  The photovoltaic and bolometric photoresponse in gapped bilayer graphene was
investigated by optical and transport measurements. A pulse coincidence
technique at 1.5 um was used to measure the response times as a function of
temperature. The bolometric and photovoltaic response times were found to be
identical implying that the photovoltaic response is also governed by hot
electron thermal relaxation. Response times of tau ~ 100 - 20 ps were found for
temperatures from 3 - 100 K. Above 10 K, the relaxation time was observed to be
tau = 25 +/- 5 ps, independent of temperature within noise.
