Ge nanoantennas for enhanced emission at telecom wavelengths

Over the last decade Ge has been proposed as one of the most promising materials for light detection, modulation, and emission in silicon-photonics architectures. Its direct band-gap, which is only about 140 meV larger than the indirect one, ensures excellent absorption and promising emission properties at telecommunication wavelengths, recently leading to the realization of integrated detectors, electroluminescent devices, and to the demonstration of optically- and electrically-pumped Ge lasers. Here we investigate germanium-on-silicon Fabry-Pérot cavity resonators working around 1.55 um (see Fig. 1) and, by properly tuning their length, we experimentally demonstrate an almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a Ge film of the same thickness. Finite-difference time-domain simulations reveal that this is the combined result of enhancement in the radiation efficiency (the nanoresonators act as efficient optical antennas), absorption enhancement at the pump wavelength, and a moderate emission enhancement (Purcell effect). These results set the basis for understanding and engineering emitting devices based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.