We investigate ionization and high harmonic generation (HHG) from the interaction of a mid infra-red laser pulse with a solid state system confined to nano-dimensions. The theory of strong field processes in solids is developed for confined quantum systems in general. Here it is applied to two-dimensional quantum wires with a driving field linearly polarised along the axis of the wires. Our findings indicate that that we are able to control the ionization and high-harmonic output by altering the width of the wire. Control of ionization leads to an increased damage threshold which has important implications for nano-engineering and realizing all solid state coherent XUV sources.

Master of Science / Department of Physics / Brian Washburn / Zenghu Chang / Attosecond pulse generation is a powerful tool to study electron dynamics in atoms and molecules. However, application of attosecond pulses is limited by the low photon flux of attosecond sources. Theoretical models predict that the harmonic efficiency scales as λ[lambda]-6 in the plateau region of the HHG spectrum, where λ [lambda] is the wavelength of the driving laser. This indicates the possibility of generating more intense attosecond pulses using short wavelength driving lasers. The purpose of this work is to find a method to generate intense single attosecond pulses using a 400 nm driving laser. In our experiments, 400 nm femtosecond laser pulses are used to generate high harmonics. First, the dependence of the high harmonic generation yield on the ellipticity of 400 nm driving laser pulse is studied experimentally, and it is compared with that of 800 nm driving lasers. A semi-classical theory is developed to explain the ellipticity dependence where the theoretical calculations match experiment results very well. Next, 400 nm short pulses (sub-10 fs) are produced with a hollow core fiber and chirped mirrors. Finally, we propose a scheme to extract single attosecond pulses with the Generalized Double Optical Gating (GDOG) method.

Orbital angular momentum (OAM) is a property of light that is widely used for applications in bioimaging, optical communication and optical manipulation, but is mainly limited to the infrared and visible spectra. Developing a table-top source of Extreme Ultraviolet (XUV) light containing an arbitrary amount of OAM is yet to be achieved. We accomplish this by exploiting high-harmonic generation (HHG), a process whereby an infrared pump beam produces high order harmonics. We experimentally demonstrate the conservation of OAM in HHG by measuring harmonics of order n containing n times the OAM of the pump (n = 11, 13, 15 in our experiment). These results agree with our theoretical model. We also show theoretically how to manipulate the HHG process to impart an arbitrary amount of OAM to the di fferent harmonics. We hence show the way to a table-top and flexible source of XUV light containing orbital angular momentum.

When intense laser pulses interact with an atomic or solid target, high order harmonics of the fundamental laser frequency are generated. In the case of atoms, this highly nonlinear optical process is initiated by ionization and terminated by the energetic recollision and recombination of the ionized electron with its correlated ion. In this thesis I demonstrate, both theoretically and experimentally, that high harmonics from bulk crystals can originate from the recollision of electrons with their associated holes, similarly to the atomic case, but where ionization is replaced by excitation of electron-hole pairs that accelerate within the material. This model is first derived from a quantum-mechanical theory of the solid-laser interaction, and then confirmed experimentally in ZnO and Si crystals. Despite the link I establish between high harmonic generation in solids and gases, there are notable dissimilarities. These include: a generalized motion of electrons and holes in their respective bands and its consequences, a more prominent role of dephasing and enhanced sensitivity to perturbing fields. These aspects are investigated throughout this thesis. Finally, I develop a method that exploits the recollision mechanism to reconstruct the momentum-dependent band structure of solids.

Orbital angular momentum (OAM) is a property of light that is widely used for applications in bioimaging, optical communication and optical manipulation, but is mainly limited to the infrared and visible spectra. Developing a table-top source of Extreme Ultraviolet (XUV) light containing an arbitrary amount of OAM is yet to be achieved. We accomplish this by exploiting high-harmonic generation (HHG), a process whereby an infrared pump beam produces high order harmonics. We experimentally demonstrate the conservation of OAM in HHG by measuring harmonics of order n containing n times the OAM of the pump (n = 11, 13, 15 in our experiment). These results agree with our theoretical model. We also show theoretically how to manipulate the HHG process to impart an arbitrary amount of OAM to the di fferent harmonics. We hence show the way to a table-top and flexible source of XUV light containing orbital angular momentum.

Doctor of Philosophy / Department of Physics / Vinod Kumarappan / In this thesis, our work on developing new techniques to measure and enhance field-free molecular alignment and orientation is described. Non-resonant femtosecond laser pulses are used to align and orient rotationally-cold gas-phase molecules. The time-dependent Schrodinger equation is solved to simulate the experimental results. A single-shot kHz velocity map imaging (VMI) spectrometer is developed for characterizing 1D and 3D alignment. Stimulated by a novel metric for 3D alignment proposed by Makhija et al. [Phys. Rev. A 85,033425 (2012)], a multi-pulse scheme to improve 3D alignment is demonstrated experimentally on difluoro-iodobenzene molecules and the best field-free 3D alignment is achieved. A degenerate four wave mixing probe is developed to overcome limitations in VMI measurement; experiments on different types of molecules show good agreement with computational results. Highly aligned linear molecules are used for high harmonic generation experiments. Due to the high degree of alignment, fractional revivals, variation of revival structure with harmonic order and the shape resonance and Cooper minimum in the photoionization cross section of molecular nitrogen are all observed directly in experiment for the first time. Enhanced orientation from rotationally cold heteronuclear molecules is also demonstrated. We follow the theory developed by Zhang et al. [Phys. Rev. A 83, 043410 (2011)] and demonstrate experimentally for the first time that for rotationally cold carbon monoxide an aligning laser pulse followed by a two-color laser pulse can increase field-free orientation level by almost a factor of three compared to using just the two-color pulse.

High Harmonic Generation (HHG) enables the production of bursts of coherent soft x-rays with attosecond pulse duration. This process arrises from the nonlinear interaction between intense infrared laser pulses and an ionizing gas medium. Soft x-ray photons are used for spectroscopy of inner-shell electron correlation and exchange processes, and the availability of attosecond pulse durations will enable these processes to be resolved on their natural time scales. The maximum or cutoff photon energy in HHG increases with both the intensity as well as the wavelength of the driving laser. It is highly desirable to increase the harmonic cutoff as this will allow for the generation of shorter attosecond pulses, as well as HHG spectroscopy of increasingly energetic electronic transitions. While the harmonic cutoff increases with laser wavelength, there is a corresponding decrease in harmonic yield. The first part of this thesis describes the experimental measurement of the wavelength scaling of HHG efficiency, which we report as lambda^(-6.3) in xenon, and lambda^(-6.5) in krypton. To increase the HHG cutoff, we have developed a 1.8 um source, with stable carrier envelope phase and a pulse duration of <2 optical cycles. The 1.8 um wavelength allowed for a significant increase in the harmonic cutoff compared to equivalent 800 nm sources, while still maintaing reasonable harmonic yield. By focusing this source into neon we have produced 400 eV harmonics that extend into the x-ray water window. In addition to providing a source of photons for a secondary target, the HHG spectrum caries the signature of the electronic structure of the generating medium. In krypton we observed a Cooper minimum at 85 eV, showing that photoionization cross sections can be measured with HHG. Measurements in xenon lead to the first clear observation of electron correlation effects during HHG, which manifest as a broad peak in the HHG spectrum centred at 100 eV. This thesis also describes several improvements to the HHG experiment including the development of an ionization detector for measuring laser intensity, as well as an investigation into the role of laser mode quality on HHG phase matching and efficiency.

Electron dynamics in atoms and molecules occurs on a time-scale of attoseconds (10⁻¹⁸s). With the availability of strong field (∼ 10¹²- 10¹³ W cm⁻²) femtosecond (10⁻¹⁵s) laser pulses with electric fields that can reach and exceed the Coulomb field strength experienced by an electron in the ground state of an atom, it is now possible to generate even shorter pulses with durations on the order of attoseconds by the process of high-harmonic generation (HHG). In this dissertation, experiments to study electron dynamics on attosecond time-scales in a helium atom using attosecond pulses generated by HHG will be described. We use extreme-ultraviolet (XUV) attosecond pulse trains and strong femtosecond near-infrared (IR) laser pulses to excite and ionize helium atoms. We first discuss an experimental technique that allows us to quantify and reduce the detrimental effects of Gouy phase slip on attosecond XUV-IR experiments. We then discuss our experiments to study the dynamic behavior of electronic states in a strong field modified helium atom where we use attosecond pulses to explore the strong-field modified atomic landscape. Using the Floquet theory to interpret our experimental observations we measure the variation in quantum phase of interferences between different fourier components of Floquet states as the IR intensity is varied and as different ionization channels dominate, in real-time. Next, we briefly discuss quantum interferences between photo-electrons ionized from XUV excited states in helium using an IR field which is polarized orthogonal to the XUV polarization. We observe variation in angular distribution of photo-electrons as a function of XUV-IR time-delay. We then discuss a new technique to measure the time-of-birth of attosecond pulses using XUV+IR photo-ionization in helium as a measurement probe. Finally, experiments to study the evolution of XUV excited wave-packets in helium on a time-scale of 100's of femtoseconds with attosecond resolution will be described.

Femtosecond time scale allows us to follow and control atomic and molecular motion. The atomic vibrations happen in the range of femtosecond scale. Thus, femtosecond technology effectively measures the atomic vibration. However, to determine electron motion, one needs to reach sub-femtosecond time scale that is in attosecond time scale. High Harmonic Generation (HHG) is a non-linear process that converts infrared light to shortest wavelength, such as in the XUV regime. HHG allows to explore electronic motion and to control electron dynamics. HHG easily reaches to XUV region and is enabling attosecond pulse generation. In this thesis we focused to generate attosecond pulses by using noble gases and their mixtures. We used only argon gas, only hydrogen molecule and their mixture with neon gas. We wanted to improve the conversion efficiency (10^-6) of the fundamental light into high harmonics. We use Ne and H2 gas mixture to look enhancement of the HHs.

High Harmonic Generation (HHG) enables the production of bursts of coherent soft x-rays with attosecond pulse duration. This process arrises from the nonlinear interaction between intense infrared laser pulses and an ionizing gas medium. Soft x-ray photons are used for spectroscopy of inner-shell electron correlation and exchange processes, and the availability of attosecond pulse durations will enable these processes to be resolved on their natural time scales. The maximum or cutoff photon energy in HHG increases with both the intensity as well as the wavelength of the driving laser. It is highly desirable to increase the harmonic cutoff as this will allow for the generation of shorter attosecond pulses, as well as HHG spectroscopy of increasingly energetic electronic transitions. While the harmonic cutoff increases with laser wavelength, there is a corresponding decrease in harmonic yield. The first part of this thesis describes the experimental measurement of the wavelength scaling of HHG efficiency, which we report as lambda^(-6.3) in xenon, and lambda^(-6.5) in krypton. To increase the HHG cutoff, we have developed a 1.8 um source, with stable carrier envelope phase and a pulse duration of <2 optical cycles. The 1.8 um wavelength allowed for a significant increase in the harmonic cutoff compared to equivalent 800 nm sources, while still maintaing reasonable harmonic yield. By focusing this source into neon we have produced 400 eV harmonics that extend into the x-ray water window. In addition to providing a source of photons for a secondary target, the HHG spectrum caries the signature of the electronic structure of the generating medium. In krypton we observed a Cooper minimum at 85 eV, showing that photoionization cross sections can be measured with HHG. Measurements in xenon lead to the first clear observation of electron correlation effects during HHG, which manifest as a broad peak in the HHG spectrum centred at 100 eV. This thesis also describes several improvements to the HHG experiment including the development of an ionization detector for measuring laser intensity, as well as an investigation into the role of laser mode quality on HHG phase matching and efficiency.