H. W. Kihm

Bethe-hole polarization analyser for the magnetic vector of light

The nature of light as an electromagnetic wave with transverse components has been confirmed using optical polarizers, which are sensitive to the orientation of the electric field. Recent advances in nanoscale optical technologies demand their magnetic counterpart, which can sense the orientation of the optical magnetic field. Here we report that subwavelength metallic apertures on infinite plane predominantly sense the magnetic field of light, establishing the orientation of the magnetic component of light as a separate entity from its electric counterpart. A subwavelength aperture combined with a tapered optical fibre probe can also serve as a nanoscale polarization analyser for the optical magnetic field, analogous to a nanoparticle sensing the local electric polarization. As proof of its functionality, we demonstrate the measurement of a magnetic field orientation that is parallel to the electric field, as well as a circularly polarized magnetic field in the presence of a linearly polarized electric field.

Control of surface plasmon generation efficiency by slit-width tuning

We demonstrate control of surface plasmon polariton (SPP) generation efficiency via varying the width of a single slit that acts as a SPP launcher. Generated SPP intensities are directly measured through a near-field scanning microscope measuring both the transmitted and the scattered light. These results demonstrate enhancement as well as suppression of surface plasmon generation efficiency at specific slit widths. The experimentally observed sinusoidal width dependence can be explained by diffraction theory.

Optical magnetic field mapping using a subwavelength aperture.

Local distribution of the optical magnetic field is a critical parameter in developing materials with artificially engineered optical properties. Optical magnetic field characterization in nano-scale remains a challenge, because of the weak matter-optical magnetic field interactions. Here, we demonstrate an experimental visualization of the optical magnetic field profiles by raster scanning circular apertures in metal film or in a conical probe. Optical magnetic fields of surface plasmon polaritons and radially polarized beam are visualized by measuring the transmission through metallic apertures, in excellent agreements with theoretical predictions. Our results show that Bethe-Bouwkamp aperture can be used in visualizing optical magnetic field profiles.

Vector field mapping of local polarization using gold nanoparticle functionalized tips: independence of the tip shape.

We have measured local electric field vectors of local polarizaton on the nanoscale using gold nanoparticle functionalized tips as local field scatterers. In our experiments, the local field induces a dipole-moment in the gold nanoparticle functionalized tip, which then radiates into the far-field, transferring the full information about the local electric field from the near into the far field. The polarization characteristics of the scattered fields are analyzed using a conventional ellipsometry method. The tip dependent scattering function- the polarizability tensor- is fully determined by far field scattering measurements. Once the polarizability tensor for each tip is correctly accounted for in the data analysis, our results show that the finally determined local field polarization vectors are essentially independent of the tip shape.

Optical and terahertz near-field studies of surface plasmons in subwavelength metallic slits

We studied the transmission of the electromagnetic waves through subwavelength slit arrays in terahertz (THz) and optical frequency regions, respectively. In the optical frequency regime, the influences of surface plasmon polaritons on the near-field distribution and on the far-field transmittance are discussed. The near-field electro-optic sampling technique combined with fast Fourier transformation is applied in measuring the THz near-field distribution in time and spectral domains. From these, we discuss the existence of highly confined surface waves in the perfect conductor regime (THz) in comparison with the finite conducting case (visible range) in metallic multi-slit arrays. Our studies provide an integrated view of surface plasmons in the optical regime, and surface-bound waves mimicking surface plasmons in the THz region.

3-dimensional local field polarization vector mapping of a focused radially polarized beam using gold nanoparticle functionalized tips.

We have measured local electric field polarization vectors in 3-dimensional space on the nanoscale. A radial polarized light is generated by using a radial polarization converter and focused by an objective lens. Gold nanoparticle functionalized tips are used to scatter the focused field into the far-field region. Two different methods, rotational analyzer ellipsometry and Stokes parameters, are used in determining the polarization state of the scattered light. Two methods give consistent results with each other. Three dimensional local polarization vectors could be reconstructed by applying back transformation of the fully characterized polarizability tensor of the tip.

Separation of surface plasmon polariton from nonconfined cylindrical wave launched from single slits

We theoretically investigate the separation positions of surface plasmon polariton (SPP) from the cylindrical nonconfined diffracted light launched from a single slit. Through an analysis of the finite difference time domain calculations and the analytical solution of a line source on a metal surface, we find the wavelength dependent SPP separation positions defined as the lateral distance from the slit with two different criteria. These results show that the SPP separation positions can be approximated by a simple relationship given by the permittivity of the metal and by the wavelength regardless of how the criteria are chosen.

Surface plasmon polariton detection discriminating the polarization reversal image dipole effects

Image dipole effects are highly dependent on the polarization direction, constructive (destructive) interference between real and image dipoles for the vertically (horizontally) aligned one in the vicinity of metal surfaces, respectively. This polarization-reversal of the image dipole effects is quantitatively investigated by using a gold nanoparticle functionalized tip as a local dipolar scatterer and a propagating surface plasmon polariton as an excitation source of dipoles. The polarization-resolved detection technique is applied to separate the radiations of the vertical and the horizontal dipoles from each other. In our study, the image dipole effects on the far-field detected signals are fully explained by the Fabry-Perot like interference between the radiations from the real and the image dipoles, and by considering the finite size effects of the gold nanoparticle.

Near-to-far-field spectral evolution in a plasmonic crystal: Experimental verification of the equipartition of diffraction orders

We report on drastic changes in the near-field spectrum as it evolves into the far field in periodically corrugated metallic nanoslit arrays. The far-field spectral minimum is located exactly at the near-field spectral maximum, where a quasimonochromatic standing wave pattern is observed in the near field. These results are in excellent agreement with the equipartition of diffraction orders recently proposed [K. G. Lee and Q-Han Park, Phys. Rev. Lett. 95, 103902 (2005)].

Surface plasmon polariton generation on metal surface: From nano-optical to terahertz frequency regime

We investigate surface wave generation by a single slit, both in nano-optical and in terahertz regimes. The wave-front in optical regime separates itself into forward propagating wave and surface-bound 90-degree diffracted wave; while, no such clear separation of modes is not observed in terahertz regime. Excitation wavelength dependency is systematically studied.