Going beyond an improved colour gamut, an asymmetric colour contrast, which depends on the viewing direction, and its ability to readily deliver information could create opportunities for a wide range of applications, such as next-generation optical switches, colour displays, and security features in anti-counterfeiting devices. Here, we propose a simple Fabry–Perot etalon architecture capable of generating viewing-direction-sensitive colour contrasts and encrypting pre-inscribed information upon immersion in particular solvents (optical camouflage). Based on the experimental verification of the theoretical modelling, we have discovered a completely new and exotic optical phenomenon involving a tuneable colour switch for viewing-direction-dependent information delivery, which we define as asymmetric optical camouflage.
We introduce a robust control method of terahertz (THz) transmission by tuning filling factors of Au nanoparticles (AuNPs) inside nano slot antennas. AuNPs in sub-100 nm diameters were spread over the nano slot antennas, followed by sweeping them into the slots. AuNPs can be efficiently localized and inserted into nano slots where the THz fields are greatly enhanced, by a "squeegee" made of the polydimethylsiloxane (PDMS). The sweeping of the AuNPs results in further dramatic reduction of THz transmission by suppressing the fundamental resonance mode of the nano slot, as compared to a typical random dropping case. It definitely works for an accurate THz transmission control, as well as the removal of unwanted ions that occasionally confuse signal accuracy from the target signals. Our approach provides a complete reinterpretation of sample deposition for further steady demands in developing ultrasensitive terahertz (THz) molecule sensors.
A liquid-permeable concept in a metal− insulator−metal (MIM) structure is proposed to achieve highly sensitive color-tuning property through the change of the effective refractive index of the dielectric insulator layer. A semicontinuous top metal film with nanoapertures, adopted as a transreflective layer for MIM resonator, allows to tailor the nanomorphology of a dielectric layer through selective etching of the underneath insulator layer, resulting in nanopillars and hollow voids in the insulator layer. By allowing outer mediums to enter into the hollow voids of the dielectric layer, such liquid-permeable MIM architecture enables to achieve the wavelength shift as large as 323.5 nm/RIU in the visible range, which is the largest wavelength shift reported so far. Our liquidpermeable approaches indeed provide dramatic color tunablility, a real-time sensing scheme, long-term durability, and reproducibility in a simple and scalable manner.
Probing the kinetic evolution of nanoparticle (NP) growth in liquids is essential for understanding complex nano‐phases and their corresponding functions. Terahertz (THz) sensing, an emerging technology for next‐generation laser photonics, has been developed with unique photonic features, including label‐free, non‐destructive, and molecular‐specific spectral characteristics. Recently, metasurface‐based sensing platforms have helped trace biomolecules by overcoming low THz absorption cross‐sectional limits. However, the direct probing of THz signals in aqueous environments remains difficult. Here, the authors report that vertically aligned nanogap‐hybridized metasurfaces can efficiently trap traveling NPs in the sensing region, thus enabling us to monitor the real‐time kinetic evolution of NP assemblies in liquids. The THz photonics approach, together with an electric tweezing technique via spatially matching optical hotspots to particle trapping sites with a nanoscale spatial resolution, is highly promising for underwater THz analysis, forging a route toward unraveling the physicochemical events of nature within an ultra‐broadband wavelength regime.
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