Showing papers by "Nirmal K. Viswanathan published in 2013"

Topological structures in the Poynting vector field: an experimental realization

Abstract: Experimental measurements of the nonplanar phase of scalar optical beams is achieved using polarization singularities and Stokes parameters. The product of beam intensity and its phase gradient is calculated to get the Poynting vector distribution in the beam cross section. Using these we propose a scheme and experimentally obtain generic fundamental spiral, node, and saddle topological structures in the Poynting vector field.

Formation and morphological transformation of polarization singularities: hunting the monstar

Abstract: The theoretical formalism and experimental measurements to form and transform between the three morphologies of the polarization singular patterns—star, lemon and monstar—are presented here. The monstar is statistically rare in isotropic random fields but its controllable realization is achieved by tuning the field anisotropy in three-beam interference.

The Pancharatnam–Berry phase in polarization singular beams

Abstract: Space-variant inhomogeneously polarized field formed due to superposition of orthogonally polarized Gaussian (LG00) and Laguerre?Gaussian (LG01) beams results in polarization singular beams with different morphology structures such as lemon, star and dipole patterns around the C-point in the beam cross-section. The Pancharatnam?Berry phase plays a critical role in the formation and characteristics of these spatially inhomogeneous fields. We present our experimental results wherein we measure the variable geometric phase by tracking the trajectory of the component vortices in the beam cross-section, by interfering with selective polarization states and by tracking different latitudes on the Poincar? sphere without the effect of a dynamic phase.

Polarization singularities and fiber modal decomposition

Abstract: Interference of fiber eigen vector modes of different phase and spatial variation of polarization gives rise to different types of polarization singularities – isolated C-point surrounded by star / lemon type polarization morphology patterns, a dipole or two C-points of same index – in 2D polarization fields. In this context, fiber modal decomposition refers to identifying the constituent modes, their relative amplitude and the phase relationship among them in the fiber output. The size and location of the L-contour and the location of Cpoints determine the relative amplitude and the orientation of the polarization morphology pattern provides information regarding the relative phase difference between the constituent vector modes. We use these aspects of polarization singularity to demonstrate a novel fiber modal decomposition method.

On-axis time-resolved spatial characterization of shock-induced refractive fringes in liquid water

Abstract: We report here the on-axis measurement of time-resolved spatial characterization of refractive fringes due to nanosecond-laser-induced shock waves in liquid water. The complex shadowgraphic fringes due to interference of multiple shock waves observed in the transverse measurements are completely avoided in the on-axis measurements due to the fact that the outermost region of the shock front acts as a radially symmetric phase object to the probe beam, refraction from which results in clean and continuous fringes observed by the intensified charge coupled detector (ICCD) detector. A detailed analysis of different types of time-resolved fringes obtained in the on-axis measurement for fixed laser pulse energy leads us to an alternate and better way to analyze the fringes to obtain the shock wave velocity and the density profile in the entire region surrounding the shock origin, which will enable 3D imaging of shock wave dynamics.

Polarimetric measurement of optically perturbed surface plasmonic field

Abstract: A surface plasmon (SP) field excited at the metal–air interface by a TM-polarized laser and perturbed by an unpolarized, weakly absorbing laser beam leads to an understanding of the SP decay field’s contribution to specular reflection at the near field. The locally perturbed near field results in a spatial variation of the magnitude of the SP decay field due to the photo-thermal effect on the excited SP wave. The SP decay field of different magnitude interferes with the specular reflected field, affecting its polarization and phase characteristics. The changes in the resulting far field are polarimetrically analyzed to extract the polarization ellipse parameters and the phase changes in the entire plasmonic field region. The obtained results are promising for potential applications in all-optical polarization modulators and switches for optical computing.

Measurement of Goos-Hänchen shift using polarimetry

Abstract: The enhanced Goos-Hänchen (GH) shift due to surface plasmons excitation at the metal-dielectric interface is measured using Stokes polarimetry and is found to agree well with theoretical calculation and position-sensitive detector based measurement.

Poynting Vector of Complex Optical Fields

Abstract: Poynting vector density of polarization singular field with well defined polarization structure such as lemon or star is experimentally measured using polarization interference and found to have unique features in contrast to constituent scalar fields.

Long-range Longitudinally-polarized Optical Needle Beam

Abstract: Longitudinally-polarized optical needle beam with spot-size 0.48λ and enhanced non-diverging range 40λ is achieved by focusing radially-polarized beam using a high numerical aperture axicon. Also presented are its Poynting vector and degree of polarization distributions.

Evolution of polarization singularities in few-mode fiber

Abstract: Polarization singularities formed due to superposition of orthogonally polarized fundamental and higher order modes evolve as it propagates in a two-mode fiber and is used to study the vector waveguide modes and their dispersion.