The mid-infrared (MIR) system presented in the thesis is based on a sub-100-fs erbium-doped fiber laser operating at 1.55 µm. The output of the laser is split in two, each arm seeding an erbium-doped fiber amplifier. The output of the first amplifier is sent to a grating-based stretcher to be stretched to 50 ps before seeding the optical parametric chirped-pulse amplifier (OPCPA). The output of the second amplifier is coupled to a highly nonlinear fiber to generate the 1 µm needed to seed the a neodymium-doped yttrium lithium fluoride (Nd:YLF) system. This work represents the first time this synchronization scheme is used, and the timing jitter between the two arms at the OPCPA is reduced to 333 fs. The pump laser for the OPCPA is a regenerative amplifier producing 1.6 W followed by a double-pass amplifier, for a final output power of 2.5 W at 1 kHz. Etalons were inserted into the cavity of the regenerative amplifier to stretch the pulses to 50 ps The OPCPA consists of two potassium titanyl arsenate crystals in a noncollinear configuration. With three passes, the gain is 3.8 · 10 6 . Using a grating compressor, the pulse duration is reduced to 140 fs, with a power of 300 mW. Because of the reduction of the timing jitter, the amplitude stability is 1 %, which is a great improvement compare to existing systems. To generate ultrafast light in the MIR, an optical parametric amplifier is used, pumped ii by the output of the OPCPA and seeded with its 3-µm idler. Two crystals were tested, both in a single-pass configuration. For the first crystal, a 4-mm thick silver thiogallate, an efficiency of 7.4 % was reached, with 8.76 mW in the signal and 7.2 mW in the idler. For the second crystal, a 2-mm thick lithium gallium selenide, the efficiency was higher, reaching 10.8 %. The power for the signal was 11.5 mW, and for the idler, 11.11 mW. Using this new scheme, energies on par with current systems are achieved with much higher efficiencies.

The mid-infrared (MIR) system presented in the thesis is based on a sub-100-fs erbium-doped fiber laser operating at 1.55 µm. The output of the laser is split in two, each arm seeding an erbium-doped fiber amplifier. The output of the first amplifier is sent to a grating-based stretcher to be stretched to 50 ps before seeding the optical parametric chirped-pulse amplifier (OPCPA). The output of the second amplifier is coupled to a highly nonlinear fiber to generate the 1 µm needed to seed the a neodymium-doped yttrium lithium fluoride (Nd:YLF) system. This work represents the first time this synchronization scheme is used, and the timing jitter between the two arms at the OPCPA is reduced to 333 fs. The pump laser for the OPCPA is a regenerative amplifier producing 1.6 W followed by a double-pass amplifier, for a final output power of 2.5 W at 1 kHz. Etalons were inserted into the cavity of the regenerative amplifier to stretch the pulses to 50 ps The OPCPA consists of two potassium titanyl arsenate crystals in a noncollinear configuration. With three passes, the gain is 3.8 · 10 6 . Using a grating compressor, the pulse duration is reduced to 140 fs, with a power of 300 mW. Because of the reduction of the timing jitter, the amplitude stability is 1 %, which is a great improvement compare to existing systems. To generate ultrafast light in the MIR, an optical parametric amplifier is used, pumped ii by the output of the OPCPA and seeded with its 3-µm idler. Two crystals were tested, both in a single-pass configuration. For the first crystal, a 4-mm thick silver thiogallate, an efficiency of 7.4 % was reached, with 8.76 mW in the signal and 7.2 mW in the idler. For the second crystal, a 2-mm thick lithium gallium selenide, the efficiency was higher, reaching 10.8 %. The power for the signal was 11.5 mW, and for the idler, 11.11 mW. Using this new scheme, energies on par with current systems are achieved with much higher efficiencies.

Mode-locked fiber lasers have attracted significant scientific and commercial interest since they offer a compact and highly stable platform with straightforward operation for exploiting ultrafast and nonlinear phenomena. They have enabled a vast range of applications that span from distinct disciplines such as medical diagnostics, molecular spectroscopy, and high-power precise mechanical cutting, to optical metrology. Various gain media have been utilized to achieve laser emission at different wavelengths. We have developed unique thulium/holmium (Tm/Ho) doped mode-locked fiber laser systems to address the needs of low-noise ultrafast optical sources in the wavelength vicinity of 2 μm at higher repetition rates. Since the 2 μm wavelength regime has recently attracted more attention with the emergence of thulium gain fibers, the rich underlying cavity dynamics, novel pulse operation regimes and nonlinear phenomena in compact fiber configurations have not been fully explored yet. In this thesis, research is conducted on novel Tm fiber laser cavity configurations and on the formation of unique, polarization-based pulsing regimes. Particularly, this research is focused on the exploration of novel ultrafast and nonlinear phenomena, and the development of optical sources emitting unprecedented ultrafast pulse trains beyond conventional equal-intensity distribution using Tm/Ho doped gain media. The research presented features four main results: 1) development of a high repetition rate and low-noise Tm/Ho doped mode-locked fiber laser platform as an attractive optical source for a wide variety of applications 2) investigation of a novel mode-locked state in which the ultrafast pulse train is composed of co-generated, consecutive, equal intensity and orthogonally polarized pulses in order to achieve dual RF comb generation for dual-comb spectroscopy applications, 3) exploration of controllable ultrafast waveform generation utilizing vector soliton and harmonic mode-locking mechanisms for optical telecommunication applications, and 4) demonstration of unique transitional mode-locked states showing exceptional features such as powerful irregular bursts of ultrafast pulses and rogue wave behavior without damaging the laser elements. The aim of these projects has been to explore the novel optical properties of Tm/Ho co-doped fiber lasers in order to achieve advanced functionalities in commonly practiced applications such as telecommunication, metrology and spectroscopic applications. / 2019-10-22T00:00:00Z

Deoxyribonucleic acid (DNA) forms the basis of all known living organisms. Despite the essential role played by DNA, its dynamic system and functional behaviour are still not completely understood. The work presented in this thesis aims to explore the structural dynamics of DNA systems, using fluorescence-based approaches, and to attempt to develop a technique for the measurement of fluorescence decays of biological molecules on the ultrafast (femtosecond) timescale. Absorption of UV radiation by DNA is known to lead to mutations and damage to DNA structure and functionality. For the majority of absorbed photons, the excitation energy dissipates harmlessly as heat, but in some instances this energy transfers to regions of DNA that are more susceptible to damage. 2-Aminopurine (2AP), a fluorescent analogue of the native DNA base adenine, can be incorporated into DNA with minimal perturbation to the DNA structure, and can be used to investigate inter-base electronic energy transfer. By selectively exciting the native DNA base in 2AP-containing dinucleotides and utilising 2AP fluorescence as an energy acceptor, the mechanism of electronic energy transfer has been investigated. Analysis of the resulting fluorescence lifetimes of 2AP has revealed that energy transfer preferentially excites conformations in which the bases are highly stacked, and the fluorescence of 2AP is highly quenched. This has led to a re-evaluation of energy transfer efficiencies between the natural bases and 2AP, and has shown that transfer efficiencies cannot be determined correctly from steady-state fluorescence measurements. To investigate the influence of base dynamics on the quenching of 2AP fluorescence in DNA, time-resolved fluorescence measurements were carried out on 2AP-containing systems in frozen solution at 77 K. These studies included dinucleotides, single–strand oligonucleotides and their corresponding duplexes. In all cases, comparison of the fluorescence decay parameters measured at room temperature with those measured at 77 K showed that elimination of base dynamics prevented rapid quenching, on the 10s of ps timescale or faster, although quenching on the 100s of ps timescale persisted for 2AP in single strands and duplexes. The multi-exponential fluorescence decay of 2AP in DNA and its high sensitivity to local environment is commonly exploited to investigate DNA-enzyme interactions. Transposases are enzymes involved in the movement of sections of DNA (transposons) within the genome. The Mos1 transposase catalyses the movement of a transposon via a cut-and-paste mechanism involving several intermediate complexes. Understanding the complex mechanism by which the transposase can remove and insert a section of DNA would allow these enzymes to be used as biomolecular tools. The structure of the intermediate Mos1 strand-transfer complex (STC) has been investigated by incorporating 2AP into several regions of the transposon and analysing the fluorescence decay. The involvement of a base-flipping-like mechanism has been identified in the mechanism of strand transfer for the Mos1 transposon. The time-resolved fluorescence measurements performed in this thesis are limited to time resolution of ~20 ps and longer using TSCPC. However, an abundance of photophysical events in DNA occur on the femtosecond timescale. Development of a methodology utilising fluorescence gating techniques (such as sum-frequency generation or diffraction from a transient grating) have been attempted, in order to construct an experimental system that enables the broadband detection of ultrafast fluorescence decays. Despite the lack of immediate success in recording the fluorescence decay from a sample, due to technical issues and time-constraints, initial characterisation of the set-up was performed and the prospect of broadband detection was demonstrated. Overall, this thesis gives insight into some of the dynamic processes taking place in DNA and presents work performed to develop a system that would allow the extension of these studies to processes occurring on the fs timescale.

Doctor of Philosophy / Department of Physics / Artem Rudenko / Christopher M. Sorensen / The motion of electrons in atoms, molecules, and solids in the presence of intense electromagnetic radiation is an important research topic in physics and physical chemistry because of its fundamental nature and numerous practical applications, ranging from precise machining of materials to optical control of chemical reactions and light-driven electronic devices. Mechanisms of light-matter interactions critically depend on the dimensions of the irradiated system and evolve significantly from single atoms or molecules to the macroscopic bulk. Nanoparticles provide the link between these two extremes. In this thesis, I take advantage of this bridge to study light-matter interactions as a function of nanoparticle size, shape, and composition. I present here three discrete, but interconnected, experiments contributing to our knowledge of nanoparticle properties and their response to intense, short-pulsed light fields. First, I investigate how individual nanoparticles interact with each other in solution, studying their temperature-dependent solubility. The interaction potential between 5.5nm gold nanoparticles, ligated by an alkanethiol was found to be -0.165eV, in reasonable agreement with a phenomenological model. The other two experiments explore ultrafast dynamics driven by intense femtosecond lasers in isolated, gas-phase metallic and dielectric nanoparticles. Photoelectron momentum imaging is applied to study the response of gold, silica, and gold-shell/silica-core nanoparticles (ranging from single to several hundred nanometers in size) with near-infrared (NIR), 25 fs laser pulses in the intensity range of 10¹¹ - 10¹⁴ W/cm². These measurements, which constitute the bulk of my graduate work, reveal the complex interplay between the external optical field and the induced near-field of the nanoparticle, resulting in the emission of very energetic electrons that are much faster than those emitted from isolated atoms or molecules exposed to the same light pulses. The highest photoelectron energies (“cutoffs”) were measured as a function of laser intensity, nanoparticle material and size. We found that the energy cutoffs increase monotonically with laser intensity and nanoparticle size, except for the gold/silica hybrid where the plasmon resonance response modifies this behavior at low intensities. The measured photoelectron spectra for metallic nanoparticles display a large energy enhancement over silica. Finally, the last part of this thesis explores the possibility to apply time-resolved x-ray scattering as a probe of the ultrafast dynamics in isolated nanoparticles driven by very intense (~10¹⁵ W/cm²) NIR laser radiation. To do this, I developed and built a nanoparticle source capable of injecting single, gas-phase nanoparticles with a narrow size distribution into the laser focus. We used femtosecond x-ray pulses from an x-ray free electron laser (XFEL) to map the evolution of the laser-irradiated nanoparticle. The ultrafast dynamics were observed in the single-shot x-ray diffraction patterns measured as a function of delay between the NIR and x-ray pulses, which allows for femtosecond temporal and nanometer spatial resolution. We found that the intense IR laser pulse rapidly ionizes the nanoparticle, effectively turning it into a nanoplasma within less than a picosecond, and observed signatures of the nanoparticle surface softening on a few hundred-femtosecond time scale.

Ce travail a été consacré à l'étude de composants optoélectroniques ultra-rapides pour le traitement de signaux RF jusqu'au domaine THz, ainsi qu'à l'étude de composants pour les faisceaux THz. Tout d'abord, le travail a porté sur des photo-commutateurs optoélectroniques fabriqués avec des semi-conducteurs ultrarapides. Le dispositif, éclairé par le battement de 2 faisceaux optiques et polarisé par une tension RF, réalise le mélange de ces fréquences. Les propriétés du dispositif (bande passante, efficacité, génération de fréquences…) ont été modélisées et les simulations ont été validées par des mesures expérimentales. Quand le signal RF est modulé par un signal « information », cette information peut être directement extraite en égalant fréquences RF et de battement optique. Le signal démodulé est très pur : ainsi nous avons mesuré une largeur spectrale à -3 dB de 11 Hz. D'autres matériaux pour la génération THz et la photo-commutation ultrarapide furent aussi étudiés, comme des boîtes quantiques en InAs. Enfin, nous avons réalisé une étude par spectroscopie THz dans le domaine temporel des propriétés de dispositifs métalliques sous-longueur d'onde pour la manipulation de faisceaux THz, comme des réseaux de trous dans une plaque métallique ou de filtres à grille, ainsi que de films nanométriques de graphène ou de nanotubes de carbone. / This PhD work was devoted to the study of ultrafast optoelectronic components for processing RF signals up to the THz range, and of related THz devices. First, we used a photoconductive switch, made of low-temperature grown GaAs, excited by the optical beating of two CW laser beams and biased by a RF signal. The switch serves as a frequency mixer, whose properties (bandwidth, efficiency, sideband generation…) were modeled and the simulation results were experimentally validated. When the RF signal is modulated by information, this information can be directly extracted by setting the beating frequency equal to the RF one. The demodulated signal exhibits a high spectral purity, 11 Hz bandwidth at -3 dB. Other materials for THz generation and fast photo-switching were also studied, like InAs quantum dots. Finally, we performed a THz time-domain spectroscopy study of metallic sub-wavelength devices for THz beam processing, like hole arrays and metallic mesh filters, as well as nanometric thin films of graphene and carbon nanotubes.

Two dimensional (2D) materials are poised to revolutionize the future of optics and electronics. The past decade saw intense research centered around graphene. More recently, the tide has shifted to a bigger class of two-dimensional materials including graphene but more expansive in their capabilities. The so called ‘2D material zoo’ includes metals, semi-metals, semiconductors, superconductors and insulators. The possibility of mixing and matching 2D materials to fabricate heterostructures with desirable properties is very exciting. To make devices with superior electronic, optical and thermal properties, we need to understand how the electrons, phonons and other quasi particles interact with each other and exchange energy in the femtosecond and nanosecond timescales. To measure the timescales of energy distribution and dissipation, I used ultrafast pump-probe spectroscopy to perform time-domain measurements of optical absorption. This approach allows us to understand the impact of manybody interactions on the bandstructure and carrier dynamics of 2D materials. After a brief introduction to femtosecond laser spectroscopy, I will explore the transient absorption dynamics of three classes of 2D materials: intrinsic graphene, graphene-hBN heterostructures and Transition Metal Dichalcogenides (TMDs). We will see that using pumpprobe measurements around the high energy M-point of intrinsicgraphene, we can extract the value of the acoustic deformation potential which is vital in characterizing the electron-acoustic phonon interactions. In the next part of the thesis, I will delineate the role of the substrate in the cooling dynamics in graphene devices. We will see that excited carriers in graphene on hBN substrates cool much faster that on SiO2 substrates due to faster decay of the optical phonons in graphenehBN heterostructures. These results show that graphene-hBN heterostructures can solve the hot phonon bottleneck that plagues graphene devices at high power densities. In the last part, I will demonstrate the role of phonon induced bandgap renormalization in the carrier dynamics of TMD materials and measure the timescale of phonon decay through the generation of low-energy phonons and transfer to the substrate. This study will help us understand carrier recombination in TMD devices under high-bias conditions which show great potential in opto-electronic applications such as photovoltaics, LEDs etc.

The ultrafast charge dynamics in a number of nanostructured materials relevant to the production of renewable energy are investigated using ultrafast transient absorption spectroscopy. The materials include mercury telluride and cadmium mercury telluride quantum dots, and gold nanoparticles loaded on titanium dioxide colloidal spheres. The analysis of the resultant pump-induced transmittance change spectra and transients allow the determination of charge relaxation routes including multiple exciton generation, trion formation and direct-surface trapping. The investigation of HgTe QDs passivated with thioglycerol, mercaptopropionic acid and dodecanethiol ligands suggests that mercaptopropionic acid ligand results in better passivation of HgTe QDs due to its carboxylic acid group. It allows more electron density donation to the QD surface to passivate the traps related with unsaturated Hg bonds and hence supresses the associated non-radiative processes. The decay lifetimes of the thioglycerol/dodecanethiol-capped QDs in addition to the photo-induced absorption feature in their spectra, are found to be consistent with surface charge trapping observed in CdSe QDs. In comparison, the transients obtained for mercaptopropionic acid passivated QDs coupled with the pump-induced transmittance change spectrum show no sign of any surface-related processes. Therefore, our analyses allow the determination of multiple exciton generation for the first time in these QDs with a quantum yield of 1.36 ± 0.04 when photo-exciting with photons of energy 3.1 times the band gap. Such result should turn researchers’ attention to those ligands which could improve the QD solar cell field. The study of exciton dynamics in CdxHg(1-x)Te alloy QDs is also presented here. Their pump-induced transmittance change spectrum show two bleaches: at the shoulder position of the steady state absorption and at the PL peak. The exciton dynamics of these materials are studied using four different wavelengths, two of them are above the MEG threshold. The resultant transmittance transients and the pump-induced transmittance change spectrum are free of any photo-induced absorption or long-lived surface trapping. Hence, the decay of the transients obtained above the MEG threshold for well-stirred samples at low pump fluences is attributed to biexciton recombination. The assessment of multiple exciton generation reveals a quantum yield value of 1.12 ± 0.01 when photo-exciting with 2.6 times the band gap. Finally, the investigation of the recovery of the plasmon bleach in TiO2 colloidal spheres decorated with different sizes of Au NPs is presented in this thesis. The pump-induced transmittance change spectra obtained for two different wavelengths show bleaches at the plasmon band maximum superimposed with two wings of absorption features at shorter and longer wavelengths. The resultant transmittance transients for these samples are well-described by bi-exponential decay with a very quick decline of a few ps associated with electron–phonon scattering, followed by a slower decay over a few 10s of ps associated with heat dissipation. Only the heat dissipation rate is found to be dependent on the size of the Au NPs as it rises from 49 ± 3 ps to 128 ± 6 ps when the diameter of the Au NPs is increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

We have built an imaging mass spectrometer adapted for ultrashort laser pulses as its ionization technique, as an alternative to other imaging techniques. Before my arrival, the mass spectrometer has only been subject to preliminary tests on noble gases. Since then, we’ve made some modifications to the system in order to properly analyze solids. This thesis shows how we obtain our ultrashort laser pulses, the inner workings of our homemade imaging mass spectrometer, and the results that we’ve obtained with it so far. We tested two modes of operation concerning the extraction of the ions from the system into the mass analyzer: continuous and pulsed. We discuss the advantages and disadvantages of each configuration. We also display preliminary imaging results with our imaging technique of a simple WO3 and ITO structure. We conclude by comparing the resolution of this image to the different techniques in imaging mass spectrometry, how we can further improve our mass spectrometer, and the future use of this machine. Nous avons construit un spectromètre de masse adapté pour les pulses de laser très courts comme technique d’ionisation, pour acquisition des images d’un échantillon. Avant je suis arrivé, le spectromètre de masse avait seulement été utilisé pour des tests préliminaires de gaz nobles. Depuis ce moment, nous avons modifié le système pour analyser les solides. Cette thèse démontre comment on obtient nos pulses de laser très courts, comment notre spectromètre fait maison fonctionne et les résultats nous avons obtenus jusqu’à présent. Nous avons testé deux configurations différentes au sujet de l’extraction des ions du système : constant et pulsé. Nous discutons aussi les avantages et désavantages de chaque mode d’opération. Nous démontrons aussi des images préliminaires d’un substrat mixte de WO3 et ITO. Nous concluons par comparer la résolution des images aux autres techniques de collection d’images, comment nous pouvons améliorer notre spectromètre de masse et les plans pour la machine dans le futur.