Third-order nonlinear optical processes

Frequency-dependent polarizability a and second hyperpolarizability y corresponding to various third-order nonlinear optical processes have been... 

Sekkat, Z., Pretre, P., Knoesen, A., Volksen, W., Lee, Y. Y., Miller, R. D., Wotxl, J., and Knoll, W. Correlation between polymer architecture and sub-glass-transiiion-temperature light-induced molecular movement in azo-polyimide polymers influence on linear and second- and third-order nonlinear optical processes. Journal of the Optical Society of America B (Optical Physics), vol. I S, (no. 1), Opt. Soc. America, m. 1998, p. 401-41.1. 

Coherent Anti-Stokes Raman Scattering (CARS) Thermometry is a technique for temperature measurement in high temperature environments using a third-order nonlinear optical process involving a pump and a Stokes frequency laser beam that interacts with the sample and generates a coherent anti-Stokes frequency beam. 

The coefficients W determine the probabilities of third-order nonlinear optical processes in an unbounded crystal. An analogous expression can be derived for the coefficients determining the probabilities of fourth-order nonlinear optical processes. As already mentioned the derivation for multilevel molecules is rather complicated and has not yet been obtained. However, the simplicity of the final result, that is the simplicity of the nonlinear Hamiltonian, determines the simplicity of the calculations of nonlinear processes. Note also that a similar polariton approach can be applied for consideration of nonlinear processes in low-dimensional nanostructures (chains, quantum wells). For such structures just resonances of the pumping radiation with polaritons of low-dimensional structure and not with excitons will determine the resonances in the absorption of light as well as resonances in nonlinear processes. 

The best way to monitor this transient absorption change is the pump-probe technique (flash photolysis), where a short laser is used to excite the material and a second pulse (or lamp) is used to probe the change in the absorption spectrum. This technique has been used for decades in the field of photochemistry, but without being generally recognized as a third-order nonlinear optical process. 

From the above discussion, it is apparent that amorphous polymeric or glassy structures are more suitable as media for third-order nonlinear optical processes. Interest recently has shifted considerably to the third-order processes because they provide mechanisms for all-optical switching and all-optical signal processing. 

In order to fully use the potential of various structures prepared by this technique, an important step is a careful characterization of L-B films. In our laboratory, a number of spectroscopic and surface-sensitive techniques are used. Both second and third-order nonlinear optical processes have been observed using L-B films. The organization of this paper is as follows. First, some of the techniques used for the characterization of L-B films are discussed. Then some interesting examples of control of order and conformation in the L-B films are presented. This is followed by a subsection, which presents results of the study of both second and third-order nonlinear optical processes in L-B films. Finally, possible applications of L-B films in nonlinear optical devices are discussed. 

Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process and therefore is most efficient at very high intensities. In the nonresonant TPA, two photons combine to bridge an energy gap larger than the energies of each photon individually, and the transition occurs without the presence of the intermediate state. This can be viewed as being due to a virtual state created by the interaction of the photons with the molecule. 

Casstevens, M. K., Samoc, M., Pfleger, J., and Prasad, P. N., Dynamics of third-order nonlinear optical processes in Langmuir-Blodgett and evaporated films of phthalocya-nines, J. Chem, Phys., 92, 2019-2024 (1990). 

Khoo IC (1981) Optically induced molecular reorientation and third-order nonlinear optical processes in nematic liquid crystals. Phys Rev A 23(4) 2077-2081 Khoo IC (1982a) Nonlinear light scattering by laser- and dc-field-induced molecular reorientations in nematic-liquid-crystal films. Phys Rev A 25(2) 1040-1048 Khoo IC (1982b) Theory of optically induced molecular reorientations and quantitative experiments on wave mixing and the self-focusing of light. Phys Rev A 25(3) 1636-1644 Khoo IC (1995) Liquid crystals physical properties and nonlinear optical phenomena. Wiley, New York... 

Khoo, I. C. 1981. Optically induced molecular reorientation and third order nonlinear optical processes in nematic liquid crystal. Phys. Rev. A. 23 2077. 

PRASAD P.N., (1989) Ultrafast Third-Order Nonlinear Optical Processes in Polymeric Films, in Nonlinear Optical Effects in Organic Polymers, J. Messier, F. Kajzar, P. Prasad and D. Ulrich eds., NATO ASI Series, Vol. 162, Kluwer Academic Publ. Dordrecht, 1989t 351 363 ... 

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