Ultrashort terahertz (THz) pulses are a powerful tool for both probing and controlling novel phenomena in quantum materials. This is particularly useful in Dirac materials, since these materials exhibit novel magnetic and lattice excitations that can potentially be used to control their properties. Here, the use of ultrashort THz pulses to reveal the circular photogalvanic effect in the Weyl semimetal TaAs and probe magnetoplasmon modes in graphene nanoribbons will be examined.
We performed THz emission spectroscopy on the (112) and (001) surfaces of the Weyl semimetal TaAs. Our data enables us to clearly distinguish between helicity-dependent photocurrents generated within the ab-plane and polarization-independent photocurrents flowing along the non-centrosymmetric c-axis. Such findings are in excellent agreement with previous static photocurrent measurements. However, by considering both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay, we can attribute these transient photocurrents to the underlying crystal symmetry of these materials.
We also used terahertz (THz) magneto-optical spectroscopy to demonstrate how a periodic array of graphene micro-ribbons can be used to control the transmission spectrum and polarization state of a THz pulse whose electric field is oriented along the pattern’s axis of periodicity (perpendicular to the long axis of the ribbons). Our results demonstrate that graphene micro-ribbon arrays are a powerful system for controlling the coupling between light and magnetoplasmonic modes. This enables the tailoring of THz transmission profiles and polarization states using applied magnetic fields.