Thermal emissions with both narrow spectral bandwidth and compressed angular distributions suggesting high temporal and spatial coherence are useful for the realization of an alternative family of highly efficient optical devices, and they play an increasingly significant role in numerous applications. Although people have endeavored to realize thermal emitters by harnessing different physics implemented in various artificial metasurfaces mainly made from metals, most of the reported results to date fail to produce emitters possessing the above merits. This work presents a proof-of-principal experimental demonstration on how to make use of the concept of quasiguided modes to realize alternative thermal mid-infrared emitters. Supported by an elaborately designed binary grating structure containing a period-doubling perturbation, these modes possess ultrahigh and perturbation-dependent Q factors. Although an amorphous form of evaporated materials is used, and the measured signal is a superposition of multiangle emission peaks, an emission bandwidth of about 25 nm is still experimentally obtained at the wavelength of 5.8 μm, which is one order of magnitude smaller than conventional thermal emitters based on metallic metamaterials. This bandwidth implies that a similar level of enhancement in the temporal coherence is achieved. Furthermore, our calculations show that the thermal emissions based on the quasiguided modes can achieve a spatial coherence length of 1.7 mm. Both of these two coherence properties are the state of the art to date in the mid-infrared thermal emitters.
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