Ultrafast Optical Measurements of Chirality Induced Spin Selectivity in Chiral Materials

While transient absorption (TA) spectroscopy is a ubiquitous tool for understanding photoexcited charge carrier dynamics that govern solar energy conversion in light-harvesting systems, it is generally insensitive to the role of circular dichroism (i.e., as exhibited in chiral or spin-polarized systems). Although its implementation alongside traditional TA experiments is conceptually simple, TRCD has yet to find widespread applications. This is due, in part, to: i) naturally weak signals (often 10^-5 to 10^-3 OD) that are at least an order of magnitude lower than those in static CD and difficult to detect against a strong TA background, ii) technical difficulties associated with broadband detection that arise due to the polarization sensitivity of dispersive and reflective optics, and iii) the presence of anisotropic artifacts in the measured signal arising from pump-induced linear birefringence (LB) and linear dichroism (LD). The most commonly reported approaches to measuring TRCD can be divided into two general categories. The first is a differential absorption method which effectively involves two sequential TA measurements with alternatively left- and right-hand circularly polarized probe pulses. The second is an ellipsometry-based approach, whereby the ellipticity imparted to a linearly polarized probe by an optically active sample is characterized. Here, we present a recently implemented ellipsometry-based technique that enables simultaneous measurement of TA and artifact-free time-resolved circular dichroism (TRCD) spectra to elucidate the role of chirality induced spin selectivity in the photoexcited state dynamics of novel chiral materials. The experiment can be run in two general configurations. While the probe pulse is always linearly polarized, the pump pulse can be either collinearly polarized with the probe or circularly polarized. The former configuration is useful for investigating pump-induced changes to the natural optical activity of a chiral ground state. In the latter configuration, the angular momentum of the pump photons may induce spin-polarized optical transitions, which in an achiral sample affords the measurement of spin coherence lifetimes. We predict the signal produced by exciting a chiral sample with a circularly polarized pump will correspond to a superposition of pump-induced modulation of its natural optical activity and spin polarization of the excited state that will require deconvolution.