Femtosecond Optical Engineering

Author:

Li, Peng

Graduate Program:

Electrical Engineering

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

May 19, 2008

Committee Members:

Zhiwen Liu, Committee Chair/Co-Chair
Shizhuo Yin, Committee Member
C Russell Philbrick, Committee Member
William O Hancock, Committee Member

Keywords:

ultrafast optics
supercontinuum
optical tweezers
scattering
beam shaping
terahertz

Abstract:

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.