High-purity reflective color filters based on thin film cavities embedded with an ultrathin Ge2Sb2Te5 absorption layer

High-purity and wide-angle reflective structural colors based on an all-dielectric Fabry-Pérot cavity structure

High-purity structural colors with low fabrication cost are in demand for their commercial applications. Here, we demonstrate an all-dielectric Fabry-Pérot cavity structure consisting of four-layer lossy and lossless dielectric films alternately stacked for producing high-purity and angle-invariant reflective colors. Multiple cavity resonances function together to significantly suppress the undesired reflection with the enhanced optical absorption, leading to a distinct and saturated color with a high efficiency of ∼70%. Besides, due to the high refractive indices of constituent materials, the color appearance of the designed structure can be maintained well at ±50° incident angle for two polarization states. The excellent color performance of the proposed device together with cost-effective manufacturing convenience opens up new avenues for their large-area applications in various areas.

Laser Dynamic Control of the Thermal Emissivity of a Planar Cavity Structure Based on a Phase-Change Material

Tailoring the thermal emission of a material in the long-wave infrared (IR) range of 8-13 μm is crucial for many IR-adaptive applications, including personal thermal management, IR camouflage, and radiative cooling. Although various materials and surface structures have been proposed for these purposes, space-selective and dynamic control of their emissivity is challenging. In this study, we present a planar surface cavity structure consisting of a Ge2Sb2Te5 (GST) film on top of a thin metal reflector to modulate its emissivity by using an ultraviolet laser beam. A laser-induced phase change in GST allowed for the local control of emissivity. The average emissivity in the long-wave IR range was tunable from 0.15 to 0.77 simply by changing the laser energy deposited on the GST film. This enabled the laser printing of high-contrast emissivity patterns, which were erasable by subsequent thermal annealing. Emissivity-modulated GST cavities could be fabricated on not only rigid substrates but also flexible plastic substrates such as polyimide. The GST surface cavity was highly flexible and remained stable upon repeated bending to a curvature radius of 0.5 cm. This study provides a promising route for realizing scalable and flexible thermal emitters with tunable surface emissivity.

Fano resonant optical coatings platform for full gamut and high purity structural colors

Structural coloring is a photostable and environmentally friendly coloring approach that harnesses optical interference and nanophotonic resonances to obtain colors with a range of applications including display technologies, colorful solar panels, steganography, décor, data storage, and anticounterfeiting measures. We show that optical coatings exhibiting the photonic Fano Resonance present an ideal platform for structural coloring; they provide full color access, high color purity, high brightness, controlled iridescence, and scalable manufacturing. We show that an additional oxide film deposited on Fano resonant optical coatings (FROCs) increases the color purity (up to 99%) and color gamut coverage range of FROCs to 61% of the CIE color space. For wide-area structural coloring applications, FROCs have a significant advantage over existing structural coloring schemes.

A review of tunable photonics: Optically active materials and applications from visible to terahertz

The next frontier of photonics is evolving into reconfigurable platforms with tunable functions to realize the ubiquitous application. The dynamic control of optical properties of photonics is highly desirable for a plethora of applications, including optical communication, dynamic display, self-adaptive photonics, and multi-spectral camouflage. Recently, to meet the dynamic response over broad optical bands, optically active materials have been integrated with the diverse photonic platforms, typically in the dimension of micro/nanometer scales. Here, we review recent advances in tunable photonics with controlling optical properties from visible to terahertz (THz) spectral range. We propose guidelines for designing tunable photonics in conjunction with optically active materials, inherent in wavelength characteristics. In particular, we devote our review to their potential uses for five different applications: structural coloration, metasurface for flat optics, photonic memory, thermal radiation, and terahertz plasmonics. Finally, we conclude with an outlook on the challenges and prospects of tunable photonics.

Structural color printing via polymer-assisted photochemical deposition

Structural color printings have broad applications due to their advantages of long-term sustainability, eco-friendly manufacturing, and ultra-high resolution. However, most of them require costly and time-consuming fabrication processes from nanolithography to vacuum deposition and etching. Here, we demonstrate a new color printing technology based on polymer-assisted photochemical metal deposition (PPD), a room temperature, ambient, and additive manufacturing process without requiring heating, vacuum deposition or etching. The PPD-printed silver films comprise densely aggregated silver nanoparticles filled with a small amount (estimated <20% volume) of polymers, producing a smooth surface (roughness 2.5 nm) even better than vacuum-deposited silver films (roughness 2.8 nm) at ~4 nm thickness. Further, the printed composite films have a much larger effective refractive index n (~1.90) and a smaller extinction coefficient k (~0.92) than PVD ones in the visible wavelength range (400 to 800 nm), therefore modulating the surface reflection and the phase accumulation. The capability of PPD in printing both ultra-thin (~5 nm) composite films and highly reflective thicker film greatly benefit the design and construction of multilayered Fabry-Perot (FP) cavity structures to exhibit vivid and saturated colors. We demonstrated programmed printing of complex pictures of different color schemes at a high spatial resolution of ~6.5 μm by three-dimensionally modulating the top composite film geometries and dielectric spacer thicknesses (75 to 200 nm). Finally, PPD-based color picture printing is demonstrated on a wide range of substrates, including glass, PDMS, and plastic, proving its broad potential in future applications from security labeling to color displays.

Compact multilayer thin-film color filters and direct integration on white-light-emitting diodes for color conversion

We present highly efficient green, yellow, and red filters based on a metal-dielectric structure. The filters encompass only five layers of alternating zinc tin oxide and silver thin films that are grown on soda lime glass and white light-emitting diodes (LEDs) using direct current magnetron sputtering at room temperature. The designed filters provide efficient color filtering in the visible spectrum. High purity colored light is obtained by direct application of filters on LEDs as color converters. The presented method offers an easy way for realizing different colors by tuning the thicknesses of layers in the structure.