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Semiconductor lasers are considered essential for the advancement in the field of photonics where compact and energy-efficient lasers are necessary. Advancements in integrated photonic technologies will help push the performance of semiconductor lasers in the coming years and expand the technology to several other applications. Semiconductor lasers offer several key features such as high energy efficiency, mass production, availability at a myriad of wavelengths, and high integration capabilities. However, limitations in noise performance, pulse energy, and duration hold back semiconductor lasers from being utilized to their full potential. This dissertation reviews the utilization and development of external techniques that enable semiconductor mode-locked lasers to be used in multi-photon imaging and microwave photonic applications. We first review a two-color external cavity mode-locked laser system operating at wavelengths 834 nm and 974 nm that can generate synchronized picosecond pulses with peak powers exceeding 80 W and 100 W respectively. We verify the feasibility of this system to induce non-linear processes by demonstrating two-photon excitation in commercially available dyes. Next, we introduce the concepts of optical injection locking and discuss the development of a multi-tone optical self-injection locking technique to improve the noise performance and optical linewidth of a chip-scale InP based mode-locked laser. We utilize a Fabry-Perot etalon as the optical comb filter, which also serves to suppress the super-mode noise that arises from external cavity feedback. In addition to this, we also implement a coupled opto-electronic loop and reference it to an external RF source demonstrating exceptional timing stability. This approach along with the usage of fully integrated and ultra-compact components in subsequent versions has the potential to realize compact frequency comb lasers for microwave photonic and other practical applications.

The ADK-PPT model of tunneling ionization is experimentally well-verified at intensity below 10¹⁹ W/cm². The relative simplicity of the tunneling model physics and the highly nonlinear ionization rate scaling with intensity make the detection of high ion charge states a promising method for directly measuring laser intensity. We present our progress toward the next generation of high-field atomic physics experiments, including modeling of the ion and electron dynamics in ultra-intense laser fields. We modeled the interaction of a highly-charged ion with a tightly-focused (f/1) laser pulse with a wavelength of 1057 nm and a pulse duration of 140 fs, characteristic of Nd:glass OPCPA laser facilities. We found that ponderomotive expulsion of the ions from the laser field necessitates new experimental methods for directly measuring or indirectly inferring the presence of high charge states. Our modeling showed ions will be accelerated to energies > 2 MeV/nucleon and ATI electron energies can exceed 1.4 GeV at the peak intensity of 3 x 10²³ W/cm² expected to be reached in the next few years. We also present our findings from two novel experimental campaigns on the Texas Petawatt Laser, an unsuccessful test of field-free ion time-of-flight and a successful detection of electrons produced by above-threshold ionization (ATI) of highly-charged neon states. We observed ATI electrons with energy greater than 10 MeV originating from the neon K-shell when laser intensity exceeds 2 x 10²⁰ W/cm².

Two-photon microscopy is often used to image in vivo neural structural due to the excellent contrast, resolution, and penetration depth available through the technique. While brain tissue can be imaged down to 500 µm routinely, there is a desire to push this even further given that the cerebral cortex extends to a 1 mm depth in mice. In this work we introduce a custom ytterbium fiber amplifier and diamond Raman laser that operate at wavelengths more suitable for deep imaging (1060 nm, 1250 nm) as compared to more traditional Ti:sapphire lasers (700-1000 nm). We prove their utility for two-photon excitation of multiple fluorophores that commonly serve as vascular and neuronal labels. Our custom oscillators are well suited for rapid imaging strategies as well given their high repetition rate (80 MHz), so additionally we present a methodology to increase our lateral imaging field of view which makes use of a resonant scanning mirror. Finally, we demonstrate a pulse gating strategy that can either increase signal generation or reduce average excitation power when imaging as to avoid excessive tissue heating. Many of the tools and strategies we present can be used in parallel to image larger tissue volumes for more comprehensive longitudinal study of neurovascular structure.

The development of few-cycle, ultra-intense laser sources in the long-wave infrared (LWIR, 8–12 μm) is of keen interest in many research fields, including molecular spectroscopy in the “fingerprint” region, laser-induced filamentation, remote sensing, and strong-field physics such as laser particle acceleration and high-harmonic generation. LWIR sources have gained their foothold due to their high transmittance in the atmosphere and their advantages granted by wavelength-scaling rules. The current absence of LWIR laser sources capable of generating ∼100-fs pulses with multi-terawatt peak power is due to the lack of broadband laser gain medium in the LWIR. The state-of-the-art CO2 lasers can produce multi-terawatt peak power, however, in a relatively long picosecond duration. Our vision for delivering such performance is through optical parametric chirped-pulse amplification (OPCPA) of ultra-broadband, carrier-envelope phase (CEP) stabilized LWIR seed. Facilitated by the coherent combining and mid-infrared Er:ZBLAN fiber technologies, the energy scaling capabilities of such architecture can be further extended. This dissertation focuses on developing few-cycle, ultra-intense LWIR laser sources in three key aspects. First, LWIR seed pulses were generated through difference-frequency generation (DFG) in birefringent nonlinear crystals. For the first time, milliwatt-level LWIR seed pulses were produced by mixing 2.9-μm Er:ZBLAN fiber amplifier/compressor outputs with their frequency-shifted replicas. Higher pulse energy (110 nJ) and ultra-broadband spectral bandwidth (supporting two-cycle pulses) were achieved at a lower repetition rate driven by two-color Ti:sapphire laser output and assisted by genetic algorithm optimization. A single-shot infrared spectroscopy study on ammonia was performed with the LWIR seed source. Second, parametric amplification of LWIR seed pulses was realized through noncollinear optical parametric amplification (NOPA) in a 5-mm thick GaSe crystal, pumped by nanosecond 2.7-μm pulses. As a consequence of crystal properties related to laser-induced damage threshold and the beam-pointing fluctuation, the attained parametric gain was limited. Lastly, an active beam-pointing control system was developed to address the beam-pointing fluctuations confronted in a laser system. Using a proportional feedback loop and piezo-actuated mirror, the short-term beam-pointing fluctuations induced by ambient vibration were significantly reduced by up to an order of magnitude in a proof-of-principle study. Long-term fluctuations due to temperature drift were also eliminated. To summarize, two broadband LWIR seed sources driven by an Er:ZBLAN fiber laser and a solid-state Ti:sapphire laser were developed, accompanied by the implementation of numerical models and optimization algorithms. Experimental and analytical efforts were presented on characterizing ultrashort LWIR pulses. Amplification of LWIR pulses was accomplished in an OPCPA apparatus with limited parametric gain. The diminished efficacy of the system was attributed to deteriorated nonlinear crystal and beam pointing. An active control system was constructed to potentially tackle the latter by compensating for the beam-pointing fluctuations using a proportional feedback-controlled piezo-actuated mirror. This dissertation research paves new paths for generating and characterizing few-cycle, CEP-stabilized ultra-intense LWIR pulses favorable by a broad spectrum of applications.

Light-matter interactions are interesting at high intensities: strong-field effects can result in ultrashort current bursts and optically-driven tunneling. However, this regime of lightmatter interactions is generally only accessible with high-power, short-pulse, exotic laser systems. In this thesis, we investigate whether we can extend this regime of light-matter interactions to low-power laser systems that are common in many applications. In short, we aim to leverage developments in nanofabrication to produce devices where we can apply enormous static electric fields that may help us reach the high fields necessary for strongfield light-matter interactions with only low-power, everyday laser systems. Such devices could offer two main benefits. Firstly, such devices could provide easy access to ultrashort electrical tunneling bursts produced in strong-field interactions. These could be useful for future sampling applications. Secondly, strong-field effects are relatively insensitive to the photon energy (since they are field-driven), so devices based on strong-field effects could be useful for broadband optical detectors. In particular, they could be extremely useful for detecting traditionally challenging to detect parts of the electromagnetic spectrum, such as the mid- to far-infrared ranges.In this thesis, we tackle the problem of simulating these strong-field light-matter interactions. After a brief introduction, we outline in Chapter 2 the exact configurations used in the simulations as well as some discussion about photoemission. Next, in Chapter 3, we ensure that the solver has enough points to accurately simulate our devices. We also discuss the benchmarking parameters we tested the solver with as well as the benchmarking process. In Chapter 4, we look at the functionality that we added to the solver to more closely represent realistic devices. Overall, we find that our devices can achieve strong field effects with low-power laser systems and low applied static fields. Finally, in Chapter 5, we discuss some future work this solver will be implemented with as well as improvements and additional questions to explore related to detecting mid- to far- infrared light.

Multiphoton microscopy is a powerful optical imaging modality renowned for its non-invasive nature and relatively affordable characteristics. In particular, it has found its niche in neuroimaging due to its ability to probe in vivo biological processes in scattering brain tissue approaching millimeter depths with cellular resolution. However, the brain is a large and complex organ, and in order to fully understand its heterogeneous architecture and associated functional roles, several distal regions must be imaged simultaneously. Moreover, due to the critical implications of organelle features in various macroscale processes, whole-brain imaging at subcellular resolution scales presents itself as one of the outstanding challenges faced by the neuroscientific community today. Primarily, this research aims to expand the depth, field-of-view, and temporal throughput of multiphoton microscopy to enable large volume imaging of microvasculature at greater acquisition speeds. To accomplish this, we combine multi-faceted efforts focused on the engineering and development of advanced multiphoton microscopy techniques and technologies. This includes the characterization of novel contrast agents, the optimization of scan system optics, and the integration of high-repetition rate lasers with a resonant galvanometer. In addition, we develop a two-color imaging system capable of enhancing excitation efficiency, improving signal-to- background ratio, and further extending imaging depth. Finally, we present a novel application for two-color non-degenerate mode mixing to effectively circumvent the diffraction-limited nature of optical resolution and enable subcellular imaging. Collectively, these efforts advance the state-of-the art of multiphoton microscopy for routine cerebrovascular and neuroimaging.

Ultrafast lasers are uniquely suited for tissue ablation in situations where precision and preservation of the surrounding tissue architecture is crucial. However, their clinical adoption has been limited by challenges to flexible endoscopic delivery, and the slow cutting speeds and material removal rates these lasers have historically produced. Solving the challenges surrounding miniaturization, fiber-optic delivery and the limited cutting speeds can provide surgeons with a viable tool for precise tissue excision in biologically sensitive regions like the vocal fold and spine. This dissertation focuses on the addressing the engineering challenges surrounding ultrafast laser surgery. The first part of the dissertation describes the design and realization of a 5 mm ultrafast laser scalpel capable of reaching material removal rates in the order of 0.6 mm³/min/W, a first for a miniaturized ultrafast laser device. Micro-Joules energy delivery at the tissue is made possible by air-core Kagome optical fibers. Diffraction limited performance is achieved via objectives with Zinc Sulfide (ZnS), and Calcium Fluoride (CaF₂) lenses respectively. The probe’s performance is tested via metal and tissue ablation studies, characterizing high-speed ablation parameters and uniformity of ablation over the scan area. The onset of three-photon absorption in our ZnS lenses limits the energy deliverable for these systems. Through additional z-scan studies, we characterize the nonlinear behavior of ZnS crystals at 776 nm and 1552 nm wavelengths, with an eye towards future endoscopic imaging systems. As a second part of the dissertation, we explore how the near field plasmonic enhancement around gold nanoparticles can be exploited to improve ultrafast laser surgery rates, especially on a cellular level. We detail bench-top pump-probe studies exploring the threshold reduction brought about in ultrafast laser bubble formation in nanoparticle suspensions. The nanoparticle’s spatial position within the focal volume is extracted from the size and frequency of bubbles generated. This information is then used to establish a relationship between the fluence experienced and size of the photo-disruptive bubble formed. With experimental results indicating nearly 20-fold reduction in threshold fluences, the use of such systems can potentially reduce treatment times as large volumes of tissue can be rapidly irradiated at a given time

Abstract: This thesis work focuses on obtaining preliminary results demonstrating the isolated femtosecond laser ablation of sub-RPE drusen-like deposits. Age-related macular degeneration (AMD) is an age-onset degenerative condition of the retina that can lead to legal blindness. The early stages of the condition are characterized by the development of hemi-spherical cellular debris accumulations, known as drusen, under the retina. This project has two specific aims: demonstrating the laser ablation of AMD related sub-RPE deposits and assembling a prototype for a clinical therapeutic beam delivery system. An articulating arm is fitted with silver mirrors for 800 nm and coupled to a slit lamp ophthalmic device for femtosecond laser delivery to the retina. A novel cell culture model using the immortalized cell line ARPE-19 is developed. The cell culture model is evaluated by confocal and transmission electron microscopy for the presence of sub-RPE drusen-like deposits. The samples are stained for two known drusen components, apolipoprotein E and cholesterol, to verify the artificial drusen-like deposits share a similar biochemical composition to drusen in vivo. Sub-RPE drusen-like deposits in the ARPE-19 model were targeted with 10 femtosecond pulse trains, 1.9-4.2 nJ/pulse, centered at 800 nm wavelength, from a Ti:Sapphire laser. Fluorescence microscopy was used to qualitatively confirm successful deposit ablation. After laser ablation more fluorescent dye was added to the dish and monitored to see if the areas targeted by the laser regain fluorescence. Although areas of the sample did regain fluorescence, the targeted deposit accumulation never regained fluorescence. Thus, eliminating the possibility of laser photobleaching being responsible for deposit removal as opposed to plasma mediated breakdown.

Optical microscopy techniques such as single-particle tracking and high-resolution (

Petawatt-level laser pulses have many potential applications in science and industry, but will require three orders of magnitude increase in pulse repetition rate from existing solid-state laser technology. Fiber lasers can operate at such repetition rates, but are limited in pulse energy. To overcome the gap between current achievable fiber-laser pulse energies (~mJ) and required pulse energies for high-energy applications (up to 10J), this dissertation work explores four novel techniques: (1) Coherent beam combining in the spatial domain; (2) Coherent spectral combining in spatial and spectral domains; (3) Coherent pulse stacking amplification in the time domain; (4) N-squared coherent combining in spatial and time domains. (1) We demonstrate coherent femtosecond pulse beam combining of up to four chirped-pulse fiber amplifier channels. Theoretical and experimental analysis of combining efficiency dependence on amplitude/phase noise shows the scalability to a large number of channels. (2) We demonstrate coherent femtosecond pulse spectral synthesis by combining three parallel fiber chirped-pulse amplifiers, each amplifying different pulse spectra. This technique simultaneously overcomes individual-amplifier energy/power limitations, and spectral gain narrowing in a single fiber amplifier. (3) We propose and demonstrate a new technique of coherent pulse stacking (CPS) amplification, which uses reflecting resonators to transform a sequence of phase/amplitude modulated optical pulses into a single output pulse. Experimental validation with a single resonator is demonstrated. We show theoretically that the extension to stacking a large number of equal-amplitude pulses can be achieved using multiple reflecting resonators, which enables the extraction of all stored energy in large-core fiber amplifiers. (4) We propose and demonstrate N-squared coherent combining using resonant optical cavities, a novel pulse combining technique based on both spatial combining and temporal stacking. Its unique feature is in an N-channel system the combined pulse energy is enhanced by N-squared times. This dissertation work provides the initial experimental and theoretical validations of several novel approaches that use coherent pulse synthesis/combining to achieve power and energy scaling using multiple small-aperture lasers, and serves as an initial step on the path towards future high average-power and petawatt peak-power laser technologies.

Attosecond pulses have been developed as a means for investigating phenomena that proceed on the order of the atomic unit of time (24 as). Unfortunately, these extreme ultraviolet (XUV) pulses by themselves contain too few photons to initiate nonlinear dynamics or dress states in an attosecond pump--attosecond probe scheme. As a result, most attosecond experiments thus far have featured complementary near infrared (NIR) femtosecond lasers for instigating electron dynamics. In order to access the benefits of all-attosecond measurements and open attosecond physics to new fields of exploration, the photon flux of these pulses must be increased. One way to boost the attosecond pulse energy is to scale up the energy of the NIR pulse responsible for driving high-harmonic generation (HHG). With generalized double optical gating (GDOG), isolated attosecond pulses can be generated with multi-cycle laser systems, wherein the pulse energy can be boosted more easily than in the few-cycle laser systems required by other gating methods. At the Institute for the Frontier of Attosecond Science and Technology (IFAST), this scalability was demonstrated using a 350 mJ, 15 fs (10 TW) Ti:sapphire laser, which was used to generate a 100 nJ XUV continuum. This represented an order-of-magnitude improvement over typical attosecond pulse energies achievable by millijoule-level few-cycle lasers. To obtain the microjoule-level attosecond pulse energy required for performing all-attosecond experiments, the attosecond flux generated by the IFAST 10 TW system was still deficient by an order of magnitude. To this end, the laser system was upgraded to provide joule-level output energies while maintaining pulse compression to 15 fs, with a targeted peak power of 200 TW. This was accomplished by adding an additional Ti:sapphire amplifier to the existing 10 TW system and implementing a new pulse compression system to accommodate the higher pulse energy. Because this system operated at a 10 Hz repetition rate, stabilization of the carrier-envelope phase (CEP) -- important for controlling attosecond pulse production -- could not be achieved using traditional methods. Therefore, a new scheme was developed, demonstrating the first-ever control of CEP in a chirped-pulse amplifier (CPA) at low repetition rates. Finally, a new variation of optical gating was proposed as a way to improve the efficiency of the attosecond pulse generation process. This method was also predicted to allow for the generation of isolated attosecond pulses with longer driving laser pulses, as well as the extension of the high-energy photon cut-off of the XUV continuum.

The advantage of fiber lasers is self-evident. Fiber is relatively low cost, flexible for compact design, has a large surface area to volume ratio for efficient heat dissipation, is alignment free, and provides good spatial beam quality due to its properties as a waveguide. It can be highly user friendly due to the minimum maintenance and much lower cost. The performance increases in fiber lasers have allowed them to reach applications in the fields of manufacturing with lasing machining, multi-photon biological imaging, and laser surgery. Moreover, fiber laser has rich nonlinearity so it provides a good play ground for nonlinear phenomena study, such as Rogue waves. Before 5 years ago, fiber laser had been dominated by soliton lasers [47] [9], which gives good performance for solid state laser but has limited performance for fiber lasers due to the accumulated large nonlinear phase due from strong confined wave propagation within the small core of the fiber and relative large local dispersion to generate short pulses according to the area theorem. The ordinary soliton fiber laser has limited pulse energy of 0.1 nJ. For dispersion managed (DM) soliton fiber lasers, although the nonlinearity could be controlled to some extend by temporal pulse broadening inside the cavity, which results an order of magnitude increase of energy, it is still hard for fiber laser to compete with solid state lasers in energy. Recent developments in the field of fiber lasers over the past five years , particularly the dissipative soliton lasers at nor- mal dispersion regime [1] showed a different nonlinear solution which does not constrained by soliton area theorem, have enabled fiber lasers to compete with solid state lasers in terms of energy around 100 fs regime. To compete with solid state lasers, fiber lasers also must be able to achieve short pulses down to a few cycles. Solid state gain media have gain bandwidths of hundreds of nanometers while the bandwidth of doped fiber is around 40 nm, which fundamentally limits the shortest pulses available from fiber source based on commonly used mode locking mechanisms. Since the gain will act like a filter to limit the possible average cavity solution both both soliton and dissipative soliton lasers. Another difficulty is the relative large net dispersion of fiber lasers compared with solid state lasers, which starts to place additional limits on the pulse durations below 100 fs. Although 100 fs pulses can be routinely generated from fiber lasers, pulses as short as a few cycles are harder to stabilize than in solid state lasers. Over the past few decades, fiber lasers were typically made with Er-doped fiber and later with Yb-doped fiber. Tm fiber lasers have drawn people s attention due to its potential applications for high harmonic generation, mid-IR pumping source and many other areas. In this thesis, fiber lasers with different gain medium, Erbium, Ytterbium and Thulium, will be presented with the effort of generating short pulses through the use of different mode locking mechanisms. The extended self-similar propagation, which is different from previous average cavity analysis, will be introduced as a route to few cycle pulse generation. Pulses as short a 6 cycles have been generated for both Yb and Er lasers. The extended self-similar propagation can occur in either a piece of highly nonlinear fiber through passive self-similar propagation of parabolic pulses in normal dispersion fiber or through a segment of dispersion decreasing fiber, which has a mathematical analogy to self- similar propagation in a gain fiber. The design guide for this type of cavity will be given, and energy scaling of this cavity with large core fibers will be discussed.

This thesis outlines the high intensity tabletop attosecond extreme ultraviolet laser source at the Institute for the Frontier of Attosecond Science and Technology Laboratory. First, a unique Ti:Sapphire chirped pulse amplifier laser system that delivers 14 fs pulses with 300 mJ energy at a 10 Hz repetition rate was designed and built. The broadband spectrum extending from 700 nm to 900 nm was obtained by seeding a two stage Ti:Sapphire chirped pulse power amplifier with mJ-level white light pulses from a gas filled hollow core fiber. It is the highest energy level ever achieved by a broadband pulse in a chirped pulse amplifier up to the current date. Second, using this laser as a driving laser source, the generalized double optical gating method is employed to generate isolated attosecond pulses. Detailed gate width analysis of the ellipticity dependent pulse were performed. Calculation of electron light interaction dynamics on the atomic level was carried out to demonstrate the mechanism of isolated pulse generation. Third, a complete diagnostic apparatus was built to extract and analyze the generated attosecond pulse in spectral domain. The result confirms that an extreme ultraviolet super continuum supporting 230 as isolated attosecond pulses at 35 eV was generated using the generalized double optical gating technique. The extreme ultraviolet pulse energy was ∼100 nJ at the exit of the argon gas target.

The resonant interaction of light with metal nanoparticles can result in extraordinary optical effects in both the near and far fields. Plasmonics, the study of this interaction, has the potential to enhance performance in a wide range of applications, including sensing, photovoltaics, photocatalysis, biomedical imaging, diagnostics, and treatment. However, the mechanisms of plasmonic enhancement often remain poorly understood, limiting the design and effectiveness of plasmonics for advanced applications. This dissertation focuses on evaluating the mechanisms of plasmonic enhancement and distinguishing between near and far field effects using simulations and experimental results. Thorough characterization of metal nanoparticle colloids shows that electromagnetic simulations can be used to accurately predict the optical response of nanoparticles only if the true shapes and size distributions are taken into account. By coupling these optical interaction calculations with heat transfer models, experimental limits for the maximum optical power before nanoparticle melting can be found. These limits are important for plasmonic multiphoton luminescence imaging applications. Subsequently, we demonstrate ultrafast laser plasmonic nanoablation of silicon substrates using gold nanorods to identify the near-field enhancement and mechanism of plasmon-assisted ablation. The experimentally observed shape of the ablation region and reduction of the ablation threshold are compared with simulations to show the importance of the enhanced electromagnetic fields in near-field nanoablation with plasmonic nanoparticles. The targeted use of plasmonic nanoparticles requires narrow size distribution colloids, because wide size distributions result in a blurring and weakening of the optical response. A new synthesis method is presented for the seeded-growth of nearly monodisperse metal nanoparticles ranging from 10 to 100 nm in diameter, both with and without dielectric shells of controlled thickness. This method is used to acquire fine control over the position and width of the plasmonic peak response. We also demonstrate self-assembled sub-monolayers of these particles with controllable concentrations, which is ideal for looking at plasmonic effects in surface and layered geometries. Finally, we present results for the spatial distribution of absorption around plasmonic nanoparticles. We introduce field-based definitions for distinguishing near-field and far-field regions and develop a new set of equations to determine the point-by-point enhanced absorption in a medium around a plasmonic nanoparticle. This set of equations is used to study plasmon-enhanced optical absorption for thin-film photovoltaic cells. Plasmonic nanoparticle systems are identified using simulations and proof-of-concept experiments are used to demonstrate the potential of this approach.

The development of ultrafast lasers has enabled a wide range of applications such as time-resolved spectroscopy, micro-machining and non-linear microscopy. For the past 20 years, solid-state lasers have been the workhorses of ultrafast science. However, they remain bulky and sensitive tools requiring careful alignment. Thus, rare-earth doped fiber lasers have generated significant interest. They can be monolithically integrated and use simple power scalable diode pumping. Thanks to recent advances in the understanding of non-linear pulse evolution in optical fiber, as well as the development of large-core fiber technologies, fiber lasers have achieved performance matching conventional solid-state lasers. This thesis explores the non-linear propagation and parametric conversion of high energy short pulses in normal dispersion optical fibers. A laser source for coherent anti-Stokes Raman scattering microscopy is demonstrated based on frequency conversion of picosecond pulses through four-wave mixing in photonic crystal fiber. The effects of vibrational dephasing on the coherence and compressibility of Raman Stokes pulses generated in chirped-pulse fiber amplifiers are investigated. Finally, all-normal-dispersion fiber lasers are scaled to high pulse energies using large-core fibers. The performance of multimode step-index fiber, chirally-coupled core fiber and photonic crystal fiber is compared. Using fibers with robust modefiltering, fibers lasers delivering up to a megawatt of peak power are demonstrated.

The beam combination of fiber lasers can achieve higher output average power and energy than a single fiber amplifier. This thesis explores fiber laser combining as a path towards increasing powers and energies from fiber-based systems. In this work two general areas are addressed: we explore passive coherent self-locking of cw fiber laser arrays, and active coherent phasing of ultrashort-pulse fiber laser arrays. Passive self-locking is technologically attractive since it does not require any active phase-control of parallel channels in the array and, therefore, could potentially lead to much simpler coherently self-locked fiber arrays. However, the array-size scalability of passive self-locking has not been well understood. In this work we (i) experimentally validate the new numerical model developed in collaboration with Prof. H. Winful group, which describes accurately the complex phenomena occurring in such arrays, and (ii) experimentally confirm the array-size scaling limitations, predicted by this theoretical model. Extensive experimental characterization of 16-channel self-locked fiber laser array agrees well with the theoretically predicted decrease of power combining efficiency with array size, occurring as a result of increasing scarcity of coherently-combined supermodes. In pulsed coherent combining, we demonstrate femtosecond pulse combining with up to 4 parallel fiber amplifier channels using LOCSET single-detector-feedback phase-locking technique. We achieve good combining efficiency and negligible distortions in the combined pulses. Extrapolating from experimental results we explore array-size scalability in relation to modulation amplitude, power amplitude, and phase error, and show that multi-channel pulse combining with LOCSET feedback can be scalable to very large combined-array sizes. Using the established 4-channel pulse coherent combining experimental test bed, we demonstrate femtosecond pulse simultaneous combining and synthesis by coherently spectrally combining 3 fiber chirped pulse amplifiers. We explore different phase-feedback strategies using linear detection schemes when sufficient partial spectral overlap is present, and using two-photon-absorption detector when this spectral overlap becomes vanishingly small. This demonstration shows a path towards simultaneous power scaling and pulse shape and spectral shape control in coherently-combined femtosecond-pulse laser arrays. For example, this could be used to overcome gain narrowing effects in a rare-earth doped fiber amplifier.

The strong field regime describes interactions between light and matter where the electric field of the laser is a significant fraction of the binding field of the atom. Short pulsed lasers are capable of producing local fields on the order of the atomic unit of electric field. Under the influence of such strong fields, the ionization regime and electron dynamics are highly dependent on the wavelength used to drive the interaction. Few studies have been performed in the mid-infrared (MIR) spectral range. Using MIR wavelengths, the ponderomotive energy, Up, imposed on the electrons is a factor of 20 greater than in the visible and near-infrared. Experiments on above threshold ionization (ATI) of cesium, nonsequential ionization (NSI) of noble gases, and high harmonic generation (HHG) in condensed phase media highlight the benefits of performing strong field investigations in the MIR.The photoelectron energy spectrum from ATI of atoms provides details about the strong field interaction. Cesium atoms driven by a 3.6 um laser indicate that excited states can play a large role in ionization from the ground state. Previous experiments on argon in the near-infrared can be compared to cesium at 3.6 um due to their similar Keldysh-scaling. The ground state of cesium experiences a large Stark shift, unlike similar experiments in argon. The low-lying 6P excited states of cesium produce a strong effect on the photoelectron energy spectrum, resulting in a splitting of each ATI peak. Enhancements in the photoelectron energy spectrum similar to those found in argon are observed in cesium. These enhancements are relatively insensitive to ellipticity of the drive laser.To take advantage of the large ponderomotive energy associated with Mid-IR lasers, ionization of argon, krypton and xenon is studied at 3.6 um. Electrons ionized by the MIR laser to return to the core with up to ~300eV in energy. Inelastic scattering of returning electrons impinging on their parent ion results in impact ionization, producing up to the 6th charge state in xenon. Scaling the impact ionization cross section by the electron return energy distribution and wavepacket spread yields excellent agreement with the observed ionization ratios. Elastic scattering of the returning electrons provides information about the structure of the parent ion, as the differential cross section can be extracted from the experimental photoelectron angular distribution.HHG in atomic gases occurs with low conversion efficiency. The advantages of a MIR laser system are used to study HHG in liquid and solid samples. The long wavelength makes possible the first demonstration of high order harmonics from the bulk of a crystalline solid. Harmonics generated in liquids appear perturbative in nature, while those generated in a crystal are highly nonperturbative. The high order harmonics yield information about the strong field response of the crystal’s band structure.The use of MIR wavelengths allows the observation of new strong field phenomena. As all of the above methods are applicable to molecules as well as atoms, MIR studies have a strong impact on understanding the interaction between light and matter.

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.

Tomographic imaging has dramatically changed science, most notably in the fields of medicine and biology, by producing 3D views of structures which are too complex to understand in any other way. Current tomographic techniques require extensive time both for post-processing and data collection. Femtosecond laser based tomographic techniques have been developed in both standard atmosphere (femtosecond laser-based serial sectioning technique - FSLSS) and in vacuum (Tri-Beam System) for the fast collection (10^5 μm^3/s) of mm^3 sized 3D datasets. Both techniques use femtosecond laser pulses to selectively remove layer-by-layer areas of material with low collateral damage and a negligible heat affected zone. To the authors knowledge, femtosecond lasers have never been used to serial section and these techniques have been entirely and uniquely developed by the author and his collaborators at the University of Michigan and University of California Santa Barbara. The FSLSS was applied to measure the 3D distribution of TiN particles in a 4330 steel. Single pulse ablation morphologies and rates were measured and collected from literature. Simultaneous two-phase ablation of TiN and steel matrix was shown to occur at fluences of 0.9-2 J/cm^2. Laser scanning protocols were developed minimizing surface roughness to 0.1-0.4 μm for laser-based sectioning. The FSLSS technique was used to section and 3D reconstruct titanium nitride(TiN) containing 4330 steel. Statistical analysis of 3D TiN particle sizes, distribution parameters, and particle density were measured. A methodology was developed to use the 3D datasets to produce statistical volume elements (SVEs) for toughness modeling. Six FSLSS TiN datasets were sub-sampled into 48 SVEs for statistical analysis and toughness modeling using the Rice-Tracey and Garrison-Moody models. A two-parameter Weibull analysis was performed and variability in the toughness data agreed well with Ruggieri et al. bulk toughness measurements. The Tri-Beam system combines the benefits of laser based material removal (speed, low-damage, automated) with detectors that collect chemical, structural, and topological information. Multi-modal sectioning information was collected after many laser scanning passes demonstrating the capability of the Tri-Beam system.

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.