Emission engineering in germanium nanoresonators

We experimentally investigate the smallest germanium waveguide cavity resonators on silicon that can be designed to work around 1.55 μm wavelength and observe an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a continuous germanium film of the same thickness. The enhancement is due to an effective combination of (i) excitation enhancement at the pump wavelength, (ii) emission enhancement (Purcell effect) at the emission wavelength, and (iii) effective beaming by the nanoresonators, which act as optical antennas to enhance the radiation efficiency. Our results set a basis for the understanding and engineering of light emission based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.