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 develop a general and self-consistent formalism to describe the thermal radiation from arbitrary optical resonators made by lossy and dispersive materials like metals and graphene-based on quasi-normal modes (QNM). Our formalism derives the fundamental limit of the spectral thermal emission power from an optical resonator and proves that this limit can be achieved when the mode losses to the emitter and the absorber (or far-field background) are matched, and meanwhile, the predominant resonant mode is electrically quasi-static. We also extend our theory to optical resonators with higher order symmetry, where degenerate and spectrally-adjacent modes are taken into consideration.
With our formalism serving as a general principle of designing the thermal emitters with maximized emission in both near and far-fields, we propose a metamaterial-based structure consisting of patterned doped silicon nanorod emitters that exhibits tunable narrow-band thermal emission. Direct numerical simulation based on the Wiener chaos expansion (WCE) method is performed to accurately investigate the heat transfer mechanism of metamaterials in the near field.
By applying group theory to the geometry of thermal emitter, we identify the existence and the upper limit to the resonance mode degeneracy and its influence on the far-field thermal emission. The existence of the degeneracy proves to be harmful to the far-field thermal. The upper limit of far-field thermal radiation is derived in terms of the coupling strength between degenerate modes. By building up the thermal emitter with higher-order symmetry group, the far-field thermal radiation intensity at certain resonance frequencies turns out to be stronger compared to the single emitter when normalized to the emitting volume. It demonstrates great potential to design the meta-surface with perfect absorption.
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.
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