Reversing Optical Energy Dissipation in Ultrathin MoS₂ Cavities at Telecommunication Wavelengths

Planar optical cavities are widely used to enhance light–matter interactions in ultrathin materials, and are readily integrated into on-chip devices. While cavity designs commonly focus on suppressing reflection, low reflectance alone does not reveal where the incident optical energy is ultimately dissipated. In metal-backed cavities, absorption can occur either in the functional target layer or parasitically in the metallic reflector, leading to a fundamental competition that is particularly detrimental for weakly absorbing ultrathin semiconductors. Here, we demonstrate that the dominant optical dissipation channel in planar cavities can be reversed through the combined control of standing-wave field overlap and target-material loss. Using a layer-resolved electromagnetic model, we show that positioning a standing-wave antinode within the target layer strengthens its absorption relative to that of the metallic reflector, enabling efficient redirection of optical energy into the functional material. Using a model system comprising molybdenum disulfide (MoS₂), aluminum oxide (Al₂O₃), and aluminum (Al) operating at telecommunication wavelengths, we introduce gallium (Ga) ion implantation to systematically modify the optical loss of MoS₂. Together, dielectric-spacer engineering and controlled loss modification enable a transition from reflector-dominated to target-dominated absorption, with the MoS₂ layer capturing up to 72% of the incident power in a structure only about λ/10 thick. For pristine MoS₂, the maximum calculated cavity-induced absorption enhancement exceeds tenfold relative to a MoS₂/quartz reference at the same wavelength. These findings establish standing-wave field overlap and material loss as two key parameters jointly governing where optical energy is dissipated, providing a general framework for directing optical energy into functional materials while minimizing parasitic metal losses.