Extreme Photothermal Generation at the Absorption–Transmission Boundary of Semi-Transparent Planar Photonic Systems

Optical absorption and transmission are intrinsically competing quantities in passive photonic systems. In reciprocal two-port architectures under single-sided illumination, achieving massive localized heat generation traditionally dictates the suppression of optical transparency. This absorption–transmission constraint has traditionally limited photothermal generation in transparent platforms. Here, we demonstrate a planar metamaterial engineered to resolve this trade-off through interference-mediated energy partitioning. By coupling a broadband lossy background to a confined epsilon-near-zero (ENZ) resonance via Fano interference, we strictly suppress reflection (R$\approx$0) while preserving controlled semi-transparency (T$\approx$50 $\%$) and maximizing the absorption (A$\approx$50 $\%$). Despite this fundamental energy constraint, the structure yields an exceptional internal photothermal conversion efficiency (40.5%) and generates macroscopic temperature rises exceeding $500\,^\circ\text{C}$ within seconds under moderate continuous-wave excitation. This extreme heating originates from enforced sub-wavelength field confinement and spatial overlap between loss channels and resonant enhancement. We further demonstrate that such photothermal engineering enables phase-transition control and spectrally programmable thermal actuation, including a transparent laser-driven surgical interface. Our results establish a general route to heat engineering under transparency constraints, transforming an apparent optical bound into a functional design paradigm for photonic energy deposition.