Interference Limited Absorption in Dense Molecular Nanolayers Near Reflecting Surfaces

TL;DR

This study explores how dense molecular nanolayers near reflecting surfaces exhibit a non-monotonic absorption response due to radiative effects, revealing fundamental limits and design principles for optimizing collective molecular absorption.

Contribution

It provides a combined analytical and simulation framework to understand and optimize absorption in dense molecular layers near reflectors, highlighting the role of radiative coupling and interference.

Findings

Absorption peaks at an optimal molecular density before decreasing due to radiative saturation.

Adding a mirror transforms the system into a one-port absorber capable of unity absorption at critical coupling.

Theoretical models accurately predict the conditions for maximum absorption and the fundamental limits involved.

Abstract

We investigate linear resonant absorption by a dense ensemble of molecules confined to a subwavelength layer in two geometries: (i) a free-standing film in homogeneous space and (ii) the same film placed at a controlled distance from a reflecting surface. In both cases, increasing the effective light-matter coupling (via molecular density/oscillator strength) produces a non-monotonic response: absorption rises to an optimum and then decreases as the film becomes increasingly radiatively bright and reflective. Finite-difference time-domain simulations and analytical transfer-matrix calculations agree quantitatively and yield compact ridge conditions for the optimum. We interpret the trends using a scattering/port picture: the isolated film is a symmetric two-port system (reflection and transmission), which bounds single-sided resonant absorption to 50% in the ultrathin limit (reflecting transition saturation), whereas adding a mirror suppresses transmission and converts the structure into an effectively one-port absorber. In the mirror-backed geometry, interference can cancel reflection and unity absorption is obtained at critical coupling, when radiative leakage is balanced by intrinsic molecular loss. These results clarify fundamental limits and design rules for collective absorption in dense molecular layers near dielectric or metallic boundaries.