Second-order NLO coefficients

Meier et al. [98] have shown that DAST crystal is also a very interesting material for phase-matched parametric oscillation around the telecommunication wavelength at A = 1318 and 1542 nm. Their results (as shown in Table 9) indicate that the second-order NLO coefficient dill = lOlOpm/V at 1318nm for DAST crystal. 

Electrostatic layer-by-layer (LBL) self-assembly techniques based on oppositely charged poipelectrolytes can be useful to create stable noncentrosymmetric order in thin films. Using this interesting technique, thermodynamically stable noncentrosymmetric multilayer films can be prepared without any need for poling. Tripathy et al. reported the fabrication stable multilayer films of epoxy-based side chain azo polymers for second-order nonlinear optics.The second-order NLO coefficients of the five-bilayer LBL films of these polymers were found to be comparable to those of spin-coated poled films. A schematic view of the procedure to fabricate polyelectrolyte-based LBL films is shown in Fig. 7. 

Comments on NLO and Electrooptic Coefficients. Typically, the Pockels effect is observed at relatively low frequencies (up to gigahertz) so that slower nonlinear polarization mechanisms, such as vibrational polarizations, can effectively contribute to the "r" coefficients. The tensor used traditionally by theorists to characterize the second-order nonlinear optical response is xijk Experimentalists use the coefficient dijk to describe second-order NLO effects. Usually the two are simply related by equation 31 (16) ... 

In the literature however, other related parameters, besides x are often used to describe the macroscopic second-order NLO properties of materials. The SHG nonlinear coefficient d and the linear electro-optic coefficient r are the parameters commonly used for second-harmonic generation and the Pockels effect respectively [3, 5]. They are related to x according to Eqs. (4) and (5). 

Table 2. Physical data and coefficients for second-order NLO polyurethanes...

Hystorically, many second order NLO data have been obtained by SHG measurements (see Chapter 3) and, even presently, SHG measurements give the easiest way to screen second order NLO active materials. SHG coefficients are indicated by the simbol dyk. The relation between dyk coefficients and the components of susceptibility tensor x is given by... 

In order to illustrate the method, we can refer to the measurements of the isothermal decay of nonlinear d33 proportional to the square root of the SHG intensity, of DR19 in a polyimide films [71]. Polyimides are good candidates for NLO devices since they offer high thermal and good chemical stability. The second order nonlinear coefficient d33 have been fitted with a double exponential function ... 

Here ai is the frequency of the first strongly absorbing electronic transition in the molecule, and la and at are the fundamental wavelengths in second harmonic generation and for electrooptic coefficient measurements, respectively. The electrooptic effect (Pockels effect) is related to the corresponding second-order NLO susceptibility and by knowing the SHG coefficients, one can also estimate the electrooptic coefficients. 

Semiempirical methods were applied to the design of a new second order nonlinear optical (NLO) dye through noncentrosymmetric modifications to the symmetric dipyrromethene boron difluoride chromophore. Computational evaluations ofi candidate structures suggested that a synthetically accessible methoxyindole modification would have second order NLO properties. This new dye consists of 4 fused rings, is soluble in polar organic solvents and has a large molar extinction coefficient (86 x 1(P). Its measured hyperpolarizability, p, is -44x 16 esu at 1367nm. The methoxyindole therefore induces moderate asymmetry to the chromophore. 

Dipolar chromophores must be assembled into a noncentrosymmetric lattice to translate molecular optical nonlinearity to maaoscopic electro-optic (EO) aaivity, which is one example of second-order NLO properties. The EO coefficient in units of pmV , rs3, is the principal element of the linear Pockel s EO effect tensor and denotes the magnitude of refractive index shift (Ai ) obtained for an applied low-frequency electric field. This... 

The corona-poled polymer exhibited an initial second-harmonic coefficient = 31 pm V and relaxed to a stabilized value of 19 pm V after 5 days at room temperature. The spin-coated thin films, with a PPNA concentration of 30% by weight, can be prepared from a NMP solution. At that time, this polymer system was considered a good candidate for NLO applications because of its good mechanical and optical properties, coupled with a very high second-order nonlinear coefficient. 

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,... 

Two of the most important nonlinear optical (NLO) processess, electro-optic switching and second harmonic generation, are second order effects. As such, they occur in materials consisting of noncentrosymmetrically arranged molecular subunits whose polarizability contains a second order dependence on electric fields. Excluding the special cases of noncentrosymmetric but nonpolar crystals, which would be nearly impossible to design from first principles, the rational fabrication of an optimal material would result from the simultaneous maximization of the molecular second order coefficients (first hyperpolarizabilities, p) and the polar order parameters of the assembly of subunits. (1)... 

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