Direct observation of widely tunable mid-infrared emission of graphene foam induced by modulated laser diode light

Tunable mid-infrared emission is highly applicable in the fields of molecular spectroscopy and atmospheric monitoring. Recent studies have demonstrated that external electric fields can induce non-zero optical bandgaps of up to 250 meV in graphene materials, thereby paving the way for mid-infrared emissions (3–14 μm) to be realized. Graphene foam (GF) possesses a unique highly porous microstructure, which allows intense reflection or scattering of incident photons and gives rise to efficient interband optical absorption. Enhanced electron–phonon coupling enables free electrons in the conduction band to relax non-radiatively to the 1s-state. Mid-infrared emission is generated owing to electron–hole recombination. In this work, we demonstrate the induction of widely tunable (2.9–8.6 μm) mid-infrared emission in GF by modulated visible laser diode (LD) light with modulation frequencies ranging from 1.5 to 0.5 kHz. The peak emission wavelength correlates with the modulation frequency and is independent of the wavelength of LD light of visible range, whereas the emission intensity is related to the alternating intensity of the LD. Our findings indicate that GF is not only a novel mid-infrared emitting material, but one that promises bandgap flexibility for widely tunable mid-infrared sources.