Doctor of Philosophy / Department of Physics / Kristan L. Corwin / This research has focused on the development, demonstration, and characterization of a new type of laser based on optically-pumped gases contained within hollow optical fibers. These novel lasers are appealing for a variety of applications including frequency metrology in the mid-infrared, free-space communications and imaging, and defense applications. Furthermore, because of the hollow core fibers used, this technology may provide the means to surpass the theoretical limits of output power available from high power solid-core fiber laser systems. Gas-filled hollow-core fiber lasers based on population inversion from acetylene ([superscript]12C[subscript]2H[subscript]2) and hydrogen cyanide (HCN) gas contained within the core of a kagome-structured hollow-core photonic crystal fiber have now been demonstrated. The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns duration pulses from a home-built optical parametric amplifier. Narrow-band laser emission peaks in the 3-μm region corresponding to the ΔJ = ±1 dipole allowed rotational transitions between the pumped vibrational overtone modes and the fundamental C-H stretching modes have been observed in both molecules. High gain resulting from tight confinement of the pump and laser light together with the active gas permits these lasers to operate in a single pass configuration, without the use of any external resonator structure. Studies of the generated mid-infrared pulse energy, threshold energy, and slope efficiency as functions of the launched pump pulse energy and gas pressure have been performed and show an optimum condition where the maximum laser pulse energy is achieved for a given fiber length. The laser pulse shape and the laser-to-pump pulse delay have been observed to change with varying pump pulse energy and gas pressure, resulting from the necessary population inversion being created in the gases at a specific fiber length dependent on the launched pulse energy. Work is on going to demonstrate the first continuous wave version of the laser which may be used to produce a single coherent output from many mutually incoherent pump sources.

De nombreuses applications militaires ou civiles, telles que la spectroscopie dans les bandes de transmission de l’atmosphère (bandes 3-5µm et 8-12µm), nécessitent de disposer de sources émettant dans le moyen infrarouge (IR). Les travaux de cette thèse portent sur la génération de rayonnement dans la bande 3-5µm par amplification paramétrique (mélange à quatre ondes) dans les fibres optiques en verres fluorés et en verres de chalcogénures. La première partie de ce travail a été consacrée à l’étude théorique et à la modélisation des conditions d’accord de phase et du gain paramétrique dans des fibres à saut d’indice monomodes en verres fluorés ZBLAN et verres de chalcogénures As2S3 et As2Se3. La nature des résultats obtenus nous a conduit à étudier théoriquement le potentiel de l’accord de phase multimode dans les fibres en verres de chalcogénures. La deuxième partie de ce travail a porté sur la modélisation de l’amplification paramétrique dans des fibres en verres de chalcogénures microstructurées à géométrie hexagonale. Pour ce faire, un modèle simplifié de la propagation dans les fibres microstructurées hexagonales a été développé : le modèle de l’indice effectif de gaine (EIM). Il a ensuite été comparé à une méthode de résolution aux éléments finis. Grâce à cette comparaison, nous avons pu améliorer la précision du modèle EIM en déterminant la valeur de plusieurs paramètres empiriques. Ce modèle nous a alors permis de prédire l’efficacité du processus d’amplification paramétrique dans les fibres microstructurées. L’ensemble de ces études théoriques a permis d’identifier les fibres les plus adaptées à la conversion de fréquence vers la bande 3-5µm. Enfin, nous avons réalisé un banc de mesure de la dispersion chromatique des fibres, ainsi que le dimensionnement d’un convertisseur de fréquence utilisant les fibres identifiées dans l’étude théorique. / Various civil or military applications, such as spectroscopy in the atmospheric transparency windows (3 – 5 µm and 8 – 12 µm ranges), require the use of mid-infrared emitting laser sources.The work presented in this thesis is about light generation in the 3 – 5 µm range by parametric amplification (four-wave mixing) in fluoride and chalcogenide fibers. The first part of the study is devoted to modelizations of phase-matching condition and parametric gain in monomode step-index ZBLAN fluoride fibers as well as As2S3 and As2Se3 chalcogenide fibers. The results obtained in this modelization led to the theoretical study of multimode phase-matching conditions in chalcogenide fibers.The second part of the study presents the modelization of parametric amplification in hexagonal microstructured chalcogenide fibers. A simplified model, called the effective index method (EIM), has been developed and compared to the finite element method. Thanks to this comparison, the accuracy of the EIM model was improved through the determination of several empirical parameters. Using the improved EIM model, we have been able to predict the parametric amplification efficiency in microstructured fibers. Thus, all those theoretical studies allowed us to identify the most adapted fibers for frequency conversion in the 3 – 5 µm range. Eventually, we realized an experimental bench to measure the chromatic dispersion of optical fibers, and we suggested an experimental architecture using the fibers we had indentified in the theoretical study.

Changes in milk fat composition influence its nutritional quality as well as the technological properties of butter. The impact of feed on fat composition is well known; however, limited information is available on the genetic variability of fatty acids in bovine milk. The overall aim of this PhD thesis was to study the genetic variability of fatty acid profile in bovine milk and fat. This type of research needs a large amount of data. Expensive reference analysis is used to measure the fatty acid contents in fat. The first objective of this thesis was to develop an alternative method that could be faster and cheaper than traditional methods. Calibration equations predicting the contents of fatty acid from mid-infrared spectrum were established. The contents of saturated and monounsaturated fatty acids, omega-9, fatty acids with short, medium and long chain were the best predicted. Thanks to the implementation of this method in the Walloon routine milk recording, more than 20,000 milk samples were analyzed. This database permitted to model the variation of fatty acid contents in milk and fat. From these models, the genetic variability of fatty acid profile was shown. The complexity of models increased throughout this project due to the increase of new available data. Differences across 7 dairy breeds were estimated using single and multi-trait mixed models. Milk fat and delta-9 desaturase activity of Jersey and dual purpose Belgian Blue differed significantly from Holsteins. Therefore, the choice of a given breed could modify the fat composition. Heritability values obtained for studied fatty acids with multi-trait mixed model ranged from 0.05 to 0.42. Higher values were observed for saturated compared to unsaturated fatty acids. Moderate heritability estimates were observed for the activity of delta-9 desaturase (0.20) and the hardness of butter (0.27). These two traits were estimated by specific fatty acid ratios. The heritability observed using a multi-trait random regressions test day model for the content of saturated fatty acids (0.42) was similar to the one observed for the percentage of fat (0.37). Considering the impact of selection on fat content, the selection could have a great impact on fat composition. High genetic correlations were observed between some fatty acids having similarities in their synthesis. Heritability and correlations varied through the duration of the lactation. Due to the large number of fatty acids, the estimation of an index, which includes the proper fatty acid profile, could be interesting for a future selection program. This PhD thesis provides the background required by future studies to estimate the impact of animal selection on milk fat composition.

This thesis describes research on near degenerate quasi phase-matched opticalparametric oscillators (OPO) where volume Bragg gratings (VBG) are used toproduce narrow oscillation bandwidth. These OPOs are then used to pump a secondOPO to generate mid-infrared radiation. The atmospheric transmission windows in the 3.5 to 5 μm wavelength region areused for seekers on infrared homing missiles. These missiles are available to guerrillaand terrorist groups and have been used in a number of attacks on military and civilianaircraft. Laser sources at the same wavelengths are an important component incountermeasure systems for aircraft self-protection. Similar laser sources also haveapplications in laser surgery. At wavelengths longer than 4 μm crystal materials for multi-Watt level averagepower nonlinear devices is a problem. The best solution so far is to use ZnGeP2(ZGP). ZGP and the available alternatives all have a problem of near-infraredabsorption, and a mid-infrared OPO thus has to use a pump wavelength near 2 μm.This pump source can be a neodymium laser at 1.06 μm with a near degenerate OPO. Nonlinear devices for low to medium pulse energies are dominated by quasi phasematchedmaterials because of their higher effective nonlinearities and lack of walkoff.In addition they allow type I interaction where signal and idler from the OPOhave the same polarization, which has the advantage that both waves can be used topump the ZGP OPO. The drawback of this is that the near-degenerate interaction hasvery wide gain bandwidth. Efficient pumping of the second OPO demands narrowbandwidth output from the first OPO.Volume Bragg gratings that are glass materials with a periodic refractive indexmodulation have emerged as high quality narrow bandwidth reflectors. By using aVBG as one cavity mirror in an OPO the feedback bandwidth and hence the OPOoscillation bandwidth can be kept very narrow. Signal and idler bandwidths of 10 and20 GHz (FWHM) at 2122 and 2135 nm, respectively, have been demonstrated. Thisshould be compared to the several hundred nanometre bandwidth from an OPO usingdielectric mirrors. Very narrow bandwidth operation has been achieved so close todegeneracy that the signal and idler are not resolvable. The total output energy generated in the PPKTP OPO (signal and idler together)has been used to pump a ZGP OPO that produced mid-IR radiation. Tuning of thesignal from a ZGP OPO from 2.9 μm to degeneracy at 4.3 μm has been shown, with acorresponding idler wavelength tuneable up to 8 μm. The highest conversionefficiency that has been reached from 1.06 μm to the mid-IR was 12 %. This setupused a PPKTP OPO with 30 % conversion efficiency and 13 nm separation of signaland idler (2122 and 2135 nm). The pulse repetition frequency was 20 kHz and thegenerated output power in the mid-IR was 3.2 W. / QC 20100517

The work described herein focuses on FT-IR and quantum cascade laser (QCL) based studies towards the development of compact and portable trace gas sensor for benzene, toluene, and xylenes (BTX). FT-IR broadband radiation was used to probe the mid-infrared fingerprint region for quantitatively detecting trace gas levels of BTX. Using direct absorption through a hollow waveguide, parts-per-million (ppm) detection limits for BTX with a response time of 39 seconds was demonstrated. Univariate calibration provided limits of detection (3σ) for benzene, toluene, and meta-xylene at 5, 17, and 11 ppm, respectively. Multivariate calibration using partial least squares regression algorithms were used to simulate real-world conditions with multiple analytes present within a complex sample. A calibration model was built with 110 training set standards enabled by using a customized gas mixing system. Furthermore, a preconcentration/thermal desorption (TD) step was added to the FT-IR HWG trace gas sensor enabling parts-per-billion detection of BTX. A univariate calibration was established in the laboratory with certified gas standards over a dynamic range of 1000 - 100 ppb for benzene, toluene, and the xylenes. The sensor was then taken to an industrial site during a field measurement campaign for the quantitative determination of BTX in field air samples. The laboratory calibration was used to predict unknown concentrations which were in close agreement with industrial hygiene standard techniques, and industrial prototype analyzers, that were simultaneously operated in the field environment. In addition to FT-IR, quantum cascade laser spectroscopy was also investigated due to enhanced spectral density and efforts to precisely overlap emission with analyte absorption. Particular efforts were dedicated on a novel principle for consistent and deliberate QCL emission wavelength selection by varying the QCL cavity length. These studies experimentally confirmed that using this straight-forward post-processing technique, emission wavelength tuning across a range of one hundred wavenumbers range may be achieved. This tuning range was experimentally demonstrated for a QCL emitting across an entire absorption feature of carbon dioxide by tailoring the length of the cavity. Additionally, using an external cavity (EC) - QCL combined with a HWG gas sensor module for the first time enabled the quantitative and simultaneous determination of ethyl chloride, trichloromethane, and dichloromethane within exponential dilution experiments at ppb limits of detection. Multianalyte detection was demonstrated utilizing partial least squares regression for quantitative discrimination of individual constituents within a mixture, yet applying a single broadly tunable QCL light source.

This dissertation is composed of seven chapters, detailing advances within the area of sulfur polymer chemistry and processing, and highlights the relevance of the work to the fields of polymer science, energy storage, and optics that are enabled through the development of novel high sulfur-content copolymers as discussed in the following chapters. The first chapter is a review summarizing both the historical forays into utilization of elemental sulfur in high sulfur-content materials and the current research on the incorporation of sulfur into novel copolymers and composites for high value added applications such as energy production/storage, polymeric optical components, and dynamic/self-healing materials. Although recent efforts by the materials and polymer chemistry communities have afforded innovative sulfur containing materials, many studies fail to take advantage of the low cost and incredible abundance of sulfur by incorporating only minimal quantities into the end products. A fundamental challenge in the preparation of sulfur-containing polymers is simultaneous incorporation of high sulfur-content through facile chemical methods, to truly use the element as a novel feedstock in copolymerizations. Contributing to the challenge are the intrinsic limitations of sulfur (i.e., low miscibility with organic solvents, high crystallinity, and poor processability). The emphasis in chapter 1 is the critical development of utilizing sulfur as both a reagent and solvent in a bulk reaction, termed inverse vulcanization. Through this methodology we can directly prepare materials which retain the advantageous properties of elemental sulfur (i.e., high electrochemical capacity, high refractive index, and liable bond character), obviate the processing challenges, and enable precise control over composition and properties in a facile manner. The second chapter focuses on advancement in colloid synthesis, specifically an example mediated by in-situ reduction of organometallic precursors (ClAu^IPPh₃) by elemental sulfur at high temperatures. In chapter 2, elemental sulfur is employed both as a reactant and novel solvent, generating composite composed of well-defined gold nanoparticles (Au NPs) fully dispersed in a sulfur matrix. While the synthesis of Au NPs in molten sulfur was a novel development the challenge of analyzing the particles directly within the sulfur composite matrix by microscopy techniques required improvement of the composites mechanical properties. To overcome this issue, a one-pot reaction in which the Au NPs were initially synthesized, was vulcanized through an ambient atmosphere-tolerant bulk copolymerization by the addition of a difunctional comonomer (divinylbenzene). The improved composite integrity enabled microtoming and transmission electron microscopy analysis of the particles within the crosslinked reaction matrix. Due to the facile capabilities of directly dissolving the comonomers within the molten sulfur the inverse vulcanization methodology provides a simple route to prepare stable, high sulfur-content copolymers in a single one-pot reaction. The third chapter expands upon the methodology for direct dissolution of difunctional comonomers into molten elemental sulfur to afford chemically stable copolymer. A major challenge associated with the high temperature (i.e., 185 °C) bulk copolymerization reactions between sulfur and vinyl comonomers (i.e., divinylbenzene, DVB) is the high volatility of the organic monomers at elevated temperatures (BP of DVB = 195 °C). To obviate this problem required a novel monomer with an increased boiling point for successful scaling of the inverse vulcanization methodology. The work presented in chapter 3 details the employment of 1,3-diisopropenylbenzene (DIB, BP = 231 °C) to enable larger scale bulk inverse vulcanization reactions, allowing facile control over thermomechanical properties by simple variation in copolymer composition (50–90-wt% S₈, 10–50-wt% DIB). Poly(Sulfur-random-1,3-diisopropenylbenzene) ((poly(S-r-DIB)) copolymers prepared via the inverse vulcanization methodology possess substantially improved processing capabilities compared with elemental sulfur. A facile demonstration of improved processability is the generation of free-standing micropatterned structures using a high sulfur content liquid pre-polymer resin that can be poured into a mold and cured into the desired final form. The highest weight percentage copolymer (i.e., 90-wt% S₈) was also demonstrated to improve cycle lifetimes and capacity retention (823 mAh•g⁻¹ at 100 cycles) of a Lithium-Sulfur (Li-S) cell when the copolymer was utilized as the active material instead of elemental sulfur. Chapter four focuses on the optimization of Li-S cell performance as a function of copolymer composition and provides a more thorough understanding of the means by which copolymer active material improves battery performance. A substantial challenge associated with Li-S cells is the fast capacity fade and short cycle lifetimes that result from loss of the active material (i.e., sulfur) during normal cycling processes. The field has generally addressed these issues by encapsulation of the sulfur in a protective shell (e.g., polymeric, carbonaceous, or metal oxide in nature) in an attempt to sequester the active material. However, encapsulation of sulfur is non-trivial and leads to low loadings of sulfur, resulting in a low energy density within the final cell. To address the challenges associated with maintaining high capacity and long cycle lifetimes while employing an active material which is low cost, generated in a facile manner, and has a high sulfur content required a novel approach. In the work presented in chapter 4 we prepared high sulfur content copolymers via the inverse vulcanization methodology, which meet all the requirements necessary of an active material, and investigated the performance of Li-S batteries as a function of the copolymer composition. A survey of several poly(S-r-DIB) copolymer compositions were prepared with DIB compositions ranging from 1-50-wt% DIB (i.e., 50-99 wt% sulfur) and screened to determine optimal compositions for optimal Li-S battery performance. From this analysis it was determined that copolymers with 10-wt% DIB (90-wt% S₈) were optimal for producing Li-S batteries with high capacity and long cycle lifetimes. 10-wt% DIB copolymers batteries ultimately achieved long cyclic lifetimes and maintained high capacity (>600 mAh/g at 500 cycles). Chapter five details the optimization of conditions necessary to generate large scale (>100 g) inversely vulcanized sulfur copolymers and their application towards Li-S batteries. As previously stated a significant challenge in the Li-S battery field is the production of a Li-S active material with improved performance that is low cost, synthesized in a facile manner, and possesses high sulfur content. To date poly(S-r-DIB) copolymers prepared via the inverse vulcanization methodology afford some of the longest cycle lifetimes and highest capacity retention for polymeric active materials. However, initial inverse vulcanization reactions investigated for preparing active materials were performed on 10 gram scales. The goal of the work presented in chapter 5 was to prepare materials on a scale applicable to fabrication of several prismatic Li-S cells, each of which requires several grams of active material. However, scaling up of the reaction to a kilogram and utilizing the traditional inverse vulcanization conditions (i.e., 185 °C) results in catastrophic degradation as a consequence of the Trommsdorf effect. To address this challenge required decreasing the radical concentration within the bulk copolymerization, which necessitated performing the kilogram scale inverse vulcanization reactions at lower temperatures (i.e., 130 °C) over a longer reaction period. Decreasing the temperature generates materials that are nearly identical in thermomechanical properties to smaller scale samples and the battery performance is likewise comparable (>600 mAh/g at 500 cycles). The key advantage of performing the inverse vulcanization reaction at lower temperatures is that additional monomers, with lower boiling points or degradation issues, can be utilized and the increased gelation time, enables facile incorporation of additives (e.g., carbon black or nanoparticles) into the reaction. Chapter six focuses on the development of poly(S-r-DIB) copolymers as novel mid-infrared (mid-IR) transmitting materials and the analysis of the optical properties as a function of copolymer composition. A challenge in the optical science community is the limited number of materials applicable to the development of innovative optical components capable of functioning in the mid and far-IR regions. Semi-conductor and chalcogenide glasses have been widely applied as device components in infrared optics due to their high refractive indices (n ~2.0–4.0) and high transparency in the infrared region (1–10 μm). However, such materials are also expensive, difficult to fabricate, and toxic in comparison to organic polymers. On the other hand organic polymers are easily processed, low cost, and generated from easily accessible raw materials. Unfortunately, polymeric materials generally have low refractive indices (n<1.65) and are prepared from monomers with functional groups that are highly absorbing at mid-IR and longer wavelengths. Chapter 6 details the realization through the inverse vulcanization methodology of the first example of a material that is high refractive index and low mid-IR absorption, but also low cost and easily processable. Critical to achieving a polymeric material which was appropriate for mid-IR applications was the high sulfur content and the absence of functional groups, both of which are afforded by the facile copolymerization process. By simply controlling copolymer composition the optical properties of the material were tailorable; allowing adjustment of the refractive index from ~1.75 (50-wt% DIB) to ~1.875 (20-wt% DIB). Finally, through facile techniques, high quality copolymers lenses were prepared and we demonstrated the high optical transparency over several regions of the optical spectrum, from the visible (400–700 nm) all the way to the mid-IR (3–5μm). Poly(S-r-DIB) copolymers demonstrated high transparency to mid-IR light, but still maintain the processing capabilities of an organic polymer, the first example of such a material to possess both qualities. Ultimately the inverse vulcanization methodology offers a novel route to low cost, high refractive index, IR transparent materials, opening up unique opportunities for polymeric optical components within the optical sciences field. The seventh chapter discusses utilization of the inverse vulcanization methodology as a means to prepare and control the dynamic behavior of sulfur copolymers for potential applications towards self-healing materials. The incorporation of dynamic covalent bonds into conventional polymer architectures, either directly within the backbone or as side-chain groups, offers the stability of covalent bonds but with the ability of stimuli-responsive behavior to afford a change in chemical makeup or morphology. Traditionally the installation of such functionality requires the use of disparate, orthogonally polymerizable functional groups (i.e., vinyl) and discrete design of the comonomers utilized to generate a responsive copolymer. Therefore, a challenge in developing novel dynamic copolymers is the ability to install stimuli-responsive functionality directly as a result of the copolymerization without the need for rigorous synthetic monomer design and complex copolymerization techniques. In chapter 7 we discuss the analysis of poly(S-r-DIB) copolymers with rheological techniques to assess the composition dependent dynamic behavior. Aided by the bulk nature of copolymerization, the feed ratio of S₈ and DIB directly dictates copolymer microstructure; thus the sulfur rank between the organic groups (i.e., DIB) was tailorable from a single sulfur (thioether) to multiple sulfurs (pentasulfide). Control over sulfur content and number of S–S enables control over the dynamic behavior, as monitored via in-situ rheological techniques. The highest sulfur-content copolymers (80-wt% S₈, 20-wt% DIB) showed the fastest response when under shear stress due to the large number of S–S bonds. On the other hand when no dynamic bonds were present in the copolymer (i.e.; 35-wt% S₈, 65-wt% DIB) there is no dynamic behavior and full recovery of the pristine mechanical properties was not observed. The facile synthesis and simple control over copolymer microstructure affords the inverse vulcanization methodology an advantage over other dynamic materials, and provides potential secondary qualities (i.e., high refractive index) built directly into the structure.

In this thesis, I have developed a tunable two-color two-stage ultrafast Yb:fiber chirped pulse amplifier for the generation of short-pulse mid-infrared (MIR) radiation in the long-wavelength side of the "molecular fingerprint" (2.5-25 μm) using difference frequency generation (DFG) technique. The two colors called blue and red are in the wavelengths 1.03-1.11 μm and are amplified simultaneously in the same Yb-doped fiber amplifier (YDFA) stages in order to reduce the induced environmental noise on the phase difference of the pulses and to minimize the complexity and system cost. I will present numerical simulations on the two-stage YDFA system to amplify a two-color spectrum in the wavelengths 1.03-1.11 μm. The first and second YDFA called preamplifier and main amplifier are single-clad, single-mode and double-clad, single-mode YDFA respectively. From numerical simulations, the optimal length of the preamplifier to have equal power at two colors centered at 1043 nm and 1105 nm are in agreement with experimental results. It is well known that the power of MIR radiation generated by difference frequency mixing of two wavelengths scales up with the product of mixing powers in a fixed-field approximation. Furthermore, for the gain narrowing effect on the short-wavelength side of the YDFA gain profile, the spectral bandwidth of the blue color decreases resulting in pulse broadening. In addition, for the two colors separated largely, the amplified spontaneous emission is intensified. Considering the cited factors, I will present the modeling results on the two-color, two-stage YDFA system that the product of the power of the two colors is maximized for a maximized wavelength separation between the two mixing colors and a minimized gain narrowing on the blue color in order to build an as broadly tunable and powerful as possible ultrafast mid-infrared source by difference frequency mixing of the two colors. In this research, I achieved a wavelength separation as broad as 71 nm between pulses centered at 1038 nm and 1109 nm from the two-color ultrafast YDFA system. I achieved combined average powers of 2.7 W just after the main amplifier and 1.5 W after compressing the two-color pulses centered at 1041 nm and 1103 nm to nearly Fourier transform limited pulses. From autocorrelation measurements, the full width at half maximum (FWHM) of the compressed two-color pulses with the peak wavelengths of 1041 nm and 1103 nm was ~500 fs. By mixing the tunable two-color pulses in a 1-mm-thick GaSe crystal using DFG technique, I achieved tunable short-pulse MIR radiation. In this research, I achieved short-pulse MIR radiation tunable in the wavelengths 16-20 μm. The MIR tuning range from the lower side was limited to the 16 μm because of the 71-nm limitation on the two-color separation and from the upper side was limited to the 20 μm because of the 20-μm cutoff absorption wavelength of GaSe. Based on measured MIR spectra, the MIR pulses have a picosecond pulse duration in the wavelengths 16-20 μm. The FWHM of measured spectra of the MIR pulses increases from 0.3 μm to 0.8 μm as the MIR wavelength increases from 16 μm to 20 μm. According to Fourier transform theory, the FWHM of the MIR spectra corresponds to the bandwidth of picosecond MIR pulses assuming that the MIR pulses are perfectly Fourier-transform-limited Gaussian pulses. In this research, I achieved a maximum average power of 1.5 mW on short-pulse MIR radiation at the wavelength 18.5 μm corresponding to the difference frequency of the 500-fs two-color pulses with the peak wavelengths of 1041 nm and 1103 nm and average powers of 1350 mW and 80 mW respectively. Considering the gain bandwidth, Ti:sapphire is a main competitor to the YDFA to be used in the two-color ultrafast laser systems. In the past, the broad gain bandwidth of Ti:sapphire crystal has resulted in synchronized two-color pulses with a wavelength separation up to 120 nm. Apart from its bulkiness and high cost, Ti:sapphire laser system is limited to a watt-level output average power at room temperature mainly due to Kerr lensing problem that occurs at high pumping powers. In comparison, YDFA as a laser amplifier has a narrower gain bandwidth but it is superior in terms of average power. Optical parametric generation (OPG) and optical parametric amplification (OPA) techniques are two competitors to DFG technique for the generation of short-pulse long-wavelength MIR radiation. Although OPG offers a tunability range as broad as DFG, the MIR output power is lower because of the absence of input signal pulses. From the OPA technique, the tunability range is not as broad as the DFG technique due to limitations with the spectral bandwidth of the optical elements. Currently, quantum cascade lasers (QCLs) are the state-of-art MIR laser sources. At the present time, the tunability range of a single MIR QCL is not as abroad as that achieved from the DFG technique. More, mode-locked MIR QCLs are not abundant mainly because of the fast gain recovery time. Thus, the generation of widely tunable short-pulse MIR radiation from DFG technique such as that developed in this thesis remains as a persistent technological solution. The application of the system developed in this thesis is twofold: on one hand, the tunable two-color ultrashort pulses will find applications for example in pump-probe ultrafast spectroscopy, short-pulse MIR generation, and optical frequency combs generation. On the other hand, the short-pulse MIR radiation will find applications for example in time-resolved MIR spectroscopy to study dynamical behavior of large molecules such as organic and biological molecules.

In this thesis, I have developed a tunable two-color two-stage ultrafast Yb:fiber chirped pulse amplifier for the generation of short-pulse mid-infrared (MIR) radiation in the long-wavelength side of the "molecular fingerprint" (2.5-25 μm) using difference frequency generation (DFG) technique. The two colors called blue and red are in the wavelengths 1.03-1.11 μm and are amplified simultaneously in the same Yb-doped fiber amplifier (YDFA) stages in order to reduce the induced environmental noise on the phase difference of the pulses and to minimize the complexity and system cost. I will present numerical simulations on the two-stage YDFA system to amplify a two-color spectrum in the wavelengths 1.03-1.11 μm. The first and second YDFA called preamplifier and main amplifier are single-clad, single-mode and double-clad, single-mode YDFA respectively. From numerical simulations, the optimal length of the preamplifier to have equal power at two colors centered at 1043 nm and 1105 nm are in agreement with experimental results. It is well known that the power of MIR radiation generated by difference frequency mixing of two wavelengths scales up with the product of mixing powers in a fixed-field approximation. Furthermore, for the gain narrowing effect on the short-wavelength side of the YDFA gain profile, the spectral bandwidth of the blue color decreases resulting in pulse broadening. In addition, for the two colors separated largely, the amplified spontaneous emission is intensified. Considering the cited factors, I will present the modeling results on the two-color, two-stage YDFA system that the product of the power of the two colors is maximized for a maximized wavelength separation between the two mixing colors and a minimized gain narrowing on the blue color in order to build an as broadly tunable and powerful as possible ultrafast mid-infrared source by difference frequency mixing of the two colors. In this research, I achieved a wavelength separation as broad as 71 nm between pulses centered at 1038 nm and 1109 nm from the two-color ultrafast YDFA system. I achieved combined average powers of 2.7 W just after the main amplifier and 1.5 W after compressing the two-color pulses centered at 1041 nm and 1103 nm to nearly Fourier transform limited pulses. From autocorrelation measurements, the full width at half maximum (FWHM) of the compressed two-color pulses with the peak wavelengths of 1041 nm and 1103 nm was ~500 fs. By mixing the tunable two-color pulses in a 1-mm-thick GaSe crystal using DFG technique, I achieved tunable short-pulse MIR radiation. In this research, I achieved short-pulse MIR radiation tunable in the wavelengths 16-20 μm. The MIR tuning range from the lower side was limited to the 16 μm because of the 71-nm limitation on the two-color separation and from the upper side was limited to the 20 μm because of the 20-μm cutoff absorption wavelength of GaSe. Based on measured MIR spectra, the MIR pulses have a picosecond pulse duration in the wavelengths 16-20 μm. The FWHM of measured spectra of the MIR pulses increases from 0.3 μm to 0.8 μm as the MIR wavelength increases from 16 μm to 20 μm. According to Fourier transform theory, the FWHM of the MIR spectra corresponds to the bandwidth of picosecond MIR pulses assuming that the MIR pulses are perfectly Fourier-transform-limited Gaussian pulses. In this research, I achieved a maximum average power of 1.5 mW on short-pulse MIR radiation at the wavelength 18.5 μm corresponding to the difference frequency of the 500-fs two-color pulses with the peak wavelengths of 1041 nm and 1103 nm and average powers of 1350 mW and 80 mW respectively. Considering the gain bandwidth, Ti:sapphire is a main competitor to the YDFA to be used in the two-color ultrafast laser systems. In the past, the broad gain bandwidth of Ti:sapphire crystal has resulted in synchronized two-color pulses with a wavelength separation up to 120 nm. Apart from its bulkiness and high cost, Ti:sapphire laser system is limited to a watt-level output average power at room temperature mainly due to Kerr lensing problem that occurs at high pumping powers. In comparison, YDFA as a laser amplifier has a narrower gain bandwidth but it is superior in terms of average power. Optical parametric generation (OPG) and optical parametric amplification (OPA) techniques are two competitors to DFG technique for the generation of short-pulse long-wavelength MIR radiation. Although OPG offers a tunability range as broad as DFG, the MIR output power is lower because of the absence of input signal pulses. From the OPA technique, the tunability range is not as broad as the DFG technique due to limitations with the spectral bandwidth of the optical elements. Currently, quantum cascade lasers (QCLs) are the state-of-art MIR laser sources. At the present time, the tunability range of a single MIR QCL is not as abroad as that achieved from the DFG technique. More, mode-locked MIR QCLs are not abundant mainly because of the fast gain recovery time. Thus, the generation of widely tunable short-pulse MIR radiation from DFG technique such as that developed in this thesis remains as a persistent technological solution. The application of the system developed in this thesis is twofold: on one hand, the tunable two-color ultrashort pulses will find applications for example in pump-probe ultrafast spectroscopy, short-pulse MIR generation, and optical frequency combs generation. On the other hand, the short-pulse MIR radiation will find applications for example in time-resolved MIR spectroscopy to study dynamical behavior of large molecules such as organic and biological molecules.

The aim of this thesis is to use a cross-match between WISE, 2MASS and SDSS to identify a large sample of M dwarfs. Through the careful characterisation and quality control of these M dwarfs I aim to identify rare systems (i.e. unresolved UCD companions, young M dwarfs, late M dwarfs and M dwarfs with common proper motion companions). Locating ultracool companions to M dwarfs is important for constraining low-mass formation models, the measurement of substellar dynamical masses and radii, and for testing ultracool evolutionary models. This is done by using an optimised method for identifying M dwarfs which may have unresolved ultracool companions. To do this I construct a catalogue of 440 694 M dwarf candidates, from WISE, 2MASS and SDSS, based on optical- and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints I isolate a sub-sample of 36 898 M dwarfs and search for possible mid-infrared M dwarf + ultracool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). I present 1 082 M dwarf + ultracool dwarf candidates for follow-up. Using simulated ultracool dwarf companions to M dwarfs, I estimate that the occurrence of unresolved ultracool companions amongst my M dwarf + ultracool dwarf candidates should be at least four times the average for my full M dwarf catalogue. I discuss yields of candidates based on my simulations. The possible contamination and bias from misidentified M dwarfs is then discussed, from chance alignments with other M dwarfs and UCDs, from chance alignments with giant stars, from chance alignments with galaxies, and from blended systems (via visual inspection). I then use optical spectra from LAMOST to spectral type a subset of my M dwarf + ultracool dwarf candidates. These candidates need confirming as true M dwarf + ultracool dwarf systems thus I present a new method I developed to use low resolution near-infrared spectra which relies on two colour similar objects (one an excess candidate, one not) having very similar spectra. A spectral difference of these two colour similar objects should leave the signature of a UCD in the residual of their differences, which I look for using the difference in two spectral bands designed to identify UCD spectral features. I then present the methods used to identify other rare systems from my full M dwarf catalogue. Young M dwarfs were identified by measuring equivalent widths of Hα from the LAMOST spectra, and by measuring rotation periods from Kepler 2 light curves. I identify late M dwarfs photometrically (using reduced proper motion and colour cuts) and spectroscopically (using the LAMOST spectra with spectral indices from the literature). Also I present common proper motion analysis aimed at finding Tycho-2 primaries for my M dwarfs and look for physically separated M dwarf + M dwarf pairs (internally within my full M dwarf catalogue).

Le moyen infrarouge est la région spectrale comprise entre 2 et 20 µm. Cette gamme de longueurs d'onde présente un fort intérêt scientifique grâce à la présence des transitions vibrationnelles fondamentales caractéristiques d'espèces moléculaires en phase liquide ou gazeuse. Les capteurs en optique intégrée sont devenus une excellente alternative pour la détection in situ car ils présentent certains avantages sur les autres types de capteurs, tels que l'intégration des éléments dans un dispositif compact. Ils sont capables d'effectuer des détections sélectives et quantitative dans divers domaines sociétaux tels que la santé, la défense et l'environnement. Dans cette thèse, nous présentons la conception, la fabrication et la caractérisation à 7.7 µm d'un transducteur spectroscopique intégré à base de verres de chalcogénures. Des couches à base de Ge-Sb-Se ont été déposées par pulvérisation cathodique RF magnétron. Des guides d'onde de type ridge ont ensuite été mise en forme, par photolithographie et gravure sèche (gravure ionique réactive) avec un plasma de CHF3. Différentes structures ont ainsi pu être réalisées : guides droits, jonctions Y, guide en spirale ou encore guides en S. En outre, la propagation guidée a été observé à 7.7 µm et des pertes de propagation égales à 2.5 dB/cm ont été mesurées à cette longueur d'onde. Enfin, des substances chimiques en phase liquide (isopropanol et acide acétique dissous dans du cyclohexane) ont été détectées par onde évanescente. Des limites de détection égales à 2 %v/v et 0.2 %v/v ont été, respectivement, démontrées à 7.7 µm pour l'isopropanol et l'acide acétique. Enfin, des simulations ont démontré le potentiel de ce capteur intégré pour la détection de substances polluantes contenues dans l'eau ou dans l'air avec des limites de détection inférieures à celles imposées par les normes internationales de l'environnement. L'ensemble de ces résultats représente une première étape prometteuse vers le développement d'applications dans le moyen Infrarouge au sein du laboratoire Foton. / The mid-infrared is a spectral range (2-20 µ m) of great scientific and technological interest. Indeed, the strong vibrational absorption bands of numerous molecules overlap this wavelength range. Thus, the mid-infrared has become in the last years a suitable solution for chemical sensing applications in gas or liquid phase. Furthermore, on-chip sensors provide several advantages over other kind of sensors, such as high integration of elements in a compact device and low fabrication cost by an easy-going to mass production. They could allow quantitative, sensitive and selective detection for health, defense and environmental applications. This thesis presents the design, fabrication and optical characterization at 7.7 µ m of a spectroscopic optical integrated sensor based on chalcogenide glasses. Ge-Sb-Se multilayered structures were deposited by RF magnetron sputtering. Using i-line photolithography and fluorine-based reactive ion etching (RIE-ICP), ridge waveguides were processed as straight waveguide, Y-junction, spiral and S-shape waveguides. Single-mode optical propagation at 7.7 µ m was observed by optical near-field imaging and optical propagation losses of 2.5 dB/cm were measured. Finally, chemical substances in liquid phase (isopropanol - and acetic acid, both dissolved in cyclohexane) were detected at 7.7 µ m by evanescent field. Limits of detection of 2 %v/v and 0.2 %v/v are demonstrated, respectively, for isopropanol and acetic acid. Furthermore, simulations were performed to assess the potential of the optical integrated sensor to achieve limits of detection lower than environmental and health standards for air and water pollutants. These results represent a first promising step towards the development of mid-infrared applications at the Foton laboratory.