Mechanically robust and transparent Janus emitter with high infrared emissivity for passive radiative cooling

Janus structures with directional thermal emission offer a promising route toward sub-ambient passive radiative cooling; however, the simultaneous realization of high visible transparency, strong mid-infrared emissivity, and mechanical robustness within a single platform remains a significant materials challenge. Here, we report a transparent Janus emitter (TJE) that integrates spectrally selective thermal radiation with excellent optical clarity and structural durability. The TJE is fabricated using a scalable microfabrication strategy involving microscale patterning of quartz, deposition of an ultrathin titanium film, and polydimethylsiloxane (PDMS) encapsulation, resulting in a dual-faced architecture with asymmetric thermal–radiative functionality. The emitter exhibits near-unity emissivity (0.97–0.98) within the 8–13 μm atmospheric window while maintaining high visible-light transmittance (∼90%). Both indoor and outdoor measurements demonstrate effective daytime radiative cooling, achieving near-ambient temperatures under strong solar irradiation. Mechanical and thermal reliability tests further confirm outstanding durability, with a flexural strength more than twice that of conventional glass and stable performance under repeated thermal cycling. By unifying spectral selectivity, optical transparency, and mechanical robustness in a single Janus platform, this work establishes a new class of multifunctional thermal-optical materials with strong potential for energy-efficient building windows, transparent optoelectronic enclosures, and advanced thermal management applications.