Subwavelength light confinement with surface plasmon polaritons

Abstract

In free space, the diffraction limit sets a lower bound to the size to which light can be confined. Surface plasmon polaritons (SPPs), which are electromagnetic waves bound to the interface between a metal and a dielectric, allow the control of light on subwavelength length scales. This opens up a rich world of opportunities in science and technology, ranging from lighting and photovoltaics to photonic circuits and quantum optics. This thesis explores new ways to tailor the properties of SPPs such that they enable the confinement of light at nanoscale dimensions. A variety of metallodielectric geometries are used that can serve as waveguides for SPPs. We show how the SPP propagation characteristics can be controlled, and how light can be concentrated in subwavelength volumes by tapering and truncating the waveguides. In Chapter 2 we use a near-field microscope to image the fields of SPPs that are squeezed into a 50~nm thick dielectric layer between two Ag surfaces, showing that the wavelength of SPPs is significantly shortened with respect to that of light. Chapter 3 focuses on specific waveguided SPP modes that can exhibit a negative effective index of refraction. This enables negative refraction of light into the waveguide at optical frequencies. Chapters 4 and 5 show that the concentration of infrared SPPs in laterally tapered Ag stripe waveguides enhances the upconversion of infrared to visible light in Er ions in the substrate. SPPs focus at the 65 nm large taper apex. Calculations show that the observed focusing effect can only occur for SPPs at the interface between the metal stripe and the high-index substrate. The focusing in tapered waveguides is explained in terms of an adiabatic transformation of a SPP mode guided by the waveguides in Chapter 6. Tapered waveguides are used to efficiently excite SPPs on metal nanowires with diameters as small as 60 nm. Phase- and polarization-sensitive near-field microscopy allows retrieval of the propagation speed and the polarization nature of the excited nanowire mode. The efficiency with which a taper couples light to a nanowire is measured to be 50%. In Chapter 7, we show that SPPs that propagate onto a Si wedge on Ag come to a halt at a specific wedge thickness, concentrating energy in a subwavelength focus. Chapter 8 investigates field enhancement at a wavelength of 1.5 m in arrays of subwavelength apertures in a metal film. We study the enhanced excitation of Er ions positioned close to the metal film by collecting Er upconversion emission. Both propagating and localized SPP resonances are studied. The field enhancement due to localized resonances is shown to be independent of the incident angle. Chapter 9 describes the enhancement of the radiative emission rate of Er ions placed inside annular apertures. An increase of both the photoluminescence intensity and the photoluminescence decay rate is observed when the aperture resonance is tuned to the Er emission wavelength. We discuss some possible applications and future research directions in Chapter 10, in integrated photonics, photovoltaics, molecular sensing and metamaterials research.