Transient absorption (TA) is the most prominent method to observe relaxation dynamics in the toolbox of ultrafast spectroscopy. Within the framework of time-dependent perturbation theory, TA is a third-order technique as it relies on two interactions with the pump and one with the probe field. Since the advent of ultrafast TA, researchers have struggled to limit or better quantify the contributions of higher orders, such as the fifth order, as they will emerge along the same phase matching direction as the desired third order. Intensity cycling is a recently demonstrated experimental approach to isolate third from higher order signals. It requires transient absorption spectra at a minimum of three different intensities. This is experimentally challenging as the conditions for these three experiments need to be as similar as possible for a meaningful comparison. We present a solution to this problem based on a Sagnac-interferometer. Our design allows for shot-to-shot variations of pump pulse intensities. An entire intensity cycling dataset with 290 delay times for three different pump excitation densities is recorded within 30 min. We demonstrate the feasibility of our setup on a cyanine dye in solution.
Binzer et al. (2026) studied this question.