Doctor of Philosophy / Department of Physics / Brett D. Esry / Strong-field ionization plays a central role in molecules interacting with an intense laser field since it is an essential step in high-order harmonic generation thus in attosecond pulse generation and serving as a probe for molecular dynamics through either the sensitivity of ionization to the internuclear separation or the laser-induced electron scattering. Strong-field molecular ionization has been studied both theoretically and experimentally, dominantly through the Born-Oppenheimer approximation and at equilibrium or small reaction distances. We have extended the theoretical studies of molecular ionization to a much broader extent. Specifically, due to the difficulty of treating ionization in Born-Oppenheimer representation especially for molecular dynamics involving strongly-correlated electron-nuclear motion, we have investigated an alternative time-independent--adiabatic hyperspherical--picture for a one-dimensional model of the hydrogen molecule. In the adiabatic hyperspherical representation, all the reaction channels--including ionization--for the hydrogen molecule have been identified in a single set of potential curves, showing the advantage of studying molecular dynamics involving multiple breakup channels coupled with each other. We have thus proposed a good candidate to study strongly-correlated molecular dynamics, such as autoionization and dissociative recombination. Moving to a time-dependent picture by numerically solving the time-dependent Schrödinger equation (TDSE), we have explored two extreme classes of strong-field ionization of hydrogen molecule ion: at large internuclear distances (R>30 a.u.) and for long-wavelength laser fields. Remarkably, we have found strong-field two-center effects in molecular ionization beyond the long-standing one-photon two-center interference as a manifestation of the double-slit interference. In particular, the total ionization probability at large internuclear distances shows strongly symmetry-dependent two-center dynamics in homonuclear diatomic molecules and two-center induced carrier-envelope phase effect in heteronuclear diatomic molecules. Such two-center effects are expected to generalize to other diatomic systems and could potentially be used to explain phenomena in multi-center strong-field physics. Moreover, we have theoretically confirmed, for the first time, the existence of low energy structure in molecular ionization in long-wavelength laser fields by solving the three-dimensional TDSE. Finally, we have performed a pump-probe study of the hydrogen molecular ion where a pump pulse first dissociates the molecule followed by a probe pulse which ionizes the dissociating wave packet, and surprisingly found a pronounced broad ionization peak at large R or large pump-probe delay (~150 fs). Numerically, we have developed and implemented new theoretical frameworks to more accurately and efficiently calculate quantum mechanical processes for small molecules--hydrogen molecule and its ion--which could readily be adapted to heavier diatomic systems.

Vertical External Cavity Surface Emitting Lasers (VECSELs) have become well established in recent years for their design flexibility and promising power scalability. Recent efforts in VECSEL development have focused heavily on expanding the medium into the ultrafast regime of modelocked operation. Presented in this thesis is a detailed discussion regarding the development of ultrafast VECSEL devices. Achievements in continuous wave (CW) operation will be highlighted, followed by several chapters detailing the engineering challenges and design solutions which enable modelocked operation of VECSELs in the ultrafast regime, including the design of the saturable absorbers used to enforce modelocking, management of the net group delay dispersion (GDD) inside the cavity, and the design of the active region to support pulse durations on the order of 100 fs. Work involving specific applications - VECSELs emitting on multiple wavelengths simultaneously and the use of VECSEL seed oscillators for amplification and spectral broadening - will also be presented. Key experimental results will include a novel multi-fold cavity design that produced record-setting peak powers of 6.3 kW from a modelocked VECSEL, an octave-spanning supercontinuum with an average power of 2 W generated using a VECSEL seed and a 2-stage Yb fiber amplifier, and two separate experiments where a VECSEL was made to emit on multiple wavelengths simultaneously in modelocked and highly stable CW operation, respectively. Further, many diagnostic and characterization measurements will be presented, most notably the in-situ probing of a VECSEL gain medium during stable modelocked operation with temporal resolution on the order of 100 fs, but also including characterization of the relaxation rates in different saturable absorber designs and the effectiveness of different methods for managing the net GDD of a device.

We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.

Coherent anti-Stokes Raman scattering microscopy is a versatile technique for label-free imaging and spectroscopy of systems of biophysical interest. Due to the coherent nature of the generated signals, CARS images and spectra can often be difficult to interpret. In this thesis, we document how distortions and enhancements can be produced in CARS hyperspectroscopy as a result of the instrument, geometrical optical effects, or unique molecular states, and discuss how these effects may be suppressed or exploited in various CARS applications.

archives@tulane.edu / 1 / Robert Mackin

Nonlinear optics is the study of interactions of materials with intense light beams made possible by the invention of laser. Arguably the most trivial but technologically most important nonlinear optical effect is the intensity-dependent nonlinear refraction: an intense light beam can temporarily and reversibly change the refractive index of a material. However, the changes to the refractive index of a material due to the presence of a strong laser beam are very weak---maximum on the order of $10^{-3}$---and tend to be a small fraction of the linear refractive index. It must be noted that at optical frequencies vacuum has a refractive index of 1 and glass has a refractive index of 1.5. Thus, one of the foundational assumptions of nonlinear optics is that the nonlinear optical changes to material properties are always a small perturbation to the linear response. In the 58-year history of nonlinear optics, one of the overarching themes of research has been to find ways to increase the efficiency of nonlinear interactions. This thesis is a collection of six manuscripts motivated by our experimental finding that at least in a certain class of materials the above long-standing view of nonlinear optics does not necessarily hold true. We have found that in a material with low refractive index, known as an epsilon-near-zero material or ENZ material, the nonlinear changes to the refractive index can be a few times larger than the linear refractive index, i.e. the nonlinear response becomes the dominant response of the material in the presence of an intense optical beam. We believe that the results presented in this thesis collectively make a convincing case that ENZ materials are a promising platform for nonlinear nano-optics.

Laser filamentation is a highly complex and dynamic nonlinear process that is sensitive to many physical parameters. The basic properties that define a filament consist of (i) a narrow, high intensity core that persists for distances much greater than the Rayleigh distance, (ii) a low density plasma channel existing within the filament core, and (iii) a supercontinuum generated over the course of filamentation. However, there remain many questions pertaining to how these basic properties are affected by changes in the conditions in which the filaments are formed; that is the premise of the work presented in this dissertation. To examine the effects of anomalous dispersion and of different multi-photon ionization regimes, filaments were formed in solids with different laser wavelengths. The results provided a better understanding of supercontinuum generation in the anomalous dispersion regime, and of how multi-photon ionization can affect the formation of filaments. Three different experiments were carried out on filamentation in air. The first was an investigation into the effects of geometrical focusing. A simplified theoretical model was derived to determine the transition of filamentation in the linear-focusing and nonlinear- focusing regimes. The second examined the effects of polarization on supercontinuum generation, where a polarization-dependent anomalous spectral broadening phenomenon due to molecular effects was identified. The third involved the characterization of filaments in the ultraviolet. The combination of physical mechanisms responsible for filamentation in the ultraviolet was found to be different from that in the near infrared.

No description available.

No description available.

No description available.