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Among fiber-based lasers, the availability of high energy and high peak-power pulsed sources is of critical importance for a variety of application ranging from remote sensing, material processing to laser-plasma EUV generation. For most of the applications, high average-power is required in preferably diffraction-limited beams. In this dissertation, we demonstrate usage of very large-mode-area (LMA) fibers to significantly exceed fundamental pulse energy and peak-power limitations of standard single-mode fibers and explore the limit of core size scaling in LMA fibers. We show systematic studies in both theoretical modeling and experimental characterization of beam quality control in extremely large core LMA fibers, including coiling induced higher-order-mode filtering effect in low numerical aperture (NA) Yb-doped fiber amplifiers and fundamental mode excitation in high NA Er-doped fiber amplifiers. We report achieving good beam qualities (M2∼1.3) in 80-mum Yb-doped and 70-mum Er-doped fiber amplifiers, corresponding to the largest mode diameters of conventional step-index LMA fibers with nearly diffraction-limited output for applications at ∼1-mum and ∼1.55mum wavelengths, respectively. Large core size ensures higher energy extraction and increases thresholds for the onset of optical damage and detrimental nonlinear effects. We demonstrate in this work that millijoule energies and megawatt peak-powers in nanosecond pulses can be achieved in nearly diffraction-limited beam using 80-mum core Yb-doped fiber amplifier, and also show the average-power scalability of this pulsed fiber laser up to 85-W. Comparable pulse energies and peak-powers are believed to be achievable in 70-mum core Er-doped fiber amplifier for applications at ∼1.55-mum wavelength. Here, we report demonstration of average-power scaling of nanosecond and ultrashort pulsed fiber lasers using 25-mum core Er/Yb co-doped fiber amplifier. In particular, high average-power (∼28-W) ultrashort (∼1.4-ps) pulsed fiber laser was realized in fiber-based chirped pulse amplification (FCPA) system using novel chirped volume Bragg grating (CVBG) pulse stretchers/compressors. These results indicate that fiber laser technology has significant potential of replacing conventional pulsed solid-state lasers and is a better alternative for applications that require high average-powers.

This dissertation discusses work in the development and characterization of narrow-bandwidth terahertz waveforms generated by optical rectification of ultrafast laser pulses in periodically-poled lithium niobate (PPLN). The sign of the second order nonlinear susceptibility, chi(2) of lithium niobate follows the sign of the local ferroelectric domain. The domain structures of the crystals are designed so that chi(2) changes sign when the optical pulse leads the locally generated terahertz polarization by one optical pulse-length. In this way, the opposite sign of terahertz polarization is generated in neighboring domains; the fields from neighboring domains connect smoothly, and an oscillatory terahertz field is radiated. The terahertz electric field is temporally resolved via free-space electro-optic sampling in a ZnTe sensor crystal. A 7.2 mm long PPLN crystal containing 240 domains of 30 microns each produced a waveform consisting of 120 cycles of the electric field. This signal was peaked at 1.7THz with a bandwidth (FWHM) of approximately 18GHz. By varying the domain length and the temperature of the crystals, nearly continuous frequency coverage from 0.8THz to 2.5THz was demonstrated. The capability of aperiodic crystals to generate more complex waveforms was also demonstrated. The terahertz radiation produced from a broad-bandwidth source (ZnTe) is used to excite 1s-2p transitions in excitons in an optically excited. In .04Ga0.96As/GaAs multiple quantum well sample. The existence of an exciton population is verified by the presence of absorption lines in the spectrum of the terahertz field at the 1s-2p resonance. This work confirms an exciton population in the sample as early as 200ps following an above-bandgap optical excitation pulse.

Mitotic chromosome movements are responsible for the correct segregation of duplicated chromosomes into the daughter cells. Errors in this process are known to play a role in some of the serious diseases such as cancer, and the little understood process of aging. A thorough comprehension of the physical basis of this process is therefore necessary. An intriguing aspect of chromosome movements during mitosis is directional instability: runs with approximately constant speed punctuated by abrupt reversal in direction of motion. I have constructed a mechanistic model that views chromosome movement as a result of interplay between poleward and antipoleward or polar ejection forces (PEF) on a chromosome; and microtubule (MT) depolymerization-coupled movement of the chromosome. Computer simulations based on this model using a single set of parameters accurately and quantitatively predict: the force, character, speed, and duration of chromosome movements, oscillations of chromosomes associated with only one spindle pole, the larger force during anaphase, the effect of MT-depolymerizing drugs on chromosome movements, and the decreased turnover of kinetochore-MTs during anaphase. The model also predicts how chromosome behavior should respond to perturbations of the PEF. These predictions could be unequivocally tested if it were possible to destroy structures smaller than the light resolution limit with minimal collateral damage. To address these requirements, I developed a methodology for ultrahigh resolution microsurgery with tightly-focused, ultrafast lasers pulses. This entailed an in-depth study of optical breakdown in dielectrics. Characterization of the single pulse damage in test dielectric materials ranging from silicon and glass to cell walls and membranes has shown that in the target regions where the laser intensity exceeds critical intensity, optical breakdown proceeds by tunneling ionization followed by a runaway avalanche ionization that ends with the ionization of all the valence electrons. Highly reproducible features on the nanometer size-scale indicate that the valence electron density is the central factor determining the critical intensity, implying that high precision can be maintained in a wide range of solids. Along with the new understanding optical breakdown, this technique will find potential applications in diverse fields ranging from MEMS fabrication to nano-fluidics, as well as cellular nanosurgery.

Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.

Two adaptive optic systems for correction of either temporal phase error and wavefront errors for ultrafast pulses are demonstrated. These systems consists of a computer controlled micromachined membrane deformable mirror (MMDM) and a genetic learning algorithm (GA). Nonlinear excitation such as two-photon fluorescence or second harmonic generation are used as feedback to the GA to determine the appropriate correction to apply to the mirror. Two MMDMs are used, a 30 x 8 mm, 39 actuator linear MMDM for pulse-shaping applications and a 15 mm diameter, 37 actuator wavefront MMDM. Linear pre-compensation of self-phase modulation (SPM) was experimentally demonstrated utilizing the linear MMDM in a linear pulse-shaper for ultrafast pulses. The nonlinear nature of SPM makes arbitrary polynomial compensation necessary. Pre-compensation of SPM generated in an optical fiber by a 10 fs pulse reduced the pulse from 30fs to 20fs. We demonstrates adaptive correction with the wavefront MMDM by corrected for coma and astigmatism in a reflective multiphoton scanning microscope. An f1, parabola produces a very tight focus with no aberration when it is perfectly aligned. However, when beam scanning is used for two-dimensional imaging the image is severely aberrated. The MMDM and the GA are able to find the best possible wavefront for aberration correction for each scanning position. The horizontal scanning range was increased from 60 mum without the adaptive correction to 170 mum, ≈3 times the uncorrected scanning range, and the vertical scanning range was increased by a comparable amount. This resulted in an increase in scanning area of 9 times. The wavefront MMDM was also used for adaptive correction of spherical aberration from focusing from air, deep into a water-based sample. This depth-based aberration results from an index of refraction mismatch between the sample and the immersion medium of the objective and occurs regardless of beam scanning or sample scanning. By dynamically correcting for the first order spherical aberration in a water-Cumarin dye solution, the depth that can be imaged with a 40x/0.6NA objective was extended from 150 mum to 600 mum. Adaptive aberration correction for photodisruption is also being investigated for ophathalmological and micromachining applications.

This dissertation investigates the fabrication and use of micromachined probes to measure picosecond signals on devices, especially on mesoscopic structures. Photoconductive sampling technique is chosen because of its high sensitivity and its ability to make direct voltage measurements. A miniature photoconductive probe is fabricated from low-temperature GaAs (LT-GaAs) using standard semiconductor processing techniques including etching, metallizing, and backside processing. From this micromachining process a probe is created with a 1-mum thickness of LT-GaAs and a tip having a 7-mum spatial resolution. The probe is mounted onto a single-mode optical fiber to couple the light onto the photoconductive switch and to allow the probe to be freely positionable. Because of the fast carrier lifetime of LT-GaAs, the probe has a 3.8-ps response time. The probe's capacitance affects its response as well as the responsivity. The sensitivity of the probe is limited to about 1-muV/(Hz) 1/2. However, when the probe is integrated with a JFET source follower, the sensitivity can be improved to tens of nV/(Hz)1/2. This higher sensitivity occurs since a higher frequency modulation can be used to suppress the effect of laser noise fluctuations. In addition to the higher sensitivity, absolute voltage measurements are also possible. An improved theoretical model explains the influence of the modulation frequency on the sensitivity and absolute voltage measurements. Other types of devices can be fabricated using micromachining techniques. A small cone tip is placed on the probe electrode to create an atomic force microscope probe with micron to sub-micron resolution to obtain topographical information of a device. Another micromachined device that can be fabricated is a miniature terahertz emitter. The device's emitted radiation is due to the magnetic moment since the electrode has closed loop geometry. A split-gate device is fabricated in order to measure the propagating signal using the probe. The current versus gate voltage shows quantized conductance under a low temperature environment. Theoretical calculations reveal that the temporal response of the split-gate transistor requires a probe with sub-microvolt sensitivity and picosecond resolution. The probe's capability to launch and detect signals in a cryogenic environment makes it an appropriate method for measuring the response of the split-gate device.

The control of atoms and molecules using coherent light fields has been a long-standing goal in chemical physics. This thesis discusses experiments which use a programmable pulse shaper to control the quantum state of electronic wavepackets in Rydberg atoms and electronic and nuclear dynamics in molecular liquids. The shape of Rydberg wavepackets was controlled by using tailored ultrafast pulses to excite a beam of cesium atoms. The quantum state of these atoms was measured using holographic techniques borrowed from optics. The experiments with molecular liquids involved the construction of an automated learning machine. A Genetic Algorithm directed the choice of shaped pulses which interacted with the molecular system inside a learning control loop. Analysis of successful pulse shapes that were found by using the GA yield insight into the systems being controlled.

In this work, I present a programmable, deformable-mirror based pulse shaper. The mirror used was a micromachined membrane deformable mirror. I used the pulse shaper to compress pulses from an ultrafast Ti:sapphire oscillator and to remove the residual phase from an ultrafast Ti:sapphire amplifier system. In both systems, I was able to obtain near transform-limited pulses. To achieve this compression I used two different methods. In one method, I calibrated the pulse shaper using frequency resolved optical gating (FROG), and calculated the settings necessary to eliminate the phase from the pulse. In the other method, I used an evolutionary strategy algorithm to optimize the second harmonic signal. I also used the pulse shaper to optimize the output of high-harmonic generation (HHG). For this I again used the evolutionary strategy algorithm to optimize the harmonic spectrum. Using this method, I was able to more than double the peak intensity of a single harmonic, and I was able to shift the cutoff of the harmonics. In subsequent work, colleagues have achieved a factor of eleven increase in the peak intensity of a single harmonic using the setup reported here.

A compact, three-stage, multipass Ti:sapphire laser amplifier system has been developed that generates >40 TW pulses, with 24 fs pulse duration, at a repetition rate of 10Hz, and with an average power of 10 W. Output intensities in excess of 1 x 1019 W/cm2 have been produced. The technique of frequency-resolved optical gating was used to fully characterize the output pulses, and to carefully compare theoretical models with experiment. High dynamic range FROG measurements have been performed for the first time to characterize the temporal wings of the output pulses. Ultrafast lasers has been used to generate high-order harmonies. The shortest wavelength we observe, at 2.7 nm, is well within the water window region of x-ray transmission. In the case of all the noble gases, excellent agreement has been obtained between theoretical predictions for the highest harmonic photon energy generated and our experimental observations. We also observe that the individual harmonic peaks near the cutoff are well resolved for positively chirped pump pulses, but are unresolved in the case of negatively chirped excitation pulses. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. We have demonstrated that the intrinsic phase of the harmonic emission can be controlled by adjusting the chirp of the excitation laser pulse. Short wavelength recombinational x-ray lasers using high peak power ultrafast laser interaction with small clusters has also been investigated. A model has been developed to simulate this process. The model predicts that this is a feasible scheme to scale the recombinational x-ray laser to short wavelength. Experimental results have verified the hydrodynamic evolution predicted by this model.