Graphene multilayers for coherent perfect absorption: effects of interlayer separation

We present a model study to estimate the sensitivity of the optical absorption of multilayered graphene structure to the subnanometer interlayer separation. Starting from a transfer-matrix formalism we derive semi-analytical expressions for the far-field observables. Neglecting the interlayer separation, results in upper bounds to the absorption of 50% for real-valued sheet conductivities, exactly the value needed for coherent perfect absorption (CPA), while for complex-valued conductivities we identify upper bounds that are always lower. For pristine graphene the number of layers required to attain this maximum is found to be fixed by the fine structure constant. For finite interlayer separations we find that this upper bound of absorption only exists until a particular value of interlayer separation (Dlim) which is less than the realistic interlayer separation in graphene multilayers. Beyond this value, we find a strong dependence of absorption with the interlayer separation. For an infinite number of graphene layers a closed-form analytical expression for the absorption is derived, based on a continued-fraction analysis that also leads to a simple expression for Dlim. Our comparison with experiments illustrates that multilayer Van der Waals crystals suitable for CPA can be more accurately modelled as electronically independent layers and more reliable predictions of their optical properties can be obtained if their subnanometer interlayer separations are carefully accounted for.(c) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

This study investigates the sensitivity of the optical absorption of multilayer graphene structure to subnanometer interlayer separation. The results show that neglecting the interlayer separation leads to upper bounds on the absorption, while finite interlayer separations greatly affect the absorption. A closed-form analytical expression for the absorption is derived for an infinite number of graphene layers. The findings highlight the importance of considering subnanometer interlayer separations for accurate modeling and prediction of the optical properties of multilayer Van der Waals crystals suitable for coherent perfect absorption.