Reason for high nonlinear absorption and refractive index
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Nonlinear Absorption
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Semiconductor
Gallium Arsenide
Nanoparticles
Nonlinearity
Photons
Wavelength
Laser Radiation
Nonlinear Refractive Index
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Phthalocyanines: Synthesis, Supramolecular Organization, and Physical Properties
Gema de la Torre, ... Tomás Torres, in Supramolecular Photosensitive and Electroactive Materials, 2001
5.2.3 Nonlinear Absorption and Refraction: Optical Limiting
Nonlinear absorption and refraction are closely related NLO effects. Several mechanisms, both parametric and nonparametric, may contribute to these nonlinearities, although the parametric contributions to the nonlinear response are very small except for very short and intense light pulses. Thus, nonparametric (optical pumping) mechanisms usually give rise to nonlinear absorption and refraction. These processes exclusively depend on the particular energetic level structure of the system. Because of its practical relevance these NLO effects are discussed in relation to optical limiting.
Optical limiting (OL) is a nonlinear effect consisting of a decrease in the transmittance of a sample under high-intensity or fluence illumination. In this way, the intensity-dependent transmission is used to limit the transmitted light intensity below a maximum value. Optical limiters find useful applications for sensor protection, including the human eye, from high intensity light sources. In a way, optical limiting is the reverse of saturable absorption (RSA), where an increase in transmittance is observed at high illumination levels. In these conditions, a photon is absorbed from a singlet level (S1) or a triplet level (T1) to a higher one. If the cross section of either the S1–S2 or T1–T2 transition is larger than that from the ground state, reverse saturation behavior occurs. Both nonlinear absorption and refraction may contribute to optical limiting, either directly (nonlinear absorption) or through the use of an aperture that limits the cross section of the illuminating beam (nonlinear refraction). Experiments are usually carried out by focusing a good-quality laser beam onto the liquid or solid sample to reach a high intensity (Z-scan experiments).
Phthalocyanines have shown to be promising metalloorganic materials for optical limiting in the visible and NIR spectral range because of their appropriate electronic structure. Moreover, the spectral bandwidth or window over which the limiter operates can be engineered by altering both the main ring and the peripheral substituents, which permits a fine-tuning of the performance parameters.
Some useful reviews on the subject, including work on phthalocyanines, have been recently published [584, 590, 664–666].
Molecular Level
Microscopic optical-limiting measurements with phthalocyanines were first reported for the chloroaluminum derivative (CAP) [667], but further experiments in solution were performed with many other phthalocyanines. Much effort is still being devoted to determine the main physical parameters and understand the relevant mechanisms [668–670]. As in the case of parametric effects, the role of metal substitution and molecular stacking is being investigated [671].
Remarkable enhancements of the RSA were achieved when a heavy atom was introduced in the Pc cavity. Thus, for example, several Pc series containing Al, Ga, In, and Si, Ge, Sn, Pb as central atoms were studied [672]. In particular, lead tetrakis(cumylphenoxy)phthalocyanine [PbPc(CP)4] (Fig. 74) showed efficient optical-limiting performance [673]. The reason for such behavior is that the intercrossing rate from S1 to T1 is enhanced for this heavy central metal ion. Indium leads to an even faster intercrossing rate than lead and so it improves the limiting threshold [674]. Paramagnetic phthalocyanines such as VO(tert-butyl)4Pc and Cu(SO3−)4Pc sodium salt were tested as optical limiters [675]. These experiments point out that the magnetic moments also enhance the intercrossing rate.
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