Robust Broadband Infrared Unidirectional Absorption Enabled by a Non-Hermitian Multilayer

Abstract: Unidirectional electromagnetic absorption provides a powerful approach for controlling light and heat, yet broadband realization in the infrared spectral region remains experimentally unexplored. Here, we report a non-Hermitian multilayer structure that enables robust broadband infrared unidirectional absorption. By combining low- and high-loss materials and engineering their thicknesses using the transfer-matrix formulation, the structure exhibits nearly perfect absorption spectrally matched to the blackbody radiation at 373 K under forward illumination, while suppressing backward absorption below 30%. Spectral analysis indicates that the observed unidirectionality originates from non-Hermitian physics near an exceptional point. Notably, broadband unidirectional absorption is achieved even without strict exceptional-point condition. This indicates that the observed unidirectionality is governed by the combination effect of loss distribution and optical interference, rather than a singular condition, ensuring robustness against film thickness variations. Furthermore, thermal shielding experiments demonstrate that the structure enables unidirectional control of thermal radiation, resulting in a temperature difference of up to 21 0C between forward and backward configurations. These results establish a robust strategy for broadband directional control of infrared radiation, with potential applications in passive thermal management, including thermal smart windows and infrared heat-shielding devices.

• The paper introduces a non-Hermitian multilayer design achieving robust broadband IR unidirectional absorption, even with fabrication variations.
• It employs alternating low-loss CaF₂ and high-loss Bi layers, optimized through electromagnetic transfer-matrix simulation for near-exceptional performance.
• Experimental validation confirms nearly perfect forward absorption (~99%) and effective thermal shielding with a 21°C rear-side temperature differential.

Robust Broadband Infrared Unidirectional Absorption via Non-Hermitian Multilayer Engineering

Introduction and Context

The paper "Robust Broadband Infrared Unidirectional Absorption Enabled by a Non-Hermitian Multilayer" (2605.21926) presents a comprehensive study on the theoretical design and experimental realization of broadband infrared (IR) unidirectional absorption in a non-Hermitian multilayer system. The work addresses a notable gap in directional thermal radiation control for IR frequencies, particularly with robust absorption properties that are spectrally matched to blackbody radiation and tolerant to fabrication-induced structural variations.

Non-Hermitian photonic systems have shown profound utility in engineering asymmetric optical responses due to the presence of loss and gain, leading to phenomena like optical unidirectionality and the skin effect. Previous demonstrations largely targeted narrowband regimes in the visible, and their sensitivity to deviations from ideal design conditions limited practical applications. This paper advances the paradigm by achieving robust, broadband directional absorption in IR, leveraging asymmetric loss and interference in a multilayer configuration.

Design Principles and Non-Hermitian Physics

The multilayer structure is engineered by alternating low-loss (CaF₂) and high-loss (Bi) layers, with thicknesses optimized using the electromagnetic transfer-matrix method. This approach allows precise control over spatial loss distribution and interference effects that drive the unidirectional absorption mechanism. Unlike designs relying solely on geometric asymmetry, the key is in the interplay of the lossy layers and interference, which together break Hermiticity and cause direction-dependent dissipation.

Theoretical modeling demonstrates that non-Hermitian behavior, characterized by complex refractive indices and asymmetric optical penetration, is critical for unidirectionality. Notably, the system operates near but not exactly at a spectral exceptional point, and pronounced unidirectional absorption persists even when the strict exceptional-point condition is unmet. This observation strongly supports the robustness of the loss-interference mechanism against structural perturbations.

Numerical Results and Optimization

The study systematically matches the absorption spectra of the structure to blackbody radiation at temperatures ranging from 1000 K to 273 K, with total film thicknesses spanning 337 nm to 1570 nm. Calculated absorption spectra exhibit tunable peak positions achieved through thickness variation, confirming thermal shielding applicability across a broad temperature range.

For the four-layer configuration with total thickness 978 nm, forward illumination yields absorption up to 92%, while backward absorption is suppressed to 40% at 7.57 μm—a modulation depth of 52%. Absorbed power density is quantified at $4.9 \times 10^8$ W m⁻³, substantiating strong IR absorption characteristics. The structure demonstrates high sensitivity to asymmetric loss distribution; symmetric stacking lacks the desired unidirectionality, underscoring the necessity of non-Hermitian engineering.

Analysis of generalized total power and its wavelength derivative reveals abrupt phase transitions near peak absorption, indicative of strong mode coupling near exceptional points and dramatic interference shifts.

Experimental Demonstration

The four-layer stacks were fabricated on Cu substrates via thermal evaporation, with forward and backward configurations realized separately due to experimental constraints. Cross-sectional SEM verified the actual layer thicknesses, which, despite minor deviations from design values, maintained the intended asymmetric distribution. The measured absorption spectra under oblique incidence (5°) and unpolarized illumination displayed nearly perfect forward absorption—99.1% and 98.4% at 4.7 μm and 7.2 μm, respectively—while backward absorption was significantly lower, achieving a 52% modulation depth at 4.7 μm. Numerical simulations confirm that these peak shifts result from fabrication-induced thickness variations and do not compromise the broadband unidirectionality.

Importantly, both polarization and incidence angle are found to have negligible influence on the absorption characteristics, enhancing the practical robustness of the structure.

Thermal Shielding and Practical Implications

Thermal shielding experiments highlight the practical effectiveness of the structure. When subjected to IR thermal radiation from a hot plate at 500°C, the rear-side temperature differential between forward and backward configurations reaches 21°C. This result is indicative of efficient unidirectional control of thermal radiation, relevant for applications in passive thermal management, smart windows, and heat-shielding devices.

Implications and Prospective Developments

The demonstrated approach provides a robust, scalable strategy for controlling broadband infrared radiation through non-Hermitian multilayer engineering. The independence from precise exceptional-point conditions, insensitivity to geometric variations, and substantial modulation depth position this framework favorably for integration in real-world thermal management components. The technique may be extended to other spectral ranges and functional material systems, driving developments in energy transport, optoelectronics, and photonic isolation.

Theoretical exploration into further optimizing loss-interference coupling and leveraging tunable materials may unlock even greater bandwidths and adaptive functionality. Additionally, the non-Hermitian physics underlying these structures—and their resilience—open pathways for topological control and dynamic response in next-generation photonic devices.

Conclusion

The paper establishes a robust non-Hermitian multilayer architecture for broadband infrared unidirectional absorption, verified through both analytical modeling and experimental fabrication. Nearly perfect forward absorption matched to blackbody spectra, strong suppression in the backward direction, and operational stability near exceptional points without exact degeneracies mark the salient strengths. The demonstrated unidirectional thermal shielding underscores the practical significance for passive thermal management. The strategic combination of asymmetric loss-engineering and optical interference broadens the landscape for functional photonic and thermally active devices.