  In the close vicinity of a hot solid, at distances smaller than the thermal
wavelength, a strong electromagnetic energy density exists because of the
presence of evanescent field. Here we explore the possibility to harvest this
energy using graphene-based pyroelectric conversion devices made with an active
layer encapsulated between two graphene field-effect transistors (GFETs)
deposited on the source and on the cold sink. By tuning the bias voltage
applied to the gates of these transistors, the thermal state and the
spontaneous polarization of the active layer can be controlled at kHz
frequencies. We demonstrate that the power density generated by these
conversion systems can reach 1300 W/m^2 using pyroelectric Ericsson cycles, a
value which surpasses the current production capacity of near-field
thermophotovoltaic conversion devices by more than five orders of magnitude
with low grade heat sources (T<500 K) and small temperature differences (DT~100
K).
