Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength . However, in current state-ofthe-art designs, a still large absorbing film thickness ( /50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus really nanoscale films, with a broad wavelength tunability using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than . With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub -/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 m.

The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of LaO and NaO can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of LaO in the guiding region with a slope of 8·10(mol.%). The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial LaO content of the glass. Maximum Δn values >10 for samples with just 5.5 mol. % of LaO have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.

COMMUNICATION 1800241 (1 of 7)

No abstract

In article number https://doi.org/10.1002/admi.201800241 by Giorgio Baraldi, Rosalía Serna, Johann Toudert, and co‐workers, the temperature of a self‐assembled subwavelength‐thin metasurface is determined optically by monitoring the phase of the reflected light in quasi‐darkness conditions, which occur at specific angles of incidence and photon energies.