Compact Multilayer Film Structures for Ultrabroadband, Omnidirectional, and Efficient Absorption

We present a novel scheme for ultrabroadband and omnidirectional perfect absorbers with compact multilayer film structure. Our proposed device shows an average absorption of similar to 98% over a wide range of wavelengths ranging from 400 to 2000 nm. The ultrabroadband characteristics are achieved with strongly overlapped optical resonances by designing a tandem structure composed of three absorptive materials, while the overall structure features a graded refractive index profile to obtain a wideband antireflection property. In addition to the high efficiency and ultrabroadband absorption, our perfect absorbers exhibit a great angular tolerance up to 60 degrees, which is attributed to not only relatively broad resonances but also negligible propagation phase shifts in ultrathin highly absorbing layers. Lastly, we explore the effect of the number of semiconductor-metal stacks on the performance of the absorber. The presented approach can have tremendous potential for various applications, such as solar-thermal energy harvesting, thermoelectrics, detection, and imaging.