| Περίληψη: | This dissertation is a collection of three (3) research papers that were published in international peer-reviewed journals during my Ph.D. studies. This thesis contains two introductory chapters that provide, a concise introduction to the basic principles of nonlinear optics (Chapter 1), and a detailed description of the experimental techniques employed (Chapter 2). An extensive review of the key background literature and how these led into the Ph.D. project are discussed in Chapter 3. Each research paper comprises a separate chapter (i.e., Chapter 4, Chapter 5, and Chapter 6). In the last Chapter (i.e., Chapter 7) the main findings of the research are summarized and discussed. Suggestions for future work are also proposed.
The field of nonlinear optics was born with the invention of the optical maser, today known as the laser. In 1961, Franken and co-workers, demonstrated second harmonic generation by tightly focusing a pulsed ruby optical maser into crystalline quartz. In the years since, a slew of nonlinear effects in light-matter interaction have been experimentally demonstrated and, in many cases, quickly found their way to market applications in a variety of fields ranging from telecommunications to imaging for health care and characterization. The rapid advancement of laser technology has enabled several nonlinear optics breakthroughs, such as frequency conversion, ultrafast optics, all-optical modulation, optical limiting, and others. As a result, nonlinear optics has emerged as a hot topic in cutting-edge science and application fields. Up to these days, materials with significant nonlinear optical response, such as graphene and its derivatives, TMDs, PAHs, and others, have found applications in photonics and optoelectronics. One of the most well-known of these materials is graphene, a two-dimensional (2D) material with sp2 hybridized carbon atoms organized in a honeycomb-like structure with a thickness equal to one atom diameter. Its structure exhibits a number of unprecedented properties which render it particularly attractive for both basic research and applications. Pure graphene has a zero-energy gap (bandgap), Eg, and behaves like metal while its electrons move at relativistic speeds and can absorb just as strongly from the ultraviolet to the far infrared of the optical spectrum. Major progress in the research of graphene’s nonlinearities is revealed by the countless recent results in the literature where graphene was employed as a saturable absorber, implemented into photonic chips, and was found to exhibit giant two-photon absorption, reverse saturable absorption, and optical limiting. The relevant forms in which graphene-based materials are encountered are pristine graphene in dispersions or deposited on various substrates and graphene hybrids.
Because of its single-atom thickness, graphene is a material in which any distortion in its interface has a significant impact on its inherent properties. As a result, its properties are heavily influenced by the method of manufacture, the environment to which it is exposed, the substrate material on which it is placed, and other factors. This also suggests that different graphene samples may have different nonlinear optical properties. As a result, from both a fundamental science and a practical application standpoint, an in-depth characterization of the graphene sample used in nonlinear-optical experiments is critical.
In that view, in Chapter 4, the ultrafast third order optical nonlinearities of single, bi- and tri- layer CVD grown graphene films are studied, under fs laser pulses at a low repetition rate regime. The measurements were performed under visible and infrared laser excitation employing 70 fs, 10 Hz laser pulses utilizing Z-scan and the Optical Kerr Effect (O.K.E.) techniques. The results are compared to those of some single and few layer graphene dispersions to evaluate the different nonlinearities that may arise in dispersed graphenes. In addition, the obtained results are compared to similar works in order to address the experimental discrepancies between the sign and magnitude of graphene's nonlinear refractive index using O.K.E. dynamics. A variation of the Z-scan technique is employed, known as Thermal lensing Z-scan, to distinguish between the possible cumulative thermal effects and the pure electronic response under high and low repetition rate regime. Results revealed that the ultrafast nonlinearities of graphene, (meaning χ(3)) highly depend on the repetition rate of the laser, type of sample (i.e., liquid or solid) spanning from 10-8 to 10-21 esu.In Chapter 5 of this dissertation, the third order nonlinear optical response of some N-octylamine modified fluorographene is studied under 35 ps laser pulses, both in the visible and infrared regime. The organic ligand N-octylamine was identified as the most efficient nucleophile ascribing to the obtained FG-octylamine derivatives (FG-OA) exceptionally high N content (up to 18%) along with a record non‑linear optical response. It is observed that as the reaction time between FG and OA proceeds, the fluorine (F) content declines at the expense of the growing aromatic regions and nitrogen centers, resulting to FG-OA materials with progressively improved nonlinear optical response. Therefore, octylamine functionalization of FG is particularly effective for switching “ON” an unprecedented non‑linear optical response on this organophilic graphene derivative. The results demonstrate the potential of fluorinated graphene as a versatile scaffold allowing the preparation of materials with custom made nonlinear optical response.
In Chapter 6, the research was extended into the systematic study of the third order nonlinear optical properties as well as into the investigation of the broadband optical limiting action of the aforementioned graphene derivatives. The purpose of this study was to investigate the effect of the degree of defects and doping percentage on the nonlinear optical response and optical limiting, as well as investigate the contribution of doping to the nonlinear optical response. The NLO response of the N-octylamine-modified fluorographene is assessed using 4 ns pulses. The NLO responses of the FG and the FG-OAx derivatives were investigated by means of the Z-scan technique, along with their OL action, under various excitation wavelengths ranging from visible (450 nm) to NIR (1750 nm). Their NLO response was turned ON shortly after the initiation of the partial nucleophilic substitution of the FG, leading to strong enhancement with the time of reaction. The FG-OAxs were found to be exhibiting strong nonlinear absorption, attributed to defect-induced states due to the C–N bonding configuration formed by the incorporation of nitrogen into the graphenic lattice. Their strong nonlinear absorption was found to result in extremely efficient OL action.
The results presented in Chapter 5 and 6 pave the way for controlled covalent functionalization of graphene, allowing for scalable access to a wide portfolio of graphene derivatives with tailored properties. The experimental results demonstrate unambiguously that fluorographene is a strong competitor to its “relative”, graphene oxide, GO. Being a suitable platform for achieving chemical functionalization results in a large collection of fluorographene derivatives, while by controlling the degree of functionalization, their nonlinear optical properties can be largely tuned, providing materials with customized properties across almost the entire optical spectrum, which is highly desirable for a variety of photonic and optoelectronic applications.
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