Graduate and Postdoctoral Studies
Electrical and Computer Engineering
Optoelectronic and Ultraviolet Plasmonics
Thursday, February 27, 2014
to 10:30 AM
300 Brockman Hall for Physics
Metallodielectric nanostructures are driving 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 to enable 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 infrared radiation. Due to their strong plasmon resonance, these devices are also 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, with significant deviations reported between experimental properties and theoretical predictions in the literature. This lack of predictability is shown to result from a remarkable sensitivity to the presence of oxide within the aluminum nanoparticles, an effect which is distinct from the well known surface oxidation of Al. For high purity Al nanoparticles, broad tuneability throughout the UV and visible has been measured. The plasmonic nature of Al nanostructures was further confirmed via direct imaging of the local density of optical states (LDOS) with a spatial resolution of <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 two contributions – electrically active nanoantenna-diodes and ultraviolet plasmonics – expand the toolkit for engineering future generations of optical devices.