Photocurrent and photoconductance of a helical edge state

We consider the helical edge state of a quantum spin Hall insulator, connected to leads and irradiated by a monochromatic and circularly polarized electromagnetic wave. The photocurrent carried by a single helical edge state is studied as a function of the characteristics of the electromagnetic radiation and electronic doping. We focus on the effect of the distance between the leads on the photocurrent. We also investigate the differential photoconductance of the helical edge state in the presence of a small voltage bias between the leads.