Nonlinear processes are of great importance in a wide range of technologies, including the conversion of solar energy to electricity. The quantification and analysis of these nonlinearities, as well as the identification of their origin (e.g., optical, chemical, or kinetic) is challenging using current optical methods. We have introduced a new class of methods to study the behavior of optical systems that exhibit nonlinear behavior. 2-beam action (2-BA) spectroscopies allow for the direct assessment of the nonlinearity contributing to a particular value of an experimental observable, such as emission intensity or photocurrent. I will discuss the application of two 2-BA spectroscopies, 2-beam constant emission intensity (2-BCEIn) spectroscopy and 2-beam constant photocurrent (2-BCoP) to gain new insights into photovoltaic processes.
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