David Jones

The success of organic photovoltaic (OPV) devices is directly related to the ability of excitations to propagate through the semiconductor and for excitons to dissociate into free charge carriers before they recombine. Some interfaces have been shown to aid in efficient exciton dissociation while others, like grain boundaries, can lead to fast recombination. Understanding the processes that occur at interfaces in these types of materials is essential for efficient device design. In this thesis, we investigate the role of two types of interfaces on excitons in the model OPV system C₆₀/Au(111). First, we develop a two-stage epitaxial growth method for well-ordered thin films of C₆₀ on Au(111). Compared to the traditional single-stage growth methods, our recipe consistently generates thin films with an extremely high degree of long-range orientational order. Next, the electronic structure at the C₆₀-Au interface are investigated using angle-resolved photoemission spectroscopy (ARPES). The Fermi surface reveals a partially filled hybridized Au-C₆₀ LUMO state dispersing across the Fermi level.The dynamics and dispersion of the lowest lying charge transfer excitonic state in thin films of C₆₀/Au(111) are investigated by time-resolved ARPES. We find a momentum-dependent reduction in the lifetime of the excitonic state that appears to be correlated with the position of the interfacial states. The dispersion shows a combination of electronic interfacial states and excitonic signatures. Together, the data implies exciton dissociation is a momentum-dependent process at the C₆₀-Au interface.Finally, we investigate the effect of a small amount of additional rotational domains present in the film - even when barely measurable via low-energy electron diffraction and ARPES - on the lifetime of the excitonic state. Using scanning tunnelling microscopy and spectroscopy, we find the grain boundaries are expected to act as exciton funnels, increasing the local exciton density at the grain boundaries and therefore the films' susceptibility to exciton-exciton annihilation. These results highlight the importance of an extremely high degree of order required to measure intrinsic exciton lifetimes, reinforcing the importance of our two-stage growth recipe. Moreover, the results suggest that disorder induced exciton quenching may play a role in limiting the efficiency of organic optoelectronics.

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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)6p²₂ ⟵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)6p²₂ 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 (200 mTorr) - aphenomenon reminiscent of helium, neon and krypton. However, further investigation is requiredto fully characterize the pressure broadening effects in ¹²⁹Xe.Overall, these results define the expected signal lineshape and relative transition frequency tothe F = 3/2 hyperfine level of ¹²⁹Xe for precision laser tuning. Collectively, this work contributesto optical magnetometry in nEDM experiments, as well as to precision spectroscopy and theories ofatom-atom interactions.

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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 1.2% over 100 Hz - 100 kHz bandwidth. In comparison to other laser based HHG systems, while the EUV flux is similar, the cumulative RMS noise is an order of magnitude lower.

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