Doctor of Philosophy in Physics (PhD)
Coherent control of ultrafast dynamics of materials studied with time and angle-resolved photoemission spectroscopy
Following a review of the theory of terahertz generation via optical rectification in nonlinear crystals, a method for enhancing the nonlinear conversion efficiency of this process is proposed. A nonlinear crystal is placed at the intracavity focus of a passive optical resonator, which is seeded by an ytterbium-doped fibre laser. Models of this arrangement indicate that an enhancement of the optical field of several orders of magnitude is possible. As the difference frequency radiation produced through the optical rectification process results from a mixing of the spectral components of the optical field, one expects a corresponding increase in the terahertz field. We present a design of optical resonator that compensates for the large group velocity dispersion of the nonlinear crystal. Our experimental results indicate that below bandgap absorption in the crystal severely limits the resulting enhancement of the optical field, and hence the terahertz field one would expect from this nonlinear process. A scanning-delay terahertz time-domain spectrometer has been constructed, using a gallium phosphide guiding structure to increase the interaction length of the optical and terahertz fields, thereby increasing the terahertz power produced. Our experiment demonstrates 20 dB signal to noise ratio over the spectral range of 0.5-1 THz. We propose a method for increasing the spectral resolution, whilst simultaneously reducing the required data acquisition time of such a terahertz spectrometer, through the use of two femtosecond optical frequency combs. One of these fields drives the nonlinear optical rectification process, whilst the second serves as a sampling local oscillator field to probe the terahertz field via electro-optic sampling in a second nonlinear crystal. By precisely controlling the relative pulse repetition rates of the two oscillators, we show that the full spectral content of the terahertz field can be acquired at rf frequencies, and without the slow mechanical delay lines associated with conventional terahertz time-domain spectroscopy. Finally, we present experimental efforts towards the demonstration of this technique, and show that, to be effective, steps must be taken to increase the strength of the expected rf signal over that of the measurement noise floor.
The goal of this work is the generation of extreme ultraviolet (EUV) radiation from a laser based source. To this end, we use high harmonic generation (HHG) to convert the near infrared output of a mode-locked Ti:Sapphire laser oscillator to the EUV. The requirement for HHG is a high peak intensity (>10¹³ W/cm²), which can be met by external amplification of the laser output. The method of amplification chosen for this work is a femtosecond enhancement cavity (fsEC), which stores and amplifies the output of a femtosecond mode-locked Ti:Sapphire laser by greater than a factor of 900 while maintaining the original repetition rate of 66 MHz. The design, benefits, and limitations of using a fsEC are discussed. The EUV light is created by the interaction of the amplified light with xenon gas delivered to the fsEC focus. The strong intracavity field leads to xenon plasma generation with detrimental effects on the HHG process, where it is shown that HHG is sensitive to the xenon gas and plasma dynamics. Methods of minimizing the plasma density and maximizing the EUV amplitude are discussed. The EUV is coupled out of the cavity, and up to the thirteenth harmonic (61 nm) of the laser is observed. The relative amplitudes of the different quantum trajectories generating the harmonics are calculated theoretically, and compared to experiment. The generated power of the eleventh harmonic (72 nm) is estimated to be 30μW, with a measured outcoupled power of 1.1μW. The relative intensity noise is also measured, with a cumulative root-mean-square (RMS) noise of
In this thesis I present my work on building a set of magnetic coils for the purpose of performing nuclear magnetic resonance (NMR) on Boltzmann polarized protons in water, and on hyperpolarized ¹²⁹Xe. The coils were designed to be used as a method for testing the degree of polarization achieved in ¹²⁹Xe, and for testing the capability of an in-house developed continuous wave (CW) ultraviolet (UV) laser to drive a 2-photon transition in ¹²⁹Xe. This laser will be used to measure the precession frequency of ¹²⁹Xe in a magnetic field, in order to precisely measure the magnitude of that field. This work is being done for the ultra-cold neutron (UCN) collaboration’s flagship experiment: to measure the neutron electric dipole moment (EDM). Previous neutron EDM experiments have only found an upper limit, and have been limited in precision largely because of systematic errors in the magnetic field strength measurement. These experiments, such as the one performed at Institut Laue-Langevin (ILL), which has given us the current lowest limit, used ¹⁹⁹Hg as a co-magnetometer. The UCN EDM experiment will add ¹²⁹Xe in addition to the ¹⁹⁹Hg, to make a dual co-magnetometer. By using multiple species of atoms in the measurement, systematic effects can be greatly reduced. I have characterized the coils that I built by performing NMR on protons in water. I measured the inhomogeneity in the B₀ field, across the sample container, to be 18.9±0.9 μT. It turns out that the homogeneity of the B₀ field can be improved significantly, and it will likely be necessary to do so in order to perform similar experiments on hyperpolarized ¹²⁹Xe. I also found the T₁ time of water in this setup to be 2.7±0.2 s.
The Optically Pumped Semiconductor Laser (OPSL) offers several advantages as a laser source for precision spectroscopy. The semiconductor gain bandwidth allows an OPSL to run continuous wave (CW) between 920 - 1154 nm and with a free running linewidth 500 kHz. High powers have been observed in OPSL, as high as 70 W. Paired with doubling crystals the wavelength range can be extended down to the ultraviolet(UV) with high power. This research presents an OPSL operating at 972 nm at 1.7 W sequentially doubled down twice to a wavelength of 243 nm at 150 mW. The linewidth is reduced by locking one OPSL to a Fabry-Poret stabilization cavity and then the relative linewidth was measured between two OPSL's locked together. The linewidth is determined to be 87 kHz, dominated mostly by technical noise. This laser is set to be used for cryogenic hydrogen spectroscopy and precision measurements of the Lamor precessional frequency of ¹²⁹Xe when it is used as a comagnetometer for measuring the electrical dipole moment (EDM) of the neutron.
Extreme ultraviolet (EUV) light has many potential applications, including spectroscopy and scattering experiments in physical chemistry and atmospheric science. The dominant method for producing high-flux coherent radiation in this spectral range is synchrotron radiation produced from highly subscribed national-scale facilities such as the Canadian Light Source. An alternative to synchrotron radiation is high harmonic generation (HHG), a nonlinear optical process requiring high optical intensities. This thesis describes the development of an optical amplifier and passive enhancement cavity in order to realize a table-top EUV source. A chirped-pulse ytterbium-doped fiber amplifier system outputs 20 W average power from an initial mode-locked laser outputting pulses at 80 MHz and 160 mW average power. The pulses, of duration ~250 fs after the amplifier, are coupled to a high-finesse cavity which further increases the power by a factor of 500. The peak intensity achieved in the cavity is over 10¹⁴ W/cm² and is an order of magnitude above the intensity required to drive HHG in xenon gas.