Molecular imaging using fluorescence spectroscopy-based techniques is generally inefficient due to the low quantum yield of most naturally occurring biomolecules. Current fluorescence imaging techniques tag these biomolecules chemically or through genetic manipulation, increasing the complexity of the system. A technique capable of imaging these biomolecules without modifying the chromophore and/or its environment could provide vital biometric parameters and unique insights into various biological processes at a molecular level. Pump probe spectroscopy has been used extensively to study the molecular properties of poorly fluorescing biomolecules, because it utilizes the known absorption spectrum of these chromophores. Optical Coherence Tomography (OCT) is an optical imaging modality that harnesses the power of low coherence interferometry to measure the 3-D spatially resolved reflectivity of a tissue sample. We plan to develop a new molecular imaging modality that combines these techniques to provide 3-D, highresolution molecular images of various important biomolecules. The system uses a Fourier domain OCT setup with a modified sample arm that combines the "pump" and "probe" beams. The pump beam drives the molecules from the ground state to excited state and the probe interrogates the population change due to the pump and is detected interferometrically. The pump and the probe beam wavelengths are optimized to maximize absorption at the pump wavelength and maximize the penetration depth at the probe wavelength. The pump-probe delay can be varied to measure the rate at which the excited state repopulates the ground state, i.e., the ground state recovery time. The ground state recovery time varies for different chromophores and can potentially be used to identify different biomolecules. The system was designed and optimized to increase the SNR of the PPOCT signals. It was tested by imaging hemoglobin and melanin samples and yielded encouraging results. Potential applications of imaging hemoglobin using this technique include the mapping of tissue microvasculature and measuring blood-oxygen saturation levels. These applications could be used to identify hypoxic areas in tissue. Melanin imaging can provide means of demarcation of melanoma in various organs such as skin, eye and intestines.

Thesis (MSc (Physics))--University of Stellenbosch, 2009. / An ultrafast pump-probe spectroscopy experiment was developed in order to investigate the fast photoinduced isomerization reaction of the molecule resveratrol. Characteristics of the resveratrol molecule are discussed, including the photoisomerization reaction from trans- to cis-resveratrol. The experimental setup for the conventional spectroscopy measurement was developed and characterized in order to investigate and understand the conventional absorption and uorescence spectroscopy of resveratrol thoroughly. The absorption spectra for both trans- and cis-resveratrol, as well as the uorescence spectra were measured, discussed and explained. This therefore forms a foundation and serves as an initial step to develop a pump-probe spectroscopy experiment for resveratrol. A general overview of ultrafast pump-probe spectroscopy is presented, as well as an explanation of the nal developed experimental setup. The principles and characteristics of the chirped pulse ampli cation (CPA) femtosecond laser source and the tunable noncollinear optical parametric ampli er (NOPA) employed as the pump pulse are discussed. The process of white light continuum (WLC) generation was investigated to utilize as the ultrashort probe pulse. Two white light continuum generation experimental setups were developed and characterized for WLC generation in a transparent medium with the fundamental CPA laser light at 775 nm (in sapphire) and with the second harmonic (SH) of the CPA light at 387 nm (in quartz). A spectrometer was designed, built and characterized in conjuction with a line focus, for simultaneous measurement of the absorption in the pumped, unpumped and reference regions in the sample. In this way the photoisomerization of resveratrol could be measured with temporal resolution as a transient absorption signal. A 420 μg/ml resveratrol solution in ethanol was investigated in this pump-probe spectroscopy experiment and the results obtained are discussed accordingly.

In the course of this thesis gold nanoparticle/polyelectrolyte multilayer structures were prepared, characterized, and investigated according to their static and ultrafast optical properties. Using the dip-coating or spin-coating layer-by-layer deposition method, gold-nanoparticle layers were embedded in a polyelectrolyte environment with high structural perfection. Typical structures exhibit four repetition units, each consisting of one gold-particle layer and ten double layers of polyelectrolyte (cationic+anionic polyelectrolyte). The structures were characterized by X-ray reflectivity measurements, which reveal Bragg peaks up to the seventh order, evidencing the high stratication of the particle layers. In the same measurements pronounced Kiessig fringes were observed, which indicate a low global roughness of the samples. Atomic force microscopy (AFM) images veried this low roughness, which results from the high smoothing capabilities of polyelectrolyte layers. This smoothing effect facilitates the fabrication of stratified nanoparticle/polyelectrolyte multilayer structures, which were nicely illustrated in a transmission electron microscopy image. The samples' optical properties were investigated by static spectroscopic measurements in the visible and UV range. The measurements revealed a frequency shift of the reflectance and of the plasmon absorption band, depending on the thickness of the polyelectrolyte layers that cover a nanoparticle layer. When the covering layer becomes thicker than the particle interaction range, the absorption spectrum becomes independent of the polymer thickness. However, the reflectance spectrum continues shifting to lower frequencies (even for large thicknesses). The range of plasmon interaction was determined to be in the order of the particle diameter for 10 nm, 20 nm, and 150 nm particles. The transient broadband complex dielectric function of a multilayer structure was determined experimentally by ultrafast pump-probe spectroscopy. This was achieved by simultaneous measurements of the changes in the reflectance and transmittance of the excited sample over a broad spectral range. The changes in the real and imaginary parts of the dielectric function were directly deduced from the measured data by using a recursive formalism based on the Fresnel equations. This method can be applied to a broad range of nanoparticle systems where experimental data on the transient dielectric response are rare. This complete experimental approach serves as a test ground for modeling the dielectric function of a nanoparticle compound structure upon laser excitation. / Im Rahmen dieser Arbeit wurden Gold-Nanopartikel/Polyelektrolyt Multischichtstrukturen hergestellt, strukturell charakterisiert und bezüglich ihrer optischen Eigenschaften sowohl statisch als auch zeitaufgelöst analysiert. Die Strukturen wurden mithilfe der Dip-coating oder der Spin-coating Methode hergestellt. Beide Methoden ermöglichen das Einbetten einzelner Partikellagen in eine Polyelektrolytumgebung. Typische Strukturen in dieser Arbeit bestehen aus vier Wiederholeinheiten, wobei jede aus einer Nanopartikelschicht und zehn Polyelektrolyt-Doppellagen (kationisches und anionisches Polyelektrolyt) zusammengesetzt ist. Die Stratizierung der Gold-Nanopartikellagen wurde mittels Röntgenreflektometrie-Messungen im Kleinwinkelbereich nachgewiesen, welche Bragg Reflexionen bis zur siebten Ordnung aufzeigen. Das ausgeprägte Kiessig Interferenzmuster dieser Messungen weist zudem auf eine geringe globale Rauheit hin, die durch Oberflächenanalysen mit einem Rasterkraftmikroskop bestätigt werden konnte. Diese geringe Rauheit resultiert aus den glättenden Eigenschaften der Polyelektrolyte, die die Herstellung von Multilagensystemen mit mehreren Partikellagen erst ermöglichen. Die Aufnahme eines Transmissionselektronenmikroskops veranschaulicht eindrucksvoll die Anordnung der Partikel in einzelne Schichten. Durch photospektroskopische Messungen wurden die optischen Eigenschaften der Strukturen im UV- und sichtbaren Bereich untersucht. Beispielsweise wird eine Verschiebung und Verstärkung der Plasmonenresonanz beobachtet, wenn eine Goldnanopartikellage mit transparenten Polyelektrolyten beschichtet wird. Erst wenn die bedeckende Schicht dicker als die Reichweite der Plasmonen wird, bleibt die Absorption konstant. Die spektrale Reflektivität jedoch ändert sich auch mit jeder weiteren adsorbierten Polyelektrolytschicht. Die Reichweite der Plasmonenresonanz konnte auf diese Art für Partikel der Größe 10 nm, 20 nm und 150 nm bestimmt werden. Die Ergebnisse wurden im Kontext einer Effektiven Mediums Theorie diskutiert. Die komplexe dielektrische Funktion einer Multilagenstruktur wurde zeitabhängig nach Laserpulsanregung für einen breiten spektralen Bereich bestimmt. Dazu wurden zuerst die Änderungen der Reflektivität und Transmittivität simultan mittels der Pump-Probe (Anrege-Abtast) Spektroskopie gemessen. Anschließend wurden aus diesen Daten, mithilfe eines Formalismus, der auf den Fresnelschen Formeln basiert, die Änderungen im Real- und Imaginärteil der dielektrischen Funktion ermittelt. Diese Methode eignet sich zur Bestimmung der transienten dielektrischen Funktion einer Vielzahl von Nanopartikelsystemen. Der rein experimentelle Ansatz ermöglicht es, effektive Medien Theorien und Simulationen der dielektrischen Funktion nach Laserpulsanregung zu überprüfen.

Transient absorption spectroscopy was used to study light induced electron transfer processes in Type 1 photosynthetic reaction centers. Flash induced absorption changes were probed at 800, 703 and 487 nm, and on multiple timescales from nanoseconds to tens of milliseconds. Both wild type and menB mutant photosystem I reaction centers from the cyanobacterium Synechocystis sp. PCC 6803 were studied. Photosystem I reaction centers from the green algae Chlamydomonas reinhardtii, and the newly discovered chlorophyll-d containing organism Acaryochloris marina, were also studied. The flash induced absorption changes obtained for menB mutant photosystem I reaction centers are distinguishable from wild type at 800 nm. MenB mutant photosystem I reaction centers displays a large amplitude decay phase with lifetime of ~50 ns which is absent in wild type photosystem I reaction centers. It is hypothesized that this ~50 ns phase is due to the formation of the triplet state of primary electron donor.

Controlling the quantum states of charge (excitons) or spin-polarized carriers in semiconductor quantum dots (QDs) has been the focus of a considerable research effort in recent years due to the strong promise of using this approach to develop solid state quantum computing hardware. The long-term scalability of this type of quantum computing architecture is enhanced by the use of QDs emitting in the telecom band, which would exploit the established photonic infrastructure. This thesis reports the use of all optical infrared experimental techniques to control exciton dynamics in two different QD samples consisting of InAs/GaAs QDs and InAs/InP QDs within a planar microcavity. An infrared quantum control apparatus was developed and used to apply optimized shaping masks to ultrafast pulses from an optical parametric oscillator. Pulse shaping protocols designed to execute a two-qubit controlled-rotation operation on an individual semiconductor QD were demonstrated and characterized. The quantum control apparatus was then implemented in simultaneous single qubit rotations using two uncoupled, distant InAs/GaAs QDs. These optimal control experiments demonstrated high fidelity optical manipulation of exciton states in the two QDs using a single broadband laser pulse, representing a step forward on the path to a scalable QD architecture and showcasing the power of pulse shaping techniques for quantum control on solid state qubits. As an alternative to single QDs, which have very low optical signals, subsets of QDs within an ensemble can be used in quantum computing applications. To investigate the mediation of inhomogeneities in a QD ensemble, pump-probe experiments were performed on InAs/InP QDs within a dielectric Bragg stack microcavity. Two different excitation geometries showed that the angle dependence of the microcavity transmission allowed for the spectral selection of QD subsets with transition energies resonant with the cavity mode. The microcavity mitigated inhomogeneities in the ensemble while providing a basis for addressing QD subsets which could be used as distinguishable quantum bits. This thesis work shows significant advances towards an optical computing architecture using quantum states in semiconductor QDs.

The study of photovoltaic materials is important so as to develop new solar energy technologies: in particular, quantum-confined semiconductors could offer increased quantum efficiencies at a much lower manufacture cost. This thesis contains results from a number of pump-probe experiments designed to probe the carrier dynamics in bulk and quantum-confined photovoltaics. A THz time-domain spectrometer was designed, built and commissioned. The THz refractive indices and absorption coefficients of toluene and hexane were determined, and the spectrometer was benchmarked using a photoexcited GaAs wafer. Results are presented of time-resolved THz spectroscopy of photoexcited bulk InP as a function of laser excitation wavelength. These data were used to extract the quantum efficiency of bulk InP in order to compare with recent results for InP quantum dots. The quantum efficiency in quantum dots increases when the incident photon energy is at least twice the band gap energy, whereasthe efficiency of the bulk material is found to decrease. This is because of surface recombination, and these measurements therefore verify the potential superiority of quantum dot materials over bulk materials for use in solar energy applications. Initial measurements of quantum dots using THz spectroscopy highlighted the various experimental challenges involved and the upgrades required to study such samples in the future.The time-dependence of the photoinduced surface photovoltage (SPV) in Si was studied on nanosecond timescales by synchronizing an ultrafast laser system to a synchrotron radiation source (the SRS at Daresbury, UK), and measuring the resulting shift in the photoelectron spectrum. The equilibrium band bending was determined, and the decay of the SPV was attributed to the recombination of charge carriers across the band gap. Results are presented for the SPV in bulk ZnO and for PbS quantum dot chemically attached to ZnO. The fact that the PbS quantum dots were chemically attached to the surface without becoming oxidized was verified using X-ray photoelectron spectroscopy (XPS). The changes caused by photoexcitation occur on much longer timescales in ZnO than Si (sub-milliseconds rather than nanoseconds), and these timescales were conveniently accessed using the time-resolved XPS facility at the TEMPO beamline at Synchrotron SOLEIL (Paris, France). This is due to oxygen adsorption and desorption processes at the ZnO surface affectingthe transfer of charge carriers. The addition of PbS quantum dots to the ZnO surface was found to increase the speed of this charge transfer due to injection of carriers directly from the PbS quantum dot to the bulk ZnO conduction band.

Insight into the nanoscale carrier transport in the rapidly developing class of solutionprocessed semiconductors known as metal halide perovskites is the focal point for these studies. Further advancement in fundamentally understanding photophysical processes associated with charge carrier transport is needed to realize the true potential of perovskites for photovoltaic applications. In this work, we study photogenerated carrier transport to understand the underlying transport behavior of the material on the 10s to 100s nanometer lengthscales. To study these processes, we employ a temporally-resolved and spatially-resolved technique, known as transient absorption microscopy, to elucidate the charge carrier dynamics and propagation associated with metal halide perovskites. This technique provides a simultaneous high temporal resolution (200 fs) and spatial resolution (50 nm) to allow for direct visualization of charge carrier migration on the nanometer length scale. There are many obstacles these carriers encounter between photogeneration and charge collection such as morphological effects (grain boundaries) and carrier interactions (scattering processes). We investigate carrier transport on the nanoscale to understand how morphological effects influence the materials transport behavior. Morphological defects such as voids and grain boundaries are inherently small and traditionally difficult to study directly. Further, because carrier cooling takes place on an ultrafast time scale (fs to ps), the combined spatial and temporal resolution is necessary for direct probing of hot (non-equilibrium) carrier transport. Here we investigate a variety of ways to enhance carrier transport lengthscales by studying how non-equilibrium carriers propagate throughout the material, as well as, carrier cooling mechanisms to extend the non-equilibrium regime. For optoelectronic devices based on polycrystalline semiconducting thin films, grain boundaries are important to consider since solution-based processing results in the formation of well-defined grains. In Chapter 3, we investigate equilibrium carrier transport in metal halide perovskite thin films that are created via the highly desired solution processing method. Carrier transport across grain boundaries is an important process in defining efficiency due to the literary discrepancies on whether the grains limit carrier transport or not. In this work, we employ transient absorption microscopy to directly measure carrier transport within and across the boundaries. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation. The results of this work provide insight into why this defect tolerant material performs so well. Photovoltaic performance (power conversion efficiency) is governed by the ShockleyQueisser limit which can be overcame if hot carriers can be harvested before they thermalize. To convert sunlight to usable electricity, the photogenerated charge carriers need to migrate long distances and or live long enough to be collected. It is unclear whether these hot carriers can migrate a long enough distance for efficient collection. In Chapter 4, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy. This work demonstrates three distinct transport regimes. (i) Quasiballistic transport, (ii) nonequilibrium transport, and (iii) diffusive transport. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers of transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites to ultimately overcome the Shockley-Queisser limit. In the next work, we investigated a way to extend non-equilibrium carrier lifetime, which ultimately corresponds to an accelerated carrier transport. From the knowledge of the hot carrier transport work, we showed a proof of concept that the excess kinetic energy corresponds to long range carrier transport. To further develop the idea of harvesting hot carriers, one must investigate a way to make the carriers stay hot for a longer period (i.e. cool down slower). In Chapter 5, we slow down the cooling of hot carriers via a phonon bottleneck, which points toward the potential to overcome the Shockley-Queisser limit. Open questions remain on whether the high optical phonon density from the bottleneck impedes the transport of these hot carriers. We show a direct visualization of hot carrier transport in the phonon bottleneck regime in both single crystalline and polycrystalline lead halide perovskites, more specifically, a relatively new class of alkali metal doped perovskites (RbCsMAFA), which has one of the highest power conversion efficiencies. Remarkably, hot carrier diffusion is enhanced by the presence of a phonon bottleneck, the exact opposite from what is observed in conventional semiconductors such as GaAs. These results showcase the unique aspects of hot carrier transport in hybrid perovskites and suggest even larger potential for hot carrier devices than previously envisioned by the initial results presented in Chapter 4. The final chapter will be divided into two sections, as we summarize and highlight our collaborative efforts towards homogenization of carrier dynamics via doping perovskites with alkali metals and our work on two-dimensional hybrid quantum well perovskites. Further studies on the champion solar cell (RbCsMAFA) were performed to elucidate the role inorganic cations play in this material. By employing transient absorption microscopy, we show that alkali metals Rb+ and Cs+ are responsible for inducing a more homogenous halide (Iand Br- ) distribution, despite the partial incorporation into the perovskite lattice. This translates into improved electronic dynamics, including fluorescence lifetimes above 3 µs and homogenous carrier dynamics, which was visualized by ultrafast microscopy. Additionally, there is an improvement in photovoltaic device performance. We find that while Cs cations tend to distribute homogenously across the perovskite grain, Rb and K cations tend to phase segregate at precursor concentrations as low as 1%. These precipitates have a counter-productive effect on the solar cell, acting as recombination centers in the device, as argued from electron beam-induced current measurements. Remarkably, the high concentration of Rb and Cs agglomerations do not affect the open-circuit voltage, average lifetimes, and photoluminescence distribution, further indicating the perovskite’s notorious defect tolerance. A new class of high-quality two dimensional organic-inorganic hybrid perovskite quantum wells with tunable structures and band alignments was studied. By tuning the functionality of the material, the strong self-aggregation of the conjugated organic molecules can be suppressed, and 2D organic-halide perovskite superlattice crystals and thin films can be easily obtained via onestep solution-processing. We observe energy transfer and charge transfer between adjacent organic and inorganic layers, which is extremely fast and efficient (as revealed by ultrafast spectroscopy characterizations). Remarkably, these 2D hybrid perovskite superlattices are stable, due to the protection of the bulky hydrophobic organic groups. This is a huge step towards the practicality of using perovskites for optoelectronics, since stability is always a huge concern with water-sensitive materials. The molecularly engineered 2D semiconductors are on par with III-V quantum wells and are promising for next-generation electronics, optoelectronics, and photonics.

Global energy demands have escalated over the past few decades, creating a necessity for alternative energy sources. Solar technologies inspired by the primary solar energy storing process known on earth, photosynthesis, have subsequently gained popularity. The natural photosynthetic apparatus comprises a network of membrane-bound pigment-protein complexes, with the main plant light-harvesting complex (LHCII) consisting of chlorophyll (Chl) and carotenoid (Car) pigments. Electronic excitation energy transfer (ET) of the harvested energy takes place amongst these pigments on ultrafast timescales. This energy is funnelled towards a photosynthetic reaction centre where charge separation is achieved, creating a Biobattery, which powers the subsequent manufacture of energy-rich chemical compounds for photosynthetic activity. Transient absorption pump-probe spectroscopy has proven to be a useful technique for monitoring the evolution of the excited state dynamics, such as electronic transitions and excitation ET amongst Car and Chl pigments of LHCII trimers isolated from spinach leaves. This method was utilized to probe samples excited under four different conditions: at pump excitation wavelengths (𝜆𝑒𝑥) of 489 nm (preferentially exciting Cars Lutein1 and Neoxanthin) and 506 nm (targeting Cars Lutein2 and Violaxanthin), each with an intensity of either 800 nJ/pulse (relatively high) or 500 nJ/pulse (comparatively low). A global analysis was applied to each dataset using the robust, open-source Glotaran software, from which three kinetic decay lifetimes for the various processes were extracted. General spectral observations encompassed a negative pump ground state bleach (GSB) at each 𝜆𝑒𝑥; negative Chl b and Chl a GSBs, superimposed with negative stimulated emission (SE) signals; and a positive excited state absorption (ESA) band. The first lifetime of a few picoseconds corresponded mainly to Car-S2 depopulation, resulting either from energy relaxation towards Car-S1, or ET to Chls. Small, but distinct Chl b signals of less than 3 mOD were also detected on this timescale. The second lifetime, which is between 10 and 12 ps, was characteristic to the Lutein Car-S1 lifetime, mainly depicting Car-S1 ET to Chl a. The third lifetime, which extended from ~200 ps to the nanosecond timescale, was attributed to Chl a fluorescence. The 𝜆𝑒𝑥 of 489 nm directly excites the Chl Soret region, whilst excitation at 506 nm shows a pump intensity-dependence. Laser pulse photon density values were ~1014 photons·cm-2·pulse-1 for these datasets. Singlet-singlet annihilation calculations performed on the samples excited at 506 nm provided low annihilation probabilities of 9.0% and 11.5% for a low and high pump intensity, respectively, limiting the possibility of sample photobleaching. Optimization and redevelopment of the experimental setup significantly improved both the data quality and various recorded parameters, concluding that pump-probe spectroscopy was successful on the prepared LHCII trimers. Results acquired and calculations performed correlated with literature, where minimal changes were noticed in the timescales and ET pathways. The robustness of plant systems was confirmed through both excitation-wavelength and intensity dependence. This work paves the way for advanced studies on the role Cars play in non-photochemical quenching (NPQ), a self-protection mechanism of plants against over-illumination; and for the tailoring of artificial light-harvesting antennas based on research conducted on their natural counterparts. / Globale energievereistes het oor die afgelope paar dekades toegeneem, wat die ontwikkeling van alternatiewe energiebronne noodsaaklik maak. Sontegnologieë, geïnspireer deur die primêre sonenergiebergingsproses op aarde, fotosintese, het daarom gewild geword. Die natuurlike fotosintetiese apparaat bestaan uit 'n netwerk van membraangebonde pigment-proteïenkomplekse, met die hoof ligversamelingskompleks in plante (LHCII) wat bestaan uit chlorofil- (Chl) en karotenoïed- (Car) pigmente. Die energie wat deur die pigmente geabsorbeer word, word tussen elektroniese opgewekte toestande op verskillende pigmente op ultravinnige tydskale oorgedra. Hierdie energie word na ʼn fotosintetiese reaksiesentrum gekanaliseer, waar 'n ladingskeiding geïnduseer word en 'n Biobattery sodoende geskep word. Die energie wat in dié battery gestoor is, word gebruik om energieryke chemiese verbindings te vervaardig — wat as brandstof vir die plant dien om sy lewensfunksies te verrig. Tydopgeloste-absorpsie-pomp-tasting-spektroskopie is 'n nuttige tegniek om die dinamika tussen opgewekte toestande te volg. ‘n Voorbeeld van sulke dinamika is die elektroniese opwekking en energie-oordrag tussen die Car- en Chl-pigmente van geïsoleerde LHCII-trimere in spinasieblare. Hierdie metode is gebruik om monsters onder vier verskillende toestande te ondersoek by pompgolflengtes (𝜆𝑒𝑥) van 489 nm (waar hoofsaaklik die Cars Luteïne1 en Neoksantine opgewek word) en 506 nm (vir Cars Luteïne2 en Violaksantine), en pompenergieë van ‘n relatief hoë 800 nJ/puls, of 500 nJ/puls vir elke golflengte. / Dissertation (MSc)--University of Pretoria, 2017. / National Research Foundation (NRF) / Physics / MSc / Unrestricted

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