Laser-Driven sp3/sp2 Hybridization of Lignin for Polarization-Dominated Microwave Absorption

Traditional sp 2 -dominant absorbers (e.g., graphene, carbon nanotubes) often require engineered conductive networks or magnetic hybrids to moderate their high conductivity and achieve impedance matching. Herein, we propose lignin—with its inherent sp 3 /sp 2 carbon mix—as an ideal precursor for developing sp 3 –sp 2 hybrid carbon materials, enabling novel absorption loss mechanisms. By simply modulating the laser power of a low-cost diode laser (450 nm), we achieved precise control over the lignin’s carbon hybridization, transforming it from an sp 3 -rich amorphous polymer into a porous hybrid architecture with tunable sp 3 /sp 2 domains. Spectroscopic analysis revealed a monotonic decrease in the sp 3 /sp 2 ratio with laser power, resulting in an optimal balance for the sample irradiated at 60 % power. Unlike conduction-dominated conventional absorbers, absorption was primarily governed by sp 3 −sp 2 C−C dipolar polarization and sp 3 /sp 2 interfacial polarization effects. This synergy resulted in optimal absorption with a minimum reflection loss of -74 dB at 4.9 mm thickness and -50 dB at 2.1 mm thickness, combined with a maximum effective absorption bandwidth of 6.73 GHz at 2.2 mm thickness. Our results not only demonstrate a green and precise laser-processing route to transform biomass waste in a single-component, single step but also establish a novel mechanism for designing absorbers based on engineered carbon hybridization states.