Second-order nonlinear optical measurement

Hayden, L.M., Sauter, G.F., Ore, F.R., and Pasillas, P.L. (1990) Second-order nonlinear optical measurements in guest-host and side-chain polymers. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

However, non-centrosymmetry does not automatically imply a dipolar molecule, or, more generally, vectorial properties. Also molecules without a dipole moment can exhibit second-order nonlinear optical properties. Tetrahedral molecules, such as CC14, and trigonal molecules, such as BC13, also lack centrosymmetry. However, they cannot be oriented in an electric field, due to the absence of a dipole moment. Therefore, they can simply not be measured by EFISHG. Also ionic species cannot be measured, since these migrate, rather than rotate, under the influence of an applied field. 

Fig-2. Correlation of ionic conductivity with poling efficiency (second-order nonlinear optical activity). The upper graph shows conductivity measured as a function of temperature while the lower graph shows second-order nonlinear optical activity (measured by second harmonic generation, SHG) as a function of temperature. Note that second-order NLO activity starts to decrease with the onset of conductivity. Conductivity in this case was shown to arise from ionic impurities... 

Assessing thermal and photochemical stability is important. Thermal stability can be readily measured by measuring properties such as second harmonic generation as a function of heating at a constant rate (e.g., 4-10 °C/min) [121]. The temperature at which second-order optical nonlinearity is first observed to decrease is taken as defining the thermal stability of the material [2,3,5,63,63]. It is important to understand that the loss of second-order nonlinear optical activity measured in such experiments is not due to chemical decomposition of the electro-optic material but rather is due to relaxation of poling-induced acentric... 

Principally two techniques are used to measure the second-order nonlinear optical susceptibilities. These are ... 

Dehu, C., Meyers, F., Hendrickx. E.. Clays, K., Persoons, A., Marder, S.R., Bredas, J.L. Solvent effects on the second-order nonlinear optical response of tr-conjugated molecules A combined evaluation through self-consistent reaction field calculations and hyper-Rayleigh scaterring measurements. J. Am. Chem. Soc. 117, 10127-10128 (1995)... 

The isotropic part of the first hypcrpolarizability, fi, may be calculated using a number of methods - ranging from a simple single-centre chiral molecular orbital approach to ab / z7/(9 alculations at varying levels of approximation (see Section 4). We have computed fi for a number of chiral molecules in order to establish the typical strength and frequency dispersion of the pseudoscalar [30, 54, 59, 71]. fi is a measure of tlie signal strength in SFG experiments and a convenient measure that facilitates direct comparison with achiral second-order nonlinear optical processes. 

In the early days, optical nonlinearity of organic materials was measured usually with powder samples, mainly because it is very difficult to isolate organic compounds in the form of molecular crystals. In the case of centrosymmetric crystal lattices, macroscopic second-order nonlinear optical characteristics are not detected. Molecular crystals are organized assemblies of individual molecules held together by intermolecular forces. Their macroscopic nonlinear optical constants are estimated as the sum of the molecular polarizability of individual molecules. Thus, neglecting intermolecular interactions in the crystal, the nonlinear optical constant, dtlK, is expressed by... 

Two complementary amphiphilic polymers woe alternatively deposited by Y-type deposition. In Polymer A, the chromophore s electron accepting end is connected to a hydrophobic bridging unit, and its electron donating end is connected to a hydrophilic bridging unit The converse is true for Polymer B. These multilayer thin films have second order nonlinear optical properties which are stable at ambient temperature in the absence of oxygen. Microstructural information on a 92-bilayer polymer film was obtained fix)m polarized optical measurements. 

I. Shogi, H. Nakamura, K. Obdaira, T. Kondo, R. Ito Absolute measurement of second order nonlinear-optical cefficient of beta-BaB204 for visible and ultraviolet second harmonic wavelengths. J. Opt. Soc. Am. B 16, 620 (1999)... 

Figure 13 shows the relationship between the measured second-order nonlinear optical susceptibility, of -field-oriented PBLG films and the strength of the applied field. A linear regression of the data excluding the 0,0 point yields... 

In summary, chiral smectic-C phases lack a center of symmetry. Hence they can be used as materials for second-order nonlinear optics [120-124], and possess piezoelectric and pyroelectric properties. Pyroelectric measurements have been performed on LC polymers [125] as well as on LCEs [126-128]. Irradiation of an FLCE sample with light usually leads to a temperature increase resulting in a pyroelectric signal [129]. More interesting are systems in which dye molecules like azobenzenes lead to a shift of the phase transition temperature upon isomerization [19]. 

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