Landau-level terahertz emission from electrically biased graphene

Terahertz (THz) emission from electrically biased graphene is studied under high magnetic fields. A quantum well (QW)-based charge-sensitive infrared phototransistor (CSIP) is used to detect weak THz emission from a graphene Hall bar. THz emission is clearly observed at around 5 T when the Hall voltage exceeds the corresponding Landau-level (LL) energy spacing between the zero-energy (N = 0) and first excited (N = +1 or N = −1) LLs, suggesting that the emission occurs primarily at the carrier entry and exit corners of the current contacts in the Hall bar. We also investigate the emission spectra through measurements of the QW spectrum of the CSIP. The emission spectra are well explained by the N = +1 → 0 (or N = −1 → 0) inter-LL radiative transition in monolayer graphene. The linewidth of the emission spectra is estimated to be on the order of 10 meV, even though no explicit LL splitting is observed in the magnetotransport at 5 T.