Thermal radiation with a narrow-band emission spectrum is of great significance in various applications such as infrared sensing, thermophotovoltaics, radiation cooling, and thermal circuits. Although resonant nanophotonic structures such as metamaterials and nanocavities have been demonstrated to achieve the narrow-band thermal emission, tuning their radiation power toward perfect emission still remains challenging. Here, based on the recently developed quasi-normal mode theory, we prove that thermal emission from nanoscale transmission line resonators can always be controlled by tuning the size and geometry of single resonator and the density of the resonator array. By use of nanoscale transmission line resonators as basic building blocks, we experimentally demonstrate a new type of macroscopic perfect and tunable thermal emitters. The transmission line resonator arrays are fabricated by standard E-beam lithography techniques and subsequent lift-off process. The emissivity of the samples is measured by using a FTIR spectrometer combined with an infrared microscope. Our experimental demonstration in conjunction with the general theoretical framework lays the foundation for designing tunable narrowband thermal emitters with applications in thermal infrared light sources, thermal management, and infrared sensing and imaging.
We investigate the directional control of narrow-band perfect thermal emission using a nanoscale Yagi–Uda antenna. Although Yagi–Uda antennas were demonstrated to achieve directional control in the optical and radio frequency regimes, they have not been applied for thermal infrared emission. Here, by coupling a nanoscale thermal emitter into a Yagi–Uda antenna, we demonstrate strong directional control of thermal emission with a narrow-band spectrum at the nanoscale. By exploring the effects of the reflector and the director of a Yagi–Uda antenna, the forward emission enhancement factor up to 8.3 is achieved through geometry optimization.