Intensity-difference effects, nonlinear optics

The effect that the light itself induces as it propagates through the medium determines the different types of nonlinear processes and optical phenomena. These phenomena are usually only observed at very high light intensities and such nonlinearity requires the use of high-power pulsed lasers [20]. 

The term solvatochromism is used to describe the change of position, intensity and shape of the UV-Vis absorption band of the chromophore in solvents of different polarity [1, 2], This phenomenon can be explained on the basis of the theory of intermolecular solute-solvent Interactions in the ground g) and the Franck-Condon excited state e). We will consider only the effect of the solute-solvent interaction on the electronic absorption and nonlinear optical response of a dilute solution of the solute. This way we avoid the explicit discussion of the solute-solute interaction, which significantly obscures the picture of the solvatochromism phenomenon. 

The tensors and 7 constitute the molecular origin of the second-and third-order nonlinear optical phenomena such as electro-optic Pock-els effect (EOPE), optical rectification (OR), third harmonic generation (THG), electric field induced second harmonic generation (EFI-SHG), intensity dependent refractive index (IDRI), optical Kerr effect (OKE), electric field induced optical rectification (EFI-OR). To save space we do not indicate the full expressions for and 7 related to the different second and third order processes but we introduce the notations —(Ajy,ui,cj2) and 7(—a , o i,W2,W3), where the frequency relations to be used for the various non-linear optical processes which can be obtained in the case of both static and oscillating monochromatic fields are reported in Table 1.7. 

The differences in behavior of AE and poe for the three directions of packing are consistent with the differences in the evolution of the effective linear and nonlinear optical properties when building one-dimensional arrays along the a, b, or c crystallographic axes. In addition, the position of ftie first intense absorption band between 3.5 and 4.0 eV is consistrat with the yellow color of the MNA crystals [17]. 

The first optical effect pointed out by Wang [13], and studied by computational simulations, is so-called dielectric confinement. Dielectric confinement is caused by the difference in refractive indices of a polymer medium (which has lower refractive index) and a semiconductor or metal particle (which usually has hi er refractive index). When illuminated by light, the field intensity near, at and inside the particle surface can be enhanced considerably compared to the inddent intensity becau of tte boundary established by the different refractive indices. This local ld enhancement eflect can have important con quences on photophysical and nonlinear optical properties of such polyn r-nancqmrtide systems. 

Nonlinear spectra are produced by monitoring the intensity of the output beam while varying some parameters, such as the frequency o) of one or more of the laser beams. When the difference in frequency between two laser beams matches the frequency of a Raman-active mode, the resulting resonance enhances the nonlinear optical effect, causing a change in the intensity of the output beam. The result is a peak in the nonlinear Raman spectrum. Some nonlinear Raman techniques that use this approach are given in Table 1. 

In conventional frequency-domain CARS, either o)j or CO2 is scanned so that o)j - (O2 passes through Raman-active resonances. As the difference in frequency between these two beams is tuned to each resonance, a resonance enhancement of the nonlinear optical effect occurs, leading to a peak in the intensity of the output beam. Spectra are produced by plotting the intensity of the output beam as a function of COj - a>2. 

Nonlinear optics concerns effects in the response of a material medium that are nonlinear in the optical field intensity. By response we refer to the radiation sources that are set up inside the medium because of the forced motion of bovmd and unbovmd charges (actually electrons). There are different ways to classify such effects, depending on the emphasis... 

A final point worth mentioning is the effect of local fields on the optical nonlinearities of strongly QC nanostructures. These arise from embedding QD s in a medium of different dielectric constant (2). One requires to know how the field intensity inside the particle varies at saturation in excitonic absorption. This has been approached theoretically by defining a local field factor f such that Em = f Eout (2). The factor f depends on the shape of the QD and the dielectric constant of the QD e = + E2 relative to that of the surrounding medium. Here... 

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