Recently, a team of scientists led by Prof. Shumin Xiao at the Harbin Institute of Technology (Shenzhen), was invited by Co-Editors-in-Chief to contribute a comprehensive review paper entitled "Recent progress on structure coloration", which was published in the second issue of Photonics Insights in 2024 and was selected as the "On the Cover". (Yingjie Li, Jingtian Hu, Yixuan Zeng, Qinghai Song, Cheng-Wei Qiu, Shumin Xiao, "Recent progress on structural coloration," Photon. Insights 3, R03 (2024)).

This review summarizes the development process of the field of structural coloration in the past ten years, focuses on the latest research progress of static structure color display technology, tunable structure color display technology and the application of structure color in various fields, and finally looks forward to the future development of this field.

The development of structure color

Structure color, also known as physical color, is an optical phenomenon that relies on the interaction of nanostructures with light waves to manifest colors. Many structure colors had evolved in nature, such as peacock feathers and butterfly wings, long before humans understood the concept of structure color. In recent years, optical research has developed towards the sub-wavelength scale, leading to new theories on light field modulation, planar optical devices, and their applications. Structured colour is considered the future of display technology due to its unique properties.

The development of advanced manufacturing technologies, such as electron beam lithography, focused ion beam, and laser direct writing, has enabled the nano-unit of structure color to reach the hundred-nanometer scale. As a result, micro- and nanostructure-based metasurface color printing technology has gained attention and been researched in various fields due to its unique advantages. Structure colors based on metal surface plasmon resonances as well as nanostructured arrays based on dielectric materials have been extensively studied (Fig. 1). Due to the superior performance advantages of structure color over traditional display schemes, such as extremely high resolution and low energy consumption, the rich design freedom of the metasurfaces can be utilized to achieve multifunctional integrated microdisplays systems. The field of structure color has undergone rapid development in the past decade, resulting in the creation of static and dynamic structural colour displays, as well as devices for encryption, sensing, and other applications.

Fig 1 The physical model for the generation of structure color in metal/dielectric nanostructures

However, structure color research still faces some challenges in three main aspects: (1) To achieve fast optimal nanostructure design, existing optimization algorithms and machine learning methods should be used with caution as they can be overly dependent on program settings and often only provide localized optimal solutions. (2) Designing structure colors based on metasurfaces requires a more cost-effective process that guarantees the resolution of nanoscale structures while also enabling faster and more convenient large-area fabrication processes. (3) The integration of structure color metasurfaces with traditional CMOS is crucial. Direct integration of structure color metasurfaces on CMOS chips can lead to compact and highly integrated devices. This approach is superior to current display methods and shows the potential of optical devices in a wide range of applications, from optical information to medical sensing.

In this article, Prof. Shumin Xiao's team from Harbin Institute of Technology (Shenzhen) summarizes recent research on structure colors and provides insight into future developments in light of these challenges.

Static structure colors

Structure color refers to the visual outcome of the combined effects of refraction, diffraction, interference, and reflection of structures on the nanoscale. Metal nanoparticles were first widely investigated for their structure color properties due to the strong light field modulation effect exhibited by the resonance of free electrons within the metal with the corresponding wavelength of electromagnetic waves. Designs using metal structures can achieve ultra-high resolution beyond the resolving limit, however, the optical loss in metallic materials results in low brightness. Therefore, researchers have turned to dielectric materials which with low extinction coefficients in the visible range. They have achieved high-purity, high-brightness display of structure colors by cleverly designing the Mie resonance of dielectric nano-arrays, as shown in Fig. 2(a), (b).

Fig 2 Static structure color display design (a) Structure color design based on metal anisotropic nanogroove. (b) Structure color design based on silicon nanopillar arrays.

Currently, researchers have widely investigated static metasurface structure colors. By combining them with the design freedom of metasurfaces, they achieve multi-dimensional modulation of electromagnetic wave amplitude, as well as phase, polarization state, and angle of incidence. Static metasurface structure colors are considered an effective means of anti-counterfeiting and encryption in the future.

Dynamic tunable structure colors

As numerous researchers delve into the geometric shapes, material properties, and spatial arrangements of micro/nanostructures on metasurfaces, an increasing number of high-performance structure colors are being designed. However, traditional metasurfaces, once fabricated, lack adjustability, making them unsuitable for the dynamic tuning demands of most devices. Consequently, dynamic tunable structure color displays have become an urgent necessity. In recent years, various materials and optical mechanisms have been employed to achieve dynamically tunable metasurfaces.

Dynamic tunable structure colors hold the potential to be integrated into next generation displays, offering reduced power consumption, enhanced readability, and resolution improvement, especially in wearable displays. Reflective displays differ from emissive/backlit displays (LCDs, LEDs, and OLEDs) in their utilization of ambient light, resulting in significantly lower power consumption. Emitting/backlit displays are often the most power-consuming components in portable devices. One advantage of reflective displays is their ability to use bright external light sources to illuminate display components. Achieving high brightness is a challenge for micro displays, and high-resolution reflective displays based on tunable structure colors can play a crucial role in portable devices. In recent years, various materials and optical mechanisms have been used to achieve dynamically tunable metasurface. Dynamic tuning schemes based on voltage, chemical processes, microfluidic systems, phase change materials, liquid crystals, and mechanical deformation have been proposed, as shown in Fig. 3.

Fig 3 Tunable Structure Colour Design (a) Structure color design for voltage variation (b) Structure color design for reversible chemical process variation (c) Tunable structure color design based on microfluidic systems (d) Structure color design for polarization state anisotropy correspondence

In addition, this review also introduces the wide range of applications of metasurface structure colors in areas other than conventional displays. Through the unique design of metasurfaces combined with the advantages of high-performance colour display, stability and resolution of structure colors, metasurface structure colors have been widely used in various fields such as optical encryption, structure color coating or ink, colorimetry, sensing, and more.

metasurface structure colors are ideal for sensing applications due to their significant color changes accompanying changes in resonance wavelength. In contrast to physical and chemical sensing, biological sensing often involves complex processes, lengthy detection times, and challenges in achieving real-time monitoring. The real-time color changes exhibited by structure colors in response to biological samples significantly reduce the complexity and time of biological detection, enabling easy implementation of real-time monitoring. Structure color-based biosensors have been widely researched and commercialized due to these advantages. Figure 4 illustrates the widespread use of these biosensors.

Fig 4 Biosensing design based on metasurface structure color

Summary and Outlook

Prof. Shumin Xiao's team has reviewed the latest advances in the field of structure color over the last decade. This review covers the most basic optical models to machine learning-based design solutions for metasurface structure color, the field of structure color has made significant progress in recent years. Compared to traditional color display technologies, metasurface structure color offers advantages such as high colour gamut, resolution, brightness and so on. This makes it ideal for future high-resolution microdisplays and advanced optical anti-counterfeiting, etc. Meanwhile, the rich design freedom of fused metasurfaces can achieve highly integrated and highly functional display devices. With the subsequent application of low-cost large-area micro-nano preparation process, the metasurface structure colors are expected to achieve a wide range of commercial applications in the near future.