Cataract surgery is one of the most common procedures performed in the entire world. Cataracts are inevitable in anyone who live to old age and as the global population continues to age, the number of cataract surgeries that are expected to be performed each year is only expected to increase. The procedure is also one of the oldest and has matured a substantial amount since its first utilization but it is still not perfect. It is crucial that we continue to improve to procedure in pursuit of improved post-operative outcomes for patients. In this study, a delayed phacoemulsification after the femtosecond laser anterior capsulotomy and lens fragmentation was introduced into the procedure with the hypothesis that this may allow the cataract to hydrate and soften, leading to an intraoperative reduced phacoemulsification power and a post-operative preservation of the corneal endothelium in grade 4 nuclear cataracts. This study is a new study with very little data collected so far but it is planned that we will initially perform the procedure on 10 patients with bilateral grade 4 cataracts. Each patient will have the new procedure done on one eye and then the typical cataract surgery procedure done on the other eye to be used as a statistical comparator. This thesis specifically explores the concept of introducing a delay between the capsulotomy and the phacoemulsification process and the safety and feasibility of such procedure in practice. The new procedure, so far, has been performed on one patient with the surgery data for the other eye currently pending. Relevant data such as cumulative dissipated energy and intraocular pressures were recorded during and after the surgery for future comparisons. Without a complete set of data, a statistical conclusion cannot be made about the efficacy of the new procedure, but as addressed in this thesis, the procedure has been concluded to be safe and feasible with a good post-operative outcome.

Ti: sapphire femtosecond laser ablation of dielectrics (fused silica and BK7 glass) and metals (Cu, Fe, Al) is presented. Results of laser -induced breakdown experiments in fused silica and BK7 glass employing 130 fs -1.7 ps, 790 nm laser pulses are reported. The fluence ablation threshold does not follow the scaling of 4>th ~ ^/2 when pulses are shorter than 1 ps. Single-shot and multi-shot (130 fs pulse) ablation of selected materials are investigated with laser wavelengths of 395 nm, 790 nm, and 1300 nm. The ablation threshold is almost independent of the laser wavelength. The surface morphologies in metals after ultrashort pulse ablation are very different from dielectrics and semiconductors. The roughness of the ablated surface depends on the thermal properties of the metal target. The preliminary TEM result from Cu single crystal that was irradiated by single laser pulses shows few defects in the center region of the ablated crater. Single-shot ablation of single-crystal Fe induces much different surface features than on selected samples of poly-crystal Fe metal. / Thesis / Master of Engineering (ME)

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Thesis advisor: Torsten Fiebig / This dissertation introduces a new field of DNA photonics centering on the electronic properties of DNA, which emerges after the initial controversies regarding the long-range conductivity and wire-type behavior of DNA have been widely settled. DNA photonics study is not solely focused on charge transfer phenomena but encompasses all possible photophysical processes and their potentially complex interplays. For instance, ultrafast electronic energy migration, dissipation, and (de)localization on the femtosecond time scale are shown to be crucial for the description of light-induced dynamics in DNA and have been thoroughly investigated in this dissertation. In addition to measurements on natural single and double-stranded DNA, this dissertation also presents experimental data on a series of functionalized DNA systems (derivatized by stilbene, ethidium, 2-aminopurine, etc.), obtained by state-of-the-art femtosecond broadband pump–probe spectroscopy. The results illustrate the distance dependence of charge transfer, emphasize the role of the initial electronic excitation on energy transfer dynamics, and highlight the influence of structural factors on both processes in DNA. Finally, as one initial step towards DNA electronics application, a DNA mimicking system of tertiary arylureas were employed to demonstrate molecular wire behavior, implying its potential use in molecular electronics. Thus, both the experimental and theoretical research accumulated for DNA π–π coupling can be translated into designing and testing various molecular systems with similar π-stacked structures. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Femtosecond laser pulse has been used for the machining of the gratings primarily due to its superior advantages over conventional continuous wave (CW) and long pulse lasers for micromachining. In this paper, we develop a novel technique for the fabrication of planar gratings by colliding two beams to generate interference fringes. This technique is simple, fast and low cost. We have successfully fabricated planar gratings on a copper substrate. / Singapore-MIT Alliance (SMA)

Photomasks are the backbone of microfabrication industries. Currently they are fabricated by lithographic process, which is very expensive and time consuming since it is a several step process. These issues can be addressed by fabricating photomask by direct femtosecond laser writing, which is a single step process and comparatively cheaper and faster than lithography. In this paper we discuss about our investigations on the effect of two types of laser writing techniques, namely, front and rear side laser writing with regard to the feature size and the edge quality of the feature. It is proved conclusively that for the patterning of mask, front side laser writing is a better technique than rear side laser writing with regard to smaller feature size and better edge quality. Moreover the energy required for front side laser writing is considerably lower than that for rear side laser writing. / Singapore-MIT Alliance (SMA)

This thesis reports on a new design of photonic crystal waveguides (PCWs) to achieve large guiding bandwidth, linear dispersion, single-mode behavior, good coupling efficiency to dielectric waveguides, and small loss. The design is based on using the linear dispersion region of one PCW in the photonic bandgap (PBG) of another PCW. While perturbing the period can result in a PCW with linear dispersion and large guiding bandwidth, it introduces an odd mode at those frequencies, as well. By using another perturbation scheme, it is shown that single-mode behavior can also be achieved. The linear dispersion of these waveguides and their operation at lower frequencies of the PBG, where the density of states of radiation modes is smaller, gives rise to very small loss coefficients as verified experimentally. Full characterization of a waveguide requires the measurement of not only the transmission coefficient, but also the dispersion and spectral phase. We have developed a real-time characterization technique based on spectral interferometry with femtosecond laser pulses at optical communication wavelengths to measure the spectral phase of waveguides. This haracterization technique can be used to study fast dynamics in timevarying structures and makes the alignment easy.

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.

The carrier envelope (CE) phase of a femtosecond laser was stabilized. The laser produces an ultra stable comb of frequency spanning the visible region and basically is an optical frequency synthesizer and ready for the frequency domain applications. In this context, the CW stability of the Ti:sapphire laser is discussed to provide a procedure for the femtosecond laser adjustments. In addition, the pulse trains emitted by the femtosecond laser are described analytically to provide a theoretical basis for carrier envelope phase stabilization. An f to 2f interferometer was used to detect the carrier envelope offset frequency, and a fast photo diode was employed to measure the repetition rate. Two similar designed phase lock loops are used to stabilize both the carrier envelope offset frequency and the repetition rate to the respective reference frequencies. The stability reaches 100mHz for the carrier envelope offset frequency and 10mHz for the repetition rate for a period of up to an hour. Doppler free iodine saturation spectroscopy was set up to provide a precise frequency reference to which a CW dye laser can be locked on. The near future goal is to accurately measure this frequency stabilized dye laser with the optical frequency synthesizer.