Terahertz Time-Domain Investigation of Axial Optical Activity from a Sub-wavelength Helix.

Chiral media are characterized by preferential interaction with either left- or right- circularly polarized radiation, whereupon an optically active medium and its enantiomorph possess rotary powers of opposing sign due to mirror handedness of their micro- or nano-structures. Here, we report on the first time-resolved investigations of few-cycle pulse propagation along the axis of a sub-wavelength size helix. Time-resolved measurements of the electric field pulse scattered from the helix enable temporal discriminations of transient scattering mechanisms within the helix. Our main finding is that polarization circularization associated with axial propagation through the helix is non-instantaneous, and requires several picoseconds to develop before reaching steady state values. Using a 3D FDTD model, we describe the field and Poynting vector dynamics within the helix leading to steady state polarization circularization. Our conclusions not only support the established picture that optical activity arises from multiple scattering within the helical structure, but also show that this operative mechanism requires a finite time to induce steady state polarization circularization.