiabatic spin-transfer torque and driving field. The large increase in the saturated velocity of the domain wall driven by current and field due to the transverse magnetic field is identified through the presence of driving field. There is no impact in the saturated velocity of the domain wall driven by current from the transverse magnetic field. For the domain wall driven by the current in the presence of the transverse magnetic field, the saturated velocity remains constant. The transverse magnetic field along with current and driving field is more advantageous that the transverse magnetic field along with current for increasing the saturated velocity of the domain wall. The numerical results showed that the saturated velocity is increased by the transverse magnetic field with the irrespective of the directions of the driving field and current further it is higher and lower when the directions of driving field and current are antiparallel and parallel respectively. The obtained analytical solutions are closely coincided with the computed numerical results.

cond-mat.mtrl-sci↗

arXiv2015

Impact of Current on Static and Kinetic Depinning Fields of Domain Wall in Ferromagnetic Nanostrip

The impact of current on static and kinetic depinning fields of a domain wall in an one dimensional ferromagnetic nanostrip is investigated by solving the Landau-Lifshitz-Gilbert equation with adiabatic and non-adiabatic spin-transfer torques analytically and numerically. The results show that in the absence of current, the static depinning field is greater than the kinetic depinning field and both the depinning fields decrease by the increase of current applied in a direction opposite to the direction of the applied field. Both the depinning fields can also be tuned by the current to make them equal.

cond-mat.mtrl-sci↗

arXiv2014

Phase control of squeezing in fluorescence radiation

We study squeezing properties of the fluorescence radiation emitted by a driven $Λ$-type atom in which the metastable lower energy levels are coupled by an additional field. We find that the relative phase of the applied fields can significantly modify the squeezing characteristics of radiation. It is shown that the additional field connecting the lower levels in the system can induce spectral squeezing in a parameter regime for which the squeezing is absent without the additional field. Moreover, the squeezing can be shifted from inner- to outer-sidebands of the spectrum by simply changing the relative phase. A dressed-state description is presented to explain these numerical results. The phase control of squeezing in the total variance of quadrature components is also examined. We show that the squeezing in total variance attains its maximal value when the system reduces to an effective two-level system.

quant-ph↗

arXiv2012

Interference-assisted squeezing in fluorescence radiation

The squeezing spectrum of the resonance fluorescence is studied for a coherently driven four-level atom in the Y-type configuration. It is found that the squeezing properties of the fluorescence radiation are modified significantly when quantum interference of the spontaneous decays channels is included. We show a considerable enhancement of steady-state squeezing in spectral components for strong and off-resonant driving fields. The squeezing may be increased in both the inner and outer sidebands of the spectrum depending upon the choice of parameters. We also show that the interference can degrade the spectral squeezing by increasing the decay rates of atomic transitions. An analytical description using dressed states is provided to explain the numerical results.

quant-ph↗

arXiv2007

Interference-induced splitting of resonances in spontaneous emission

We study the resonance fluorescence from a coherently driven four-level atom in the Y-type configuration. The effects of quantum interference induced by spontaneous emission on the fluorescence properties of the atom are investigated. It is found that the quantum interference resulting from cascade emission decays of the atom leads to a splitting of resonances in the excited level populations calculated as a function of light detuning. For some parameters, interference assisted enhancement of inner sidebands and narrowing of central peaks may also occur in the fluorescence spectrum. We present a physical understandingof our numerical results using the dressed state description of the atom-light interaction.

quant-ph↗

arXiv2006

Atom Lithography with Near-Resonant Light Masks: Quantum Optimization Analysis

We study the optimal focusing of two-level atoms with a near resonant standing wave light, using both classical and quantum treatments of the problem. Operation of the focusing setup is considered as a nonlinear spatial squeezing of atoms in the thin- and thick-lens regimes. It is found that the near-resonant standing wave focuses the atoms with a reduced background in comparison with far-detuned light fields. For some parameters, the quantum atomic distribution shows even better localization than the classical one. Spontaneous emission effects are included via the technique of quantum Monte Carlo wave function simulations. We investigate the extent to which non-adiabatic and spontaneous emission effects limit the achievable minimal size of the deposited structures.

quant-ph↗

arXiv2005

Atom Nano-lithography with Multi-layer Light Masks: Particle Optics Analysis

We study the focusing of atoms by multiple layers of standing light waves in the context of atom lithography. In particular, atomic localization by a double-layer light mask is examined using the optimal squeezing approach. Operation of the focusing setup is analyzed both in the paraxial approximation and in the regime of nonlinear spatial squeezing for the thin-thin as well as thin-thick atom lens combinations. It is shown that the optimized double light mask may considerably reduce the imaging problems, improve the quality of focusing and enhance the contrast ratio of the deposited structures.

quant-ph↗

arXiv2002

Dark States and Interferences in Cascade Transitions of Ultra-Cold Atoms in a Cavity

We examine the competition among one- and two-photon processes in an ultra-cold, three-level atom undergoing cascade transitions as a result of its interaction with a bimodal cavity. We show parameter domains where two-photon transitions are dominant and also study the effect of two-photon emission on the mazer action in the cavity. The two-photon emission leads to the loss of detailed balance and therefore we obtain the photon statistics of the cavity field by the numerical integration of the master equation. The photon distribution in each cavity mode exhibits sub- and super- Poissonian behaviors depending on the strength of atom-field coupling. The photon distribution becomes identical to a Poisson distribution when the atom-field coupling strengths of the modes are equal.

quant-ph↗

arXiv2001

Tunneling and Traversal of Ultra-cold Atoms through Vacuum Induced Potentials

We examine the passage of ultra-cold two-level atoms through the potential produced by the vacuum of the cavity field. The peak of the transmitted wave packet generally occurs at the instant given by the expression for phase time even if that instant is earlier than the instant at which incident wave packet's peak enters the cavity and thus the phase time can be considered as the appropriate measure of the time required for the atom to traverse the cavity. We show that the phase tunneling time for ultra-cold atoms could be both super - and sub - classical time and we show how this behaviour can be understood in terms of the momentum dependence of the phase of transmission amplitude. The passage of the atom through the cavity is unique as it involves a coherent addition of the transition amplitudes corresponding to both barrier and well. New features such as the splitting of the wave packet arise from the entanglement between the center of mass motion and the electronic as well as field states in the cavity.

quant-ph↗