Abstract:Covert polarization displays provide a barrier to the inadvertent viewing of stored optical information. For security and anti‐counterfeiting purposes, access to concealed information without compromising packaging aesthetics is required in certain situations. However, optical conversion with polarized light typically requires sophisticated nanostructures only possible with limited materials, which are not appropriate for application to objects requiring flexibility or conformability, and color selectivity. A … Show more
“…[11,12] Another strategy to obtain wide color selectivity in a metal film is the construction of sophisticated nanostructures to realize various colors by means of polarization conversion. [13] Despite numerous attempts to modulate color by oxidation and nanostructuring efforts, [14][15][16] the complexity associated with conversion of the Cu lattice into an oxide remains an obstacle for coherent control of the interface between metal and metal oxide, which is necessary to obtain a full, well-defined color spectrum.…”
Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single‐crystal Cu thin film is demonstrated to achieve a full‐color spectrum for Cu by precisely controlling its oxide‐layer thickness. Grain‐boundary‐free and atomically flat films prepared by atomic‐sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full‐color red/green/blue indices is realized by precise control of the oxide‐layer thickness; the samples cover ≈50.4% of the standard red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from the oxide “pigment” (complex dielectric functions of Cu2O) and light‐layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser‐oxide lithography is demonstrated with micrometer‐scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.
“…[11,12] Another strategy to obtain wide color selectivity in a metal film is the construction of sophisticated nanostructures to realize various colors by means of polarization conversion. [13] Despite numerous attempts to modulate color by oxidation and nanostructuring efforts, [14][15][16] the complexity associated with conversion of the Cu lattice into an oxide remains an obstacle for coherent control of the interface between metal and metal oxide, which is necessary to obtain a full, well-defined color spectrum.…”
Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single‐crystal Cu thin film is demonstrated to achieve a full‐color spectrum for Cu by precisely controlling its oxide‐layer thickness. Grain‐boundary‐free and atomically flat films prepared by atomic‐sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full‐color red/green/blue indices is realized by precise control of the oxide‐layer thickness; the samples cover ≈50.4% of the standard red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from the oxide “pigment” (complex dielectric functions of Cu2O) and light‐layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser‐oxide lithography is demonstrated with micrometer‐scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.
“…Narrowband perfect absorbers are perceived to facilitate applications in diverse fields including imaging, sensors, spectroscopy, and absorption filtering in the visible band 25 – 27 . In light of its salient benefits like lithography-free production, highly scalable thin-film design, and angle invariant operation, the proposed perfect absorber will be able to categorically catalyze the development of eminent applications such as smart camouflage and stealth 56 , 57 . Figure 7 Angular dependence.…”
We developed a flexible perfect absorber based on a thin-film nano-resonator, which consists of metal–dielectric–metal integrated with a dielectric overlay. The proposed perfect absorber exhibits a high quality (Q-)factor of ~ 33 with a narrow bandwidth of ~ 20 nm in the visible band. The resonance condition hinging on the adoption of a dielectric overlay was comprehensively explored by referring to the absorption spectra as a function of the wavelength and thicknesses of the overlay and metal. The results verified that utilizing a thicker metal layer improved the Q-factor and surface smoothness, while the presence of the overlay allowed for a relaxed tolerance during practical fabrication, in favor of high fidelity with the design. The origin of the perfect absorption pertaining to zero reflection was elucidated by referring to the optical admittance. We also explored a suite of perfect absorbers with varying thicknesses. An angle insensitive performance, which is integral to such a flexible optical device, was experimentally identified. Consequently, the proposed thin-film absorber featured an enhanced Q-factor in conjunction with a wide angle of acceptance. It is anticipated that our absorber can facilitate seminal applications encompassing advanced sensors and absorption filtering devices geared for smart camouflage and stealth.
“…This conventional film could show a difference in brightness depending on the direction of the incident LPL because almost all dichroic dyes have only one absorption axis, except in special cases [20,21]. To overcome this limitation, chiral photonic crystals [11] or materials that use refractive index change [22] have been proposed, but this method requires expensive and complicated fabrication methods. In this study, we suggest the use of a bilayer dichroic dye-doped LC (BDLC) film as an anti-counterfeiting film with enhanced security.…”
We report on bilayer dichroic dye-doped liquid crystal (BDLC) films for patternable color codes, which can be used in anti-counterfeiting applications such as security codes and, unlike the conventional single-layer LC color films, show unique color changes depending on the direction of the polarization of the incident light that passes through them. This is a facile way to enhance the security level of security codes. BDLC films are fabricated by laminating two single-layer LC polymer network films with crossed optical axes. We believe that BDLC films can be used as a new platform for a securityenhanced anti-counterfeiting code that can replace complex chiral materials or expensive plasmonic particles.
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