Second-order NLO

Materials for Frequency Doubling. Second-order NLO materials can be used to generate new frequencies through second harmonic generation (SHG), sum and difference frequency mixing, and optical parametric oscillation (OPO). The first, SHG, is given in equation 3. 

For second-order NLO applications, the films need to be noncentrosymmetric. 4-Di(2-hydroxyethyl)amino-4 -a2oben2enephosphonate was used to form SAMs on 2irconium-treated phosphorylated surfaces. Further reaction with POCl and hydrolysis created a new phosphorylated surface that could be treated with 2irconium salt (341—343). The principal advantage of the phosphate systems is high thermal stabiUty, simple preparation, and the variety of substrates that can be used. The latter is especially important if transparent substrates are required. Thiolate monolayers are not transparent, and alkyltrichlorosilanes have a serious stabiUty disadvantage. 

NLO effects result when the polarization response of the valence electrons becomes significantly anharmonic, usually in intense light beams where the magnitude of E is very large. The magnitudes of the coefficients of the terms in equation 2 diminish rapidly at higher orders, and thus readily observable NLO effects are either second-order third-order (X ) processes. Most NLO appHcations rely on second-order processes. However,... 

An important goal of this study is to characterize the noncentrosymmetric arrangement of the azobenzene branches in dendrimers and the effect of these dendrimers on macroscopic second-order susceptibihty. The NLO activity of the dendrimers can be characterized by their molecular hyperpolarizabihty. The hyper-Rayleigh scattering (HRS) method is a reliable way of measuring the mol-... 

G2, to G3, and to G4, the effective enhancement was 10%, 36%, and 35% larger than the value estimated by the simple addition of monomeric values. The enhancement included the local field effect due to the screening electric field generated by neighboring molecules. Assuming the chromophore-solvent effect on the second-order susceptibility is independent of the number of chro-mophore units in the dendrimers, p enhancement can be attributed to the inter-molecular dipole-dipole interaction of the chromophore units. Hence, such an intermolecular coupling for the p enhancement should be more effective with the dendrimers composed of the NLO chromophore, whose dipole moment and the charge transfer are unidirectional parallel to the molecular axis. 

Second-order NLO processes, including SFG, are strictly forbidden in media with inversion symmetry under the electric dipole approximation and are allowed only at the interface between these media where the inversion symmetry is necessarily broken. In the IR-Visible SFG measurement, a visible laser beam (covis) and a tunable infrared laser beam (cOir) are overlapped at an interface and the SFG signal is measured by scanning cOir while keeping cOvis constant. The SFG intensity (Isfg) is enhanced when coir becomes equal to the vibration levels of the molecules at the interface. Thus, one can obtain surface-specific vibrational spectra at an interface... 

The proportionality constants a and (> are the linear polarizability and the second-order polarizability (or first hyperpolarizability), and x(1) and x<2) are the first- and second-order susceptibility. The quadratic terms (> and x<2) are related by x(2) = (V/(P) and are responsible for second-order nonlinear optical (NLO) effects such as frequency doubling (or second-harmonic generation), frequency mixing, and the electro-optic effect (or Pockels effect). These effects are schematically illustrated in Figure 9.3. In the remainder of this chapter, we will primarily focus on the process of second-harmonic generation (SHG). 

Second-order nonlinear optics (NLO) has several applications in the field of optoelectronics.11 Several of these nonlinear processes are straightforward to experimentally demonstrate but their application in devices has been hampered by the lack of appropriate materials. Necessary requirements for second-order nonlinear optical materials include the absence of centrosymmetry, stability (thermal and mechanical), low optical loss, and large and fast nonlinearities.8... 

The electro-optic property of EO polymers comes from the NLO chromophores. When these chromophores are preferentially aligned to break the centrosymmetry of the material, the molecular level microscopic NLO effect of the molecules translates to the macroscopic second-order NLO effect of the polymer material. The poled material exhibits a strong macroscopic electro-optic effect. 

One of the more interesting applications of non-linear optical effects is the generation of the second harmonic. This phenomenon results when a laser beam passes through a material having second-order NLO properties (hence, composed by non-centrosymmetric molecules) the light emitted has a frequency double that of the incident radiation (or the wavelength has been halved). 

It is also worth noting that, due to their dipolar and conjugated donor-acceptor nature, all these amino-terminated Group 6 metaUacumulenes exhibit a strong negative solvatochromic effect and they show significant second-order NLO properties [68]. 

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