Nonlinear optical properties of carbon nanostructures for photonic applications

The topics of this Ph.D. thesis are Nonlinear (NL) optical properties of materials for photonic application, in particular mechanisms leading to them. In fact in order to obtain photonic devices with optimized characteristics the understanding of mechanisms from which nonlinearities arise is a necessary step.
Mainly Reversal Saturable Absorption (RSA) and NL scattering phenomena, giving both rise to Optical Limiting (OL) toward nanosecond pulsed lasers have been investigated.
The materials here investigated are carbon nanostructures, since are particularly promising for photonic applications. Buckminsterfullerene C60 and related higher fullerenes, Carbon Black Suspensions (CBS) and Carbon Nanotubes (CNs) either Single Walled (SWNTs) or Multiple Walled (MWNTs) belong to this family. Here C60 derivatives and SWNTs have been studied and it has been turned out to be evident that the structural analogy between those allotropes of carbon is reflected in their NL optical properties too.
A preliminary problem faced within this thesis has been whether RSA properties are kept upon including an RSA molecule in a solid host, since this is a necessary step in order to realize photonic devices for a practical use. A theoretical model has been developed, suitable to describe the behavior of an ideal RS absorber. Numerical simulation results lead us to infer that the RSA efficiency loss of such a system will not be dramatic from a liquid surrounding to a disordered solid one.
In the following section, a study about the sequential two photon absorption phenomenon in C60 fullerene derivative solutions is shown. The triplet photophysics of fulleropyrrolidines (C60 6,6 closed ring derivatives) has been investigated by nanosecond time resolved spectroscopy. First pump-probe experiments have been performed at low fluence; condition that makes triplet excited state phenomena to be predominant. In order to interpret the experimental data a theoretical model accounting for the role of triplet excimers, whose formation is involving bimolecular pathways, has been formulated. Then investigations about photochemical effects strongly connected with the triplet photophysics of those systems are reported as well. Pump-probe experiments were carried out at higher fluence, while photochemical reactions have been occurring. The photoproduct was identified as a C60 photopolymer by combined analyses by Raman and UV-Vis techniques. Moreover one-beam transmission measurements allowed us to establish a relationship between OL performance and photochemical effects. In particular a NL scattering mechanism owing to C60 photopolymers was found to play a role in limiting nanosecond laser pulses. Therefore in fullerene solutions OL effectiveness arise both from RSA and from NL scattering mechanisms, acting in a synergic way.
In the latter part a study of photophysical properties of SWNTs is presented. Linear and NL optical properties have been investigated by different spectroscopic techniques. Raman, Fluorescence and UV-Vis techniques have been employed with the purpose of obtaining structure to property relationships, specifically concerning electronic properties. However the research interest is focused on OL properties of SWNTs. A variety of samples have been studied either native or chemically modified samples at different stages of the process of functionalization with PEG 2000. OL properties, besides, have been studied varying the concentration of solutions, and in different detection modes: at a wider detection solid angle and in a closed aperture configuration. Furthermore pump-probe measurements have been carried out both with a monochromatic probe source, consisting of a He-Ne laser at 632.8 nm, and a polychromatic one, consisting of a Tungsten lamp. These experiments have allowed us to gain insights into mechanisms leading to OL. In fact it has been found that a NL scattering mechanism, arising from vapor bubbles formation is active together with an RSA one, likely due to a triplet. Therefore in SWNTs as well as in C60’s more than one mechanism is playing a role in determining the OL effectiveness, making their performances to be improved.