Optoelectronic and Ultraviolet Plasmonics
Knight, Mark William
Halas, Naomi J.
Doctor of Philosophy
Metallodielectric nanostructures are enabling a revolution in optics. For centuries optical techniques were constrained by the diffraction limit, which prevents the control over light at the nanoscale (~200 nm or less). This constraint is lifted when the electric field of light interacts with metals at the nanoscale, exciting collective, longitudinal oscillations in the metallic valence electrons. These oscillations, known as surface plasmons, allow the capture and manipulation of light within nanoscale volumes significantly below the diffraction limit. Originally predicted by Rufus Richie in 1957, plasmons have since been engineered into a zoo of resonant geometries that exploit their spectral tuneability, enhanced local fields, and controllable absorption for a new generation of optical research and applications. This thesis presents two advances that extend plasmonics into both electrically active devices, and the high energy ultraviolet regime. In the first part of this thesis, two distinct concepts – optical nanoantennas and Schottky diodes – are combined to demonstrate an active optical nanoantenna diode. Photons coupled into a metallic nanoantenna drive resonant plasmonic oscillations which decay, yielding energetic (‘hot’) electrons. These electrons can then undergo ballistic transport over the potential barrier formed at the nanoantenna/semiconductor interface, yielding photocurrent. The photoresponse of nanoantenna-diodes extends well below the semiconductor band edge, enabling silicon-based optical detectors for the infrared. Due to their strong plasmon resonance, these devices are capable of sensing both the wavelength and polarization of monochromatic radiation. The second half of this thesis will demonstrate the use of aluminum as a plasmonic material in the ultraviolet, exceeding the spectral limitations imposed by silver and gold. The extended spectral response of aluminum, combined with its natural abundance, low cost, and complementary metal–oxide–semiconductor (CMOS) compatibility, make it a highly promising material for commercial applications. However, fabricating Al-based nanostructures with controllable resonances has proven nontrivial, and literature reports show significant deviations can occur between experimental properties and theoretical predictions. This lack of predictability is the result of a remarkable sensitivity to the presence of oxide within the aluminum. We show that this bulk oxidization, which is distinct from the well known surface oxidation of Al, is the result of contamination during the Al deposition process itself. Using high purity Al nanoparticles, broad tuneability throughout the UV and visible has been demonstrated. The plasmonic nature of these Al nanostructures was further confirmed through direct imaging of the local density of optical states (LDOS) with a spatial resolution better than 20 nm. This foundational work paves the way toward the use of aluminum as a low-cost plasmonic material with complementary properties to the coinage metals, including access to short-wavelength regions of the spectrum, low-cost, and the possibility for mass production of plasmonic devices. Together, these contributions – electrically active nanoantenna-diodes and ultraviolet plasmonics – expand the toolkit for engineering future generations of optical devices.
Optoelectronics; Plasmonics; Nanoantennas; Ultraviolet; Aluminum