Experimental realization and near-unity optimization of ultra-broadband ultrathin metamaterial absorbers across the long-to-very-long infrared band (8-30 µm)

Ultrathin metal/insulator/metal (MIM) metamaterial absorbers are in demand for long-wavelength infrared (LWIR, 8-14 µm) imaging and uncooled detection and for very-long-wavelength infrared (VLWIR, 14-30 µm) space-based target detection, where they must combine near-unity broadband absorption with small thermal mass and simple, scalable patterns. Zhou et al. (Light: Science & Applications 10, 138 (2021)) proposed four-layer Ti/Si/SiO$_2$/Ti absorbers whose simulations predict average absorptions of 92% over 14-30 µm and 87% over 8-30 µm, but these designs were never fabricated or measured, the authors noting that very few accurate VLWIR measurement devices exist, and the simulated 8-30 µm spectrum retains dips near 10 µm and 22 µm that are hard to remove while keeping a thin dielectric and simple top-metal pattern. Subsequent ultra-broadband VLWIR designs to date remain simulation-only, while experimental demonstrations either stop at 20 µm and use thick structured surfaces (doped black silicon), or cover only 8-14 µm. The problem is therefore open: experimentally verify, and push toward near-unity, ultra-broadband absorption over 8-30 µm in an ultrathin, simply patterned absorber, including the VLWIR metrology required to do so.

Background

A metamaterial perfect absorber (MPA) is a subwavelength-patterned artificial structure engineered to absorb incident electromagnetic radiation nearly completely; the canonical architecture is a metal/insulator/metal (MIM) stack in which a patterned top metal layer sits on a dielectric spacer above an optically thick metal film, so that transmission is blocked and absorption is $A=1-R$ once reflection is suppressed. Broadband MPAs typically couple several resonances: propagating surface plasmon resonances (PSPRs), surface waves that travel along the interface between the spacer and the bottom metal film and are phase-matched by the periodic top pattern, and localized surface plasmon resonances (LSPRs), subwavelength charge oscillations confined to individual top-metal resonators. The classic route to broadband response superposes resonators of many different sizes, which enlarges and complicates the unit cell, raises fabrication cost, and — for a thick dielectric spacer — increases thermal capacity, i.e. the noise-equivalent temperature difference (NETD) of an uncooled thermal detector built on the absorber. Most experimental broadband MPA demonstrations up to 2021 concentrated on the visible, short-infrared and THz bands, while the long-wavelength infrared (LWIR, roughly 8-14 µm) and especially the very-long-wavelength infrared (VLWIR, roughly 14-30 µm) regimes, needed for LWIR imaging and for interstellar/missile and satellite detection in space, were sparsely covered and poorly served by measurement instrumentation. A further design lever is the choice of metal: refractory, intrinsically lossy metals such as titanium (Ti) support strongly damped, low-Q plasmon resonances that broaden absorption bands, unlike high-Q noble metals (Au, Ag) that give narrow resonant selectivity. In this context Zhou and co-workers (2021) analysed a Ti/Ge/Ti three-layer absorber (about 90% average absorption simulated over 8-12 µm, 80% measured), then experimentally demonstrated two four-layer Ti/Ge/Si$_3$N$_4$/Ti absorbers in which a thin lossless high-index Ge layer sets the PSPR/LSPR resonance conditions and a lossy Si$_3$N$_4$ layer adds intrinsic absorption, reaching nearly 95% average absorption over 8-14 µm in simulation (78% measured over 6.5-13.5 µm, the measured-simulated gap being attributed to the unknown actual refractive index of the deposited Si$_3$N$_4$, the 50×50 µm measurement diaphragm, and film-thickness imprecision). To extend the band they further proposed two Ti/Si/SiO$_2$/Ti absorbers with the same strategy, each patterned with periodic irregular nano-cross top-metal resonators, simulated to absorb 92% on average over 14-30 µm and 87% over 8-30 µm. These VLWIR designs were not fabricated: the paper's Fourier-transform infrared (FTIR) reflectometry spanned 600-7500 cm$^{-1}$ (about 1.33-16.7 µm) and the authors state that very few accurate measuring devices exist for VLWIR applications. The simulated 8-30 µm spectrum also shows two dips near 10 µm and 22 µm, and the authors note it is difficult to increase the number of absorption peaks under a small dielectric thickness and an easy top-metal pattern, suggesting as remedies increasing the dielectric thickness or using more complicated top-metal geometries — both of which sacrifice the low-thermal-mass and low-fabrication-complexity goals. The unresolved target is therefore a coherent one: an ultrathin, simply patterned MIM-type absorber whose near-unity average absorption across the whole 8-30 µm band is not merely predicted but measured, together with the VLWIR optical metrology that such a measurement requires.

Problem Statement

Realize and experimentally verify an ultrathin, low-thermal-mass MIM-type metamaterial absorber whose average absorption is near unity across the entire long-to-very-long infrared band, 8-30 µm. This comprises three jointly linked tasks that the source work (Zhou et al. 2021) leaves unresolved: (1) experimental verification and metrology — fabricate an ultra-broadband absorber that keeps the source design's small dielectric thickness (the Ti/Si/SiO$_2$/Ti design uses a 300 nm Si layer plus a 1300 nm SiO$_2$ layer, a ~1.6 µm total dielectric spacer) and its single-size top-metal pattern — the simulated stacks (92% average absorption simulated over 14-30 µm, 87% over 8-30 µm) being one admissible instance rather than a mandatory target — and measure its absolute absorption spectra across the full 8-30 µm band, including the 14-30 µm very-long-wavelength infrared (VLWIR) range where accurate measurement devices are scarce, and manage the known experimental error sources (actual refractive indices of deposited dielectric films, finite measurement apertures, and film-thickness imprecision) identified in the measured 8-14 µm Ti/Ge/Si$_3$N$_4$/Ti prototypes; (2) performance optimization — close the gap between the simulated 87-92% averages and near-unity average absorption over 8-30 µm, where near-unity denotes a measured, band-averaged absorption across the full 8-30 µm band measurably greater than the simulated 87% baseline (any increase toward unity counts; no absolute threshold is imposed), in particular removing the spectral dips near 10 µm and 22 µm, under the explicit constraint that the total dielectric spacer thickness is not increased beyond the source design's ~1.6 µm and the top-metal pattern stays a single resonator size (rather than enlarging the unit cell with multi-sized resonators or otherwise increasing thermal mass and fabrication complexity); and (3) the practical envelope needed for the intended applications — large-incidence-angle response, thermal and environmental stability, and scalable, low-cost fabrication of a single-size top-metal pattern over large areas. The three sub-tasks are jointly required rather than alternatives. The overall scientific objective is a single one: a measured, deployable, ultrathin absorber covering LWIR plus VLWIR for uncooled infrared detectors and space-based detection, not merely a simulated spectrum.

References

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