Critical Coupling in Semi‐Trasparent Fano Resonators for Photothermal Converters

ABSTRACT Efficient photothermal conversion and localized thermal field modulation are powerful design axes for next‐generation energy, electronic, and biomedical materials. Conventional photothermal platforms typically rely on opaque broadband absorbers, enforcing a fundamental trade‐off precluding simultaneous optical transparency and high thermal performance. Here, we report a semi‐transparent Fano nano‐cavity resonator, fabricated via lithography‐free deposition, that overcomes this limitation by operating under a critically coupled, resonant interaction regime. By harnessing asymmetric Fano interference between a continuous lossy background and a discrete cavity mode, the resonator suppresses reflection to achieve a zero‐reflectance () state. Under this coupling condition, the system reaches a balanced 50%–50% optical transmission absorption, respectively. Under continuous‐wave laser excitation, this optimal energy partition drives extreme, spatially confined temperature rises exceeding 500 within seconds, with an internal photothermal efficiency of 50%. Its functional versatility is demonstrated across diverse applications, including rapid phase‐transition defrosting and localized biothermal tissue cauterization, along with the proof of concept of an optical scalpel.