David Jones
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Organic photovoltaics (OPVs) have the potential to serve as flexible and transparent solar cells, comprised of inexpensive materials and created through scalable and energy efficient processes. However, poor power conversion efficiency (largely due to inefficient charge separation) prevents them from being a commercial competitor to today’s conventional solar cells. This thesis marks the first results of a collaborative research project at the University of British Columbia’s Stewart Blusson Quantum Matter Institute investigating the physics underlying poor charge transfer in OPVs.The first step in engineering more efficient charge separation is understanding the fundamentals of the charge transfer process. When an electron is excited in an OPV, it remains coulombically bound with its hole, forming an exciton. Excitons in organic solar cells are more tightly bound with larger binding energies than those in conventional solar cells, making them harder to dissociate into free charges. The nature of these excitons plays a role in the generation of free carriers in OPVs, but it remains an open question that can benefit from the temporal and spatial information provided by time- and angle-resolved photoemission spectroscopy (TR-ARPES).We work with neat C60 thin films due to the material’s foundational status as a prototypical OPV electron-acceptor material and because it is a photo-absorbing excitonic semiconductor, with a manifold of optically-accessible excitonic states. The importance of long-range order to conducting spectroscopic analysis of a thin film’s electronic structure prompted our development of a generalizable growth technique that consistently results in single-domain C60 films of controllable thicknesses, using Au(111) as an epitaxially well-matched substrate.To better understand the evolution of the optically-excited carriers, we use TR-ARPES (supported by low-energy electron diffraction and scanning tunneling microscopy) to investigate and to map the energy landscape of C₆₀ thin films in both space and time (with ångström and femto- to picosecond resolution, respectively). This thesis presents the time-evolution of excitonic states and their angular dispersion. Our results include what is, to our knowledge, the first angular-resolved data within the first Brillouin zone of the excited states of C₆₀.
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The electron-phonon interaction is ubiquitous in crystalline materials, leaving fingerprints on both physical and electronic properties. In the study of materials, angle-resolved photoemission spectroscopy (ARPES) can uniquely access the electronic band structure, and the electron interactions encoded within. However, disentangling the contributions from different degrees of freedom -- such as electron-electron and electron-phonon – can be very challenging. Extension of ARPES into the time domain via pump-probe spectroscopy allows one to access the electronic structure on an ultrafast timescale: this is advantageous as the intertwined interactions in equilibrium become separated in the time domain. In this thesis, we use time-resolved (TR)-ARPES to study the electron-phonon interaction in graphite. Specifically, we observe spectral features arising from the photoexcitation of electrons from the valence band to the conduction band, followed by quantized energy-loss processes corresponding to the emission of strongly-coupled optical phonons. The transfer of spectral weight from an identifiable initial state to a final state is the direct manifestation of a microscopic two-body scattering process from which we can extract the mode-projected electron-phonon matrix element. The spectral features observed in this study arise from the non-thermal (i.e. non-Fermi-Dirac) occupation of electrons. We use Boltzmann simulations to map out various regimes in graphite where non-thermal features arise. These non-thermal signatures are not unique to graphite but are ubiquitous in pump-probe experiments and intrinsically tied to the dominant scattering processes, their timescales, and corresponding bottlenecks. Our results were made possible by a custom-built state-of-the-art laser source featuring cavity-enhanced high-harmonic generation. The source has three key features: First, high photon energies capable of mapping the whole Brillouin zone of materials; second, a high repetition rate that minimizes the space-charge effect; and last, a balanced time and energy resolution capable of studying subtle spectral features. All three elements were crucial in discerning the spectral features related to electron-phonon scattering in graphite, the first observation of its kind. With these results, we demonstrate that the maturation of high-harmonic sources can now offer a tunable table-top source with unprecedented intensity, repetition rate and resolution.
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This dissertation presents high precision two-photon xenon spectroscopy of the 5p₅(²P₃/₂)6p²[3/2]₂ ⟵5p⁶(¹S₀) transition. Specific attention is paid to the F = 3/2 hyperfinelevel of ¹²⁹Xe, motivated by the new experiment at TRIUMF to measure the electric dipole momentof the neutron. A non-zero value of the nEDM would partially confirm the existence of the baryonasymmetry in the universe as predicted by theories beyond the standard model. To achieve this measurement, ¹²⁹Xe is proposed for use in a cohabiting ¹⁹⁹Hg/¹²⁹Xe optical comagnetometer. To date,no laser system has existed to probe the specific xenon transition required for this measurement.We developed a novel continuous-wave 252.4 nm ultra-violet (UV) laser system with the powerand precision to selectively probe the hyperfine levels of ¹²⁹Xe. Using this laser, we observed thefirst high resolution two-photon transition spectrum of xenon, which is comprised of ten transitionpeaks across the six most abundant isotopes, including the hyperfine levels of the ¹²⁹Xe and ¹³¹Xe.Detailed analysis of this spectrum revealed the hyperfine constants of the 5p₅(²P₃/₂)6p²[3/2]₂ excitedstate and other constants relating to the isotope shift. Furthermore, we describe initial observationsinto the pressure dependencies of the spectral lineshape from 15–980 mTorr. The ¹²⁹Xepressure in the nEDM experiment is limited to 3 mTorr, making it essential to characterize the xenonsignal at low pressures to maximize comagnetometer sensitivity. Intriguingly, our results suggestthat ¹²⁹Xe qualitatively exhibits nonlinear pressure broadening at low pressure (
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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.
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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
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
We detail the steps towards a tunable vacuum ultraviolet light source for time-– and angle–resolved photo emission spectroscopy (TR-ARPES); specifically, the construction and characterization of a ytterbium–fiber based pump source, and a nonlinear broadening and compression stage using gas–filled hollow–core fiber (HCF). The ultimate goal is generating sub–30 fs pulses with photon energies between 5 eV and 11 eV via resonant dispersive wave(RDW) generation in a succeeding HCF. The interim goal discussed herein is producing 30 fs, infrared (∼ 1 µm), pulses with sufficient energy (µW level) for RDW generation. Using second harmonic generation (SHG) frequency resolved optical gating (FROG) we currently measure 174 fs, 4.0 µJ pulses. We will provide explanations for the shortcoming and suggest several possible solutions to address it.
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Topological insulators are widely studied class of materials with fascinating electronic properties and a great potential for application to spintronics. The property that makes these materials interesting is the existence of linearly dispersing electronic states on the surface, occurring due to strong spin-orbit coupling and time-reversal symmetry. The electrons in these topological surface states (TSSs) exhibit helical spin-texture and are forbidden from backscattering due to time-reversal symmetry. Bi₂Se₃ is one such topological insulator that hosts two TSSs: one near the Fermi level and another in the unoccupied states. The optical excitation of the electrons from the occupied TSS to the unoccupied TSS has been an interesting avenue for exciting spin-polarized surface currents in Bi₂Se₃. Several transport experiments have demonstrated the current generation by optical means, confirming the idea, and ARPES experiments have helped in understanding the mechanism of optical excitation. However, several questions remain unanswered including the involvement of the bulk states in the generation scheme, the symmetry of the optical excitation, and therefore the directionality of the injected currents and possibilities to improve the efficiency and selectivity of these excitations. Identifying and controlling these optical transitions, understanding their nature, and isolating the TSS-TSS transition to injected current while suppressing the contribution from the bulk is the essence of the presented work. In particular, we perform a pump-probe ARPES experiment on Bi₂Se₃ single crystals using a 1.55 eV circularly-polarized pump and 6.2eV linearly-polarized probe pulse with two different in-plane orientations of the sample. The pump-induced circular dichroism maps were analyzed at different binding energies to deduce the direction of injected currents and the type of underlying optical transitions associated with them: bulk-bulk, bulk-TSS, or TSS-TSS. We classify the optical transitions into three categories based on the symmetry of the bulk and the TSS alone. As an attempt to explain our findings and simulate our experimental results, we also build a tight-binding model using the Chinook software, starting from the density functional theory calculation on Bi₂Se₃. Finally, we give a prescription to calculate the pump-probe intensity and the pump-induced circular dichroism pattern using the resultant tight-binding model.
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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.
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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.
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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.
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