Optical micro- and nanofibres: Higher mode generation and particle manipulation studies

This thesis is centred on two experimental fields of optical micro- and nanofibre research; higher mode generation/excitation and evanescent field optical manipulation. Standard, commercial, single-mode silica fibre is used throughout most of the experiments; this generally produces high-quality, single-mode, micro- or nanofibres when tapered in a flame-heated, pulling rig in the laboratory. Single mode fibre can also support higher transverse modes, when transmitting wavelengths below that of their defined single-mode regime cut-off. To investigate this, a first-order Laguerre-Gaussian beam, LG01 of 1064 nm wavelength and doughnut-shaped intensity profile is generated free space via spatial light modulation. This technique facilitates coupling to the LP11 fibre mode in two-mode fibre, and convenient, fast switching to the fundamental mode via computer-generated hologram modulation. Following LP11 mode loss when exponentially tapering 125μm diameter fibre, two mode fibre with a cladding diameter of 80μm is selected fir testing since it is more suitable for satisfying the adiabatic criteria for fibre tapering. Proving a fruitful endeavour, experiments show a transmission of 55% of the original LP11 mode set (comprising TE01, TM01, HE21e,o true modes) in submicron fibres. Furthermore, by observing pulling dynamics and progressive mode-lass behaviour, it is possible to produce a nanofibre which supports only the TE01 and TM01 modes, while suppressing the HE21e,o elements of the LP11 group. This result provides a basis for experimental studies of atom trapping via mode-interference, and offers a new set of evanescent field geometries for sensing and particle manipulation applications. The thesis highlights the experimental results of the research unit’s Cold Atom subgroup, who successfully integrated one such higher-mode nanofibre into a cloud of cold Rubidium atoms. This led to the detection of stronger signals of resonance fluorescence coupling into the nanofibre and for light absorption by the atoms due to the presence of higher guided modes within the fibre. Theoretical work on the impact of the curved nanofibre surface on the atomic-surface van der Waals interaction is also presented, showing a clear deviation of the potential from the commonly-used flat-surface approximation. Optical micro- and nanofibres are also useful tools for evanescent-field mediated optical manipulation – this includes propulsion, defect-induced trapping, mass migration and size-sorting of micron-scale particles in dispersion. Similar early trapping experiments are described in this thesis, and resulting motivations for developing a targeted, site-specific particle induction method are given. The integration of optical nanofibres into an optical tweezers is presented, facilitating individual and group isolation of selected particles, and their controlled positioning and conveyance in the evanescent field. The effects of particle size and nanofibre diameter on pronounced scattering is experimentally investigated in this systems, as are optical binding effects between adjacent particles in the evanescent field. Such inter-particle interactions lead to regulated self-positioning and particle-chain speed enhancements.

Prehistoric burnt mound archaeology in Ireland

It is apparent from the widespread distribution of burnt mounds that Ireland was the most prolific user of pyrolithic technology in Bronze Age Europe. Even though burnt mounds are the most common prehistoric site type in Ireland, they have not received the same level of research as other prehistoric sites. This is primarily due to the paucity of artefact finds and the unspectacular nature of the archaeological remains, compounded by the absence of an appropriate research framework. Due to the widespread use of the technology and the various applications of hot water, narratives related to these sites have revolved around discussions of age and function. This has resulted in a generalised classification, where the term ‘fulacht fia’ covers several site types that have similar features but differing functions and age. The study presents a re-evaluation of fulachtaí fia in light of some 1000 sites excavated in Ireland. This is the most comprehensive study undertaken on the use of pyrolithic technology in prehistoric Ireland, dealing with different aspects of site function, chronology, social role and cultural context. A number of key areas have been identified in relation to our understanding of these sites. Previous investigations of burnt mounds have provided little information on the temporality of individual sites. It has been established that appropriate sampling strategies can provide important information about the formation of individual sites, their relationships to each other and to other monuments in the same cultural landscape. The evidence suggests that considerable caution should be exercised with regard to certain single radiometric dates from burnt stone deposits, based on the degree of certainty of the dated sample and its association with pyrolithic activity. Previously regarded as Bronze Age in date, there are now numerous examples of pyrolithic-type processes in earlier contexts, with the origins of the water-boiling phenomenon now considered to be Early Neolithic. A review of recent excavation evidence provides new insights into the use of pyrolithic technology for cooking. This is based on the discovery of faunal remains at several sites, combined with insights gained through experimental studies. The model proposed here is of open-air communal feasting and food sharing hosted by small family groups, as a medium for social bonding and the construction of community. It is also argued that if cooking was the primary activity taken place at these sites, this should not be viewed as a mundane functional activity, but rather one that actively contributed to the constitution of social relations. The formality of the technology is also supported by the presence of possible specialised structures, some of which were used for cooking/feasting while others were for ritualised sweat-bathing. The duration and frequency of activities associated with burnt mounds and the opportunities they provided for social interaction suggest that these sites contributed some familiar frames of reference to contemporary discourse.