Two colour pumping was used to investigate the short-pulse technique of Multifrequency Raman Generation (MRG) in the transient regime of Raman scattering. In the course of this study we have demonstrated the ability to generate over thirty Raman orders spanning from the infrared to the ultraviolet, investigated the dependence of this generation on the pump intensities and the dispersion characteristics of the hollow-fibre system in which the experiment was conducted, and developed a simple computer model to help understand the exhibited behaviours. These dependence studies have revealed some characteristics that have been previously mentioned in the literature, such as the competition between MRG and self-phase modulation, but have also demonstrated behaviours that are dramatically different than anything reported on the subject. Furthermore, through a simple modification of the experimental apparatus we have demonstrated the ability to scatter a probe pulse into many Raman orders, generating bandwidth comparable to the best pump-probe experiments of MRG. By using a numeric fast Fourier transform, we predict that our spectra can generate pulses as short as 3. 3fs, with energies an order of magnitude larger than pulses of comparable duration that are made using current techniques.

Methods of controlling molecular rotations using linearly polarized femtosecond and picosecond pulses are considered and analyzed theoretically. These laser pulses, typically in the infrared, are highly non-resonant with respect to the electronic degrees of freedom of the molecules and have intensities of &sim; 10^13 to 10^14 W/cm&sup2;. It is shown how these laser pulses can force small linear molecules to align with the direction of the electric field vector of the laser both in the presence of the laser field as well as after the application of a short laser pulse. Recent experiments on laser-induced molecular alignment are modeled and excellent agreement between experiment and theory is found. Additional methods of controlling molecular rotational dynamics are outlined. The first method considers the forced rotational acceleration of diatomic molecules, called the <i>optical centrifuge</i>. Here, the direction of polarization of a linearly polarized laser field is made to smoothly rotate faster and faster. The molecules, which tend to align with the polarization vector of the laser field, follow the rotation of the laser polarization and are accelerated to high angular momentum. The second method considers the control of field-free rotational dynamics by applying phase shifts to the molecular wave function at select times called <i>fractional revivals</i>. At these select moments, an initially localized wave function splits into several copies of the initial state. Adding phase shifts to the copies then induces interference effects which can be used to control the subsequent evolution of the rotational wave function. This same control scheme has a close link to quantum information and this connection is outlined. Finally, a recently proposed method of controlling the quantum dynamics of the classically chaotic kicked rotor system [J. Gong and P. Brumer, Phys. Rev. Lett. 86, 1741 (2001)] is analyzed from a phase space perspective. It is shown that the proposed quantum control can be linked to small islands of stability in the classical phase space. An experimentally feasible variant of this control scenario using wave packets of molecular alignment is proposed. Two applications of molecular alignment are discussed. The first application uses field-free aligned molecules as a non-linear medium for compression of a laser pulse to the 1 fs regime at optical wavelengths. At such durations, these laser pulses contain nearly a single oscillation of the electric field and represent the shortest laser pulses physically achievable for such frequencies. The second application uses molecular alignment to create a sort of gas phase "molecular crystal" which forms a basis for laser-induced electron diffraction and imaging of the aligned molecules. Here, a first laser pulse aligns the molecules in space. A second laser pulse is then used to ionize outer-shell electrons, accelerate them in the laser field, and steer them back to collide with the parent ion creating a diffraction image with sub-femtosecond and sub-Angstrom resolution.

Two colour pumping was used to investigate the short-pulse technique of Multifrequency Raman Generation (MRG) in the transient regime of Raman scattering. In the course of this study we have demonstrated the ability to generate over thirty Raman orders spanning from the infrared to the ultraviolet, investigated the dependence of this generation on the pump intensities and the dispersion characteristics of the hollow-fibre system in which the experiment was conducted, and developed a simple computer model to help understand the exhibited behaviours. These dependence studies have revealed some characteristics that have been previously mentioned in the literature, such as the competition between MRG and self-phase modulation, but have also demonstrated behaviours that are dramatically different than anything reported on the subject. Furthermore, through a simple modification of the experimental apparatus we have demonstrated the ability to scatter a probe pulse into many Raman orders, generating bandwidth comparable to the best pump-probe experiments of MRG. By using a numeric fast Fourier transform, we predict that our spectra can generate pulses as short as 3. 3fs, with energies an order of magnitude larger than pulses of comparable duration that are made using current techniques.

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.

Diffractive optical elements (DOEs) serve an important function in many dynamic and static optical systems. The theory and design of surface diffractive structures are well understood and practically applied at high spatial and phase resolution for a wide range of optical applications in science and industry. However, these structures normally only harness phase modulation of uniform fields for the beam diffraction and therefore limit their range of application, as well as being susceptible to surface damage. Multilayered volume diffractive elements offer a powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. The advent of femtosecond laser writing inside transparent media has enabled the facile embedding of optical devices such as waveguides and diffractive optics into novel three-dimensional geometries that offer advanced functionality with compact design. In this work, femtosecond laser writing is pushed to the limits of forming high resolution phase elements with sufficiently strong refractive index contrast on which to develop volume phase gratings with the highest diffractive efficiency. The formation of both positive and negative zones of refractive index contrast together with rapid Talbot self imaging inside weakly contrasting phase gratings are major challenges here diminish the efficiency of assembled gratings. A method of strategic layering of otherwise weakly diffracting gratings onto Talbot planes is introduced to demonstrate, in FDTD models, the definitive enhancement of overall diffraction efficiency. A systematic optimization of laser writing in fused silica verify this enhancement or diminishment with weak volume gratings assembled on aligned or misaligned on Talbot planes. Advanced laser beam control methods were further demonstrated that underpin new direction for the facile assembly of highly functional DOEs that can exploit coherent light diffraction for opportunities in improving the performance of holographic devices and extend further to the powerful combination of phase and amplitude modulation control that is potentially available in a single optical device, thereby opening new directions for the design and fabrication of robust and strongly diffracting volume optical devices.

Diffractive optical elements (DOEs) serve an important function in many dynamic and static optical systems. The theory and design of surface diffractive structures are well understood and practically applied at high spatial and phase resolution for a wide range of optical applications in science and industry. However, these structures normally only harness phase modulation of uniform fields for the beam diffraction and therefore limit their range of application, as well as being susceptible to surface damage. Multilayered volume diffractive elements offer a powerful opportunity to harness both phase and amplitude modulation for benefits in diffraction efficiency and beam shaping. However, multilayered combinations have been difficult to fabricate and provide only weak diffraction for phase gratings with low refractive index contrast. The advent of femtosecond laser writing inside transparent media has enabled the facile embedding of optical devices such as waveguides and diffractive optics into novel three-dimensional geometries that offer advanced functionality with compact design. In this work, femtosecond laser writing is pushed to the limits of forming high resolution phase elements with sufficiently strong refractive index contrast on which to develop volume phase gratings with the highest diffractive efficiency. The formation of both positive and negative zones of refractive index contrast together with rapid Talbot self imaging inside weakly contrasting phase gratings are major challenges here diminish the efficiency of assembled gratings. A method of strategic layering of otherwise weakly diffracting gratings onto Talbot planes is introduced to demonstrate, in FDTD models, the definitive enhancement of overall diffraction efficiency. A systematic optimization of laser writing in fused silica verify this enhancement or diminishment with weak volume gratings assembled on aligned or misaligned on Talbot planes. Advanced laser beam control methods were further demonstrated that underpin new direction for the facile assembly of highly functional DOEs that can exploit coherent light diffraction for opportunities in improving the performance of holographic devices and extend further to the powerful combination of phase and amplitude modulation control that is potentially available in a single optical device, thereby opening new directions for the design and fabrication of robust and strongly diffracting volume optical devices.

This thesis describes an experimental investigation of optical frequency conversion in isotropic semiconductor waveguides by use of several phase-matching approaches. Efficient, type I second harmonic generation of femtosecond pulses is reported in birefringently-phase-matched GaAs/Alox waveguides pumped at 2.01 μm. Practical second harmonic average powers of up to ~ 650 μW are obtained, for an average launched pump power of ~ 5 mW. This corresponds to a waveguide conversion efficiency of ~ 20 % and a normalized conversion efficiency of greater than 1000 % W−1cm−2. Pump depletion of more than 80 % is recorded. Second harmonic generation by type I, third order quasi-phase-matching in a GaAs- AlAs superlattice waveguide is reported for fundamental wavelengths from ~1480 to 1520 nm. Quasi-phase-matching is achieved through modulation of the nonlinear coefficient χ[sub](zxy)(2), which is realised by periodically tuning the superlattice bandgap. An average output power of ~25 nW is obtained for a launched pump power of < 2.3 mW. Type I second harmonic generation by use of first order quasi-phase-matching in a GaAs/AlAs symmetric superlattice waveguide is also reported, with femtosecond fundamental pulses at 1.55 μm. A periodic spatial modulation of the bulk-like second- order susceptibility χ[sub](zxy)(2) is realized using quantum well intermixing by As+ ion implantation. A practical second harmonic average power of ~1.5 μW is detected, for a coupled pump power of ~11 mW. Second harmonic generation through modal-phase-matching in GaAs/AlGaAs semiconductor waveguides is reported. Using femtosecond pulses, both type I and type II second harmonic conversion is demonstrated for fundamental wavelengths near 1.55 μm. An average second harmonic power of ~10.3 μW is collected at the waveguide output for a coupled pump power of < 20 mW. For a complete characterisation, the optical loss is measured in these nonlinear GaAs- based waveguides over the spectral range 1.3-2.1 μm in the infrared, by deploying a femtosecond scattering technique. Typical losses of ~5-10 dB/cm are measured for the best of the waveguides, while a systematic intensity and wavelength dependent study revealed the contribution of Rayleigh scattering and two photon absorption in the overall transmission loss.

This thesis outlines the high intensity tabletop attosecond extreme ultraviolet laser source at the Institute for the Frontier of Attosecond Science and Technology Laboratory. First, a unique Ti:Sapphire chirped pulse amplifier laser system that delivers 14 fs pulses with 300 mJ energy at a 10 Hz repetition rate was designed and built. The broadband spectrum extending from 700 nm to 900 nm was obtained by seeding a two stage Ti:Sapphire chirped pulse power amplifier with mJ-level white light pulses from a gas filled hollow core fiber. It is the highest energy level ever achieved by a broadband pulse in a chirped pulse amplifier up to the current date. Second, using this laser as a driving laser source, the generalized double optical gating method is employed to generate isolated attosecond pulses. Detailed gate width analysis of the ellipticity dependent pulse were performed. Calculation of electron light interaction dynamics on the atomic level was carried out to demonstrate the mechanism of isolated pulse generation. Third, a complete diagnostic apparatus was built to extract and analyze the generated attosecond pulse in spectral domain. The result confirms that an extreme ultraviolet super continuum supporting 230 as isolated attosecond pulses at 35 eV was generated using the generalized double optical gating technique. The extreme ultraviolet pulse energy was ∼100 nJ at the exit of the argon gas target.

No description available.

At the heart of high harmonic generation lies a combination of optical and collision physics entwined by a strong laser field. An electron, initially tunnel-ionized by the field, driven away then back in the continuum, finally recombines back to rest in its initial ground state via a radiative transition. The emitted attosecond (atto=10^-18) XUV light pulse carries all the information (polarization, amplitude and phase) about the photorecombination continuum-to-ground transition dipolar field. Photorecombination is related to the time-reversed photoionization process. In this perspective, high-harmonic spectroscopy extends well-established photoelectron spectroscopy, based on charged particle detection, to a fully coherent one, based on light characterization. The main achievement presented in this thesis is to use high harmonic generation to probe femtosecond (femto=10^-15) chemical dynamics for the first time. Thanks to the coherence imposed by the strong driving laser field, homodyne detection of attosecond pulses from excited molecules undergoing dynamics is achieved, the signal from unexcited molecules acting as the reference local oscillator. First, applying time-resolved high-harmonic spectroscopy to the photodissociation of a diatomic molecule, Br2 to Br + Br, allows us to follow the break of a chemical bond occurring in a few hundreds of femtoseconds. Second, extending it to a triatomic (NO2) lets us observe both the previously unseen (but predicted) early femtosecond conical intersection dynamics followed by the late picosecond statistical photodissociation taking place in the reaction NO2 to NO + O. Another important realization of this thesis is the development of a complementary technique to time-resolved high-harmonic spectroscopy called LAPIN, for Linked Attosecond Phase INterferometry. When combined together, time-resolved high-harmonic spectroscopy and LAPIN give access to the complex photorecombination dipole of aligned excited molecules. These achievements lay the basis for electron recollision tomographic imaging of a chemical reaction with unprecedented angstrom (1 angstrom= 0.1 nanometer) spatial resolution. Other contributions dedicated to the development of attosecond science and the generalization of high-harmonic spectroscopy as a novel, fully coherent molecular spectroscopy will also be presented in this thesis.