An emitter for emitting radiation in a first range of frequencies comprising: a photoconductive material (11); and first and second contact elements (12, 13, 14) separated by a photoconducting gap provided by said photoconducting material (11), for applying a bias across said photoconducting gap, wherein at least one of said first and second contact elements (12, 13, 14) comprises a resistive element (14) for restricting current flow between said first and second contact elements in a second range of frequencies lower than the first range of frequencies.