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Title: Fabrication and characterisation of asymmetric plasmonic structures : material and structural effects
Author: Einsle , Joshua Franz
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2013
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The current momentum in plasmonics research was spurred on by the discovery of enhanced optical transmission. This phenomena results from the resonate coupling of electromagnetic surface waves on either side of a metal film. These waves are known as surface plasmon polaritons. By leveraging their ability to control light on the nanometric scale plasmonic structures are beginning to be integrated into a new generation of opto-electronics devices such as LEDS, photodetectors, photovoltaics and biosensors. This research has been made possible through the use new nanofabrication technologies. In particular, the Focused Ion Beam(FIB) has been key in the quick development and fabrication of the structures known as plasmonic crystals. These crystals are a periodic arrangements of apertures and gratings whose critical dimensions are of the same scale as the wavelengths of visible light. This is a requirement for efficient and controlled coupling between the incident electromagnetic wave and the resulting surface wave. By being able to selectively and precisely mill complicated geometries, the FIB allows for a mask less fabrication process of a variety of plasmonic devices. The structures fabricated in this manner offer the fast prototyping of systems examining fundamental optical interactions. The plasmonic crystals studied in this thesis look at systems where the signature feature is some form of asymmetry. In the first set of structures, study coaxial apertures in gold film on the high-index of refraction substrate, GaP. The GaP- gold substrate does not support SPP Bloch modes, resulting in an asymmetry in the transmission mechanisms. This asymmetry arises out of the interaction of the air-metal surface mode with the cavity mode associated with the sub-wavelength apertures. When the same coaxial structures are fabricated on glass substrates, the presence of a second set of surface modes creates a more complex system where the surface plasmon modes on each metal-dielectric interface interact via the resonate cavity mode. For both of the systems examined it is shown that this aperture geometry offers a significant improvement in transmission when compared to simple cylindrical apertures. The studies in the optical behaviors of GaP based plasmonic crystals showed that FIB patterning damages the optical transmission properties of the substrate. To improve the optical transmission, a FIB patterning strategy is developed which leads to over sevenfold increase in optical transmission for GaP based plasmonic devices. This is achieved through the use of low kV FIB patterning routines. The devices presented demonstrate the first example of fine structuring using a low kV FIB to produce functional devices. These devices demonstrate a 7x optical transmission enhancement when compared to standard fabrication approaches. Leveraging advances in FIB patterning capabilities to create complex structures allows for the exploration into the effects asymmetry on the transmission mechanisms possible in subwavelength apertures. By deforming the basic geometry of the coaxial aperture the boundary conditions of the optical system are altered. These new conditions open the possibility to excite different orders of surface modes on each side of the metal film. The structures fabricated demonstrate complex optical behaviors that provide for challenging experimental and numerical studies. The plasmonic saturates fabricated in this work, only begin to touch on the optical phenomena that can be explored through the selective application of material of structural asymmetry. Further, by the continual refinement of nanofabrication techniques, the ability to control the system studied is enhanced. Both of these processes work off each other to create the next generation of technologies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available