Electromagnetic radiation in the frequency range between about 100 GHz and 5 THz can be used for spectroscopy and microscopy, but it is also promising for security screening and even wireless communication. In the present thesis a planar photoconducting large-area THz radiation source is presented. The device exhibits outstanding properties, in particular high THz field strength and generation efficiency and large spectral bandwidth with short THz pulse length. The THz emission is based on acceleration and deceleration of photoexcited carriers in semiconductor substrates. A metallic interdigitated structure at the surface of semi-insulating GaAs provides the electrodes of an Auston switch. In a biased structure photoexcited charge carriers are accelerated. Hence electromagnetic waves are emitted. An appropriately structured second metallization, electrically isolated from the electrodes, prevents destructive interference of the emitted waves. The structure investigated here combines several advantages of different conventional photoconducting THz sources. First, it provides high electric acceleration fields at moderate voltages owing to the small electrode separation. Second, the large active area in the mm2 range allows excitation by large optical powers of some mW. Optical excitation with near-infrared femtosecond lasers is possible with repetition rates in the GHz range. The presented results point out the excellent characteristics regarding the emitted THz field strength, average power, spectral properties, and easy handling of the interdigitated structure in comparison to various conventional emitter structures. Various modifications of the semiconductor substrate and the optimum excitation conditions were investigated. In the second part of this thesis the dynamic conductivity of GaAs/AlxGa1-xAs superlattices in an applied static electric field was investigated with time-resolved THz spectroscopy. The original goal was to explore whether the predicted effect of gain of electromagnetic radiation at THz frequencies is present in such structures. Superlattice samples were grown according to the experimental requirements, which include high specific resistance and sufficient THz transparency. The characterization of the superlattices by Fourier transform infrared spectroscopy and photoluminescence spectroscopy confirms the pronounced miniband properties of the bandstructure. Furthermore indications of Bloch oscillations were found by transport measurements. However, we could not measure a change of the dynamic conductivity when the electric field was toggled. Specific reasons for this and related experiments of other groups are discussed. / Elektromagnetische Strahlung im Frequenzbereich zwischen etwa 100 GHz und 5 THz wird für verschiedene Anwendungen wie Spektroskopie und Mikroskopie genutzt, kann aber auch für Sicherheitstechnik oder sogar Datenübertragung interessant sein. In der hier vorgestellten Forschungsarbeit wird eine großflächige fotoleitende THz-Strahlungsquelle beschrieben, die sich durch eine große THz-Feldstärke und große spektrale Bandbreite auszeichnet. Die THz-Emission basiert auf der Beschleunigung und Verzögerung fotogenerierter Ladungen in Halbleitersubstraten. Eine metallische Interdigitalstruktur auf der Oberfläche von semi-isolierendem GaAs bildet die Elektroden eines Fotoschalters. Ist an diese Struktur eine Spannung angeschlossen, werden optisch generierte Ladungsträger beschleunigt und strahlen elektromagnetische Wellen ab. Eine geeignet strukturierte und isolierte zweite Metallisierung verhindert destruktive Interferenzen der abgestrahlten Wellen. Die vorgeschlagene Struktur vereinigt dabei die Vorteile verschiedener herkömmlicher fotoleitender THz-Quellen. Einerseits ermöglicht der kleine Elektrodenabstand große elektrische Felder zur Beschleunigung fotogenerierter Ladungen schon bei moderaten Spannungen. Andererseits kann die große aktive Fläche von einigen mm2 mit großen optischen Leistungen im Bereich einiger mW angeregt werden. Die optische Anregung mit Nahinfrarot-Femtosekunden- Lasern kann mit Wiederholraten bis in den GHz-Bereich geschehen. Bedingt durch die Eigenschaften der Anregungspulse entstehen kurze spektral breite THz-Pulse. Die vorliegenden Ergebnisse verdeutlichen die hervorragenden Eigenschaften der Interdigitalstruktur im Vergleich zu verschiedenen herkömmlichen Geometrien bezüglich der Feldstärke der abgestrahlten Wellen, der mittleren Leistung und der spektralen Eigenschaften. Dabei ist die Struktur sehr einfach zu handhaben. Es wurden verschiedene Modifikationen des Substrates und die optimalen Bedingungen der optischen Anregung untersucht. Der zweite Teil dieser Arbeit behandelt die Erforschung der dynamische Leitfähigkeit von GaAs/AlxGa1-xAs-Übergittern in Abhängigkeit von einem elektrischen Feld mit Hilfe der zeitaufgelösten THz-Spektroskopie. Es sollte geklärt werden, ob der vorhergesagte Effekt der Verstärkung elektro-magnetischer Strahlung in solchen Strukturen möglich ist. Dazu wurden Übergitterproben gemäß den experimentellen Anforderungen hergestellt. Zu den Vorgaben gehört ein hoher spezifischer Widerstand und ausreichende Transparenz im THz-Bereich. Die Charakterisierung der Übergitter mit Fotolumineszenz- und Fourier-Transformations-IR-Spektroskopie bestätigte die ausgeprägten Minibandeigenschaften der Bandstruktur. Hinweise auf Bloch-Oszillationen wurden durch Ladungstransportmessungen gefunden. Dennoch war eine Änderung der dynamischen Leitfähigkeit beim Schalten des elektrischen Feldes nicht messbar. Gründe dafür und ähnliche Experimente anderer Gruppen werden diskutiert.

This thesis deals with infrared studies of impurity states, ultrafast carrier dynamics as well as coherent intersubband polarizations in semiconductor quantum structures such as quantum wells and superlattices, based on the GaAs/AlGaAs material system. In the first part it is shown that the 2pz confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, showing that at low temperatures in the low doping density regime all optical transitions originate from impurity transitions. These results also require reinterpretation of previous experimental data. The relaxation dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices in the mid-IR are studied. This involves single-color pump-probe measurements to explore the dynamics at different wavelengths, which is performed with the Rossendorf freeelectron laser (FEL), providing picosecond pulses in a range from 3-200 µm and are used for the first time within this thesis. In these experiments, a fast bleaching of the interminiband transition is observed followed by thermalization and subsequent relaxation, whose time constants are determined to be 1-2 picoseconds. This is followed by an additional component due to carrier cooling in the lower miniband. In the second part, two-color pump-probe measurements are performed, involving the FEL as the pump source and a table-top broad-band tunable THz source for probing the transmission changes. These measurements allow a separate specification of the cooling times after a strong excitation, exhibiting time constants from 230 ps to 3 ps for different excitation densities and miniband widths. In addition, the dynamics of excited electrons within the minibands is explored and their contribution quantitatively extracted from the measurements. Intersubband absorption experiments of photoexcited carriers in single quantum well structures, measured directly in the time-domain, i.e. probing coherently the polarization between the first and the second subband, are presented. From the data we can directly extract the density and temperature dependence of the intersubband dephasing time between the two lowest subbands, ranging from 50 up to 400 fs. This all optical approach gives us the ability to tune the carrier concentration over an extremely wide range which is not accessible in a doped quantum well sample. By varying the carrier density, many-body effects such as the depolarization and their influence on the spectral position as well as on the lineshape on the intersubband dephasing are studied. Also the difference of excitonic and free-carrier type excitation is discussed, and indication of an excitonic intersubband transition is found.

<p>The main theme covered in this thesis is experimentalstudies of quantum dynamics and coherent control in homonuclearalkali diatomic molecules by ultrafast laser spectroscopy iththe implementation of pump-probe techniques.</p><p>A series of experiments have been performed on the Rb2molecules in a molecular beam as well as in a thermal oven. Thereal-time molecular quantum dynamics of the predissociatingelectronically excited D(3)¹Πu state of Rb₂, which couples to/intersects several otherneighbouring states, is investigated using wavepackets. Thepredissociation of the D state, explored by this wavepacketmethod, arises from two independent states, the (4)³Σ_u⁺and (1)³∆_u, for which the second corresponds to a much fasterdecay channel above a sharp energy threshold around 430 nm. Thelifetime of the D state above the energy threshold is obtained,τ ≈ 5 ps, by measuring the decay time of thewavepacket in a thermal oven. Further experimentalinvestigation performed in a molecular beam together withquantum calculations of wavepacket dynamics on the D state haveexplored new probe channels of wavepacket evolution: theD′(3)1Σu+ channel, which exhibits vibrational motionin a shelf state and the (4)³Σu+ channel, where direct build-up of thewavefunction is observed due to its spin-orbit oupling to the Dstate.</p><p>The real-time quantum dynamics of wavepackets confined totwo bound states, A¹Σ_u⁺(0_u⁺) and b³Π_u(0_u⁺), have been studied by experiment andcalculations. It is shown that these two states are fullycoupled by spin-orbit interaction, characterised by itsintermediate strength. The intermediate character of thedynamics is established by complicated wavepacket oscillationatterns and a value of 75 cm^-1is estimated for the coupling strength at thestate crossing.</p><p>The experiments on the Li₂molecule are performed by coherent control ofrovibrational molecular wavepackets. First, the Deutsch-Jozsaalgorithm is experimentally demonstrated for three-qubitfunctions using a pure coherent superposition of Li₂rovibrational eigenstates. The function’scharacter, either constant or balanced, is evaluated by firstimprinting the function, using a phase-tailored femtosecond(fs) pulse, on a coherent superposition of the molecularstates, and then projecting the superposition onto an ionicfinal state using a second fs pulse at a specific delay time.Furthermore, an amplitude-tailored fs pulse is used to exciteselected rovibrational eigenstates and collision induceddephasing of the wavepacket signal, due to Li₂-Ar collisions, is studied experimentally. Theintensities of quantum beats decaying with the delay time aremeasured under various pressures and the collisional crosssections are calculated for each well-defined rovibrationalquantum beat, which set the upper limitsfor ure dephasingcross sections.</p><p><b>Keywords:</b>Ultrafast laser spectroscopy, pump-probetechnique, predissociation, wavepacket, pin-orbit interaction,coherent control, (pure) dephasing</p>

This thesis deals with various functional materials from first-principles methods and is divided into two major parts according to the underlying atomic structure of the system under study. The first part of the thesis deals with the temperature-induced structural phase transitions in metallic  β'-AuZn and perovskite oxide LiOsO3. The former one, i.e. binary AuZn, belongs to a class of shape-memory alloys that regain their initial shape due to a reversible martensitic phase transformation. Here, by means of density functional and density functional perturbation theories, we show that the martensitic transition is due to coupling between the Fermi surface nesting and anomalies in the phonon dispersion relations. The other metallic system, perovskite LiOsO3, exhibits a ferroelectric-like transition and is currently the first and sole realization of the Anderson and Blount idea. By means of ab initio molecular dynamics simulations, we investigate the mechanism behind this structural phase transformation. Another part of the thesis is dedicated to modelling and characterization of topologically disordered materials on atomic level. The structural and electronic properties of amorphous W-S-N are addressed regarding its outstanding tribological properties, i.e. almost vanishing friction coefficient. Molecular dynamics “melt-and-quench” technique has been employed in order to construct a model structure of amorphous W-S-N. Further analysis of the atomic structure revealed a formation of quasi-free N2 molecules trapped in S cages, which, together with the complex atomic structure of W-S-N, is the key to ultra-low-friction in this functional material. In the last chapter of the thesis a magnetic class of amorphous materials is addressed. Magnetic order in amorphous Gd-Fe ferrimagnet has been shown to undergo magnezation switching driven by a femtosecond laser pulse. Here, we combine first-principles density functional theory and atomistic spin dynamics simulations to explore this phenomena. A possible mechanism behind magnetization reversal in Gd-Fe based on a combination of the Dzyaloshinskii-Moriya interaction and exchange frustration is proposed.

Topological insulators (TI) are a new phase of matter with very exotic electronic properties on their surface. As a direct consequence of the topological order, the surface electrons of TI form bands that cross the Fermi surface odd number of times and are guaranteed to be metallic. They also have a linear energy-momentum dispersion relationship that satisfies the Dirac equation and are therefore called Dirac fermions. The surface Dirac fermions of TI are spin-polarized with the direction of the spin locked to momentum and are immune from certain scatterings. These unique properties of surface electrons provide a platform for utilizing TI in future spin-based electronics and quantum computation. The surface bands of 3D TI can be directly mapped by angle-resolved photoemission spectroscopy (ARPES) and the spin polarization can be determined by spin-resolved ARPES. These types of experiments are the first to establish the 3D topological order, which demonstrates the power of ARPES in probing the surface of strongly spin-orbit coupled materials. Extensive investigation of TI has ranged from understanding the fundamental electronic and lattice structure of various TI compounds to building TI-based devices in search of more exotic particles such as Majorana fermions and magnetic monopoles. Surface-sensitive techniques that can efficiently disentangle the charge and spin degrees of freedom have been crucially important in tackling the multi-faceted problems of TI. In this thesis, I show that laser-based ARPES in combination with a time-of-flight spectrometer is a powerful tool to study the spin structure and charge dynamics of the Dirac fermions on the surface of TI. Chapter 1 gives a brief introduction of TI. Chapter 2 describes the basic principles behind ARPES and time-resolved ARPES (TrARPES). Chapter 3 provides a detailed account of the experimental setup to perform laser-based ARPES and TrARPES. In Chapters 4 and 5, how these two techniques are effectively applied to investigate two unique electronic properties of TI is elaborated. Through these studies, I have obtained a complete mapping of the spin texture of several prototypical topological insulators and have uncovered the cooling mechanism governing the hot surface Dirac fermions. / Physics

Decoherence is central to our understanding of the transition from the quantum to the classical world. It is also a way of probing the dynamics of interacting many-body systems. Photoexcited semiconductors are such systems in which the transient dynamics can be studied in considerable detail experimentally. Recent advances in spectroscopy of semiconductors provide powerful tools to explore many-body physics in new regimes. An appropriate theoretical framework is necessary to describe new physical effects now accessible for observation. We present a possible approach in this thesis, and discuss results of its application to an experimentally relevant scenario. The major portion of this thesis is devoted to a formalism for the multi-dimensional Fourier spectroscopy of semiconductors. A perturbative treatment of the electromagnetic field is used to derive a closed set of differential equations for the multi-particle correlation functions, which take into account the many-body effects up to third order in the field. A diagrammatic method is developed, in which we retain all features of the double-sided Feynman diagrams for bookkeeping the excitation scenario, and complement them by allowing for the description of interactions. We apply the formalism to study decoherence between the states of optically excited excitons embedded in an electron gas, and compare it with the decoherence between these states and the ground state. We derive a dynamical equation for the two-time correlation functions of excitons, and compare it with the corresponding equation for the interband polarization. It is argued, and verified by numerical calculation, that the decay of Raman coherence depends sensitively on how differently the superimposed exciton states interact with the electron gas, and that it can be much slower than the decay of interband polarization. We also present a new numerical approach based on the length gauge for modeling the time-dependent laser-semiconductor interaction. The interaction in the length gauge involves the position operator for electrons, as opposed to the momentum operator in the velocity gauge. The approach is free of the unphysical divergences that arise in the velocity gauge. It is invariant under local gauge symmetry of the Bloch functions, and can handle arbitrary electronic structure and temporal dependence of the fields.

Decoherence is central to our understanding of the transition from the quantum to the classical world. It is also a way of probing the dynamics of interacting many-body systems. Photoexcited semiconductors are such systems in which the transient dynamics can be studied in considerable detail experimentally. Recent advances in spectroscopy of semiconductors provide powerful tools to explore many-body physics in new regimes. An appropriate theoretical framework is necessary to describe new physical effects now accessible for observation. We present a possible approach in this thesis, and discuss results of its application to an experimentally relevant scenario. The major portion of this thesis is devoted to a formalism for the multi-dimensional Fourier spectroscopy of semiconductors. A perturbative treatment of the electromagnetic field is used to derive a closed set of differential equations for the multi-particle correlation functions, which take into account the many-body effects up to third order in the field. A diagrammatic method is developed, in which we retain all features of the double-sided Feynman diagrams for bookkeeping the excitation scenario, and complement them by allowing for the description of interactions. We apply the formalism to study decoherence between the states of optically excited excitons embedded in an electron gas, and compare it with the decoherence between these states and the ground state. We derive a dynamical equation for the two-time correlation functions of excitons, and compare it with the corresponding equation for the interband polarization. It is argued, and verified by numerical calculation, that the decay of Raman coherence depends sensitively on how differently the superimposed exciton states interact with the electron gas, and that it can be much slower than the decay of interband polarization. We also present a new numerical approach based on the length gauge for modeling the time-dependent laser-semiconductor interaction. The interaction in the length gauge involves the position operator for electrons, as opposed to the momentum operator in the velocity gauge. The approach is free of the unphysical divergences that arise in the velocity gauge. It is invariant under local gauge symmetry of the Bloch functions, and can handle arbitrary electronic structure and temporal dependence of the fields.

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.

I present a detailed study of many-body effects associated with the interband 1s transition and intraband 1s-2p transition in two- and three-dimensional photo-excited semiconductors. I employ a previously developed excitonic model to treat effects of exchange and phase space filling. I extend the scope of the model to include static free-carrier screening. I also develop a factorization scheme to obtain a consistent set of excitonic dynamical equations. The exciton transition energies are renormalized by many-body interactions, and the excitonic dynamical equations provide simple expressions for the individual contributions of screening, phase space filling and exchange. The effects of exchange and phase space filling are quantified by a set of excitonic coefficients. I first calculate these coefficients analytically by omitting screening effects. In contrast, the screened coefficients involve multi-dimensional integrals which must be evaluated numerically. I present a detailed discussion of the numerical methods used to evaluate these integrals, which include a novel algorithm for segmenting multi-dimensional integration regions. The excitonic model correctly predicts the blue shift and bleaching of the 1s exciton resonance due to exchange and phase space filling. Free-carrier screening is found to enhance these effects by lowering the exciton binding energy. In contrast, the effects of free-carrier screening on the 1s-2p transition energy are more subtle. In the absence of free-carrier screening, exchange and phase space filling lead to a blue shift of the transition energy. However, screening decreases the 1s binding energy faster than the 2p binding energy, which in turn decreases the transition energy. Thus, screening effects oppose exchange and phase space filling, and the overall magnitude and sign of the 1s-2p transition energy shift depends on the free-carrier density. Specifically, for low-moderate excitation densities exchange and phase space filling can be dominated by screening, leading to a net red shift of the transition energy. The results for two- and three-dimensional systems are qualitatively similar, although the magnitudes of the shifts are much smaller in three dimensions. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2011-05-31 15:58:27.222

Yb-ion doped gain media have become the material of choice for reliable generation of ultrashort pulses at wavelength around 1 μm. At present, however, operation at high average power (>1 W) with sub-100 fs pulses still remains challenging. The efforts of developing an Yb-ion oscillator towards this goal, therefore, are the main focus of this thesis. In this work, the Yb:KGd(WO4)2 (Yb:KGW) crystals were chosen to serve as the gain media. To achieve high power operation, two fundamental issues have been carefully considered: 1) a new pumping scheme was proposed to alleviate the thermal issues in the Yb:KGW crystals; 2) a new method was introduced to characterize intracavity losses in the broadband Yb-ion oscillators. As a side effect observed during the optimization of the CW operation, simultaneous two-wavelength emission was also discussed. With the knowledge and experimental understanding of the fundamental issues in laser oscillators operated in the continuous-wave regime, the next step of this work demonstrated their operation in a pulsed regime. The dual action of the Kerr-lens and saturable absorber (KLAS) mode locking was proposed in this work and resulted in greatly enhanced laser performance. The laser delivered pulses with 67 fs duration at a repetition rate of 77 MHz. The average output power reached 3 W, which, to the best of our knowledge, is the highest average output power produced to date from the Yb-ion based bulk lasers with such a short pulse duration. The scalability of pulse energy and peak power was also demonstrated by reducing the repetition rate to either 36 MHz or 18 MHz. The cavity with the latter repetition rate produced 85 fs pulses with the pulse energy up to 83 nJ, which corresponds to a peak power as high as 1 MW. As required by many biomedical applications, the wavelength of the generated pulses (~1 μm) can be tuned in the near-infrared region by coupling them into an optical parametric oscillator (OPO). The feasibility of this approach was demonstrated in the last part of this thesis, through a thorough theoretical analysis of two OPO materials suitable for excitation at 1.04 μm.