A Path Toward Achieving Perfect Absorption in 2D Heterostructures

MoS₂ is a promising material for atomically thin perfect absorbers due to the high oscillator strength of its excitonic transitions and excellent band nesting. As MoS₂ is thinned from multilayer to monolayer, the magnitude and energy of its optical conductivity were exponential attenuation in magnitude and manifested by a gradual blueshift in energy with decreasing layer number due to interlayer orbital hybridization. The intrinsically poor light absorption of monolayer MoS₂, which varies by about 12 % in visible spectral range, induces the weak light-matter interaction. Although this absorption is surprising for the atomic thickness of MoS₂, these values must be increased to design MoS₂-based absorbers. Most approaches to improving the light-matter interaction of MoS₂ have been devoted to plasmonic nanostructure and photonic crystal. The photonic crystal slab with a perfect conductor mirror achieves near-unity absorption at 450 nm and average absorption of 51 % over the spectrum from 400 to 700 nm in the MoS₂ [1].<br/>Here, we demonstrate a new approach to achieving perfect light absorption in MoS₂/graphene/MoS₂ (MGM) heterostructure. By putting graphene in the intermediate layer to reduce the interlayer interaction of MoS₂ with each other, the doubling effect of light absorption can be obtained. As a demonstration of the concept, we experimentally show the optical contrast of MGM heterostructure can be improved by ~28 % compared to the bilayer MoS₂ at λ = 442 nm near the band nesting region. This result is in excellent agreement with the density functional theory of absorbance of a freestanding MGM heterostructure. Based on this structure, we also demonstrate a method based on the insertion of a silver (Ag) mirror layer between the dielectric layer and the substrate to enhance the optical absorption, resulting from Fabry-Perot cavity reflection. The interband absorption of MGM heterostructure is maximized by the cavity resonance and can reach the near-perfect absorption for frequencies satisfying the cavity resonance condition. With various 2D materials and optimization strategies, we can realize ideal absorption over the wide frequency range from UV to IR. Our study points to a new opportunity to combine 2D heterostructure with cavity optics to enable novel device applications such as high-efficiency solar cells with nanometer-scale thickness.<br/><br/><b>References</b><br/>[1] J. Piper and S. Fan, <b><i>ACS Photonics </i>3</b>(4),(2016) 571–577