Single shot Positronium annihilation lifetime spectroscopy (SSPALS) is a technique that records 1¡Á¡¼10¡½^7 Positronium(Ps) atoms annihilating into gamma rays in a single shot from a positron accumulator. Here, SSPALS is combined for the first time with ns tunable lasers to study the electronic structure and emission of Ps atoms from solid surfaces. In our experiment the injection of a high density positron pulse into a target surface produces Ps atoms in vacuum which interacts immediately with the pulsed dye laser at 0.16T at 243nm. In our investigation the Ps kinetic energy along the surface of the sample decreased with increasing implantation energies and a lower limit of Ex=42¡À3 meV was discovered. The lower limit of the Ps kinetic energies corresponds to the region in which the Ps de Broglie wavelength approaches the pore diameter¡¯s of the target sample. As a result a classical model can no longer be used to describe the energy loss mechanism. Instead we find the lower limit values in agreement with the quantum confinement model.

In additional studies, the excitation of the 1S-2P transition for Ps was explored in the Paschen Back Regime (high magnetic field) of 2T. This investigation has found that Zeeman mixing of the 2P states is reduced in high magnetic fields. In the event of reduced mixing in the 2P state, which reduces conversion of triplet state Ps atoms into singlet state Ps atoms, this may result in the possibility of laser cooling Ps atoms using multiple pulsed laser events. We also measured the Positronium speed distribution perpendicular to the sample using copper and silicon as our target with time of flight. We used a two step, two photon excitation to the Rydberg states to enable single Rydberg atom annihilation to be detected using a multi-channel plate located 1.7m away from the target surface. We observed the Ps atom¡¯s kinetic energy to have a thermal Maxwell-Boltzmann speed distribution centered around 350,000 m/s, which is non- monoenergetic. This constitutes the first measurement of the kinetic energies perpendicular to the sample and demonstrates the need for further studies of Ps kinetic energies perpendicular and parallel to the sample.

The dielectric spectrum of D₂O between 15 GHz and 2 THz was measured using Terahertz Time-Domain Spectroscopy. The motivation of this work is to gain an understanding of liquid water dynamics on a molecular level. To achieve this, we use a correction to the dielectric response of polar molecules known as the reduced polarization. This correction allows us to relate the macroscopic quantity of the permittivity to the microscopic correlation function in a manner appropriate for polar liquids. Similar to previous studies on H₂O, evidence is shown of correlated and anti-correlated dipole-dipole interactions in liquid D₂O. More interestingly, the spectra also reveal dynamics that could be intimately related to the density anomaly of water.

Silicon is a popular semiconductor in spintronics. Advancements in research into

this material has only happened through contacts. There hasn’t been any research done

by means of optical injection due to its indirect bandgap. In this dissertation, I

demonstrate the optical injection of spin polarized electrons into silicon using a muon

spin relaxation technique that utilizes spin-polarized muons to probe for conduction

electron spin polarization.

When an antimuon, a positive muon, is implanted into a semiconductor, it

captures a conduction electron and forms a muonium atom. The spin of the electron can

either be parallel or antiparallel to the muon spin. If the spin is parallel, the spins are

fixed. But, if the spins are antiparallel, then they are in a superposition of both

configurations; muon spin-up with electron spin-down plus muon spin-down plusv

electron spin-up. This spin flipping is the basis of how the muons can probe for the

conduction electron spin polarization.

The experiment was completed on both n-type and intrinsic silicon to prove the

existence in two different types. The wavelength was scanned over an interval slightly

above the bandgap since there is virtually no absorption at the bandgap. Since silicon is

an indirect bandgap, the photons alone can’t be absorbed into the bottom of the bandgap

due to the momentum shift, so phonons are required to either be absorbed or emitted to

preserve momentum conservation. Due to the low temperature of the experiment,

phonon emission is the only practical path for absorption. Several parameters, including

wavelength, applied magnetic field, and laser power, were varied to find the and analyze

signal. The experiment was carried out over the course of several years, and trips, to the

ISIS pulsed muon source with a successful detection of the conduction electron spin

polarization in both the n-type and intrinsic silicon samples

The world is silicon based technology is about to come to a close as it rapidly approaches the quantum limit. New materials are required for further technological advances. The optical properties of two of these new materials, germanane (GeH) and molybdenum disulfide (MoS2) are studied in this thesis.

Germanane is synthesized using MBE co-deposition, resulting in atomic control of thickness with wafer scale size. Unfortunately, germanane is grown on germanium substrates which are conductive and opaque to light near the band edge. We have developed a large area transfer method using electrochemical delimitation (ie. Bubble transfer) to arbitrary substrates. Germanane films with thickness ranging from 1 nm to 600 nm and areas up to 1 cm2 have been reliably transferred. Characterization by photoluminescence, x-ray diffraction, and energy-dispersive X-ray pectroscopy invidicate that the films quality is preserved after transfer. The optical and electro-optical properties of germanane can now be measured. Temperature dependent photoluminescence excitation spectroscopy (PLE), absorption and photo conductivity show a band edge near 1.9 eV with a large Urbach tail. Photoluminescence (PL) measurements show a broad luminescence that is shifted by 700 meV from the absorption edge. We also observe an increase of the PL intensity of several orders of magnitude

from room temperature to 4 K. Time resolved photoluminescence (TRPL) reveal many time scales ranging from a few ns to a s.

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures

between 175 and 300 K show quenching of almost 80% for an 10% ecrease

in sulfur content.

Positronium (Ps) is an exotic hydrogen-like atom consisting of an electron and its antiparticle—positron. As a pure leptonic atom, positronium is a perfect source to test quantum electrodynamics (QED). The leptonic hydrogen-like structure also makes it an ideal test bench for the proton radius puzzle—the discrepancy in the measurement of proton charge radius using different methods and atoms. Recently, hydrogen spectroscopy has been conducted to relative uncertainties of 10-15. Benefitting from the unique structure of Ps atoms, Ps spectroscopy only needs a precision three orders of magnitude lower for the same level of impact. However, the path to this target is still full of challenges. Due to annihilation, Ps’s lifetime is relatively short. The limited lifetime of Ps significantly increased the challenge for Ps spectroscopy. Excitation of Ps atoms into Rydberg states can effectively extend the lifetime of Ps atoms for detection. However, many other challenges exist, such as the uncertainty in the energy level induced by its transient lifetime.The last positronium 13S1-23S1 spectroscopy was performed by Fee, Mills, Chu, et al. almost thirty years ago. It reached a relative uncertainty of 3.2 ppb, and the measured 13S1-23S1 interval was 1.7σ away from the theoretical calculation. In 2020, Gurungs et al. measured the Ps n=2 fine structure and also found a disagreement with the theoretical value. This paper presents the development of the instrument we built for positronium precision spectroscopy and our current experiments. The apparatus includes an ultrahigh finesse optical interferometer for positronium two-photon excitation, a frequency stabilizing system, a frequency comb for ultra-precise spectroscopy, and pulsed lasers for 2P/2S to Rydberg excitation. We have achieved a positronium 1S-2P excitation with a 7× improvement in yield compared with the last experiment in 2018. Ps 13S1-23S1 spectroscopy experiments are under operation, and we expect a 10-1000 improvement in precision from the previous experiment.

Positronium (Ps) is a purely leptonic atom comprising an electron and its antimatter equivalent, the positron, in a quasi-stable bound state. Due to its fundamental nature, Ps is an ideal test bed for bound-state QED.

Recent high-precision spectroscopic experiments reveal a discrepancy in the measurement of the proton charge radius rp, known as the Proton Charge Radius Puzzle. Spectroscopic measurments carried out on hydrogen and muonic hydrogen, the bound state of a muon and a proton, differ from other scattering and other spectroscopic experiments by 3.3σ. The measurement of rp comes from fitting the resulting measurement of either the 1S-2S interval of hydrogen or the Lamb Shift in muonic hydrogen to theory.

Neither of these atoms are governed purely by quantum electrodynamics (QED) alone as nuclear structure has a role to play. The ratio of the masses of the orbiting particle m to that of the nucleus M is a coefficient in a number of a QED corrections to the energy levels of hydrogen (m/M = 1/1836) and muonic hydrogen (m/M = 207/1836) and reveals the importance of performing a complementary spectroscopic measurement in Ps, where m/M = 1.

The last measurement of the 1S-2S interval was carried out by Fee, Chu, Mills, et al. in 1993 to a precision of 3.2 ppb. The state-of-the-art measurement on hydrogen is now at an uncertainty of 4.2 × 10^−15. While the simplicity of Ps causes it to be appealing to test bound-state QED, its antiparticle-particle nature makes it difficult to work with: the ground state lifetime of the triplet state is 142 ns, and whereas the 2S lifetime in Ps is 1.14 µs, the 2S lifetime in hydrogen is 10^5x longer.

We have designed and constructed an apparatus and experiment to measure the 1S-2S interval in Ps at precision levels that we expect to immediately improve upon the previous measurements by factor of 2x and pave the way for ultimate comparison to the hydrogenic measurements. The apparatus also opens the doors to a new frontier in high-precision spectroscopy: the sub-µs regime.

Van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for next generation scientific and technological advances. While previously realized in 3D ultrathin films where the magnetism is stabilized via substrate-assisted magnetic anisotropy, recent reports have shown intrinsic ferromagnetism at low temperatures (< 60 K) in isolated µm-sized flakes mechanically exfoliated from a bulk single-crystal down to a single-atomic layer. This opens up the possibility to truly study magnetism in free-standing 2D layers without direct effects from the underlying substrate and being intrinsically susceptible to surface effects such as atomic adsorbates, electrostatic gating, and proximity-induced phenomena. This dissertation examines the molecular beam epitaxy (MBE) growth and characterization of new 2D magnets, monolayers of MnSe2 and VSe2, that show ferromagnetic ordering above room temperature. Growth on different substrates and varying the substrate temperature during growth affects the growth mode and morphology of the deposited 2D magnet and also affects the measured magnetization. Direct atomic and magnetic imaging via scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) show stable 2D magnetic layers.

Due to the lack of dangling bonds at the surface of these 2D magnets, applying external epitaxial strain is a challenge. Later in this dissertation, the magnetic, electronic, and structural properties of vdW-layered, Fe-deficient Fe3−xGeTe2 are systematically investigated by

he application of high pressure. Fe3GeTe2 is of particular interest due to its high Curie temperature, Tc, strong perpendicular magnetic anisotropy, and tunable magnetic properties depending on the concentration of Fe and its thickness. Electrical and magneto-transport measurements show a suppression in Tc with an increasing pressure up to 20 GPa. The decrease in Tc is due to the lattice shrinkage from pressurization which leads to a weakening of the exchange interaction. These observations showcase the tunability in vdW magnets via pressure which can complement other external stimuli such as chemical doping, making them candidates for future spintronic, electronic and photonic devices.