Tunable spin-polarized terahertz excitations in graphene nanoribbons

Graphene nanoribbons have an energy gap that is tunable from zero to terahertz (THz) regime by an external gate field. The indirect energy gap in a nanoribbon of infinite length, however, is unsuitable for optical excitations. We report a theoretical investigation of such nanoribbons with a finite, nanoscale length. We show that such nanoribbons can be excited optically and exhibit unique electronic excitations in the THz regime. The results unveil THz radiation-induced edge standing spin waves with different wavelengths at the two edges and a resonant frequency that can be controlled by an external gate voltage, opening the possibility of THz ``opto-spintronic'' applications.