Determination of the Saturation Length and Study of Its Effects in Optical Gain Measurements of Organic Semiconductors Using the Variable Stripe Length Method

Three indicators for the determination of the saturation length limiting the validity of the variable stripe length method are presented. Depending on the maximum stripe length being used for the nonlinear fit, the resulting modal gain, its confidence interval, and the spectral full width at half maximum vary and give clear indications on the saturation length. Exemplarily, a blend of organic semiconducting spiro-compounds has been measured to present the importance of this length and demonstrate its determination. Subsequently, the optical modal gain is calculated with consideration of the saturation length. Both the saturation length and modal gain are shown to be dependent on the pump energy but result in a constant gain-length product of 2.2 ± 0.2, independent of the pump energy. Additionally, spectral gain profiles for different pump energies are presented with a maximum net modal gain of (128±4) cm−1 found at (2.98±0.09) mJ/cm2.