  Graphene is known to show a significant photo-thermoelectric effect that can
exceed its photovoltaic contribution. Here, by utilizing this effect, we
demonstrate a photovoltage measurement of cyclotron resonance in a
double-back-gated h-BN/graphene/h-BN two-terminal device. A graphite local
bottom gate was fabricated in addition to a p-doped Si global back gate. By
tuning the two gate voltages, an in-plane graphene junction having an
asymmetric carrier-doping profile was created. With the help of this asymmetric
structure, the photo-thermoelectric voltage generated in the vicinity of the
metal-electrode/graphene junction was detected. At a low temperature and in the
presence of a magnetic field, a photo-induced voltage was measured under the
irradiation of an infrared laser (Wavelength= 9.28 to 10.61 micron). We
observed a strong enhancement of the photovoltage signal under the cyclotron
resonance condition, at which the energy of excitation coincides with a
transition between Landau levels. These results highlight the possibility of
using the photo-thermoelectric effect in graphene for THz photo-detection.
