When two metal nanostructures are placed nanometres apart, their optically driven free electrons couple electrically across the gap. The resulting plasmons have enhanced optical fields of a specific colour tightly confined inside the gap. Many emerging nanophotonic technologies depend on the careful control of this plasmonic coupling, including optical nanoantennas for high-sensitivity chemical and biological sensors, nanoscale control of active devices, and improved photovoltaic devices. But for subnanometre gaps, coherent quantum tunnelling becomes possible and the system enters a regime of extreme non-locality in which previous classical treatments fail. Electron correlations across the gap that are driven by quantum tunnelling require a new description of non-local transport, which is crucial in nanoscale optoelectronics and single-molecule electronics. Here, by simultaneously measuring both the electrical and optical properties of two gold nanostructures with controllable subnanometre separation, we reveal the quantum regime of tunnelling plasmonics in unprecedented detail. All observed phenomena are in good agreement with recent quantum-based models of plasmonic systems, which eliminate the singularities predicted by classical theories. These findings imply that tunnelling establishes a quantum limit for plasmonic field confinement of about 10(-8)λ(3) for visible light (of wavelength λ). Our work thus prompts new theoretical and experimental investigations into quantum-domain plasmonic systems, and will affect the future of nanoplasmonic device engineering and nanoscale photochemistry.
The brightest and most vivid colours in nature arise from the interaction of light with surfaces that exhibit periodic structure on the micro-and nanoscale. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex colour mixing. Although the physics of structural colours is well understood, it remains a challenge to create artificial replicas of natural photonic structures [1][2][3] . Here we use a combination of layer deposition techniques, including colloidal self-assembly, sputtering and atomic layer deposition, to fabricate photonic structures that mimic the colour mixing effect found on the wings of the Indonesian butterfly Papilio blumei. We also show that a conceptual variation to the natural structure leads to enhanced optical properties. Our approach offers improved efficiency, versatility and scalability compared with previous approaches [4][5][6] .The intricate structures found on the wing scales of butterflies are difficult to copy, and it is particularly challenging to mimic the colour mixing effects displayed by P. blumei and P. palinurus 7,8 . The wing scales of these butterflies consist of regularly deformed multilayer stacks that are made from alternating layers of cuticle and air, and they create intense structural colours (Fig. 1). Although the P. blumei wing scales have previously been used as a template for atomic layer deposition (ALD) 9 , such an approach is not suitable for accurate replication of the internal multilayer structure on large surface areas.The bright green coloured areas on P. blumei and P. palinurus wings result from a juxtaposition of blue and yellow-green light reflected from different microscopic regions on the wing scales. Light microscopy reveals that these regions are the centres (yellow) and edges (blue) of concavities that are 5-10 mm wide, clad with a perforated cuticle multilayer 7 (Fig. 1d,e). For normal light incidence, the cuticle-air multilayer shows a reflectance peak at a wavelength of l max ¼ 525 nm, which shifts to l max ¼ 477 nm for light incident at an angle of 458. Light from the centre of the cavity is directly reflected, whereas retro-reflection of light incident onto the concavity edges occurs by double reflection off the cavity multilayer (Fig. 1g). This double reflection induces a geometrical polarization rotation 10 . If light that is polarized at an angle c to the initial plane of incidence is retro-reflected by the double bounce, it will pick up a polarization rotation of 2c and the intensity distribution through collinear polarisers is therefore given by cos 2 (2c). This leads to an interesting phenomenon: when placing the sample between crossed polarizers, light reflected off the centres of the cavities is suppressed, whereas retro-reflected light from four segments of the cavity edges is detected 10,11 (Fig. 1d, right). In microstructures without this double reflection, both the colour mixing and polarization conversion are absent...
Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We show how this affects the self-assembly process, and hence multidomain color formation in such bioinspired structural films.
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