Bright Mid-Infrared Photoluminescence in Thin-Film Black Phosphorus*

In this work, we report bright mid-infrared photoluminescence (PL) emission from thin-film black phosphorus (BP). The intensity of the PL emission from a 46-nm thick BP is only seven times weaker than that of an indium arsenide (InAs) multiple quantum well (MQW) structure grown by molecular beam epitaxy, in which the total thickness of the quantum wells is of similar thickness to BP. The PL emission is further tunable by the temperature and layer number of thin-film BP. In the 46-nm thick BP, the PL spectra indicates a bandgap of 0.334±0.003 eV at 300 K, which decreases to 0.308±0.003 eV at 80K. The anomalous redshift of the BP bandgap with decreasing temperature agrees with previous theoretical and experimental results, while the overall energy shift is about 60% of the previously reported value obtained from photocurrent spectroscopy. We also measure the layer number dependence of PL spectra of the thin-film BP (6-to 46-nm), and we show that the emission peaks from around 3.3 to 4 mm at 80 K. Our results reveal the promising future of thin-film BP in mid-infrared light emitting and lasering applications.