Miglė Kuliešaitė
“Investigation of nonlinear coherent phenomena in photonic crystal fibers”
Photonic crystal fibers (PCFs) open up new opportunities for the study of nonlinear phenomena, thanks to the ability to control their waveguiding properties – such as dispersion, nonlinearity, birefringence, and single-mode operation – during the manufacturing process. In this doctoral dissertation, the dynamics of coherent nonlinear phenomena occurring in PCFs are investigated and new methods are presented for characterizing the fiber’s nonlinear properties and the behavior of ultrashort optical pulses.
In this dissertation supercontinuum generation in a highly nonlinear polarization-maintaining PCF is first discussed, where controlled bursts of two orthogonally polarized femtosecond pulses are used for pumping. Then, an investigation of UV-VIS light generation in a short PCF is presented.
To better understand the nonlinear phenomena occurring in PCFs, it is important to have reliable methods for characterizing both the nonlinear medium and the pulses propagating through it. Therefore, this dissertation presents a new method for measuring the nonlinear refractive index of a polarization-maintaining PCF, based on measurements of the polarization state variation of the light exiting the fiber as the average power of the pump pulses is varied: the first method for determining the nonlinear refractive index of an already manufactured PCF. Finally, a non-iterative semi-analytical algorithm is presented, which enables reliable reconstruction of the intensity profile and phase of the investigated pulse from cross-correlation frequency-resolved optical gating (XFROG) measurements.
Dominyka Stonytė
“Ultrashort UV Pulse-Material Interaction for Laser Microfabrication Applications”
The rapid advancement in solid-state femtosecond laser system development and constant increase in output power have opened the path for laser processing while using femtosecond pulses in the deep-UV spectral range. While ultrashort pulses offer highly temporally confined energy deposition, short wavelength ensures high spatial localization leading to precise shallow material modifications.
This dissertation investigates the interaction of ultrashort deep-UV laser pulses with high bandgap dielectrics, such as crystalline sapphire, silicate glasses, barium fluoride. It explains the underlying mechanisms of material removal and analyzes the outcomes while using different parameters, such as pulse overlap, fluence, number of pulses, wavelength. It was demonstrated for the first time that the ablation quality of c-cut sapphire can be significantly increased (surface roughness reduced up to 7 times, heat-affected zones eliminated) when using deep-UV instead of IR pulses. Additionally, the first successful experimental demonstration of femtosecond deep-UV pulse application in multi-level diffractive optical element fabrication is shown. Furthermore, diffractive photon sieves having the smallest focal length of 9 mm were presented and characterized in this work. Consequently, the successful fabrication of a binary axicon on Barium Fluoride is presented. The fabricated element was used to extend the depth of the field in the IR imaging of silk samples at the Australian Synchrotron facility.
Vadzim Haronin
“Pulsed Laser Deposition and Broadband Dielectric Spectroscopy of Complex Perovskite Materials”
This dissertation explores the applications of the pulsed laser deposition (PLD) method and broadband dielectric spectroscopy to study thin films and bulk materials of some complex perovskite materials such as PbZr₀.₂Ti₀.₈O₃, 0.8(Na₀.₅Bi₀.₅TiO₃)-0.2(BaTiO₃), ZrSnSe₃, Nb-doped and undoped BiFeO₃–BaTiO₃ (BF–BT), and PbHf₀.₉₂Sn₀.₀₈O₃ (PHS) crystal. The dissertation analyzes how different PLD parameters (such as fluence, deposition temperature, ambient pressure, etc.) affect the quality and structure of thin films. Broadband dielectric spectroscopy reveals the dynamics of phase transitions and the dielectric response of different ferroelectric materials at various temperatures. The obtained results show that in BF–BT ceramics, the relaxor-type dielectric dispersion dominates, with frequency-dependent permittivity peaks and thermally induced transitions between ordered and disordered states. In PbHf₀.₉₂Sn₀.₀₈O₃ crystals, the dielectric dispersion maximum shifts towards higher frequencies (to the GHz range) as it approaches the Curie temperature. A very high residual polarization (93 μC/cm²) was obtained in the PbZr₀.₂Ti₀.₈O₃ thin layer, exceeding even the value of the bulk material, which is due to the stresses that have arisen between the layer and the substrate.