Tunable spectral-directional infrared emittance with voltage-gated metal–oxide–semiconductor nanowires

Active control of thermal emission is increasingly important for applications in thermal management, infrared sensing, and energy conversion. While various approaches have been taken to achieve dynamic change of emissivity, electrical tuning in particular holds great promise for the benefits of analog control and programming. In this work, we propose a voltage-gated metal–oxide–semiconductor (MOS) nanowire device for achieving active tunable spectral-directional emittance within an analog voltage range. With negative voltages from 0 to −8.5 V, numerical simulation shows the non-uniform carrier concentration depleted within most of the nanometric semiconductor layer. By considering the semiconductor layer as a gradient index medium upon depletion at a given gating voltage, full-wave optical simulation demonstrates a spectral emittance peak whose amplitude and wavelength can be tuned from 0.80 at 8 μm to 0.40 at 13 μm for transverse-magnetic polarized waves at a 45° emission angle. The underlying mechanism is elucidated as the Berreman mode associated with the near-zero effective dielectric function of the extraordinary component of the nanowire structures, which changes significantly with depleted carrier concentration upon gating. In comparison, the planar MOS counterpart device does not show any tunable infrared emittance as the penetration depth of the semiconductor thin film is much larger than the nanometric depletion length.