Electronic Steering of Directional Thermal Emission based on Delocalized Photonic Mode Resonator Combined with Graphene-Metal Hybrid Metasurface

There have been various studies to control the thermal emission of hot objects to obtain a narrowband or directional emission spectrum that is difficult to achieve by the conventional blackbody emitter. Recent studies have reported that elaborately designed metasurface/metamaterial combined with active materials enables dynamic control of the emission spectrum by external stimuli. However, previous studies have focused on emissivity modulation for the frequency spectrum, and the steering of directional thermal emission has not yet been demonstrated.<br/><br/>In this report, we experimentally demonstrate an electrically tunable directional thermal emission from a delocalized optical mode resonator combined with a graphene-metal hybrid metasurface. The wavelength scale thick dielectric layer is sandwiched by the metal electrode layer and the metasurface, supporting vertically oscillating Fabry-Perot mode. The delocalized Fabry-Perot mode has a sufficiently long coherence length, resulting in phase alignment of random thermal dipoles to obtain sufficient interference effect between emissions of each metasurface element.<br/><br/>The dynamical behavior of emissivity is estimated by calculating the absorption of the proposed structure for various incident angles of TM polarized light and Fermi levels of graphene. Due to the metasurface's non-resonant scattering, the proposed structure's resonant absorption spectrum could be completely explained by the Fabry-Perot reflection model. The phase of the reflection coefficient of the metasurface is modulated by altering the Fermi level of graphene, resulting in the resonance frequency shift of Fabry-Perot reflection. The maximum absorption of the proposed structure is achieved by nearly complete destructive interference of direct and Fabry-Perot reflections. From the constraint of wavevector for the direction of oscillation imposed by the Fabry-Perot resonance condition, the difference of resonance frequencies for different Fermi levels is equivalent to the steering angle of thermal emission. For a fixed frequency, the angle of directional thermal emission is continuously modulated by varying the Fermi level of graphene. We measured the emissivity of the device by collecting the thermal emission of the device on the 250°C heating stage through Fourier transform infrared spectroscopy. The Fermi level of graphene is modulated by applying the voltage on the backside metal reflector and the graphene layer. The experimental measurement of the device shows maximum emissivity over 0.9 for each resonance frequency. For the change of 1.15 x 10¹³ cm^-2 carrier density in graphene, the steering angle at 1508 cm^-1 is larger than 10°. This work inspires new approaches to controlling thermal emission and pave the way for expanding applications of thermal emitters beyond conventional limitations