Material is a fundamental element in human society. Cotton is used to make comfortable clothes, and gold, as a general equivalent, has a long history in commerce. To meet the surging demand for multi-functional materials, metamaterials have been developed, featuring synthetic, three-dimensional, periodic cellular architectures. These metamaterials derive their properties and multi-functional capabilities from their structure rather than directly from their composition. This is why metamaterials have been widely utilized in various fields, such as mechanics, thermodynamics, and optics. A remarkable early example in optics is the achievement of negative refraction using gold-based metamaterials, which broke the traditional Snell’s Law. Consequently, the regulation of multi-degree-of-freedom, multi-physical fields, and functional integration of metamaterials have emerged as prominent research areas in physical state manipulation and nanotechnology frontiers. Recognizing sunlight as the primary energy source for life on Earth and in response to the growing energy crisis, we aim to explore the use of plasmonic trans-scale metamaterials in the multi-physical field and multifunctional integrated regulation, including photothermal, photochemical, and photomechanical interactions. Due to the multi-degree-of-freedom in nanostructure and the plasmonic effect, plasmonic metamaterials can enhance the manipulation of light, such as achieving high absorption over a wide spectrum, similar to a blackbody. However, the absorption bandwidth of conventional 2D metamaterials is limited by efficiency. Specifically, conventional metamaterials based on 2D planar ordering (or assembling) of metamaterial atoms with metal matrix elements tend to have narrow absorption bandwidths or low absorption efficiency, making it difficult to balance in-plane coupling and surface impedance. In contrast, 3D self-assembled photothermal metamaterials expand in-plane lateral coupling to an out-of-plane longitudinal coupling system, replacing the top-down traditional nano-optical fabrication technique with a nanoparticle self-assembled process. This innovation opens new design pathways for cross-wavelength, multi-functional, large-area photothermal synergistic modulation in photothermal applications. Apart from the goal of achieving a blackbody through 3D self-assembled metamaterials, various spectra across wide bandwidths must also be considered for applications such as solar evaporation, radiative cooling, surface-enhanced Raman scattering, photocatalysis, and spectral camouflage. For instance, in radiative cooling, the ideal spectrum, in the absence of a heat source, is highly reflective in the solar band (400‒2500 nm), reducing solar energy absorption, and highly emissive in the atmospheric window (8‒windm), radiating heat into space and achieving sub-environmental cooling. In the presence of a heat source, the ideal spectrum is highly reflective in the solar band but is also highly emissive in the infrared band (2.5‒20.0 μm), enabling sub-environmental cooling by radiating high levels of infrared energy. In this study, we review the fundamental principles and recent applications of 3D self-assembled plasmonic metamaterials for photothermal manipulation.

The advancement in 3D self-assembled plasmonic metamaterials has been significant, progressing from the basic mechanisms of photothermal manipulation to diverse applications. First, we introduce the mechanism of interaction between light and 3D self-assembled metamaterials in terms of absorption (Fig. 1). These 3D metamaterials represented by the self-assembled metal nanoparticles in anodic aluminum oxide (AAO), achieve high absorption across a broad wavelength range, which originates from the localized surface plasmon (LSP) effect of metal nanoparticles and the intrinsic absorption of alumina. Modulating the number, size, and spatial distribution of nanoparticles and the diameter and period of AAO provides a rational and reliable way to design this high absorption spectrum. Besides this high absorption spectrum, we also discuss several other ideal absorption spectra suitable for different applications (Fig. 2). Third, combined with ideal spectra, we categorize these different applications into five parts: solar evaporation, radiative cooling, surface-enhanced Raman scattering, photocatalysis, and spectral camouflage. For example, in solar evaporation, Lin Zhou’s research group from Nanjing University has proposed an all-dielectric insulated plasmonic absorber, demonstrating an efficient self-floating interfacial solar evaporator with an efficiency of approximately 80% under one sun (Fig. 3). In the field of radiative cooling, Gil Ju Lee’s group from Pusan National University proposed a deep learning model to inversely design thin-film solar-transparent and solar-opaque radiative coolers (Fig. 4). In spectral camouflage, Shujiang Ding’s research group from Xi’an Jiaotong University introduced a nanostructured composited film based on ovulate-rich porous alumina for visible-to-infrared compatible camouflage with simultaneous thermal management (Fig. 7). Studies on 3D self-assembled plasmonic metamaterials are still limited and need further exploration.

3D self-assembled plasmonic metamaterials are gradually becoming significant due to their multi-functional capabilities across various fields. In summary, while these plasmonic metamaterials show great promise, more in-depth research, and detailed exploration are necessary to advance the study of photothermal manipulation and promote the development of related technologies for the benefit of human society.