Compact and low-cost ultrashort-pulse Ti:sapphire lasers

The subject matter of this thesis centres around the design and development of ultrafast Ti:sapphire lasers in a compact and low-cost context. Using 450 nm laser diodes as the pump source, both semiconductor saturable absorber mirror (SESAM) mode-locking and Kerr-lens mode-locking (KLM) techniques are used. Broad wavelength tunability while maintaining femtosecond pulse operation at 100’s of mW of average power and 10’s of kW of peak power was demonstrated. In the SESAM mode-locked configuration, a wavelength tunability range of 37 nm (788-825 nm) was demonstrated, with average output powers up to 433 mW, and with shortest pulse duration of 62 fs at 812 nm. In the KLM regime, a wavelength tunability range of 120 nm (755-875 nm) was demonstrated, with average output powers up to 382 mW, and with shortest pulse duration of 54 fs at 810 nm. Various cavity configurations were proposed and analysed with the intention of realising GHz pulse repetition rates in an ultrafast diode-pumped Ti:sapphire laser. Two different cavity configurations were chosen: KLM in a ring resonator configuration and SESAM mode-locking in a Z-shape standing-wave resonator configuration. Efficient continuous wave operation was achieved, however, mode-locked operation was not reached with either configuration. Graphene saturable absorbers for femtosecond pulse generation in diode-pumped Ti:sapphire were also investigated. Monolayer graphene samples were fully characterised in a differential transmission setup. This resulted in a saturation fluence of (41± 27) μJ/cm2, a saturable loss of (1.01 ± 0.15)% and a non-saturable loss of (0.42 ± 0.09)%, in broad agreement with values reported in the literature. The diode-pumped Ti:sapphire laser sources developed during the course of this thesis have demonstrated important performance parameters that bring them closer to matching the performance of their conventionally pumped counterparts, namely a wide wavelength tunability while maintaining femtosecond pulse operation at 100’s of mW of average power and 10’s of kW of peak power, suitable for many applications.The subject matter of this thesis centres around the design and development of ultrafast Ti:sapphire lasers in a compact and low-cost context. Using 450 nm laser diodes as the pump source, both semiconductor saturable absorber mirror (SESAM) mode-locking and Kerr-lens mode-locking (KLM) techniques are used. Broad wavelength tunability while maintaining femtosecond pulse operation at 100’s of mW of average power and 10’s of kW of peak power was demonstrated. In the SESAM mode-locked configuration, a wavelength tunability range of 37 nm (788-825 nm) was demonstrated, with average output powers up to 433 mW, and with shortest pulse duration of 62 fs at 812 nm. In the KLM regime, a wavelength tunability range of 120 nm (755-875 nm) was demonstrated, with average output powers up to 382 mW, and with shortest pulse duration of 54 fs at 810 nm. Various cavity configurations were proposed and analysed with the intention of realising GHz pulse repetition rates in an ultrafast diode-pumped Ti:sapphire laser. Two different cavity configurations were chosen: KLM in a ring resonator configuration and SESAM mode-locking in a Z-shape standing-wave resonator configuration. Efficient continuous wave operation was achieved, however, mode-locked operation was not reached with either configuration. Graphene saturable absorbers for femtosecond pulse generation in diode-pumped Ti:sapphire were also investigated. Monolayer graphene samples were fully characterised in a differential transmission setup. This resulted in a saturation fluence of (41± 27) μJ/cm2, a saturable loss of (1.01 ± 0.15)% and a non-saturable loss of (0.42 ± 0.09)%, in broad agreement with values reported in the literature. The diode-pumped Ti:sapphire laser sources developed during the course of this thesis have demonstrated important performance parameters that bring them closer to matching the performance of their conventionally pumped counterparts, namely a wide wavelength tunability while maintaining femtosecond pulse operation at 100’s of mW of average power and 10’s of kW of peak power, suitable for many applications

Electrostatic stability of insulating surfaces: Theory and applications

We analyze the electrostatic stability of insulating surfaces in the framework of the bulk modern theory of polarization. We show that heuristic arguments based on a fully ionic limit find formal justification at the microscopic level, even in solids where the bonding has a mixed ionic/covalent character. Based on these arguments, we propose simple criteria to construct arbitrary non-polar terminations of a given bulk crystal. We illustrate our ideas by performing model calculations of several LaAlO3 and SrTiO3 surfaces. We find, in the case of ideal LaAlO3 surfaces, that cleavage along a higher-index (n10) direction is energetically favorable compared to the polar (100) orientation. In the presence of external adsorbates or defects the picture can change dramatically, as we demonstrate in the case of H2O/LaAlO3(100).Comment: 18 pages, 10 figure

Evolution of the Surface Structures on SrTiO3_3(110) Tuned by Ti or Sr Concentration

The surface structure of the SrTiO$_3$(110) polar surface is studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Monophased reconstructions in (5$\times$1), (4$\times$1), (2$\times$8), and (6$\times$8) are obtained, respectively, and the evolution between these phases can be tuned reversibly by adjusting the Ar$^{+}$ sputtering dose or the amount of Sr/Ti evaporation. Upon annealing, the surface reaches the thermodynamic equilibrium that is determined by the surface metal concentration. The different electronic structures and absorption behaviors of the surface with different reconstructions are investigated.Comment: 10 pages, 14 figure

Synthesis-Dependent First-Order Raman Scattering in SrTiO 3 Nanocubes at Room Temperature

Raman spectroscopy was used to demonstrate that the lattice dynamics of SrTiO 3 (STO) nanoparticles strongly depends on their microstructure, which is in turn determined by the synthetic approach employed. First-order Raman modes are observed at room temperature in STO single-crystalline nanocubes with average edge lengths of 60 and 120 nm, obtained via sol-precipitation coupled with hydrothermal synthesis and a molten salt procedure, respectively. First-order Raman scattering arises from local loss of inversion symmetry caused by surface frozen dipoles, oxygen vacancies, and impurities incorporated into the host lattice. The presence of polar domains is suggested by the pronounced Fano asymmetry of the peak corresponding to the TO2 polar phonon, which does not vanish at room temperature. These noncentrosymmetric domains will likely influence the dielectric response of these nanoparticles

Epitaxial Stabilization of Face Selective Catalysts

Abstract Selective, active, and robust catalysts are necessary for the efficient utilization of new feedstocks. Faceselective catalysts can precisely modify catalytic properties, but are often unstable under reaction conditions, changing shape and losing selectivity. Herein we report a method for synthesizing stable heterogeneous catalysts in which the morphology and selectivity can be tuned precisely and predictably. Using nanocrystal supports, we epitaxially stabilize specific active phase morphologies. This changes the distribution of active sites of different coordination, which have correspondingly different catalytic properties. Specifically, we utilize the different interfacial free-energies between perovskite titanate nanocube supports with different crystal lattice dimensions and a platinum active phase. By substituting different sized cations into the support, we change the lattice mismatch between the support and the active phase, thereby changing the interfacial free-energy, and stabilizing the active phase in different morphologies in a predictable manner. We correlate these changes in active phase atomic coordination with changes in catalytic performance (activity and selectivity), using the hydrogenation of acrolein as a test reaction. The method is general and can be applied to many nanocrystal supports and active phase combinations. Keywords Epitaxy Á Perovskite Á Platinum Á Heterogeneous catalysis Á Hydrogenation Á Acrolein Controlling the morphology of catalytic metal nanoparticles has incredible potential for improving selectivity and yield. This is because catalytic properties often depend upon the coordination of active site atoms We have recently observed that oriented oxide nanocrystal supports can epitaxially stabilize a specific orientation and morphology of the active phas

Exploring the transition from staff nurse to ward sister/manager – An exploratory case study

Background The ward sister/manager figure has traditionally been considered the ward based clinical leader. This role has evolved over time in response to professional and political demands; despite or because of this, reports of role ambiguity exist and the ward sister/manager position has become increasingly difficult to recruit to, with nurses arguably looking to roles perceived to have greater influence and status, but less onerous managerial responsibility. Understanding the nature of this role and the factors that may impact upon the transition from staff nurse to ward sister/manager is of great significance. The gaps in understanding related to development in preparation for and in the early stages of this role, led to this research, with the aim of understanding the experience of the transition and the impact of organisation factors, and whether they facilitate or hinder the process of transition for this group of nurses. Study design A case study approach consisting of interviews and documentary analysis was undertaken in one metropolitan National Health Service hospital, underpinned by a critical realist approach. The case study comprised interviews with six nurses, repeated over time with three of those nurses, who had recently participated in a leadership development programme and key informants who were senior practitioners within the organisation or who were recommended by participants, in combination with strategy and policy scrutiny and website analysis. Ritchie and Spencer’s Framework approach was used to support the management and subsequent analysis of the data. Findings The ward sister/manager role was identified as a vital role, but the managerial components of the role served as a significant disincentive to participants. Participants required support from significant role models during the transition process, although the degree of support, both required and available, varied. Motivating factors and the sense of job satisfaction were essential for developing a sense of self-fulfilment. A range of support mechanisms were present and utilised across the organisation but these appeared disparate and the lack of a unifying vision for nursing services was apparent. Contribution to knowledge Developing into the ward sister/manager role involves a significant transition that has not previously been acknowledged. Job satisfaction in the ward sister/manager role is significantly affected by organisational factors, as well as individual factors. The organisation itself contributes to the role legitimacy of this role