Coupled Tamm Phonon and Plasmon Polaritons for Designer Planar Multiresonance Absorbers.

Wavelength-selective absorbers (WS-absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography-free fabrication of WS-absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality-factors (Q-factors) and/or multiband TPP-absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q-factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q-factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow-band and nondispersive WS-absorbers are needed. Moreover, an open-source algorithm is developed to inversely design THP-absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP-absorbers can realize same spectral resonances with fewer DBR layers than a TPP-absorber, thus reducing the fabrication complexity and enabling more cost-effective, lithography-free, wafer-scale WS-absorberss for applications such as free-space communications and gas sensing.

Deterministic inverse design of Tamm plasmon thermal emitters with multi-resonant control.

Wavelength-selective thermal emitters (WS-EMs) are of interest due to the lack of cost-effective, narrow-band sources in the mid- to long-wave infrared. WS-EMs can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors on metals. However, the design of multiple resonances is challenging as numerous structural parameters must be optimized simultaneously. Here we use stochastic gradient descent to optimize TPP emitters (TPP-EMs) composed of an aperiodic distributed Bragg reflector deposited on doped cadmium oxide (CdO) film, where layer thicknesses and carrier density are inversely designed. The combination of the aperiodic distributed Bragg reflector with the designable plasma frequency of CdO enables multiple TPP-EM modes to be simultaneously designed with arbitrary spectral control not accessible with metal-based TPPs. Using this approach, we experimentally demonstrated and numerically proposed TPP-EMs exhibiting single or multiple emission bands with designable frequencies, line-widths and amplitudes. This thereby enables lithography-free, wafer-scale WS-EMs that are complementary metal-oxide-semiconductor compatible for applications such as free-space communications and gas sensing.

Long-lived modulation of plasmonic absorption by ballistic thermal injection.

Light-matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis1,2, energy harvesting3 and photodetection4, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer5,6 or when off-resonance excitations are employed7, which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited8. This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection9-12. In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations.