  We demonstrate the controlled enhancement of photoresponsivity in a graphene
photodetector by coupling to slow light modes in a long photonic crystal linear
defect cavity. Near the Brillouin zone (BZ) boundary, spectral coupling of
multiple cavity modes results in broad-band photocurrent enhancement from 1530
nm to 1540 nm. Away from the BZ boundary, individual cavity resonances enhance
the photocurrent eight-fold in narrow resonant peaks. Optimization of the
photocurrent via critical coupling of the incident field with the
graphene-cavity system is discussed. The enhanced photocurrent demonstrates the
feasibility of a wavelength-scale graphene photodetector for efficient
photodetection with high spectral selectivity and broadband response.
