Refractive nonlinear

At this time, many of the parameters listed above have not been measured for nonlinear organic materials. Measurement will be necessary for an assessment of the full potential of such materials. To date, only a nonlinear directional coupler has been implemented in a nonlinear polymer, (poly-4BCMU). For this device at 1060 nm, the nonlinear response was dominated by two-photon absorption, and some limited switching attributable to absorption changes was observed (27). More recent work at 1030 nm appeared to show switching attributable to refractive nonlinearities (28). Clearly there is much progress to be made. 

Preliminaty studies have however allowed us to compare the PSi-SLM to photo-refractive nonlinear materials, which have been used up to now. The results show that one can expect good performances from this new type of modulator (see table). Many characteristics are in favour of the new PSi-SLM correlator, such as ... 

Electrooptic General Refractive Nonlinear dielectric snscep ability... 

Consider the reflection of a normally incident time-harmonic electromagnetic wave from an inhomogeneous layered medium of unknown refractive index n(x). The complex reflection coefficient r(k,x) satisfies the Riccati nonlinear differential equation [2] ... 

A RIKES experunent is essentially identical to that of CW CARS, except the probe laser need not be tunable. The probe beam is linearly polarized at 0° (—>), while the polarization of the tunable pump beam is controlled by a linear polarizer and a quarter waveplate. The pump and probe beams, whose frequency difference must match the Raman frequency, are overlapped in the sample (just as in CARS). The strong pump beam propagating tlirough a nonlinear medium induces an anisotropic change in the refractive mdices seen by tlie weaker probe wave, which alters the polarization of a probe beam [96]. The signal field is polarized orthogonally to the probe laser and any altered polarization may be detected as an increase in intensity transmitted tlirough a crossed polarizer. When the pump beam is Imearly polarized at 45° y), contributions... 

Applications Involving Nonlinear Index Phenomena. The index of refraction, n, can be expressed for nonlinear optical materials as... 

Nonlinear refraction phenomena, involving high iatensity femtosecond pulses of light traveling in a rod of Tfsapphire, represent one of the most important commercial exploitations of third-order optical nonlinearity. This is the realization of mode-locking ia femtosecond Tfsapphire lasers (qv). High intensity femtosecond pulses are focused on an output port by the third-order Kerr effect while the lower intensity continuous wave (CW) beam remains unfocused and thus is not effectively coupled out of the laser. 

The relatively simple study of fluorescence and phosphorescence (based on the action of colour centres) has nowadays extended to nonlinear optical crystals, in which the refractive index is sensitive to the light intensity or (in the photorefractive variety (Agullo-Lopez 1994) also to its spatial variation) a range of crystals, the stereotype of which is lithium niobate, is now used. 

Figure 7-13. (a) Linear absorption of DOO-PPV, (b) imaginary part of ) (proportional to two-photon absorption), and (c) real part ol (proportional to the nonlinear index of refraction. .). 

The linear polarizability, a, describes the first-order response of the dipole moment with respect to external electric fields. The polarizability of a solute can be related to the dielectric constant of the solution through Debye s equation and molar refractivity through the Clausius-Mosotti equation [1], Together with the dipole moment, a dominates the intermolecular forces such as the van der Waals interactions, while its variations upon vibration determine the Raman activities. Although a corresponds to the linear response of the dipole moment, it is the first quantity of interest in nonlinear optics (NLO) and particularly for the deduction of stracture-property relationships and for the design of new... 

For the application of QDs to three-dimensional biological imaging, a large two-photon absorption cross section is required to avoid cell damage by light irradiation. For application to optoelectronics, QDs should have a large nonlinear refractive index as well as fast response. Two-photon absorption and the optical Kerr effect of QDs are third-order nonlinear optical effects, which can be evaluated from the third-order nonlinear susceptibility, or the nonlinear refractive index, y, and the nonlinear absorption coefficient, p. Experimentally, third-order nonlinear optical parameters have been examined by four-wave mixing and Z-scan experiments. 

Figure 9.2 illustrates a typical example of normalized transmittance, T(z), of CdTe QDs against the sample position z from the focusing point vdth and vithout aperture [17]. Since the peak ofthe normalized transmittance for the closed aperture precedes the valley, the sign ofthe nonlinear refractive index of CdTe QDs is negative. 

Figure 9.2 Normalized transmittance measured by the Z-scan with and without the collecting aperture for CdTe QDs with the diameter of 4.1 nm excited at 803 nm (0.4 pj pulse ). The open aperture Z-scan corresponds to the nonlinear absorption and the closed aperture Z-scan to the nonlinear refractive index.

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