Generation of reproducible attosecond (10-18s) pulses is an exciting goal: in the same way as femtosecond pulses were used to make "movies" of the atomic motion in molecules, attosecond pulses could "uncover" the motion of electrons around nuclei. In this dissertation, we have suggested new ideas that will allow improving one scheme for obtaining such ultra-short pulses: the molecular modulation technique. In a theoretical proposal called Raman Additive technique, we have suggested a method that will allow (with a proper phase stabilization of generated sidebands) to obtain reproducible waveforms of arbitrary shape. An exciting range of possibilities could open up - not only for absolute phase control or sub-cycle shape control, but also for investigation of multiphoton ionization rates as a function of the sub-cycle shape. We have elaborated on the latter subject in another theoretical project, where we have exploited the unique feature of such ultrashort laser pulses, which is synchronization with molecular motion (rotational or vibrational), in order to investigate photoionization of molecules. From experimental point of view, a different construction of driving lasers than previously employed led to establishment of larger molecular coherences at higher operating pressure than in previous experiments. This resulted in simultaneous generation of rotational and vibrational sidebands with only two fields applied. In another experimental proposal using rotational transition in deuterium we have shown that employing a hollow waveguide instead of normal Raman cell improves the efficiency of the generation process. By optimizing gas pressure and waveguide geometry to compensate the dispersion, the method can be extended to efficiently generate Raman sidebands at a much lower energy of driving fields than previously employed. At the end, a very exciting possibility for controlling the molecular motion in a Raman driven system will be shown. Based on the interference effects (EITlike) that take place inside of a molecule, selectivity of different degrees of freedom can be achieved (for example switching from rotational-vibrational motion to pure rotational).

The spectral phase of high harmonic and attosecond pulses is typically shaped by the interaction of the recollision electron with the strong field in the continuum. However, the phase of the transition moment coupling bound and continuum states can be significant in shaping the emitted radiation. The measurement of transition moment phase shifts can reveal information about attosecond electron dynamics and structure. Here, I demonstrate that all-optical approaches to attosecond measurement, based on perturbing recollision with a weak infrared field, are sensitive to transition moment phase shifts arising from electronic structure and multielectron interaction using analytical theory, ab initio simulation, and experiment. The insensitivity of all-optical approaches to transition moment phase shifts arising from ionic structure is found to be a result of a first-order cancellation of the effect of the perturbing field on the recollision electron wave packet and the transition moment. Prior to these findings, it was widely believed that all-optical methods were insensitive to the transition moment phase. The insensitivity of all-optical measurement to both ionic structure and propagation effects will permit for the unambiguous isolation of electron structure and multielectron interaction in attosecond measurement. These results will allow any laboratory capable of generating attosecond pulses to perform measurements of the transition moment phase without an additional experimental apparatus, even at wavelengths where the single photoionization cross-section becomes small.

No description available.

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.

Doctor of Philosophy / Department of Physics / Brett DePaola / Zenghu Chang / Extremely broad bandwidth attosecond pulses (which can support 16as pulses) have been demonstrated in our lab based on spectral measurements, however, compensation of intrinsic chirp and their characterization has been a major bottleneck. In this work, we developed an attosecond streak camera using a multi-layer Mo/Si mirror (bandwidth can support ~100as pulses) and position sensitive time-of-flight detector, and the shortest measured pulse was 107.5as using DOG, which is close to the mirror bandwidth. We also developed a PCGPA based FROG-CRAB algorithm to characterize such short pulses, however, it uses the central momentum approximation and cannot be used for ultra-broad bandwidth pulses. To facilitate the characterization of such pulses, we developed PROOF using Fourier filtering and an evolutionary algorithm. We have demonstrated the characterization of pulses with a bandwidth corresponding to ~20as using synthetic data. We also for the first time demonstrated single attosecond pulses (SAP) generated using GDOG with a narrow gate width from a multi-cycle driving laser without CE-phase lock, which opens the possibility of scaling attosecond photon flux by extending the technique to peta-watt class lasers. Further, we generated intense attosecond pulse trains (APT) from laser ablated carbon plasmas and demonstrated ~9.5 times more intense pulses as compared to those from argon gas and for the first time demonstrated a broad continuum from a carbon plasma using DOG. Additionally, we demonstrated ~100 times enhancement in APT from gases by switching to 400 nm (blue) driving pulses instead of 800 nm (red) pulses. We measured the ellipticity dependence of high harmonics from blue pulses in argon, neon and helium, and developed a simple theoretical model to numerically calculate the ellipticity dependence with good agreement with experiments. Based on the ellipticity dependence, we proposed a new scheme of blue GDOG which we predict can be employed to extract intense SAP from an APT driven by blue laser pulses. We also demonstrated compression of long blue pulses into >240 µJ broad-bandwidth pulses using neon filled hollow core fiber, which is the highest reported pulse energy of short blue pulses. However, compression of phase using chirp mirrors is still a technical challenge.

Dynamics occurring on microscopic scales, such as electronic motion inside atoms and molecules, are governed by quantum mechanics. However, the Schroedinger equation is usually too complicated to solve analytically for systems other than the hydrogen atom. Even for some simple atoms such as helium, it still takes months to do a full numerical analysis. Therefore, practical problems are often solved only after simplification. The results are then compared with the experimental outcome in both the spectral and temporal domain. For accurate experimental comparison, temporal resolution on the attosecond scale is required. This had not been achieved until the first demonstration of the single attosecond pulse in 2001. After this breakthrough, "attophysics" immediately became a hot field in the physics and optics community. While the attosecond pulse has served as an irreplaceable tool in many fundamental research studies of ultrafast dynamics, the pulse generation process itself is an interesting topic in the ultrafast field. When an intense femtosecond laser is tightly focused on a gaseous target, electrons inside the neutral atoms are ripped away through tunneling ionization. Under certain circumstances, the electrons are able to reunite with the parent ions and release photon bursts lasting only tens to hundreds of attoseconds. This process repeats itself every half cycle of the driving pulse, generating a train of single attosecond pulses which lasts longer than one femtosecond. To achieve true temporal resolution on the attosecond time scale, single isolated attosecond pulses are required, meaning only one attosecond pulse can be produced per driving pulse. Up to now, there are only a few methods which have been demonstrated experimentally to generate isolated attosecond pulses. Pioneering work generated single attosecond pulse using a carrier-envelope phase-stabilized 3.3 fs laser pulse, which is out of reach for most research groups. An alternative method termed as polarization gating generated single attosecond pulses with 5 fs driving pulses, which is still difficult to achieve experimentally. Most recently, a new technique termed as Double Optical Gating (DOG) was developed in our group to allow the generation of single attosecond pulse with longer driving pulse durations. For example, isolated 150 as pulses were demonstrated with a 25 fs driving laser directly from a commercially-available Ti:Sapphire amplifier. Isolated attosecond pulses as short as 107 as have been demonstrated with the DOG scheme before this work. Here, we employ this method to shorten the pulse duration even further, demonstrating world-record isolated 67 as pulses. Optical pulses with attosecond duration are the shortest controllable process up to now and are much faster than the electron response times in any electronic devices. In consequence, it is also a challenge to characterize attosecond pulses experimentally, especially when they feature a broadband spectrum. Similar challenges have previously been met in characterizing femtosecond laser pulses, with many schemes already proposed and well-demonstrated experimentally. Similar schemes can be applied in characterizing attosecond pulses with narrow bandwidth. The limitation of these techniques is presented here, and a method recently developed to overcome those limitations is discussed. At last, several experimental advances toward the characterization of the isolated 25 as pulses, which is one atomic unit time, are discussed briefly.

Doctor of Philosophy / Department of Physics / Chii-Dong Lin / Isolated attosecond pulse (IAP) is a tool of probing electronic dynamics occurring in atoms, molecules, clusters and solids, since the time scale of electronic motion is on the order of attoseconds. The generation, characterization and applications of IAPs has become one of the fast frontiers of laser experiments. This dissertation focuses on several aspects of attosecond physics. First, we study the driving wavelength scaling of the yield of high-order harmonic generation (HHG) by applying the quantum orbit theory. The unfavorable scaling law especially for the short quantum orbit is of great importance to attoseond pulse generation toward hundreds of eVs or keV photon energy region by mid-infrared (mid-IR) lasers. Second, we investigate the accuracy of the current frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) and phase retrieval by omega oscillation filtering (PROOF) methods for IAP characterization by simulating the experimental data by theoretical calculation. This calibration is critical but has not been carefully carried out before. We also present an improved method, namely the swPROOF which is more universal and robust than the original PROOF method. Third, we investigate the controversial topic of photoionization time delay. We find the limitation of the FROG-CRAB method which has been used to extract the photoionization time delay between the 2s and 2p channels in neon. The time delay retrieval is sensitive to the attochirp of the XUV pulse, which may lead to discrepancies between experiment and theory. A new fitting method is proposed in order to overcome the limitations of FROG-CRAB. Finally, IAPs are used to probe the dynamic of electron correlation in helium atom by means of attosecond transient absorption spectroscopy. The agreement between the measurement and our analytical model verifies the observation of time-dependent build up of the 2s2p Fano resonance.

We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms, including laser-induced phase (LIP), time-varying (AC) Stark shift, quantum path interference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which account for the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.

Quantum mechanical motion of electrons in atoms and molecules is at the heart of many photophysical and photochemical processes. As the natural timescale of electron dynamics is in the range of femtoseconds or shorter, ultrashort pulses are required to study such phenomena. The ultrashort pulse light-matter interaction at high intensity regime can however dramatically alter the atomic and molecular structures. Our current understanding of such transient electronic modification is far from complete, especially when complicated light-induced couplings are involved. In this dissertation, we investigated how a femtosecond strong-field pulse can control or modify the evolution of atomic or molecular polarization, representing electric dipole excitation in various systems. Extreme ultraviolet (XUV) attosecond pulse trains are used to coherently prepare superposition of excited states in various atomic and molecular systems. A subsequent phase-locked infrared (IR) femtosecond pulse is applied to perturb the dipoles, and transient changes in the transmitted XUV spectra are measured. This scheme is termed as XUV attosecond transient absorption spectroscopy. In the first study, we applied this technique to study the modification of Rydberg states in dilute helium gas. We observed several transient changes to the atomic structure, including the ac Stark shift, laser-induced quantum phase, laser-induced continuum structure, and quantum path interference. When the experiments were extended to the study of a dense helium gas sample, new spectral features in the absorption spectra emerged which cannot be explained by linear optical response models. We found that these absorption features arise from the interplay between the XUV resonant pulse propagation and the IR-imposed phase shift. A unified physical model was also developed to account for various scenarios. Extending our work to argon atoms, we studied how an external infrared field can be used to impulsively control different photo-excitation pathways and the transient absorption lineshape of an otherwise isolated autoionizing state. It is found that by controlling the field polarization of the IR pulse, we can modify the transient absorption line shape from Fano-like to Lorentzian-like profiles. Unlike atoms, in our study of autoionizing states of the oxygen molecule, we observed both positive and negative optical density changes for states with different electronic symmetries. The predictions of two distinct and simplified dipole perturbation models were compared against both the experimental results and a full theoretical calculation in order to understand the origin of the sign of absorption change. We relate this symmetry-dependent sign change to the Fano parameters of static photoabsorption. The same approach was applied to study molecular nitrogen, in which we observed the decay dynamics of IR perturbed doubly-excited Rydberg states with many vibrational progressions. In addition, we also conducted experiments to investigate Rydberg state dynamics of other molecular systems such as carbon dioxide. In summary, we experimentally explored the ephemeral light-induced phenomena associated with excited states of atoms and molecules. These studies provide real-time information on ultrafast electronic processes and provide strategies for direct time-domain control of the light-matter interaction.

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.