Ankit K. Singh

Transverse spin and transverse momentum in scattering of plane waves

We study the near field to the far field evolution of spin angular momentum (SAM) density and the Poynting vector of the scattered waves from spherical scatterers. The results show that at the near field, the SAM density and the Poynting vector are dominated by their transverse components. While the former (transverse SAM) is independent of the helicity of the incident circular polarization state, the latter (transverse Poynting vector) depends upon the polarization state. It is further demonstrated that the interference of the transverse electric and transverse magnetic scattering modes enhances both the magnitudes and the spatial extent of the transverse SAM and the transverse momentum components.

Polarization-Tailored Fano Interference in Plasmonic Crystals: A Mueller Matrix Model of Anisotropic Fano Resonance

Fano resonance is observed in a wide range of micro- and nano-optical systems and has been a subject of intensive investigations due to its numerous potential applications. Methods that can control or modulate Fano resonance by tuning some experimentally accessible parameters are highly desirable for realistic applications. Here we present a simple yet elegant approach using the Mueller matrix formalism for controlling the Fano interference effect and engineering the resulting asymmetric spectral line shape in an anisotropic optical system. The approach is founded on a generalized model of anisotropic Fano resonance, which relates the spectral asymmetry to physically meaningful and experimentally accessible parameters of interference, namely, the Fano phase shift and the relative amplitudes of the interfering modes. The differences in these parameters between orthogonal linear polarizations in an anisotropic system are exploited to desirably tune the Fano spectral asymmetry using pre- and postselection of optimized polarization states. The concept is demonstrated on waveguided plasmonic crystals using Mueller matrix-based polarization analysis. The approach enabled tailoring of several exotic regimes of Fano resonance in a single device, including the complete reversal of the spectral asymmetry, and shows potential for applications involving control and manipulation of electromagnetic waves at the nanoscale.

Effects of mode mixing and avoided crossings on the transverse spin in a metal-dielectic-metal sphere

We study transverse spin in a sub-wavelength metal-dielectric-metal (MDM) sphere when the MDM sphere exhibits avoided crossing due to hybridization of the surface plasmon with the Mie localized plasmon. We show that the change in the absorptive and dipersive character near the crossing can have significant effect on the transverse spin. An enhancement in the transverse spin is shown to be possible associated with the transparency (suppression of extinction) of the MDM sphere. The effect is attributed to the highly structured field emerging as a consequence of competition of the electric and magnetic modes.

Mueller matrix spectroscopy of fano resonance in plasmonic oligomers

Abstract Fano resonance in plasmonic oligomers originating from the interference of a spectrally broad superradiant mode and a discrete subradiant mode is under intensive recent investigations due to numerous potential applications. In this regard, development of experimental means to understand and control the complex Fano interference process and to modulate the resulting asymmetric Fano spectral line shape is highly sought after. Here we present a polarization Mueller matrix measurement and inverse analysis approach for quantitative understanding and interpretation of the complex interference process that lead to Fano resonance in symmetry broken plasmonic oligomers. The spectral Mueller matrices of the plasmonic oligomers were recorded using a custom designed dark-field Mueller matrix spectroscopy system. These were subsequently analyzed using differential Mueller matrix decomposition technique to yield the quantitative sample polarimetry characteristics, namely, polarization diattenuation (d) and linear retardance ( δ ) parameters. The unique signature of the interference of the superradiant dipolar plasmon mode and the subradiant quadrupolar mode of the symmetry broken plasmonic oligomers manifested as rapid spectral variation of the diattenuation and the linear retardance parameters across the Fano spectral dip. The polarization information contained in the Mueller matrix was further utilized to desirably control the Fano spectral line shape. The experimental Mueller matrix analysis was complemented with finite element based numerical simulations, which enabled quantitative understanding of the interference of the superradiant and the subradiant plasmon modes and its link with the polarization diattenuation and retardance parameters preparation.

Enhancing transverse spin and transverse spin momentum in micro- and nano-optical systems

We show that scattering of plane waves leads to helicity-independent transverse spin angular momentum (SAM) and helicity-dependent transverse Poynting vector components. The in uence of plasmon resonance and avoided crossing for a sphere on these quantities is studied.

Quantitative polarimetric studies of Plasmonic quasicrystals

Plasmonic quasicrystals stand out as the center of cynosure behind the many potential applications which emerges due to the quasi-periodic structure and metal dielectric patterns. The rotational symmetry elicits the optical properties resembling like crystals and and the metal dielectric nanostructure are being probed and explored in various disciplines of science and even in engineering also. Plasmonic quasicrystals composed of quasi- periodic and metal-dielectric patterns furnish efficacious benefits in improving the efficiency of solar cells, broadband transmission enhancement, and bio-sensing applications etc. Due to the intriguing properties of plasmonic crystals such as periodicity and short range ordering, the excitation of the surface Plasmon polaritons is restricted by a few fewer techniques such as polarization, launch angle dependence. Polarization contains wealth of information and holds the potential to control the interaction of light with metal Nano particles. Therefore, an exhaustive and thorough information regarding incident and scattered light is necessary for the examining the spectral response of the quasi crystal. Here, we report to the best of our knowledge the first ever quantitative polarimetric studies on the extremely complex plasmonic quasicrystal by recording a full 4x4 spectral Mueller matrix from the same and tried to explore the fascinating and interesting properties of quasi crystals. A homebuilt comprehensive Mueller Matrix platform (integrated with dark field microscope) is utilized to record the conventionally weak, intermixed polarization signal from plasmonic quasicrystals. These studies probed the enthralling phenomena of Fano resonance, explored and probed the presence of phase anisotropy in the plasmonic quasicrystals using the Mueller matrix derived retardance (δ) parameter. Additionally polarization mediated tuning of Fano Resonance is achieved too. Moreover it is demonstrated that the Mueller matrix derived diattenuation, retardance parameters probes the Fano resonance, phase and amplitude anisotropy from such complex plasmonic nanostructure and proved instrumental in polarization controlled tuning of Fano resonance.