Thermal Imaging through Hot Emissive Windows

The challenging problem of enhancing thermal emission in some directions while suppressing it in others is exacerbated for hot windows because they must remain transparent in the same spectral window. This complex problem has prevented applications such as infrared imaging through hot windows, whose own thermal emission is strong enough to blind the camera [1]. Here, we demonstrate a solution to this long-standing challenge by replacing the hot emissive window with one coated with an asymmetrically emitting thermal metasurface. By engineering the imaginary index of refraction to produce an asymmetric spatial distribution of absorption losses in its constituent nanoscale resonators, the metasurface suppresses thermal emission towards the camera while being sufficiently transparent for thermal imaging.<br/>We create the constituent resonators of the asymmetric loss metasurface by coupling nanoscale Si discs with distinct losses [2, 3, 4]. These silicon nano-discs separated by SiO₂ spacer support a quasi-bound state in the continuum (q-BIC) mode. These photonic modes couple to each other under normally incident input excitation with coupling strength that can be tuned by varying the spacer thickness. We identify that, when the resonators are weakly coupled, the eigenmodes of the system are distributed asymmetrically across the resonators with a large fraction of photon energy confined in the lossless resonator and hence causes strong asymmetry in the far-field thermal radiation from the metasurface [4].<br/>The asymmetry in energy confinement also causes the resonance dip in the transmission spectra to be narrow and as a result, enhances the overall transmission in the spectral bandwidth of the thermal camera. Thus our metasurface design, inspired by non-Hermitian optics, balances the need for good transmittance and emissivity asymmetry required to achieve thermal imaging through an emissive hot window. Our metasurface window, operating at 873 K, enhanced the thermal imaging contrast by nearly 2 times when compared to a conventional window at the same temperature. This demonstration illustrates the power of using the imaginary index as a design parameter for nanophotonic thermal devices, thereby enabling novel functionalities for energy, imaging, and sensing applications.<br/><br/>Reference<br/><br/>1. L. Lorah, E. Rubin, The Infrared Handbook, WL Wolfe and GJ Zissis, eds., Office of Naval Research, Washington, DC (1978).<br/>2. Chloe F Doiron and Gururaj V Naik. Non-hermitian selective thermal emitters using metal–semiconductor hybrid resonators. Advanced Materials, 31(44):1904154, 2019.<br/>3. Frank Yang, Ciril S Prasad, Weijian Li, Rosemary Lach, Henry O Everitt, and Gururaj V Naik. Non-hermitian meta-surface with non-trivial topology. Nanophotonics, 11(6):1159–1165, 2022.<br/>4. Frank Yang, Alexander Hwang, Chloe Doiron, and Gururaj V. Naik. Non-Hermitian metasurfaces for the best of plasmonics and dielectrics. Opt. Mater. Express, OME, 11(7):2326–2334, Jul 2021.