Third-order nonresonant optical

Table II. Third-order nonresonant optical nonlinearity of...

Third-order nonlinear optical properties of CdTe QDs were examined by Z-scan and FWM experiments in the nonresonant wavelength region. We found that the two-photon absorption cross section, a, is as high as 10 GM, although this value decreases with decreasing size. In addition, the nonlinear response is comparable to the pulse width of a fs laser and the figures of merit (FOM = Re Xqd/ Xqd)... 

Kuzyk MG, Dirk CW (1990) Effects of centrosymmetry on the nonresonant electronic third-order nonlinear optical susceptibility. Phys Rev A 41 5098-5109... 

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. 

Third-order nonlinear optical susceptibility esu Nonresonant (A = 602 nm) (19)... 

On the other hand, the nonlinear optical properties of nanometer-sized materials are also known to be different from the bulk, and such properties are strongly dependent on size and shape [11]. In 1992, Wang and Herron reported that the third-order nonlinear susceptibility, of silicon nanocrystals increased with decreasing size [12]. In contrast to silicon nanocrystals, of CdS nanocrystals decreased with decreasing size [ 13 ]. These results stimulated the investigation of the nonlinear optical properties of other semiconductor QDs. For the CdTe QDs that we are concentrating on, there have been few studies of nonresonant third-order nonlinear parameters. 

Through this mechanism, the nonlinear response is produced by the changes on the electronic cloud around the atom or molecule through the optical field. It is related to the microscopic third-order molecular polarizability y. Typically, nonresonant electronic processes in non-absorbing media yield values of 10 esu. The... 

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. 

The third-order optical nonlinearity of the conjugated polymers is based on the electron delocalization, which makes the electron cloud easily polarizable. Because of this nonresonant origin, the nonlinearity is very fast with subpicosecond response times [120]. Fukaya etal [121] have measured the intensity-dependent index of refraction of P30T using a surface plasmon resonance method. TTie result was nj = 5-10 i m /W at the wavelength 1.06 Xm, which is of the same order of magnitude as the values obtained for polydiacetylenes. 

One of our major contributions to the experimental techniques used in nonlinear spectroscopy has been the wide-ranging use of diffractive optics. Simple period gratings have been used in third-order resonant [22,72,73] and nonresonant... 

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