Emission engineering in germanium nanoresonators

Over the last decade Ge has been proposed as one of the most promising materials for light detection, modulation, and emission for silicon-photonics architectures [1]. Its direct band-gap, which is only about 140 meV larger than the indirect band-gap, ensures excellent absorption and promising emission properties, which recently led to the realization of integrated detectors [2], electroluminescent devices [3], and to the first demonstration of optically-pumped [4] and electrically-pumped [5] Ge lasers. Here we investigate the smallest germanium-on-silicon Fabry-Pérot cavity resonators working around 1.55 m [see Panel (a)] and, by properly tuning their cavity length, demonstrate experimentally almost 30-fold enhancement in the collected spontaneous emission per unit volume when compared to a Ge film of the same thickness. This can be described as the combined result of the nanoresonator effective beaming (acting as optical antennas), laser excitation enhancement, and fluorescence emission enhancement (Purcell effect). These results are in excellent agreement with finite-difference time-domain simulations [see Panel (b)] and set the basis for understanding and engineering emitting devices based on subwavelength, CMOS-compatible nanostructures operating at telecommunication wavelengths.