Wavelength-Tunable, Low-Angular Dispersion, and Narrowband Thermal Emitters by Incorporating Ge₂Sb₂Te₅ Layer into...

High-quality-factor (Q-factor) mid-infrared (MIR) light sources with spectral tunability and minimal angular dependence are highly desirable for practical sensing applications. In this work, a continuously tunable, low-angular dispersion, and narrowband thermal emitter is realized by incorporating a Ge₂Sb₂Te₅ (GST) layer into the grating-assisted distributed Bragg reflector (DBR)-like structure. The multilayer back reflector is designed to achieve high reflectance over a broad DBR-like stopband, while simultaneously avoiding direct contact with the metallic back reflector, as in conventional metal-dielectric-metal (MDM) structures. Upon illumination with transverse-magnetic (TM)-polarized light, the hybrid structure is found to support a hybridized localized surface plasmon (h-LSP) with a Q-factor of 30.64 in simulations (in contrast to the LSP mode in MDM structures with a Q-factor of 6.17) due to the reduction of ohmic loss. Meanwhile, the h-LSP mode shows a similar geometric dependence on grating width and exhibits a negligible wavelength variation under varying oblique incidence angles. Samples with three grating widths of 1.4 μm, 1.7 μm, and 2.0 μm were fabricated to support the h-LSP mode at distinct wavelengths. Upon thermal annealing, both the measured reflection and emission spectra of these samples exhibit a continuous redshift, with an average shift of 140 nm corresponding to an approximately 40% crystalline fraction of GST. Meanwhile, the Q-factors of the emission peaks remain above 8.70 for w = 1.4 μm, 12.72 for w = 1.7 μm, and 15.68 for w = 2.0 μm. Such a tunable, narrowband, and small angular-dependent MIR light source is promising for enhancing the accuracy in the discrimination of molecular fingerprints.