Nirmal K. Viswanathan

Generation of optical vector beams using a two-mode fiber

We present the generation of optical vector beams using a two-mode fiber (TMF)-based beam converter. The TMF converts the input Gaussian (TEM(00)) beam into linearly polarized Hermite-Gaussian (HG(10), HG(01)) beams, a radially polarized Laguerre-Gaussian (LG(1)(0)) beam with single helical charge or coherent linear combinations of the different vector modes guided in the fiber, depending on the input beam polarization, the fiber length, and the launch condition. Polarization and two-beam interference analyses of the output beam characterize the electric field orientations of the output beam and the presence of transverse and longitudinal optical vortex in the generated HG and LG beams.

Manifestation of the Gouy phase in vector-vortex beams

Experimental measurements of the twirl and changes in the anisotropy of the constant intensity ellipse, and the rotation of the polarization singular lemon pattern a generalized vector-vortex beam experiences around the two foci due to the converging and diverging conical waves and in between, are presented and interpreted as being due to the universal form of the Gouy phase, φ(G)=mπ/2.

Generation of isolated asymmetric umbilics in light's polarization

Polarization-singularity C-points, a form of line singularities, are the vectorial counterparts of the optical vortices of spatial modes and fundamental optical features of polarization-spatial modes. Their generation in tailored beams has been limited to lemon and star C-points that contain symmetric dislocations in state-of-polarization patterns. In this article we present the theory and laboratory measurements of two complementary methods to generate isolated asymmetric C-points in tailored beams, of which symmetric lemons and stars are limiting cases; and we report on the generation of monstars, an asymmetric C-point with characteristics of both lemons and stars.

Topological structures in vector-vortex beam fields

Optical singularities of index Iu=±1/2 associated with lemon, monstar, and star topological structures in π-symmetric fields and singularities of index Ic=±1 associated with radial, circulation (elliptic), spiral, node, and saddle structures in 2π-symmetric vector fields are studied in detail here. The topological structures in the polarization ellipse orientation field, the Stokes field, and the Poynting vector field are derived from the same vector-vortex beam fields, and their interdependencies are explored using a designed experimental setup. We find that the inherently stable topological approach is much more informative for a deeper understanding of complex vector-vortex beam fields.

Spectral anomalies due to temporal correlation in a white-light interferometer.

We present what we believe to be the first experimental demonstration of anomalous spectral behavior such as spectral shifts and spectral switches due to temporal correlation around the intensity minima in a white-light interferometer. Unusual behavior in the number of spectral fringes, measured within the source bandwidth, as a function of path delay between the interfering beams is also reported. Experimental observations match well with the spectra calculated by using the interference law in the spectral domain.

Spin-hall effect and circular birefringence of a uniaxial crystal plate

We demonstrate theoretically and experimentally the fine lateral circular birefringence of uniaxial crystal plates, an example of the spin — Hall effect of light. We report experimental observations this effect using polarimetric and quantum-weak-measurement techniques.

Detection of phase transitions from the study of whispering gallery mode resonance in liquid crystal droplets

We report studies on the whispering gallery mode (WGM) resonance of liquid crystal (LC) droplets across the smectic-A (SmA) to nematic phase transition. The quality factor (Q) in the SmA phase decreases rapidly with a characteristic slope change at the SmA-N transition. In the SmA phase, Q-factor is linearly proportional to the birefringence (Δn). We discuss the effect of topological defect transformation on the WGM resonance. The study of WGM resonance is expected to be very useful for detecting subtle phase transitions among LC mesophases.

Polarimetric measurement method to calculate optical beam shifts

Stokes polarimetry measurements are carried out to calculate the spatial and angular Goos-Hänchen and Imbert-Fedorov shifts of a Gaussian beam reflected at glass-air interface, by measuring the phase difference between the TE and TM components and the amplitude of reflection. Variation of the beam shifts as a function of input beam polarization is also measured. The results obtained here are in good agreement with the theoretical predictions and the results obtained using a position sensitive detector. The polarimetric measurement method is accurate, independent of the intensity distribution of the beam, and opens up a new method to study the beam shift problem.

Formation and morphological transformation of polarization singularities: hunting the monstar

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.

Generation of spirally polarized propagation-invariant beam using fiber microaxicon

We present here a fiber microaxicon (MA)based method to generate spirally polarized propagation-invariant optical beam. MA chemically etched in the tip of a two-mode fiber efficiently converts the generic cylindrically polarized vortex fiber mode into a spirally polarized propagation-invariant (Bessel-type) beam via radial dependence of polarization rotation angle. The combined roles of helico-conical phase and nonparaxial propagation in the generation and characteristics of the output beam from the fiber MA are discussed.