Orbital Angular Momentum Imprints Studied Using Optical Vortex Pump-probe Spectroscopy

We introduce a second generation magneto-optical spectroscopy based on orbital angular momentum of light. Our technique is analogous to methods such as time resolved Faraday/Kerr rotation, but instead of utilizing photon spin, we use holographic gratings to pump and probe materials using photons carrying \emph{orbital} angular momentum (OAM). We will discuss our first time resolved experiments studying pump-induced OAM dichroism in bulk semiconductors. 100 fs pump pulses with alternating orbital angular momentum $\pm\hbar$ create OAM imprints whose momentum distributions resemble right- or left-handed ``whirlpools'' or vortices. The OAM memory of the sample is then measured using a probe beam whose $\pm\hbar$ OAM components are detected in a balanced photodiode bridge. We find that in n-GaAs, the dynamical OAM signal shows a unique timescale when compared to population and spin dynamics and, surprisingly, lasts considerably longer than the momentum scattering time. This method should be of further interest for studying non-equilibrium dynamics in a variety of orbitally coherent systems.