A nearly perfect absorber across an ultrabroad frequency range from 1 to 1200 THz is reported that combines a silicon moth‐eye structure with a thin layer of pyrolytic carbon (PyC). The moth‐eye design provides a perfect impedance match with air via gradual change of the refractive index at low frequencies and simultaneously serves as a multiple scatterer at high frequencies while coating the moth‐eye structure with PyC enables ultrabroad band absorption. The combination of the strong absorption in the PyC conductive film and moth‐eye surface allows to create a material capable of absorbing over 98% incident radiation across the entire THz–deep ultraviolet (DUV) frequency range. This makes this moth‐eye/PyC structure a promising material for a wide range of applications including energy harvesting, electromagnetic shielding, and the development of an ideal blackbody radiation source.

Achieving high-Q resonances in the terahertz frequency range is crucial for applications such as sensors, filters, and emitters. One effective strategy for achieving such resonances involves the use of metamaterials. However, the performance of terahertz (THz) metamaterials is often limited by radiation losses in their meta-atoms and in substrates. In this paper, we present both simulation and experimental results of a dual-layer free-standing metamaterial, exhibiting Q-factor of 3570 in the sub-terahertz range, achieved by integrating two complementary planar metamaterial layers that induce an isolated dominant electric dipole moment. This unique metamaterial displays the potential for advances in antennas, quantum computing, THz sensors, offering efficient energy confinement and low losses.

According to the Babinet principle, the diffraction pattern from an opaque body is identical to that from a hole of the same shape and size. Intuitively, placing two complementary structures such as an opaque metal body and its transparent counterpart one by one may result in destructive or constructive interference leading to unexpected electromagnetic response. We propose a Babinet principle-based metamaterial made of two complementary metal/hole checkerboards. The unit cell of each layer is either a metal square with 1/4 of 8 neighboring squares, four of which are made of metal, whereas the other four are square holes, or vice versa. Being placed complementary at optimal distance equal to three-unit cell length, the compound bi-layered Babinet structure demonstrates absolute transparency in a very broad frequency range. The observed absolute transparency of the bi-layered Babinet metasurface is the result of the modified multipole interaction of layers with shifted centers of radiation. We demonstrate both theoretically and experimentally absolute transmission of 0 dB for the Babinet metamaterial made of 3 cm sized Cu squares and complementary holes in the broad frequency range from 4.5 to 6.62 GHz in simulations and from 4.6 to 6.4 GHz in the experiment when the distance between two layers is 1.2 cm. Moving layers toward each other leads to blurring of the resonances. The proven concept of simple, reproducible, and scalable design of the Babinet metamaterial paves the way for the fabrication of broadband transparent devices at any frequency, including THz and optical ranges. The main advantage of broadband Babine metamaterials is applications in optical switching, sensing, filtering, and slow light devices.

The emergence of radiation sources in different frequency ranges fuels the demand for techniques for spectrally resolved calibrations of their intensity. Thermal radiation of a perfectly absorbing object (black-body radiation) is used as an etalon in the near-infrared–visible ranges. However, to date, no suitable material exists as a reference for thermal radiation in the frequency range 3–30 THz. In this work, we demonstrate a thermal source that can serve a broad THz frequency range. We employ a moth-eye-structured silicon coated with conductive graphitic film exhibiting absorbance above 99.5%. By using the thermal emission of a silicon-based Salisbury screen heated at a temperature of 60 °C we demonstrate that the moth-eye structure can be used for calibrating THz devices. We also show that the developed approach may allow one to engineer the spectrum of the frequency comb source using a two-layer heterostructure.

We report on the design, fabrication, and experimental characterization of a free‐standing terahertz metasurface supporting quasi‐bound states in the continuum (quasi‐BICs). The metasurface, realized by laser cutting double‐slot apertures in a thin brass foil, exhibits a tunable transition from symmetry‐protected BICs to sharp Fano‐type resonances when structural asymmetry is introduced. Terahertz time‐domain spectroscopy measurements confirm high‐Q resonances in good agreement with simulations. Multipole decomposition reveals that the trapped mode response is dominated by the in‐plane electric quadrupole component, which governs the confinement and radiation properties of the quasi‐BIC. The free‐standing design eliminates substrate‐induced losses and opens a robust route toward versatile, high‐performance terahertz devices. These findings demonstrate the potential of quasi‐BIC metasurfaces as compact, tunable high‐Q resonators for applications in sensing, filtering, and terahertz photonics.