Simultaneous Control of Spectral and Directional Emissivity with Gradient Epsilon-Near-Zero InAs Photonic Structures

Controlling the spatial distribution of broadband far-field radiation is a challenging, but a fundamentally enabling capability in a broad range of applications such as thermophotovoltaics, thermal imaging and IR sensing. Here, we discuss a III-V semiconductor-based broadband directional thermal emitter where the angular and spectral response of the nanophotonic structure can be decoupled, in contrast to conventional beliefs. We demonstrate this capability by using the concept of plasmonic gradient ENZ materials, where the epsilon-near-zero (ENZ) frequency of the constituent plasmonic thin films vary spatially along the depth dimension. To show that plasmonic thin film based gradient ENZ materials enable arbitrary and simultaneous tuning of the spectral peak and operation range of the thermal emitter by changing the doping concentration range of the gradient ENZ layer, we compare two plasmonic gradient ENZ structures with the same total thickness of 300nm but different doping profiles. We compare two structures, structure 1 with a doping concentration of the gradient ENZ thin film ranging from 1.010¹⁸ cm^-3 to 1.910¹⁸ cm^-3 and structure 2 ranging from 2.010¹⁸ cm^-3to 4.510¹⁸ cm^-3: the first structure demonstrating thermal beaming between 17.5 to 19.5 m and the second structure between 12.5 to 15m. Here, we also observe that it is possible to control the operational bandwidth of the thermal emitter by introducing a larger spatial gradient of the doping concentration of the gradient ENZ layer. This highlights the remarkable control over spectral emissivity bandwidth that plasmonic gradient ENZ structures can provide by controlling the doping concentration profile of the gradient ENZ layer. Furthermore, we demonstrate the directional tunability of plasmonic thin film based gradient ENZ materials by controlling the total thickness of the gradient ENZ layer, where a third structure with the same doping concentration range as structure 2 but larger thickness of 900nm is used as the test sample: the high emission angular range of the 300nm structure was centered at 74, whereas the 900nm structure was centered at 66. This response originates fundamentally from the spatial shift of the optical mode supported by the thicker gradient ENZ photonic structure relative to the thinner one. As the total thickness of the gradient ENZ layer increases, the dispersion curve of the broadband Berreman mode moves to the left and thus will couple to modes from angles of incidence that are closer to normal incidence. We emphasize that in our approach a directional emitter has emissivity that is highly directional to the <i>same</i> set of angles, across an arbitrary bandwidth. By constraining directional emission to particular angular ranges over arbitrary spectral ranges, improved performance may be possible for a range of applications, including thermophotovoltaics, radiative cooling, and waste heat recovery. It is noteworthy that the lithography/patterning free, chip-scale geometry suggests that the plasmonic gradient ENZ structure can be incorporated with conventional scattering structures, opening up exciting possibilities to observe new optical phenomena where the narrowband resonance condition is relaxed. Ultimately, since these plasmonic gradient ENZ materials exhibit extraordinary modulation of their complex refractive indices as their carrier concentrations can be controlled by orders of magnitude by applying external fields, we believe that this configuration provides an avenue for on-demand control of broadband directional thermal emission, beam steering, as well as broadband non-reciprocal thermal emission.