Third harmonic generation in liquid core optical fibres

The objective of this thesis is to investigate third harmonic generation in liquid core fibres. Such fibres are formed by injection of liquid into a hollow, solid cladding by capillary forces. Carbon disulphide and tetrachloroethylene are identified as most promising liquid candidates. Such liquids offer a strong nonlinearity whose major contribution is non-instantaneous arising from the molecular structure. The effect of this material response during harmonic generation is investigated numerically by solving coupled evolution equations and causes distinct spectral shifts and broadening of both harmonic and fundamental wave. Both liquids offer excellent transparency and a high index of refraction enabling intermodal phase matching in a step-index geometry without requiring a complex microstructure. Aspects of fibre design and experimental realisation are presented in detail. Using sub-picosecond pump pulses of different duration the harmonic is generated in a higher order fibre mode and resulting signals are analysed in the spectral domain. Modification of the fibre cross-section towards an elliptical core is investigated. Besides the induced birefringence, harmonic generation in further sets of higher order modes is possible due to their transformation of electric fields. Design considerations of spatially modified fibres were confirmed experimentally and adaptive phase matching by controlling fibre temperature could be realised. Feasibility of long term exposure of liquid filled fibres to high average powers of femtosecond pulses is demonstrated underpinning that liquid core fibres withstand practical applications beyond laboratory use. Finally, possible routes to enhance the currently achieved conversion efficiencies for tetrachloroethylen of 2 ∙ 10^-5, and carbon disulphide of 10^-7, are identified and future prospects of this fibre platform are discussed

Understanding Nonlinear Pulse Propagation in Liquid Strand-Based Photonic Bandgap Fibers

Ultrafast supercontinuum generation crucially depends on the dispersive properties of the underlying waveguide. This strong dependency allows for tailoring nonlinear frequency conversion and is particularly relevant in the context of waveguides that include geometry-induced resonances. Here, we experimentally uncovered the impact of the relative spectral distance between the pump and the bandgap edge on the supercontinuum generation and in particular on the dispersive wave formation on the example of a liquid strand-based photonic bandgap fiber. In contrast to its air-hole-based counterpart, a bandgap fiber shows a dispersion landscape that varies greatly with wavelength. Particularly due to the strong dispersion variation close to the bandgap edges, nanometer adjustments of the pump wavelength result in a dramatic change of the dispersive wave generation (wavelength and threshold). Phase-matching considerations confirm these observations, additionally revealing the relevance of third order dispersion for interband energy transfer. The present study provides additional insights into the nonlinear frequency conversion of resonance-enhanced waveguide systems which will be relevant for both understanding nonlinear processes as well as for tailoring the spectral output of nonlinear fiber sources

Taming ultrafast laser filaments for optimized semiconductor–metal welding

Ultrafast laser welding is a fast, clean, and contactless technique for joining a broad range of materials. Nevertheless, this technique cannot be applied for bonding semiconductors and metals. By investigating the nonlinear propagation of picosecond laser pulses in silicon, it is elucidated how the evolution of filaments during propagation prevents the energy deposition at the semiconductor–metal interface. While the restrictions imposed by nonlinear propagation effects in semiconductors usually inhibit countless applications, the possibility to perform semiconductor–metal ultrafast laser welding is demonstrated. This technique relies on the determination and the precompensation of the nonlinear focal shift for relocating filaments and thus optimizing the energy deposition at the interface between the materials. The resulting welds show remarkable shear joining strengths (up to 2.2 MPa) compatible with applications in microelectronics. Material analyses shed light on the physical mechanisms involved during the interaction

Numerical and Experimental Demonstration of Intermodal Dispersive Wave Generation

Abstract Evidence of intermodal dispersive wave generation mediated by intermodal cross‐phase modulation (iXPM) between different transverse modes during supercontinuum generation in silicon nitride waveguides is presented. The formation of a higher‐order soliton in one strong transverse mode leads to phase modulation of a second, weak transverse mode by iXPM. The phase modulation enables not only supercontinuum generation but also dispersive wave generation within the weak mode, that otherwise has insufficient power to facilitate dispersive wave formation. The nonlinear frequency conversion scheme presented here suggests phase‐matching conditions beyond what is currently known, which can be exploited for extending the spectral bandwidth within supercontinuum generation.Intermodal dispersive wave generation mediated by intermodal cross‐phase modulation between different transverse modes during supercontinuum generation in silicon nitride waveguides is demonstrated. The phase modulation enables dispersive wave generation within a weak mode, that otherwise has insufficient power to facilitate dispersive wave formation. This process suggests new phase‐matching conditions for frequency conversion beyond to what is currently known. imag

Measurement of the cross-section and charge asymmetry of WW bosons produced in proton-proton collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

This paper presents measurements of the $W^+ \rightarrow \mu^+\nu$ and $W^- \rightarrow \mu^-\nu$ cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables, submitted to EPJC. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13

Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb−1 of √ s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente

Search for chargino-neutralino production with mass splittings near the electroweak scale in three-lepton final states in √s=13 TeV pp collisions with the ATLAS detector

A search for supersymmetry through the pair production of electroweakinos with mass splittings near the electroweak scale and decaying via on-shell W and Z bosons is presented for a three-lepton final state. The analyzed proton-proton collision data taken at a center-of-mass energy of √s=13  TeV were collected between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 139  fb−1. A search, emulating the recursive jigsaw reconstruction technique with easily reproducible laboratory-frame variables, is performed. The two excesses observed in the 2015–2016 data recursive jigsaw analysis in the low-mass three-lepton phase space are reproduced. Results with the full data set are in agreement with the Standard Model expectations. They are interpreted to set exclusion limits at the 95% confidence level on simplified models of chargino-neutralino pair production for masses up to 345 GeV

Search for High-Mass Resonances Decaying to τν in pp Collisions at √s=13 TeV with the ATLAS Detector

A search for high-mass resonances decaying to τν using proton-proton collisions at √s=13 TeV produced by the Large Hadron Collider is presented. Only τ-lepton decays with hadrons in the final state are considered. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb−1. No statistically significant excess above the standard model expectation is observed; model-independent upper limits are set on the visible τν production cross section. Heavy W′ bosons with masses less than 3.7 TeV in the sequential standard model and masses less than 2.2–3.8 TeV depending on the coupling in the nonuniversal G(221) model are excluded at the 95% credibility level

Search for the direct production of charginos and neutralinos in final states with tau leptons in √s=13 TeV collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with at least two hadronically decaying tau leptons is presented. The analysis uses a dataset of pp collisions corresponding to an integrated luminosity of 36.1 fb−1, recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13TeV.Nosignificant deviation from the expected Standard Model background is observed. Limits are derived in scenarios of ˜χ+1 ˜χ−1 pair production and of ˜χ±1 ˜χ02 and ˜χ+1 ˜χ−1 production in simplified models where the neutralinos and charginos decay solely via intermediate left-handed staus and tau sneutrinos, and the mass of the ˜ τL state is set to be halfway between the masses of the ˜χ±1 and the ˜χ01. Chargino masses up to 630 GeV are excluded at 95% confidence level in the scenario of direct production of ˜χ+1 ˜χ−1 for a massless ˜χ01. Common ˜χ±1 and ˜χ02 masses up to 760 GeV are excluded in the case of production of ˜χ±1 ˜χ02 and ˜χ+1 ˜χ−1 assuming a massless ˜χ01. Exclusion limits for additional benchmark scenarios with large and small mass-splitting between the ˜χ±1 and the ˜χ01 are also studied by varying the ˜ τL mass between the masses of the ˜χ±1 and the ˜χ01