Controlling light with resonant plasmonic nanostructures

Abstract

Plasmons are collective oscillations of free electrons in a metal. At optical frequencies plasmons enable nanoscale confinement of light in metal nanostructures. This ability has given rise to many applications in e.g. photothermal cancer treatment, light trapping in photovoltaic cells, and sensing. Metal nanostructures also allow for manipulation of optical energy, providing such functionalities as guiding or redirecting light at the nanoscale.

This thesis focuses on nanoscale control of light using three types of metal nanostructures: 1) arrays of coupled metal nanoparticles, 2) metal nanowires, and 3) (coupled) coaxial waveguides.

Chapter 3 describes the optical behavior of a metal nanoparticle array illuminated sideways along the array axis. Confocal microscopy shows that field concentrates on just a few particles at the front or back side of the particle chain. By changing the illumination wavelength we can control which side of the chain becomes excited.

In chapter 4 we discuss angle resolved transmission measurements to determine the dispersion relation of plasmon modes in Au and Ag particle arrays. Our results confirm that far-field dynamic interactions between particles are important, even for structures five times smaller than the wavelength. Taking into account dynamic interactions we calculate that the propagation length in plasmon particle arrays can be as much as 10 m, which is an order of magnitude higher than previously estimated using quasi-electrostatic theory.

In chapter 5 we show that metal nanowires behave as plasmon resonators. We use cathodoluminescence imaging spectroscopy to determine the eigenmodes in 500-1200-nm-long Au nanowires at a resolution less than 10 nm. By combining spectral and spatial information we determine the dispersion relation for plasmons confined within the metal nanowires.

Chapter 6 focuses on coaxial plasmon waveguides. Optical transmission measurements on coaxial apertures in a Ag film point out that the plasmon dispersion in coaxial waveguides depends greatly on the refractive index and thickness of the dielectric channel. A plasmon phase shift up to  occurs upon reflection off the aperture ends. The phase shift depends greatly on the coax geometry, thereby providing further tunability of the optical behavior of coaxial nanostructures.

In chapter 7 we report on calculations of plasmon dispersion relations in coaxial waveguides. We find that certain coax geometries sustain modes of negative refractive index at optical frequencies. The spectral region of negative index can be shifted throughout the entire visible spectral range by changing the dielectric channel width. Furthermore, by fine-tuning the dielectric width, the special cases n=-1 and n=0 can be achieved in the coax, while maintaining a propagation length of 500 nm or more.

Finally, in chapter 8 we present a novel optical metamaterial, composed of a single functional layer containing coupled coaxial Ag/GaP/Ag channels, that exhibits a negative index of refraction n=-2 in the blue. Using time-domain simulations we find that the metamaterial index is independent of the angle-of-incidence over an angular range 50 degrees and is independent of polarization.

Altogether, this thesis demonstrates several new opportunities for resonant plasmonic nanostructures to control optical fields at the nanoscale. The presented concepts and insights hold great promise for new applications in integrated optics, photovoltaics, solid-state lighting, imaging below the diffraction limit, and even invisibility cloaking.