Time Resolved Ultrafast Circular Dichroism Spectroscopy Based on 4th Generation Diffraction Gratings

Circular dichroism and its temporal evolution is critical in understanding conformational dynamics of biomolecules, chemical reactions, and the electronic structure of quantum materials. In the emerging fields of quantum computing and spintronics, chiral phenomena such as electron spin can be exploited leading to advances in data storage and transfer efficiency, cryptography, computational biology and drug design, and many other technologies. Despite these promising applications, time resolved circular dichroism spectroscopy remains challenging due to weak signals and polarization sensitivity of dispersive and reflective optics. Furthermore, circular dichroism is often measured by time averaging left- and right- polarized components separately leading to integrated optical and electronic noise. Liquid crystal polymer based cycloidal diffractive waveplates (CDW) make feasible high efficiency, broadband diffraction gratings. The optical function of the gratings is obtained by spatially modulating the optical axis orientation, so-called geometrical phase modulation. The waveplate nature of the grating provides spatially separated spectral distributions of left and right circularly polarized light. Instead of using a Pockels cell or photo-elastic modulator to change the polarization across multiple probe pulses in a transient circular dichroism spectrometer, a CDW provides instantaneous and simultaneous formation of spatially separated spectral distributions of the left and right circularly polarized components such that real-time circular dichroism measurements become possible within a single pump-probe shot. This eliminates errors in transient circular dichroism spectra due to variations in probe power from shot to shot and does not limit the shot frequency to the speed of a Pockels cell. In the proposed effort, a high-speed circular dichroism spectrometer will be designed as an addon option to a commercial pump-probe ultrafast spectrometer. High efficiency, broadband CDWs will be designed to maximize contrast between the left and right circularly polarized diffraction orders for both the visible and NIR bands. An optical layout for the spectrometer, including the option for a separate reference beam to account for laser power and polarization fluctuations, will be designed and demonstrated.