Dynamic Control of Broadband Directional Thermal Emission with InAs-Based Gradient ENZ Metasurfaces

Dynamic nanophotonic control of the spectral content, directionality or polarization of broadband far-field thermal radiation is a challenging, but a fundamentally enabling capability where these thermal emission devices can operate at switching speeds much higher than the conventional ‘static’ nanophotonic components where the modulation mechanism is bound to the thermal mass of the object. Thus far, several strategies have been investigated: III-V semiconductor based quantum wells for dynamic thermal emission control by coupling the in-plane resonance of a photonic crystal slab to the quantum well intersubband tansition. The thermal emission spectrum of this apporach had to be narrowband in nature, centered at a single wavelength with suppressed emission at all other wavelengths, which is attributed to the intersubband transition frequency set by the quantum well design. Gap plasmon polariton based micro strip cavities were also explored in the context of dynamic tuning of thermal emission via electrical gating. The thermal emission peak was also centered at a single wavelength corresponding to the excitation frequency of the gap plasmons. Here, we discuss dynamic electrical control of the spectral emissivity bandwidth of thermal radiation where the spectral emissivity modulation is broadband and angularly selective. We demonstrate this capability by using the concept of III-V semiconductor based gradient epsilon-near-zero (ENZ) materials, with a doping concentration of the gradient ENZ layer ranging from 1.010¹⁸ cm^-3 to 1.910¹⁸ cm^-3, such that the resonance frequency of the constituent plasmonic thin films vary spatially along the depth dimension, creating a broadband directional thermal emission spectra: demonstrating thermal beaming between 17.5 to 19.5 m with the high emission angular range centered at 81. The epsilon-near-zero (ENZ) modes lying on the right of the light line are excited via subwavelength metal gratings designed using a genetic-algorithm based optimization method so that the gradient ENZ layer critically couples to free-space radiation at the ENZ wavelengths of the constituent plasmonic thin films over a range of angles. We use a rigorous coupled-wave analysis (RCWA) solver in conjunction with a semiconductor device solver to co-simulate the electric field dependent thermal emission spectra of the gradient ENZ structure. Here, we observe that it is possible to control the operational bandwidth of the thermal emitter by introducing an externally applied electric field and thus modulating the carrier concentration profile of the gradient ENZ layer. We observe a spectral emissivity modulation above 0.2 over a broad operational wavelength range (~2 microns) with maximum absolute emissivity modulation of 0.25 and relative modulation of 50% near the center wavelength (~18.0 m) of the zero-bias operational wavelength range. This highlights the remarkable control over spectral emissivity bandwidth that plasmonic gradient ENZ structures can provide by dynamically controlling the doping concentration profile of the gradient ENZ layer. We emphasize that in our approach the high emission angular range stays constant which has emissivity that is highly directional to the <i>same</i> set of angles, across a dynamically tunable spectral emissivity bandwidth. By constraining directional emission to particular angular ranges over arbitrarily controllable spectral ranges, improved performance may be possible for a range of applications in chemistry, health care, thermal imaging, IR sensing and spectroscopy. Ultimately, as these doped semicontuctor based platforms can be used to break time-reversal symmetry and reciprocity by externally applying a static magnetic field, we believe that this gradient ENZ materials framework can also open an avenue to explore devices for on-demand control of the spectral bandwidth of unequal absorptivity and emissivity for specific angular channels.