Multiphoton absorption, nonlinear optics

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

LeBozec and co-workers have reported nonlinear behavior in a series ofterpyri-dyl and dipicolinic acid complexes, with further studies on these complexes by Maury and co-workers [83, 84]. Their research was on new molecular materials for optoelectronics, with studies based on octupolar nonlinear optical molecules showing that molecular quadratic hyperpolarizability values were strongly influenced by the symmetry of the complexes [85]. Other studies on organic-lanthanide complexes with nonlinear optics have also reported second- and third-harmonic generation behavior with simultaneous multiphoton absorption properties [50]. Such studies have shown the importance of coordination chemistry as a versatile tool in the design of nonlinear materials. 

Nonlinear optical responses of molecules near fractal metal clusters are expected to be enhanced by many orders of magnitude. They are proportional to a high-order function of the local field [361], Strong enhancement of multiphoton absorption potentially exists because simultaneous absorption of n photons scales the intensity (/) to the power of n (/"). Thus, net multiphoton absorption is proportional to the average value of /" over a given volume. It can therefore approach a value that is orders of magnitude larger. 

We have seen how the molecular properties in nonlinear optics are defined by the expansion of the molecular polarization in orders of the external electric field, see Eq. (5) beyond the linear polarization this definition introduces the so-called nonlinear hyperpolarizabilities as coupling coefficients between the two quantities. The same equation also expresses an expansion in terms of the number of photons involved in simultaneous quantum-mechanical processes a, j3, y, and so on involve emission or absorption of two, three, four, etc. photons. The cross section for multiphoton absorption or emission, which takes place in nonlinear optical processes, is in typical cases relatively small and a high density of photons is required for these to occur. 

Nonlinear Absorption. The nonlinear absorption contains the saturation of strongly allowed one-photon transitions and multiphoton absorption. These nonlinear optical processes were detected as an anharmonic thermal grating [116, 117]. 

Therefore, a nonresonant third-order process can be overcome by a resonantly enhanced higher order process. Strong two-photon excitation or absorption saturation at 2ct), would generate strong fifth or higher order nonlinearities. Therefore, a careful characterization of a nonlinear optical response necessitates the investigation of the possible roles of higher order nonlinearities enhanced by multiphoton resonances or saturation processes. 

This article introduces the field of nonlinear optics and the electronic nonlinear optical (NLO) response of polymers and pol5mier composites. Both second- and third-order NLO phenomena are included, with primary emphasis on harmonic generation, the intensity-dependent refractive index, and nonlinear (multiphoton) absorption effects. The beginning sections introduce the phenomena and explain how the order of the nonlinearity can be understood from a series expansion of the polarization in powers of the electric-field. In addition to listing the variety of nonlinear optical phenomena and some applications, some of the advantages of polymeric materials for NLO applications are also surveyed. 

The various photochemical processes in tissue components, initiated by absorption of light occur even at very low power densities (typically at 1 W/cm ) when flic absorption process is a simple one-photon absorption (linear absorption). These processes arc dependent on fluence (irradiance) rather than intensity. Even fliough a conventional lamp source can be used for this purpose, one often uses a laser beam as a convenient light source. Recent interest has focused on nonlinear optical excitations such as multiphoton absorption, particularly two-photon absorption, to induce photochemical processes, particularly photosensitzed oxidation in photodynamic therapy (Prasad, 2003). The advantage offered by two-photon absorption is that the same photochemistry can be affected deeper inside flie tissue, compared to that induced by one-photon absorption which remains localized wifliin microns of depth from the surface. The most important photochemical process in tissues, from the biophotonics perspective, is the photosensitized oxidation discussed earlier. 

L. X. Zheng and A. K. Y. Jen, Multiphoton Absorption and Nonlinear Transmission Processes Materials, Theory and Applications, Seattle, July 7, 2002, Spie-Int. Soc. Optical Engineering, Bellingham, 2003, pp. 163-170. 

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