arXiv subjects

Anirban Ghosh

Publications and source records attributed to Anirban Ghosh.

44 records · Page 3Linked to original sources

arXiv2018

Controlled generation, manipulation and frequency conversion of broadband orbital angular momentum spectrum of asymmetric optical vortex beams

We report on generation and control of tunable, broad orbital angular momentum (OAM) spectrum of a vortex beam. Using two spiral phase plates (SPPs), we have converted the Gaussian beam of Yb-fiber femtosecond laser at 1064 nm into optical vortices of orders, l=1-6, with conversion efficiency >95%. By adjusting the transverse shift of the SPPs with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order, l, into an asymmetric vortex beam of broad OAM spectrum of orders, l, l-1, l-2,..,0. While the position of the SPPs determines the weightage of the OAM modes, however, the weightage of the higher OAM modes transfer to lower OAM modes with the shift of SPPs and finally resulting a Gaussian beam (l=0). Using single pass frequency doubling of asymmetric vortices in a 5-mm-long bismuth triborate crystal, we have transferred the pump OAM spectra, l, l-1, l-2, ..., 0, into the broad spectra of higher order OAM modes, 2l, 2l-1, 2l-2, ...,0 at green wavelength, owing to OAM conservation in nonlinear processes. We have also observed that the conversion efficiency of the frequency-doubled vortices of all orders increases with the increase in the asymmetry of the pump vortices. Using pump power of 4.6 W, we have generated symmetric vortices at green wavelength of output powers, 1.2, 0.76, 0.47, 0.4, 0.32, and 0.3 W and orders of ls=2, 4, 6, 8, 10, and 12, respectively.

physics.optics↗

arXiv2017

Viscous dynamics of vortices in a ferromagnetic film

We derive viscous forces for vortices in a thin-film ferromagnet. The viscous force acting on vortex $i$ is a linear superposition $\mathbf F_i = - \sum_{j} \hat{D}_{ij} \mathbf V_j$, where $\mathbf V_j$ is the velocity of vortex $j$. Thanks to the long-range nature of vortices, the mutual drag tensor $\hat{D}_{ij}$ is comparable in magnitude to the coefficient of self-drag $D_{ii}$.

cond-mat.mes-hall↗

arXiv2017

Annihilation of Domain Walls in a Ferromagnetic Wire

We study the annihilation of topological solitons in the simplest setting: a one-dimensional ferromagnet with an easy axis. We develop an effective theory of the annihilation process in terms of four collective coordinates: two zero modes of the translational and rotational symmetries $Z$ and $Φ$, representing the average position and azimuthal angle of the two solitons, and two conserved momenta $ζ$ and $φ$, representing the relative distance and twist. Comparison with micromagnetic simulations shows that our approach captures well the essential physics of the process.

cond-mat.mes-hall↗

arXiv2016

Lower bounds on the dilation of plane spanners

(I) We exhibit a set of 23 points in the plane that has dilation at least $1.4308$, improving the previously best lower bound of $1.4161$ for the worst-case dilation of plane spanners. (II) For every integer $n\geq13$, there exists an $n$-element point set $S$ such that the degree 3 dilation of $S$ denoted by $δ_0(S,3) \text{ equals } 1+\sqrt{3}=2.7321\ldots$ in the domain of plane geometric spanners. In the same domain, we show that for every integer $n\geq6$, there exists a an $n$-element point set $S$ such that the degree 4 dilation of $S$ denoted by $δ_0(S,4) \text{ equals } 1 + \sqrt{(5-\sqrt{5})/2}=2.1755\ldots$ The previous best lower bound of $1.4161$ holds for any degree. (III) For every integer $n\geq6 $, there exists an $n$-element point set $S$ such that the stretch factor of the greedy triangulation of $S$ is at least $2.0268$.

cs.CG↗

arXiv2016

Lattice spanners of low degree

Let $δ_0(P,k)$ denote the degree $k$ dilation of a point set $P$ in the domain of plane geometric spanners. If $Λ$ is the infinite square lattice, it is shown that $1+\sqrt{2} \leq δ_0(Λ,3) \leq (3+2\sqrt2) \, 5^{-1/2} = 2.6065\ldots$ and $δ_0(Λ,4) = \sqrt{2}$. If $Λ$ is the infinite hexagonal lattice, it is shown that $δ_0(Λ,3) = 1+\sqrt{3}$ and $δ_0(Λ,4) = 2$. All our constructions are planar lattice tilings constrained to degree $3$ or $4$.

math.MG↗

arXiv2016

Tunable and temporally stable ferroelectric imprint through polarization coupling

Here we demonstrate a method to tune ferroelectric imprint, which is stable in time, based on the coupling between the non-switchable polarization of ZnO and switchable polarization of PbZrxTi(1-x)O3. SrRuO3/PbZrxTi(1-x)O3 /ZnO/SrRuO3 heterostructures were grown with different ZnO thicknesses. It is shown that the coercive voltages and ferroelectric imprint varies linearly with the thickness of ZnO. It is also demonstrated that the ferroelectric imprint remains stable with electric field cycling and electric field stress assisted aging.

cond-mat.mtrl-sci↗

arXiv2014

On Fence Patrolling by Mobile Agents

Suppose that a fence needs to be protected (perpetually) by $k$ mobile agents with maximum speeds $v_1,\ldots,v_k$ so that no point on the fence is left unattended for more than a given amount of time. The problem is to determine if this requirement can be met, and if so, to design a suitable patrolling schedule for the agents. Alternatively, one would like to find a schedule that minimizes the \emph{idle time}, that is, the longest time interval during which some point is not visited by any agent. We revisit this problem, introduced by Czyzowicz et al.(2011), and discuss several strategies for the cases where the fence is an open and a closed curve, respectively. In particular: (i) we disprove a conjecture by Czyzowicz et al. regarding the optimality of their Algorithm ${\mathcal A_2}$ for unidirectional patrolling of a closed fence; (ii) we present an algorithm with a lower idle time for patrolling an open fence, improving an earlier result of Kawamura and Kobayashi.

cs.DS↗

arXiv2012

Greedy is good: An experimental study on minimum clique cover and maximum independent set problems for randomly generated rectangles

Given a set ${\cal R}=\{R_1,R_2,..., R_n\}$ of $n$ randomly positioned axis parallel rectangles in 2D, the problem of computing the minimum clique cover (MCC) and maximum independent set (MIS) for the intersection graph $G({\cal R})$ of the members in $\cal R$ are both computationally hard \cite{CC05}. For the MCC problem, it is proved that polynomial time constant factor approximation is impossible to obtain \cite{PT11}. Though such a result is not proved yet for the MIS problem, no polynomial time constant factor approximation algorithm exists in the literature. We study the performance of greedy algorithms for computing these two parameters of $G({\cal R})$. Experimental results shows that for each of the MCC and MIS problems, the corresponding greedy algorithm produces a solution that is very close to its optimum solution. Scheinerman \cite{Scheinerman80} showed that the size of MIS is tightly bounded by $\sqrt{n}$ for a random instance of the 1D version of the problem, (i.e., for the interval graph). Our experiment shows that the size of independent set and the clique cover produced by the greedy algorithm is at least $2\sqrt{n}$ and at most $3\sqrt{n}$, respectively. Thus the experimentally obtained approximation ratio of the greedy algorithm for MIS problem is at most 3/2 and the same for the MCC problem is at least 2/3. Finally we will provide refined greedy algorithms based on a concept of {\it simplicial rectangle}. The characteristics of this algorithm may be of interest in getting a provably constant factor approximation algorithm for random instance of both the problems. We believe that the result also holds true for any finite dimension.

cs.CG↗