Abstract
The infrared spectrum (0.78--30 μm) plays a vital role in materials detection, energy harvesting, environmental sensing, and national defense security. However, traditional materials, limited by the strong coupling between intrinsic optical constants and thermal properties, struggle to achieve independent and precise tailoring of the infrared spectrum. Recently, the emergence of hierarchical photothermal metamaterials has provided a novel paradigm to address this challenge. This paper explores a multi-dimensional, multi-physics cross-band infrared spectral modulation mechanism based on micro-nano structure design, focusing on the evolution from microscopic electromagnetic resonance to macroscopic spectral and thermal field management. It reviews the progress of photothermal metamaterials in constructing ideal blackbodies and achieving infrared camouflage, and delves into their breakthroughs in energy applications such as radiative cooling and thermophotovoltaics. Finally, we outline the challenges and opportunities facing this field in both theoretical research and engineering applications, including large-area manufacturing processes and adaptability to extreme environments.
0 Introduction
Infrared thermal radiation is a common physical phenomenon in nature, and its spectral distribution follows Planck's blackbody radiation law. For example, at room temperature(300 K), the peak wavelength of thermal radiation is located in the mid-far infrared band(8–14 μm), which constitutes the physical basis for infrared detection and thermal imaging under far-field conditions. On the other hand, the interaction between infrared photons and matter has played an important role in the fields of material detection [1] and energy utilization [2-13]. For a long time, the modulation of infrared spectra has mainly relied on the intrinsic properties of materials [14], such as the band gap of semiconductors and [14], such as the band gap of semiconductors and the plasmon frequency of metals. However, this dependence has led to the single function and non-tunability, which can not meet the urgent need of modern optical systems for"on-demand" spectral response.
The advent of metamaterials has broken this constraint [15-23]. By artificially designing subwavelength structural units, researchers can obtain equivalent electromagnetic parameters that do not exist in nature, thereby achieving arbitrary manipulation of the light field [24-25]. When metamaterials are deeply coupled with thermal effects, a new branch of photothermal metamaterials emerges [26-27]. In particular, the cross-scale design strategy—controlling photon density of states and local fields at the nanoscale, and managing heat conduction and radiation paths at the nano, micro, and macro scales—provides a powerful tool for achieving efficient and broadband infrared spectral trimming. This article aims to review the recent progress in this cutting-edge field, clarify its core physical mechanisms, summarize design strategies, and look forward to future directions.
1 Design principles and application areas of multi-scale photothermal metamaterials
The core of cross-scale photothermal metamaterials lies in the collaborative design and optimization of multiple dimensions including wavelength and intensity, as well as multiple physical fields such as optical and thermal fields. At the optical level, microstructures(nano/micron scale) achieve efficient capture and absorption of incident light in specific wavelength bands through localized surface plasmon resonance, Mie resonance, or the construction of Fabry-Perot resonators. At the thermal level, it is necessary to comprehensively consider the heat conduction, convection, and radiation processes at both the macroscopic structure(submillimeter to centimeter scale) and the micro/nano scale to ultimately achieve energy utilization and conversion.
Drawing on the concepts of near-field and far-field in light field propagation, this paper introduces near-field and far-field in energy, distinguished by whether the radiation of an object satisfies Planck's blackbody radiation law [28-30]. When the distance between objects is much greater than the characteristic wavelength of the radiation, the radiation satisfies Planck's law, which is the far-field of energy, analogous to the far-field of optics(François diffraction region); for the near-field of energy, the distance between objects is less than or much less than the characteristic wavelength of the radiation, causing the radiation intensity to exceed Planck's law, analogous to the near-field of optics(Fresnel diffraction region). In the far-field region of energy, the focus is mainly on the imaging of the detected object on the detector. Taking intensity as an example of far-field information, the following condition is satisfied:
（1）
In the formula，ε(λ, T) the absorptivity or emissivity of the object at a certain temperature; and are the radiative exitance of the object and the blackbody in the far-field region, respectively, satisfying Planck's law, i.e.
（2）
In the formula, λ is the wavelength; T is the temperature; h is Planck's constant, h=6.626×10-34J∙s; c is the speed of light; kB is Boltzmann's constant, kB=1.38×10-23J∙K-1.
In the near-field region of energy, due to the enhanced radiation caused by plasmon resonances [31-32], this paper mainly introduces the energy conversion from the optical field to the thermal field and then to the electric field. The energy conversion efficiency η(T) is formally expressed as
（3）
In the formula, Pinc denotes the incident power, which irradiates the device surface in the form of electromagnetic waves. Subject to radiation, conduction, convection and other effects, the device undergoes a temperature rise and then radiates power outward in the form of electromagnetic radiation. The spectral radiant exitance , near of an object in the near field does not obey Planck's law and thus needs to be explained by the theory of near-field radiation [33]. The radiated power is converted by thermoelectric conversion devices and utilized by humans in the form of electrical energy.
As shown in Figure 1, the infrared band can be subdivided into near-infrared, short-wave infrared, mid-wave infrared, long-wave infrared, very long-wave infrared and far-infrared sub-bands [34]. Among them, thermophotovoltaics [35-36] are mainly concentrated in the visible to mid-wave infrared band; the spectral control of radiation cooling also needs to consider the long-wave infrared band [37]. From the perspective of control, the application in the energy field mainly focuses on intensity trimming; while infrared stealth technology faces a multi-dimensional detection background [38], and not only needs to control intensity [39-40], but also needs to take into account dimensions such as angle [41, 49-50] and polarization[42]. It is particularly noteworthy that the 0.78-30 µm band is the focus of this paper. This paper will focus on the series of progress of photothermal metamaterials in this band in optical blackbody, infrared stealth, radiation cooling and thermophotovoltaics, and make a brief discussion and outlook.
Fig.
1
Application areas of cross-band infrared spectral tailoring (including optical blackbody, infrared camouflage, radiative cooling, and thermophotovoltaics)
2 Research Progress
2.1 Optical blackbody
Optical blackbodies play an important role in stray light suppression [43], radiation metrology [44], and solar interface water evaporation [26-27,45]. Furthermore, precision equipment such as spaceborne infrared remote sensors operating in harsh environments like outer space requires optical blackbodies to possess not only high emissivity and high stability but also non-shedding, radiation-resistant, and lightweight properties. As shown in Figure 2(a), the ideal absorptivity or emissivity of an optical blackbody is 1. At this time, the object generally reaches thermal equilibrium with the environment, that is, the object temperature(Tobj) and the ambient temperature(Tenv) are equal, and the detector receives the reflected signal R or transmitted signal T from the surface of the object. The emissivity of a normal temperature object is calculated according to 1-R-T=A. The emissivity of a high temperature object should refer to Equation(1).
Multiscale metamaterials effectively suppress interface reflection by constructing gradient refractive index structures or light-trapping structures, achieving near-ideal blackbody radiation properties. In 2016, Zhou L et al. constructed a multiscale porous metal structure based on an alumina template using the self-assembly of gold nanoparticles(see Figure 2(b)). This structure achieved ultrawideband absorption similar to a blackbody in the range of 0.4-10 µm through local surface plasmon resonance, multiple scattering effects, and intrinsic absorption of alumina [26-27].
Fig.
2
(a)Absorption/reflection spectra of objects under far-field conditions (η is the absorptivity or emissivity of the object);(b) Three-dimensional metallic photothermal metamaterials based on gold nanoparticles [27];(c) Gradient resonators based on multilayer films and conical structures [46];(d) Metal microcavity blackbodies fabricated using femtosecond fabrication technology [47]
In 2024, Ren Z et al. designed a gradient resonator and achieved a high absorption efficiency of about 93% in the 0.2-5 µm band by utilizing the synergistic effect of multilayer film interference and conical cavity, as shown in Figure 2(c) [46]. In 2025, Ng C K et al. used femtosecond fabrication technology to prepare a metal microcavity with a diameter of 100 µm and a thickness of about 100 µm, as shown in Figure 2(d), and achieved an average absorption efficiency of 94% in the 2.5-20 µm band [47]. These works not only verified the advantages of cross-scale structures in omnidirectional light capture, but also laid the physical foundation for high-resolution thermal imaging printing and infrared encryption technology.
2.2 Infrared Stealth
When the temperature of an object is above absolute zero, it spontaneously radiates electromagnetic waves due to the random thermal motion of its charged particles [48]. The thermal radiation energy of most objects at room temperature(around 300 K) is mainly concentrated in the mid-to far-infrared band(8 to 14 µm). As the temperature increases, the radiation peak will also blue shift to the near-infrared and even the visible light band(see Figure 3(a)). Correspondingly, infrared stealth technology also faces the challenge of coordinated control of multi-band optical response. Specifically, visual camouflage in the visible light band requires materials to have high absorptivity to achieve low brightness display, while thermal signal suppression in the infrared band requires materials to have low emissivity to match the background temperature(as shown in Figure 3(b)). This inherent conflict between photothermal and optical properties makes it difficult for a single material to simultaneously meet the stealth requirements of both bands.
To resolve this contradiction, as shown in Figure 3(c), Lin Z et al. designed a cross-scale metamaterial based on the self-assembly of porous alumina template and gold nanoparticles, achieving a low-brightness, multi-background compatible camouflage effect in the visible light band; at the same time, by controlling the radiation impedance of the micro-nano structure, an ultra-low emissivity(about 3.8%) was achieved in the 3-14 μm band. The material also has the characteristics of high temperature resistance and UV radiation resistance [39]. Fang S et al. proposed a photothermal metamaterial based on the self-assembled hollow column structure of gold nanoparticles(see Figure 3(d)), achieving synergistic control of high absorption rate(about 0.947) in visible light and low emissivity in infrared(about 0.074 in the mid-infrared band and about 0.045 in the far-infrared band). These breakthroughs show that through precise cross-scale structural design, the optical response of visible light and infrared bands can be effectively decoupled, providing a feasible technical path for dual-band stealth in nighttime or space environments [40].
Fig.
3
(a)Curves of blackbody radiative exitance versus wavelength at different temperatures; (b) Spectrum of ideal infrared stealth; (c) Metamaterial tape based on self-assembly of porous alumina templateand gold nanoparticles [39]; (d) Skin-like photothermal metamaterial based on self-assembled hollowcolumn structure of nanoparticles [40]
2.3 Radiative Cooling
Radiation cooling is a technology that uses a space cold source(about 3 K) to achieve zero-energy cooling, the core of which lies in the design of selective radiators. Transscale photothermal metamaterials can flexibly cut the spectrum in the near-infrared to very long-wave infrared range, effectively radiating the heat of objects to the cold outer space [37]. At the same time, the low-cost meter-scale material preparation also lays the technological foundation for the large-area radiation cooling application needs such as building coatings.
When the object temperature is lower than the ambient temperature, the ideal emission spectrum is shown in Figure 4(a). It has high emissivity in the atmospheric transparent window(8-14 µm) and high reflectivity in the solar radiation band(0.3-2.5 µm), thereby minimizing solar heating and maximizing thermal radiation. Chen Z et al. constructed a Si3N4(70 nm)/Si(700 nm)/Al(150 nm)/Si(substrate) cross-scale photothermal material with a diameter of about 5 cm as shown in Figure 4(b), achieving selective emission through the atmospheric window. Through a 24-hour day-night cycle, the photothermal metamaterial has achieved an average temperature reduction of 37 ̊C compared to the ambient air temperature in densely populated areas at sea level [51]. Studies have also shown that 16-30 µm [52] and atmospheric windows of 3 to 5 µm can be used as high emission windows.
When the object temperature is higher than the ambient temperature, as shown in Figure 4(c), the spectrum of an ideal radiator should have high emissivity across the entire infrared band. Experiments have shown that such a spectrum can save up to 63% of the power for the active cooler of the shell [52]. In addition, considering thermal insulation performance, as shown in Figure 4(d), Chan K Y et al. developed an anisotropic cooling aerogel plate with large-area solar band reflection and high infrared emissivity by utilizing cross-scale scattering and micro-nano scale reflection [53].
If camouflage functionality is further considered, as shown in Figure 4(e), the ideal spectrum should have low emissivity in at least two atmospheric windows(3-5 µm and 8-14 μm) to suppress detectable thermal radiation signals [55]. For example, Qin B et al. proposed an air-to-ground camouflage strategy: the material has high absorptivity in the near-infrared band to minimize the reflected signal of solar radiation; it has low emissivity in the mid-to-long-wave infrared band, thereby effectively suppressing thermal radiation signals; in addition, it has high emissivity in the very-long-wave infrared band, ensuring efficient thermal management. In a simulated space environment(1200 W·m-2 heat input), the temperature of the material was reduced by 39.8 ℃ compared with the reference metal [54].
Fig.
4
Several ideal spectra of radiation cooling (adapted to different scenarios): (a) Ideal absorptivity/emissivity spectrum when the object temperature is lower than the ambient temperature; (b) SEM interfacediagram and selective emission spectrum of Si3N4/Si/Al/Si(substrate) sample [51];(c) Ideal absorptivity/emissivity spectrum when the object temperature is higher than the ambient temperature;(d) Schematic diagram of hierarchical scattering and reflection mechanism in aerogel [53];(e) Ideal emissivity/absorptivity spectrum(green curve) of infrared camouflage considering background sky radiation(red area), atmospherictransmittance(blue area) and space-to-ground radiation heat dissipation [54]
2.4 Solar thermal and photovoltaic
Faced with the global challenges of climate change and energy transition, developing efficient and clean renewable energy technologies has become a consensus. Furthermore, when fossil fuels are used for industrial production(industrial energy consumption accounts for over 70% of China's total energy consumption), approximately 60% of the energy is ultimately converted into waste heat, while nearly 70% of industrial waste heat remains unutilized [56-57]. Therefore, utilizing the widely distributed solar energy or abundant industrial waste heat to convert thermal energy into green electricity plays a crucial role in achieving China's"carbon neutrality" goal.
Thermophotovoltaic technology is a technique that converts radiant energy into electrical energy. As shown in Figure 5(a), the core of this technology lies in a high-temperature spectrally selective absorption/emission system. When the sun is used as the heat source, an ideal absorber should capture solar radiation over a broad spectrum(approximately 0.3–2.5 µm) and exhibit low emissivity in the mid-and far-infrared bands to reduce radiation losses. The emitter is then heated to a high temperature(typically above 1300 K) using the near-field radiative enhancement effect, generating narrowband thermal radiation whose spectrum is precisely tailored to match the optimal bandgap of the rear-end photovoltaic cell. Theoretically, the system efficiency can be boosted to more than 85% [58], significantly exceeding the Shockley–Queisser limit of single-junction solar cells(approximately 32%) [59].
The key challenge in achieving a high-efficiency near-field thermophotovoltaic system lies in the precise and stable control of spectrally selective thermal radiation under high-temperature conditions. Multiscale photothermal metamaterials, with their artificially designed micro-and nano-structures, provide an ideal platform for solving this challenge; their optical response can be designed independently of the intrinsic properties of the material, thereby enabling the active"programming" of the high-temperature thermal radiation spectrum. Under near-field conditions, radiative heat transfer is affected by the characteristics of materials [60-61], structure [62], temperature [63-64], spacing [65], etc. Microstructures are used to control thermophotovoltaic cells to reduce radiation loss and improve the energy conversion efficiency of thermophotovoltaic systems, including multilayer films [66-67], nanowires [68], and surface gratings [69-70].
A variety of high-performance thermal emission structures based on photothermal metamaterials have emerged in recent studies. For instance, Yang S et al. designed a HfO2/SiO2 multilayer film structure by exploiting the optical Tamm state, achieving a narrowband high emission(emissivity of 0.97) with a central wavelength of 1.9 µm and a full width at half maximum(FWHM) of only 48 nm on a molybdenum metal substrate, with a systematic theoretical efficiency of 33.7% [11]. Wang Y et al. constructed a HfO2/Mo/HfO2 composite multifunctional structure based on the coherent perfect absorption mechanism, integrating the functions of broadband solar absorption(an average absorptivity of approximately 80% in the visible-near infrared region) and narrowband thermal emission(an emissivity of approximately 97% at 1.8 µm) on the same surface. This structure maintains stability at temperatures above 1373 K and exhibits excellent angular insensitivity [71].
To further improve performance and controllability, as shown in Figure 5(b), Zhang S et al. proposed a Si/Mo/ AlN multilayer structure based on a dual coherent enhanced absorption mechanism. By coupling dual resonant channels, a stepped emission spectrum with a center wavelength of about 1.4 µm and a bandwidth of about 530 nm was achieved. The system efficiency reached 31% under 1000 times concentration conditions. The structure showed excellent stability at a high temperature of 973 K and the emissivity in the mid-infrared band of 3-10 µm was less than 10%, effectively suppressing heat loss[35]. As shown in Figure 5(c), Lapotin A et al. used band-edge spectral filtering and a back reflector, combined with a double-junction thermal photovoltaic cell(band gap of 1.0-1.4 eV, corresponding to a temperature of 1900-2400 ℃), to achieve a system efficiency of 40% [36]. When the thickness and gap between the emitter and receiver are much smaller than the thermal photon wavelength, near-field radiation dominates. As shown in Figure 5(d), Tang L et al. achieved near-field radiative heat transfer 1400 times that of the blackbody limit by utilizing the electromagnetic corner mode and electromagnetic edge mode of the edge and corner structures [72].
Fig.
5
(a) Thermophotovoltaic devices and their corresponding ideal spectra; (b) In-situ temperature-variableinfrared spectra of Si/AlN/Mo [35];(c) Schematic diagram of cascaded thermophotovoltaic devices [36]; (d) Schematic diagram of near-field radiative heat transfer of SiC flat film [72]
3 Outlook and Conclusion
Multiscale photothermal metamaterials, through their multiscale structural design and multiphysics coupling capabilities, provide a powerful tool for achieving cross-band spectral trimming from visible light to very long-wave infrared. This paper systematically reviews the principles and application progress of this material system in key fields such as optical blackbodies, infrared stealth, radiative cooling, and solar thermophotovoltaics. Currently, optical blackbodies based on micro-nano resonances and light-trapping structures have achieved ultrawideband near-perfect absorption from ultraviolet to far-infrared, laying the foundation for thermal radiation metrology and photothermal utilization. In the field of infrared stealth, through spectral decoupling design, the compatibility of visible light camouflage and infrared low detectability has been successfully achieved, demonstrating potential in multi-band countermeasures. In radiative cooling applications, photothermal metamaterials can achieve daytime cooling below ambient temperature through selective radiation matching with atmospheric windows, and are developing towards large-scale and multifunctional applications. In addition, photothermal metamaterials adapted to extreme high-temperature environments also provide a key solution for overcoming the efficiency bottleneck of thermophotovoltaic systems.
Despite significant progress, the field still faces a series of challenges in moving from basic research to large-scale application. In the field of thermophotovoltaics, although some research has achieved a thermophotovoltaic efficiency of 40% [36], this technology is still in the laboratory research stage, and it remains extremely challenging to achieve comprehensive control of the light field and the thermal field. On the one hand, there is still room for improvement in the spectral selectivity of the absorber/emitter; on the other hand, in addition to radiative heat transfer, the interfacial thermal resistance between the emitter and the absorber should also be considered, and intrinsic materials that are resistant to high temperatures(greater than 1300 K) and compatible with existing micro-nano fabrication processes should be screened [73].
Photothermal metamaterials have completed application verification in the field of radiative cooling and are now moving towards industrialization. Multiple technological approaches [37,74] are still competing. It is also a practical issue to be addressed in this field to systematically investigate the performance evolution and failure mechanisms of photothermal metamaterials under harsh conditions such as long-term high-temperature oxidation, thermal cycling, hygrothermal aging, and intense ultraviolet/particle irradiation, and to develop novel material systems with intrinsic stability or self-healing capabilities to ensure their long-term service in real scenarios including aerospace, military, and outdoor energy applications.
The key scientific issue in the field of optical black bodies lies in the trade-off between bandwidth and thickness. There remains room for progress in theoretically explaining such constraints, rather than relying merely on empirical trial-and-error.
In the above fields, photothermal metamaterials mainly perform cross-band spectral trimming from the dimension of intensity. However, in the face of multi-band and multi-dimensional(wavelength, polarization, etc.) detection [38], optical stealth not only needs to achieve cross-band trimming of frequency domain intensity, but also needs to consider the control of radiation angle in the spatial domain. In addition, by combining reverse design [75-76], transformation optics [77-78 ] and transformation thermodynamics [79], the wide angle and cross-band spectral control of photothermal metamaterials may meet the application requirements of practical scenarios. However, the actual implementation still faces the problem of standardized preparation from laboratory principle verification to low cost, cross-scale, and large area. Therefore, it is necessary to further promote the development of advanced processes such as nanoimprinting [80-81], roll-to-roll printing [82-85], directional self-assembly and cross-scale additive manufacturing to solve the manufacturing problems of low cost, high uniformity and high fidelity from nano-feature structure to macro-device integration.
In summary, multiscale photothermal metamaterials, as a cutting-edge interdisciplinary field connecting micro/nano photonics and macroscopic thermal management, have not only deepened our understanding of the interaction between light and matter but also spawned numerous disruptive application technologies. By continuously overcoming the aforementioned challenges, they are expected to play a more crucial role in fields such as energy, information, and defense, injecting new impetus into sustainable development and high-tech industries.
Acknowledgments
The author thanks Zhu Changhao, Liu Chenshuaiyu, Ren Siyun, Yang Zhengwei and Jiang Yi of Nanjing University for their discussions, suggestions and help.