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Nonlinear optical materials play a major role in the technology of photonics. To further advance the performance of optical devices, researchers have sought for materials with enhanced nonlinear optical properties, including inorganic semiconductors, polymeric systems and other organic molecules. The latter, specifically, are of great interest because they present high nonlinear optical properties combined with versatility of available routes of synthesis, used to alter and optimize molecular structure to maximize nonlinear responses and other properties. To select a material for a specific application, it is important to characterize its nonlinear optical absorption in a broad spectral range, in order to choose the optimized excitation wavelength. This book chapter describes resonant nonlinear optical absorption spectrum of the organic molecules Chlorophyll A, Indocyanine Green, Ytterbium Bisphthalocyanine and Cytochrome C, which are materials with potential applications in nonlinear optical devices. These organic molecules present distinct nonlinear optical absorption, such as saturable absorption (SA) and reverse saturable absorption (RSA), depending on the laser excitation wavelength and pulse duration. In order to investigate the excited state absorption and the population dynamics of the molecules, two distinct Z-scan methodologies were used, both based on measurements of the sample’s transmittance under laser excitation. The main difference between the methods employed is the possibility to monitor nonlinear effects in distinct temporal regimes, and then observe effects related with singlet and triplet electronic transitions, which, in most cases, can only be discriminated by the temporal dynamic of the system. While one technique uses a sequence of picoseconds pulses to induce and probe nonlinear absorption dynamics between singlet and triplet states, the other one employs femtosecond white-light supercontinuum pulses to map the spectrum of the excited state absorption in the visible and near infrared. By using both methods, one can determine and quantify nonlinear effects from femtoseconds up to hundreds of nanoseconds, including singlet excited state absorption cross-section spectrum and other spectroscopic features, such as deca ying time (ranging from femtoseconds to a few nanoseconds) and triplet excited state cross-section. Once one is able to temporally and spectrally map the nonlinearity of materials, it can be predicted how a given nonlinear optical device will act in distinct regimes (temporal, spectral and of intensity). Specifically, Chlorophyll A and Ytterbium Bisphthalocyanine present large RSA or SA depending on the excitation wavelength. Indocyanine Green presents high singlet and triplet excited state absorptions. Cytochrome C also presents saturable absorption, with short singlet-state

Nonlinear Optical Absorption of Organic Molecules for Applications in Optical Devices