Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna

A critical challenge for the convergence of optics and electronics is that the micrometre scale of optics is significantly larger than the nanometre scale of modern electronic devices. In the conversion from photons to electrons by photodetectors, this size incompatibility often leads to substantial penalties in power dissipation, area, latency and noise(1 - 4). A photodetector can be made smaller by using a subwavelength active region; however, this can result in very low responsivity because of the diffraction limit of the light. Here we exploit the idea of a half-wave Hertz dipole antenna (length similar to 380 nm) from radio waves, but at near-infrared wavelengths (length similar to 1.3 mu m), to concentrate radiation into a nanometre-scale germanium photodetector. This gives a polarization contrast of a factor of 20 in the resulting photocurrent in the subwavelength germanium element, which has an active volume of 0.00072 mu m(3), a size that is two orders of magnitude smaller than previously demonstrated detectors at such wavelengths.