Ultra-narrowband and rainbow-free mid-infrared thermal emitters enabled by a flat band design in distorted photonic lattices

Thermal radiation, emitted by all objects above absolute zero, is typically broadband, omnidirectional, incoherent, and unpolarized. Tailoring thermal emissions through micro/nanostructures enables control over properties like polarization and coherence, leading to applications in infrared sensing, thermal management, radiative cooling, thermo-photovoltaics, imaging, and infrared camouflage. Narrowband thermal emitters are particularly desirable for energy efficiency, as seen in the limitations of traditional non-dispersive infrared (NDIR) systems that use broadband sources and filters. Existing methods for achieving narrowband thermal emission, such as using surface phonon polaritons (SPhPs) in SiC or the moiré effect in twisted SiC gratings, have limitations in operating range or fabrication complexity. Metal-insulator-metal (MIM) structures are simpler to fabricate but suffer from high losses and broad linewidths. Coupling broadband thermal fluctuations to high-Q resonances in all-dielectric nanostructures is a promising approach, with bound states in the continuum (BICs) and quasi-guided modes (QGMs) being explored. However, these typically reside within steep dispersion bands, leading to the rainbow effect where emission wavelength is highly sensitive to the output angle. This necessitates spatial filters, increasing system complexity and reducing energy efficiency. A flat band design, with many high-Q resonances at the same frequency but a wide range of wavevectors, could address this. While flat bands have been studied for their compatibility with wide-angle illumination and slow light effects, simultaneously achieving high Q and flat band dispersion remains a challenge. Existing methods like using moiré structures, symmetry breaking, plasmonic BICs, or Mie resonances face limitations in fabrication complexity, low Q-factors, or high losses. This research presents a simple yet effective approach to achieve a flat band around the Γ point, enabling ultra-narrowband and angle-insensitive mid-infrared thermal emission.

Several approaches have been investigated to achieve narrowband thermal emitters. Early work demonstrated the use of surface phonon polaritons (SPhPs) in silicon carbide (SiC) to achieve coherent thermal emission, but these are limited to the Reststrahlen band. The moiré effect in dual-layer twisted SiC gratings offers tunability, but fabrication is challenging. Metal-based structures like bull's eye gratings utilize surface plasmon polaritons (SPPs), but propagation losses limit coherence. Metal-insulator-metal (MIM) structures are simpler to fabricate but suffer from high losses and low Q factors. More recently, coupling thermal fluctuations to high-Q resonances in all-dielectric nanostructures using bound states in the continuum (BICs) and quasi-guided modes (QGMs) has shown promise, but the inherent steep dispersion leads to the rainbow effect. Previous attempts to achieve flat bands involved moiré structures (high fabrication accuracy requirements, low tunability), symmetry breaking (low Q-factor), plasmonic BICs (low Q-factor due to metal losses), and Mie resonances (low Q-factor). This study addresses the limitations of these prior approaches by introducing a new design strategy.

The proposed approach uses a simple structure to realize a flat band around the Γ point. A perturbation is introduced into a square lattice of single-disk structures (SDSs) to double the period along one direction, transforming it into a double-disk structure (DDS). This causes the folding of guided modes (GMs) to the Γ point, and strong coupling between these folded modes (QGMs) and existing guided-mode resonances (GMRs) leads to band splitting. By adjusting the coupling strength, a flat band with low group velocity is obtained. Introducing asymmetry between the two disks in the DDS creates a quasi-BIC (QBIC) resonance, resulting in a high yet finite Q-factor. Numerical simulations using the finite element method (FEM) in COMSOL Multiphysics are performed. Floquet periodic boundary conditions are applied laterally, and perfect matching layers (PMLs) are used in the z-direction. Emissivity is calculated from absorptivity (E(λ) = A(λ) = 1 − R(λ) − T(λ)), simplified to E(λ) = A(λ) = 1 − R(λ) due to the optically thick gold layer. The dispersion bands of SDS and DDS are analyzed. The effect of the asymmetry (Δ = A-B, difference between long and short axes of elliptical disks) on the Q-factor is studied, considering both material losses (amorphous materials obtained by electron beam evaporation) and ideal lossless conditions. The total Q-factor is analyzed, considering contributions from radiation and absorption losses. The relationship between Q_rad and Q_abs is determined, and the critical coupling condition (perfect emission) is identified. The dependence of the Q-factor on wavevector and output angle is investigated. The polarization characteristics of the emission are also analyzed. Experimental demonstration involves fabricating the DDS using electron beam lithography, followed by deposition of Al2O3 and Ge layers. Inductive coupled plasma-enhanced reaction ion etching is used to transfer the pattern to the Ge layer. The emission characteristics are measured using Fourier-transform infrared spectroscopy (FTIR) with a liquid-nitrogen-cooled MCT detector and a wire-grid MIR polarizer. A 3D rotating stage is used to measure the angular dependence of the emission spectra. A comparison is made with a traditional symmetry-breaking SDS (SB-SDS) structure to highlight the benefits of the flat-band design. The impact of finite-size effects is evaluated using simulations with a Gaussian beam excitation to model lens collection.

Numerical simulations and experimental results demonstrate the successful creation of an ultra-narrowband and rainbow-free mid-infrared thermal emitter. The flat band design in the distorted photonic lattice leads to a high Q-factor (experimentally measured Q ~ 224, FWHM = 23 nm at 5.144 μm) and angle-insensitive emission over a wide range of angles (-17.5° to 17.5°). The experimental linewidth is significantly smaller (more than two orders of magnitude) than that of metamaterials-based thermal emitters. The emission is linearly polarized in the y-direction. The central emission wavelength shows a near-linear dependence on temperature, enabling convenient spectral tuning for precise matching with target substances in applications like NDIR sensing. The comparison with a traditional symmetry-breaking SDS structure clearly shows the advantage of the flat band design, highlighting the negligible spectral shift and stable narrow linewidth over a broad angular range. Simulations using a focused Gaussian beam, mimicking lens collection, confirm that the ultra-narrow linewidth is maintained even with wider angular collection.

The results demonstrate the successful realization of a high-performance thermal emitter with features highly desirable for various applications. The ultra-narrow bandwidth combined with the angle-insensitive emission greatly enhances the efficiency and practicality of infrared devices, eliminating the need for spatial filters and enabling efficient energy collection using optical lenses. The ability to precisely tune the emission wavelength with temperature provides additional flexibility for applications requiring specific spectral matching. Compared to previous designs that exhibited high coherence but suffered from the rainbow effect, this approach offers a substantial improvement, as the flat band design allows for the use of lenses to collect emissions from a wide range of angles without significantly broadening the linewidth. The robustness of the linewidth against both output angle and temperature variation is a key advantage. This research represents a significant step forward in the design and development of advanced thermal emitters.

This work successfully demonstrates ultra-narrowband and rainbow-free mid-infrared thermal emitters through a novel flat band design in distorted photonic lattices. Both numerical and experimental results validate the superior performance, demonstrating a significant advancement over previous high-coherence thermal emitters that suffer from the rainbow effect. The ability to utilize optical lenses for efficient power collection, combined with temperature-tunable emission wavelength, opens up new possibilities for practical applications, especially in NDIR sensing. Future research could explore more complex polarization outputs (circular polarization) using this design and investigate the use of higher-quality materials to further reduce linewidth.

The current study utilizes amorphous films obtained through electron beam evaporation, which lead to higher absorption losses than would be observed with higher quality materials. While this demonstrates feasibility, future research could explore advanced deposition techniques to further reduce the linewidth. Another potential limitation is the minor spectral shift caused by fabrication errors, though this can be compensated using temperature tuning. Additionally, the study focuses on a specific wavelength range; further research could explore the applicability of this flat band design to other spectral regions. Finally, the computational cost limited the analysis of the finite size effects to smaller structures. Further research is needed to confirm that the scattering losses remain negligible in larger-scale devices.