Time: 2023-09-18

News Source & Photographer: 

School of Physics

Editor: Wang Lirong

Reviewer: Chen Ke

Translator: Shao Yikang & Qin Yao

Recently, the research group led by Professor Wang Yang and Professor Li Jiafang of the School of Physics of Beijing Institute of Technology has made important progress in the field of dynamic thermal emission metasurfaces. Their work focuses on the in-situ regulation of infrared the thermal emission intensity and peak wavelength through the manipulation of multi-physics fields using nano-kirigami structures. They have also explores its application potential in dynamic thermal management and energy conversion. The research findings were published in the journal Small [Small, 2305171 (2023)" under the title "Thermal Emission Manipulation Enabled by Nano-Kirigami Structures". The research work was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, and the Beijing Municipal Natural Science Foundation.

Thermal emission is a ubiquitous basic physical phenomenon that is widely studied due to its important applications in the fields of lighting, temperature management, energy utilization, and thermal imaging. According to Kirchhoff's law, the thermal emissivity of any object is determined by its light absorption rate. This equivalent relationship between emissivity and absorption rate suggests that the temporal, spatial and spectral properties of thermal emission can be controlled through material selection and micro-nanostructure design. With the rapid development of nanotechnology in the past two decades, scientists have made great progress in designing the static properties such as spectral distribution, directionality, and polarization of thermal emission using metasurface materials. However, the means of dynamically manipulating thermal emission in response to time are still very limited. Most dynamic control methods have a narrow range and dimension of control in terms of radiation intensity and spectral distribution, and at the same time limited by the material or structure of the device itself and the control mode is limited to a specific way, and it is urgent to develop a new thermal emission control method with strong modulation ability, multiple modulation dimensions and many modulation methods.

Among the many metasurface materials, nano-kirigami metasurfaces have emerged as an ideal platform for dynamic thermal emission manipulation due to their unique driving modes, reconfigurable geometry, and optical properties[Nat. Commun. 12, 1299 (2021); Adv. Opt. Mater. 11, 2202150 (2023)]. The nano-kirigami technology, based on focused ion beam, has evolved over the past decade as a novel three-dimensional mico-nano processing technique [Sci. Adv. 4, eaat4436 (2018); Light Sci. Appl. 9, 75 (2020); Nanophotonics 12,1459 (2023)]. This technique enables the fabrication of intricate two-dimensional flat patterns and the transformation of flat film structures into complex three-dimensional configurations, overcoming the limitations of traditional micro-nano processing methods in terms of structural continuity, complexity, dynamic tuning, etc. It provides a new means for designing novel micro-nano optical structures. However, remain in the selective design of radiation characteristics and the manufacturing of large- challenges scale unit structures for thermal emission manipulation applications.

In this work, the research team explored the three-dimensional mechanical deformation characteristics of paper-cutting structures, including spiral and windmill patterns, under electrostatic and mechanical forces. They also observed their dynamic spectral responses in the mid-infrared range. As shown in Figure 1, the nano-kirigami array was fabricated on a metal/silicon oxide/silicon substrate. The researchers meticulously etched curves on a metal film, akin to kirigami. Subsequently, they selectively wet-etched the underlying silicon oxide to obtain a suspended two-dimensional kirigami structure. This planar structure can be "folded" both upwards and downwards. Electrostatic forces induce downward deformation, while upward deformation can be achieved through ion beam irradiation. Structural recovery modulation is then accomplished by applying downward compressive stress. The thermal emissivity modulation of metasurface primarily relies on dynamic plasmon resonance in the metal slits. External physical forces induce substantial deformation and distortion in the kirigami structure. These alterations in the dimensions of linear slits lead to significant plasmonic optical modulation effects within mid-infrared range.

Fig 1. Design of nano-paper-cutting thermal emission metasurface devices

The three-dimensional deformation of the nano-kirigami structures can not only induces variations in infrared resonance absorption/emission intensity, but also alters the wavelengths of resonance absorption and emission. These tendencies are closely related to the shape, structural parameters and drive patterns of kirigami patterns. The research team empirically validated this potent  thermal emission control capability through two approaches. Figure 2 shows a helical thermal emission kirigami structure design, adept at electrical modulation to fine-tune emission intensity. With optimized adjustments to period, line length and line width, it achieves narrowband thermal emission across any desired wavelength in the mid-infrared range. Upon applying voltage between the gold layer and underlying silicon layer, the helical structure undergoes downward deformation due to electrostatic force, leading to shifts in thermal emissivity and radiation intensity. As the voltage increases, the peak emissivity of the spiral structure decreases with a slight blue-shifted wavelength until the maximum voltage is reached, at which point the yield strength and electrostatic force of the nanostructure reach the maximum equilibrium state.

Nano-kirigami thermal emitters possess the ability to dynamically control thermal emission wavelength range and the transmission/absorption of visible light, enabling efficient thermal management. To demonstrate this capability, the research team designed a polymer-chimerized nano-kirigami thermal management device with a windmill-shaped nanostructure (Figure 3). Windmill structures can achieve large torsional deformations under guided stress. In the visible spectrum, they function akin to windows, dynamically reflecting or capturing solar energy. With increasing deformation height, the nanowindow opens, expanding the window area, resulting in heightened visible light transmission and absorption. By incorporating doped silicon or other absorbing materials at the base, the device can effectively control the absorption of solar energy. In the infrared band, on the other hand, the windmill structure can regulate the peak wavelength of thermal radiation. The thermal emission spectra of windmill structures with different deformation heights are shown in Figure 3. When the emission peak of the nanostructure can be located in the wavelength range of the atmospheric window in the two-dimensional state, and the emission wavelength outside the atmospheric window in the three-dimensional state, this dynamic regulation is achieved by embedding the paper-cut structure in the elastic polymer.

Fig 2. Implementation of helix-shaped dynamic thermal emission metasurface devices

Fig 3. Implementation of linear dynamic thermal management device for windmills

The dynamic thermal emission control system based on Nano-kirigami metasurface offers versatile control in multiple physical fields, adjustable optical properties in various dimensions, and compatibility with a range of materials. This work proves that the infrared response of nano-kirigami structures can be dynamically regulated by electrostatic force or mechanical stress, and the structure is also suitable for thermogenic, aerodynamic, magnetogenic stress and other control methods. On the other hand, the nano-kirigami thermal emission control system provides multi-dimensional regulation capabilities in terms of wavelength, intensity and other optical characteristics. In this work, it has been demonstrated that the device can independently five-tune both the radiation intensity and the peak wavelength over a broad range. Due to the three-dimensional torsional transformation brought by the special linear design, the nano-paper-cutting structure also has great potential in adjusting the phase, chirality and direction of thermal emission. In addition, the nano-kirigami micromechanical system is not confined to a single material, with options including gold, silver, aluminum, semiconductor and phase change materials. This versatility empowers tailored device design, catering to diverse application scenarios such as radiative cooling, solar energy utilization, and thermal camouflage. In summary, the metasurface thermal emission control device based on nano-paper-cut structure has flexible spectral adjustment capabilities and unique and diverse dynamic adjustment methods, which is an ideal control platform for reconfigurable thermal emission, and is expected to play an important role in energy conversion and thermal camouflage.

Yinghao Zhao and Qinghua Liang, doctoral students at the School of Physics, BIT, co-first authors of the paper, and researcher Wang Yang and Professor Li Jiafang of BIT are co-corresponding authors.

Original links: https://doi.org/10.1002/smll.202305171