Flexible ultrabroadband near-perfect absorber enabled by synergistic effects of cavity mode overlap and broadband anti-reflection

Achieving ultrabroadband, polarization- and angle-insensitive light absorption in a flexible structure is critical for advanced applications in thermoelectrics, detection, and imaging, yet remains a significant challenge. Here, we present a flexible ultrabroadband near-perfect absorber (UNPA) that addresses this challenge by leveraging the synergistic effects of overlapping resonances in multiple cavities and broadband anti-reflection (AR) properties in slanted columnar nanostructures. The graded-index (GRIN) distribution of the structure facilitates efficient light trapping, enabling an average absorption of ∼98 % across 400–2000 nm. To optimize material combinations and layer thicknesses, we employ an inverse design method integrating an exhaustive search with a quasi-Newton approach, ensuring optimal absorption performance. The UNPA also demonstrates exceptional angle insensitivity, maintaining 92 % average absorption at incidence angles up to 60°, regardless of polarization. Additionally, it exhibits remarkable mechanical robustness, retaining its absorption efficiency after 5000 bending cycles and sustaining performance at a bending radius of 5 mm. By combining ultrabroadband absorption, mechanical flexibility, and angle insensitivity, this work provides a scalable and practical solution for next-generation energy harvesting, sensing, and optical applications.