Measurement Techniques for 2PA Cross-section

Since this chapter is focused on electrooptic materials and two-photon absorption, we will not consider measurement techniques for y and y(31 in detail, but refer the interested reader to Ref. [19]. In this section, we will briefly describe some of the techniques that have been used for the measurement of the two-photon cross-section, d. We first recall that the 2PA behavior of a molecule is characterized by a spectrum entirely analogous to a one-photon absorption (1PA) spectrum. Although, from the point of view of an application at a particular wavelength, measurements of d at only that wavelength may contribute to a figure-of-merit for that application, more generally, to understand structure-property relationships, it is helpful to acquire 2 PA data over as wide a frequency range as permitted by the lasers available and by the onset of 1PA. 

Nonlinear transmission (NLT) measurements have frequently been used for single wavelength measurements (although some studies have exploited frequency-dependent NLT, for example, see Ref. [20]) here a transmitted light intensity, It, is simply measured as a function of incident light intensity, I0. In the case 

A variety of quantum-chemical calculations have also been used to predict 2 PA spectra for molecules. Most frequently, semi-empirical SOS calculations are employed using few-state terms such as those discussed in Section 11.3.1 [Eqs. (20) and (21)], with damping terms chosen to afford linewidths for the transitions in accordance with known experimental data. To describe 2PA adequately using the SOS methodology, it is typically necessary to take account of extensive configuration interaction. 

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