Undergraduate Honors Theses

Thesis Defended

Spring 2011

Document Type

Thesis

Department

Physics

First Advisor

Prof. Thomas Schibli

Abstract

Over the last decade, a variety of exciting applications have been found for lasers that generate ultrashort pulses of light with durations of just a few femtoseconds, known as femtosecond lasers (fs-lasers) [1]. People now routinely measure optical frequencies [2, 3], atomic and molecular spectra, lengths, distances [4], and displacements [5] with fs-lasers, and new applications are constantly being discovered. Pulses of such short duration can be achieved from passively mode-locked lasers|that is, lasers in which the longitudinal electromagnetic waves in the laser cavity, or \modes," are locked into phase with each other [6, 7, 8, 9]. To lock the phase of the modes, a saturable absorber|a device which absorbs some percentage of low-intensity light, but which allows high-intensity light to pass through with reduced absorption|is used [6, 7, 8, 9]. To produce short pulse-width, high repetition-rate (many pulses per second) lasers, a saturable absorber that becomes opaque quickly after being "saturated"by light and that saturates very easily is needed [6]. In this work, the potential for single atomic-layer graphene|a honeycomb lattice of carbon atoms only one atom thick, which has already proven itself to be an extraordinary material [10, 11, 12, 13, 14, 15, 16, 17]-as a saturable absorber is explored, and a method for producing high-quality graphene saturable absorbers is developed. This high-quality graphene's nonlinear (saturable) absorption was probed optically by di erential transmission and pump-probe measurements and the possibility of tuning graphene's optical properties by chemical doping is explored by Raman spectroscopy and compared to doping concentration and measurements made in di erential transmission and spectrophotometry. It is concluded that while graphene could be a highly desirable saturable absorber, it is currently limited by its relatively high saturation fluence compared to its damage threshold. The possibility of a graphene-based high-speed electro-optic modulator is also briefly discussed. This work is a step in the development of graphene as a saturable absorber comparable to but substantially cheaper than semiconductor saturable absorber mirrors (SESAMs), and towards the development of graphene-based optical and electro-optical devices for lasers.

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