  Graphene is an ideal material for hot-electron bolometers, due to its low
heat capacity and weak electron-phonon coupling. Nanostructuring graphene with
quantum dot constrictions yields detectors with extraordinarily high intrinsic
responsivity, higher than 1x10^9 V/W at 3K. The sensing mechanism is bolometric
in nature: the quantum confinement gap causes a strong dependence of the
electrical resistance on the electron temperature. Here we show that this
quantum confinement gap does not impose a limitation on the photon energy for
light detection and these quantum dot bolometers work in a very broad spectral
range, from terahertz, through telecom to ultraviolet radiation, with
responsivity independent of wavelength. We also measure the power dependence of
the response. Although the responsivity decreases with increasing power, it
stays higher than 1x10^8 V/W in a wide range of absorbed power, from 1 pW to
0.4 nW.
