Triple Narrowband Mid-Infrared Thermal Emitter Based on a Au Grating-Assisted Nanoscale Germanium/Titanium Dioxide Distributed Bragg Reflector: Implications for Molecular Sensing

Mid-infrared (MIR) light sources with selective multiple narrowband and minimal angular dependence have desired properties for practical sensing applications. In this work, a triple narrowband MIR thermal emitter is realized by a hybrid structure composed of gold (Au) gratings on top of an alternate stacking germanium/titanium dioxide (Ge/TiO₂)-distributed Bragg reflector (DBR) and a Au reflector. Upon transverse-electric (TE)-polarized light illumination, three types of resonant modes including the hybridized Tamm plasmon polaritons (TPPs), the gap-cavity (GC) mode, and the middle-guided (MG) mode were excited simultaneously. While the hybridized TPPs are mainly determined by the composed nanolayers of the DBR, our results show that the GC mode has a strong dependence on the top-Ge-layer thickness as well as on the gap distance between metallic stripes, and the MG mode mainly relies on the grating periodicity. In addition, both the hybridized TPPs and the GC mode exhibit a small wavelength deviation when varying the oblique incident angle. Thus, these two modes are applied to contribute triple radiation peaks at the wavelength range of 3-6 μm upon heating of the device. The distinct geometric dependence among the excited modes in the hybrid structure provides an additional degree of freedom in independently adjusting the spectral positions of the emission bands. Such a selective multiwavelength, narrowband, and small angular-dependent MIR light source is promising for enhancing the accuracy in discrimination of molecular fingerprint.