A quantum/semiclassical theory for the internal nuclear dynamics of a small molecule and the induced small-amplitude coherent motion of a low-temperature host medium is developed, tested and applied to simulate and interpret ultrafast optical signals. Linear wave-packet interferometry and time-resolved coherent anti-Stokes Raman scattering signals for a model of molecular iodine in a 2D krypton lattice are calculated and used to study the vibrational decoherence and energy dissipation of iodine molecules in condensed media. The total wave function of the whole model is approximately obtained instead of a reduced system density matrix, and therefore the theory enables us to analyze the behavior and the role of the host matrix in quantum dynamics. This dissertation includes previously published co-authored material.

This thesis focuses on femtosecond studies of relaxation and photo-induced dissociation processes in the liquid environment. Measurements are performed using both polarization sensitive and magic angle transient absorption spectroscopy with excitation wavelengths of 387 nm and 258 nm and a white light continuum probe. In the first three papers the photodissociation of the trihalides I3- in acetonitrile and methanol as well as I2Br- in acetonitrile solution is investigated. These studies address such issues as the time scale for the production of the main photoproduct I2-, rotational dynamics of the formed diatomic anions, the subsequent wavepacket dynamics of the coherently excited diiodide anion and vibrational relaxation in, and the geminate recombination of, the I2- fragment. A nearly equal, bent geometry for the parent anion at the moment of bond breakage is proposed in all three solutions. However, the rotational temperature of the diiodide anion produced in the various solutions, reveals that motion along the bending coordinate of the dissociating triatomic anions plays an important role. The first signs of I2- fragments can already be observed at delays of approximately 130 – 190 fs which indicates a faster dissociation than suggested in earlier publications. The production of I2- seems fastest for I3- in methanol, followed by I2Br- in acetonitrile and is slowest for I3- in acetonitrile. It appears that vibrational relaxation of newly formed I2- fragments happens on a short time scale of a few hundred femtoseconds from initially excited vibrational states centered around v = 60 to v = 20. This fast relaxation was never directly observed before in solution. After that it relaxes with a slower time constant of approximately 2 ps which is shorter than most former reported values. This biexponential behavior agrees well with earlier molecular dynamics simulations. The dependence of the dissociation product formation on excitation energy, parent anion and solvent is found to be relatively strong. These findings lead us to believe that the photo-induced dissociation of the triatomic anions I3- and I2Br- in solution may very well resemble the gas phase process more than previously thought. In paper IV electronic and vibrational relaxation rates of the cyanine dye Methyl-DOTCI are determined after excitation to high lying electronic states. The measurements are performed with two different excitation wavelengths and in various solvents. They reveal a fast electronic relaxation to the second excited electronic state which subsequently relaxes to the first excited electronic state with a time constant of about 10 ps. This relatively long relaxation time may partly be explained by the badly overlapping electronic wavefunctions obtained from theoretical calculations. Vibrational relaxation proceeds with a similar time constant of 10 ps but shows a marked solvent dependence with faster relaxation rates in alcohol solutions.

Ultrafast electron diffraction (UED) is a method of directly imaging system dynamics at the atomic scale with picosecond time resolution. In this thesis I present theoretical analyses of the experimental processes, and construct models in order to better understand UED experiments and to guide future refinements. In particular, I derive a model of electron bunch propagation and a model of electron bunch diffraction, where both models take into account all bunch parameters. To analyse the propagation of electron bunches, I present a mean-field analytic Gaussian (AG) model. I derive a system of ordinary differential equations that are solved quickly and easily to give the bunch dynamics. The AG model is compared to N -body numerical simulations of initially Gaussian bunches, and I demonstrate excellent agreement between the two result sets. I also present a comparison of the AG model with numerical simulations of quasi-Gaussian and non-Gaussian distributions, extending the applicability of the AG model to the propagation of ``real-world'' bunches. During propagation, electron bunches can be shaped by electron-optic devices, which are necessary to attain high brightness, sub-100 fs bunches. I investigate two types of electron-optic devices: one is a magnetic lens used for collimating or focusing bunches, the other is a bunch compressor. I derive bunch parameter transformations for each of the electron-optic devices, and present numerical calculations using these transformations along with the AG model showing the effects of the devices on the evolution of the bunch parameters. To analyse electron bunch diffraction in UED experiments, I present a general scattering formalism. Using single-scattering and far-field approximations, I derive an expression for the diffracted signal that depends on the electron bunch properties just before scattering. Using this expression I identify the transverse and longitudinal coherence lengths and discuss the importance of these length scales in diffraction pattern formation. I also discuss the effects of different bunch parameters on the measured diffracted flux, and present sample numerical calculations for scattering by nanosize particles based on this model. This simulation demonstrates the cumulative effects of the bunch parameters, and shows the complex interplay of the bunch and target properties on the diffracted signal.

The structure and structural dynamics of hydrogen bonded liquids were studied experimentally and theoretically with coherent two-dimensional infrared (2DIR) spectroscopy. The resonant intermolecular interactions within the fully resonant hydrogen bond networks give access to spatial correlations in the dynamics of the liquid structures. New experimental and theoretical tools were developed that significantly reduced the technical challenges of these studies. A nanofluidic flow device was designed and manufactured providing sub-micron thin, actively stabilized liquid sample layers between similarly thin windows. A simulation protocol for nonlinear vibrational response calculations of disordered fluctuating vibrational excitons was developed that allowed for the first treatment of resonant intermolecular interactions in the 2DIR response of liquid water. The 2DIR spectrum of the O-H stretching vibration of pure liquid water was studied experimentally at different temperatures. At ambient conditions the loss of frequency correlations is extremely fast, and is attributed to very efficient modulations of the two-dimensional O-H stretching vibrational potential through librational motions in the hydrogen bond network. At temperatures near freezing, the librational motions are significantly reduced leading to a pronounced slowing down of spectral diffusion dynamics. Comparison with energy transfer time scales revealed the first direct proof of delocalization of the vibrational excitations. This work establishes a fundamentally new view of vibrations in liquid water by providing a spatial length scale of correlated hydrogen-bond motions. The linear and 2DIR response of the amide I mode in neat liquid formamide was found to be dominated by excitonic effects due to largely delocalized vibrational excitations. The spectral response and dynamics are very sensitive to the excitonic mode structure and infrared activity distributions, leading to a pronounced asymmetry of linear and 2DIR line shapes. This was attributed to structurally different species in the liquid characterized by their degree of medium range structural order. The response is dominated by energy transfer effects, sensitive to time-averaged medium range structural order, while being essentially insensitive to structural dynamics. This work is the first to recognize the importance of energy transfer contributions to the 2DIR response in a liquid, and provides additional proof of the well-structured character of liquid formamide.

Ultrafast electron diffraction (UED) is a method of directly imaging system dynamics at the atomic scale with picosecond time resolution. In this thesis I present theoretical analyses of the experimental processes, and construct models in order to better understand UED experiments and to guide future refinements. In particular, I derive a model of electron bunch propagation and a model of electron bunch diffraction, where both models take into account all bunch parameters. To analyse the propagation of electron bunches, I present a mean-field analytic Gaussian (AG) model. I derive a system of ordinary differential equations that are solved quickly and easily to give the bunch dynamics. The AG model is compared to N -body numerical simulations of initially Gaussian bunches, and I demonstrate excellent agreement between the two result sets. I also present a comparison of the AG model with numerical simulations of quasi-Gaussian and non-Gaussian distributions, extending the applicability of the AG model to the propagation of ``real-world'' bunches. During propagation, electron bunches can be shaped by electron-optic devices, which are necessary to attain high brightness, sub-100 fs bunches. I investigate two types of electron-optic devices: one is a magnetic lens used for collimating or focusing bunches, the other is a bunch compressor. I derive bunch parameter transformations for each of the electron-optic devices, and present numerical calculations using these transformations along with the AG model showing the effects of the devices on the evolution of the bunch parameters. To analyse electron bunch diffraction in UED experiments, I present a general scattering formalism. Using single-scattering and far-field approximations, I derive an expression for the diffracted signal that depends on the electron bunch properties just before scattering. Using this expression I identify the transverse and longitudinal coherence lengths and discuss the importance of these length scales in diffraction pattern formation. I also discuss the effects of different bunch parameters on the measured diffracted flux, and present sample numerical calculations for scattering by nanosize particles based on this model. This simulation demonstrates the cumulative effects of the bunch parameters, and shows the complex interplay of the bunch and target properties on the diffracted signal.

The structure and structural dynamics of hydrogen bonded liquids were studied experimentally and theoretically with coherent two-dimensional infrared (2DIR) spectroscopy. The resonant intermolecular interactions within the fully resonant hydrogen bond networks give access to spatial correlations in the dynamics of the liquid structures. New experimental and theoretical tools were developed that significantly reduced the technical challenges of these studies. A nanofluidic flow device was designed and manufactured providing sub-micron thin, actively stabilized liquid sample layers between similarly thin windows. A simulation protocol for nonlinear vibrational response calculations of disordered fluctuating vibrational excitons was developed that allowed for the first treatment of resonant intermolecular interactions in the 2DIR response of liquid water. The 2DIR spectrum of the O-H stretching vibration of pure liquid water was studied experimentally at different temperatures. At ambient conditions the loss of frequency correlations is extremely fast, and is attributed to very efficient modulations of the two-dimensional O-H stretching vibrational potential through librational motions in the hydrogen bond network. At temperatures near freezing, the librational motions are significantly reduced leading to a pronounced slowing down of spectral diffusion dynamics. Comparison with energy transfer time scales revealed the first direct proof of delocalization of the vibrational excitations. This work establishes a fundamentally new view of vibrations in liquid water by providing a spatial length scale of correlated hydrogen-bond motions. The linear and 2DIR response of the amide I mode in neat liquid formamide was found to be dominated by excitonic effects due to largely delocalized vibrational excitations. The spectral response and dynamics are very sensitive to the excitonic mode structure and infrared activity distributions, leading to a pronounced asymmetry of linear and 2DIR line shapes. This was attributed to structurally different species in the liquid characterized by their degree of medium range structural order. The response is dominated by energy transfer effects, sensitive to time-averaged medium range structural order, while being essentially insensitive to structural dynamics. This work is the first to recognize the importance of energy transfer contributions to the 2DIR response in a liquid, and provides additional proof of the well-structured character of liquid formamide.

The ability to perceive the dynamics of nature is ultimately limited by the temporal resolution of the instruments available. With the help of the ultrashort optical pulse, people now are able to observe and steer the electronic dynamics on the atomic scale. Meanwhile, high power attainable in such short time scale helps to boost the study of nonlinear physics. Most commercial femtosecond lasers are based on Ti:sapphire, but such systems can only be tuned in a spectral range around 800 nm. Few applications need only a single wavelength in this spectral region and pulses tunable from the UV to the IR are highly desirable. Based on the soliton characteristics of ultrashort laser pulses, we are the first ones who propose to make use of resonant dispersive waves, which are phase-matched non-solitonic linear waves, to extend the spectral tuning range of ultrashort laser without involving complicated amplifiers. Experimentally, we achieve the tuning of dispersive wave wavelengths by changing the dispersion parameters of the laser cavity, and confirm dispersive waves are ultrashort pulses under appropriate conditions. We successfully apply such a system into a multi-wavelength operation Ti:sapphire laser. The proposed idea is general, and can be applied to systems where solitons dominate, for example fiber lasers. Thanks to the newly developed novel fiber -photonic crystal fiber- we obtain widely tunable and gap-free femtosecond pulse by extending this mechanism to waveguides. This is the largest reported tuning range for efficient nonlinear optical frequency conversion obtained with such a simple and low energy laser. We apply such a Ti:sapphire laser to Raman micro-spectroscopy. Because of the different temporal behaviors of the Raman process and other parametric processes, we can efficiently separate the coherent Raman signal from the unwanted background, and obtain a high chemical contrast and high resolution image. This high repetition rate and low energy laser oscillator makes it very suitable for biological Raman micro-spectroscopy, especially living samples for which damage is a big concern.

In this study¡Awe apply the broad bandwidth and high energy pulse of ultrafast laser to experiment on RF OBIC and second harmonic generation. In this paper a novel method is presented for characterizing high frequency response and behavior of ultra high-speed photosensitive semiconductor devices and the set-up is capable of generating excitation at RF bandwidths of greater than 1.8 THz. In addition¡Athe collagen of dentine is able to generate the second harmonic in the ultraviolet region, so we develop a high performance transmission mode laser scanning microscope for obtaining SHG images of a tooth slice. We also study wavelength dependence and polarization dependence.

Pulse Compression and Dispersion Control in Ultrafast Optics Vikrant K. Chauhan 116 Pages Directed by Dr. Rick P. Trebino In this thesis, we introduced novel pulse compressors that are easy to align and which also compensate for higher order dispersion terms. They use a single dispersive element or a combination of dispersive elements in single-element-geometry. They solve the problem of extra-cavity pulse compression by providing control of the pulse width in almost all of the experiments performed using ultrashort pulses, and they even compensate for higher order dispersion. We performed full spatiotemporal characterization of these compressors and demonstrated their performance. We also developed a theoretical simulation of pulse compressors which is based on a matrix based formalism. It models the full spatiotemporal characteristics of any dispersion control system. We also introduced a simple equation, in its most general form, to relate the total dispersion and magnification introduced by an arbitrary sequence of dispersive devices. Pulse compressor characterization was done using interferometric measurements in the experiments presented in this work, but we also developed a method to measure pulses that uses polarization gating FROG for measuring two unknown pulses. In the last part, we briefly discuss the designing of a high energy chirped pulse amplification system.

Plasmonic Laser Nanosurgery (PLN) is a novel photodisruption technique that exploits the large enhancement of ultrafast laser pulses in the near-field of gold nanoparticles for the nanoscale manipulation of biological structures. Excitation of surface plasmons on spherical nanoparticles by pulsed irradiation provides a platform for the confinement of photoactivated processes, while functionalized nanoparticle targeting methods provide the highest level of therapeutic selectivity. In this dissertation, we demonstrate and characterize the in vitro plasmonic optoporation of MDA-MB-468 human epithelial breast cancer cells labeled with plasmonic gold nanoparticles using NIR, femtosecond laser pulses. Using a 10 kDa FITC-Dextran probe dye, we find that the PLN can optoporate nanoparticle-labeled cellular membranes at fluences down to just a few mJ/cm², providing a 50-fold reduction in pulse energy necessary to induce membrane dysfunction as compared with unlabeled cells. Limited membrane dysfunction was found to lead to transient optoporation of cells as a possible transfection method, while more extensive, non-recoverable membrane dysfunction lead to cellular death as a possible plasmonic treatment of malicious cells. In the first regime, we found a maximum optoporation efficiency of approximately 31% ± 5.4% with 2 to 2.5 mW laser light having 80 MHz repetition rate. In the second regime, we were able to necrotically kill greater than 90% of irradiated cells with as little as 5 mW average power. We found that particle aggregation along the cellular surface is crucial for the success of PLN. High particle loadings were required, suggesting that particle aggregates provide large enhancements, leading to reduced PLN threshold energies. We provide experimental evidence suggesting photodisruption with ultra-low energy pulses is directly dependent upon the emission of electrons from the particle surface, which seed the formation of free radicals in the surrounding water. These free radicals mediate membrane dysfunction by polyunsaturated lipid and protein peroxidation. / text