Student Dissertations and Theses

May 19, 2008

Femtosecond Optical Engineering

Dr. Peng Li, Dissertation, Electrical Engineering

The last a few decades have witnessed the rapid progress of ultrafast optics, which has been driving the technology revolutions in scientific research and engineering applications. For instance, several recent Nobel Prize winners have made their discoveries using femtosecond optics applications in physics, chemistry and biomedicine. This dissertation covers three important applications of ultrafast femtosecond lasers. First, we study the single particle scattering spectroscopy using supercontinuum white light tweezers. We have developed supercontinuum white light optical tweezers by using the ultrabroad band supercontinuum (SC) generated in highly nonlinear photonic crystal fibers pumped by ultrafast laser pulses and for the first time studied the scattering spectra in tightly focused supercontinuum. When the scatterer is of spherical shape, we describe modeling based on Mie's scattering theory and angular spectrum decomposition. For the non-spherical-shaped scatterers, when the size of the scatterer is small or the refractive index of the scatterer is close to the surrounding medium, another modeling based on Born approximation and Green's function is derived. The calculation results are provided as well. This work has built the foundation to understand optical scattering spectroscopy of single particles in the supercontinuum white light optical tweezers and further application to probe the single particle's physical and chemical properties via the linear and nonlinear optical scattering spectra, which can lead to many important applications particularly in nanoparticle characterization and sensing. The second part of this dissertation is dedicated to the study of the femtosecond pulse beam shaping. For femtosecond laser pulses, which cover bandwidths of tens of nanometers, the performance of beam-shaping a Gaussian beam into a flat-toped rectangular intensity profile is discussed for a practical realization. In the meantime, the convergence of the improved Gerchberg¨CSaxton algorithm has been studied and the number of iterations for phase element design is optimized. The temporal and spatial properties of femtosecond laser pulses during beam shaping are studied. This study is expected to benefit many industrial, medical and military applications where specified beam shaping profiles are desirable. The last part of this dissertation is terahertz (THz) generation by optical rectification of femtosecond laser pulses. A method using optical rectification of supercontinuum is proposed to improve the conversion efficiency over a broadband range. Highly efficient broadband terahertz will accelerate the development of terahertz technologies and their applications to areas such as biomedical imaging and remote security screening.

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September 24, 2007

Supercontinuum Imaging and Spectroscopy

Dr. Kebin Shi, Dissertation, Electrical Engineering

Developing new optical imaging and spectroscopy techniques based on supercontinuum (SC) light sources is the major theme of this thesis. As a complex nonlinear optical phenomenon, SC generated in highly nonlinear photonic crystal fiber (PCF) exhibits broad spectral bandwidth (~1000nm) and laser-like spatial coherence. These novel properties open new opportunities for imaging and spectroscopy applications. More specifically, as fast data acquisition rate has always been desired in three-dimensional optical imaging system, optical imaging part of this thesis discusses new methods of improving imaging scanning speed in both confocal and two-photon excitation fluorescence imaging systems by using wavelength division multiplexing (WDM) technique under the illumination of supercontinuum and femto-second laser sources. In spectroscopy part, we focus on broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy with SC light source. The high spatial coherence of SC enables the capability to optically trap microscopic objects while the broad bandwidth allow us to perform broad-band CARS measurement over a range up to 3000 cm-1. We also developed a technique to suppress non-resonant background, which is the major factor limiting the sensitivity of a CARS measurement. The first chapter gives the overview. Chapter three and four discuss imaging applications including reflection-type confocal microscopy and two-photon excitation fluorescence imaging. Chapter five discusses the topic of CARS and technique to suppress non-resonant background. In chapter two, I include a review of SC generation as well as a brief description on SC generation in PCF. SC generated in imaging fiber taper and spectrally smoothed SC generated by modulated femto-second pulses are also reported in this chapter. Generalized nonlinear Schrodinger equation (GNLS) is numerically solved for theoretical modeling of SC generation in PCF. Chapter three focuses on the topic of WDM confocal microscopy by using broadband SC light source. Both lateral and axial WDM systems are investigated, which can potentially speed up the data acquisition rate significantly. Chapter four describes WDM scanning for two-photon excitation fluorescence imaging system. It is experimentally demonstrated that the mechanical scanning along axial direction in two-photon imaging system can be replaced by tuning the incident laser wavelengths. A lateral WDM scanning also is demonstrated. In chapter five, CARS spectroscopy is discussed. Broad band (up to 3000cm-1) CARS spectroscopy in supercontinuum trap is demonstrated. Non-resonant background can be suppressed by applying a polarization discriminated and time-resolved technique. A spectrally smoothed SC source is also investigated to obtain a smoothed-out non-resonant background so that the resonant Raman peaks can be easily observed.

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The Pennsylvania State University    Department of Electrical Engineering