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摘要:
热辐射是一种基本的物理过程，指温度高于绝对零度的物体由于粒子的热运动自发地发射电磁能量。大多数热辐射体缺乏方向性，导致能量在不必要的方向上损失，从而降低热器件和应用的效率。在实际应用中，通常需要热辐射器在不同方向上表现出不同的热辐射能力，因此，在高效热传输中，控制热发射的方向性至关重要。定向热辐射的研究对于热成像和传感、辐射冷却、红外加密和能源利用等领域具有重要意义。本文首先阐述了传统热辐射与定向热辐射的区别，以及后者在科学前沿的潜在价值。随后，基于热辐射的特性，从窄带定向热辐射和宽带定向热辐射两个方面系统整理了国内外学者在定向热辐射方面的研究进展。最后，展望了该领域未来的研究趋势，剖析了其面临的重大挑战，旨在为定向热辐射技术的进一步发展提供理论指导与实践启示。
Abstract:Thermal radiation is a fundamental physical process that refers to the spontaneous emission of electromagnetic energy from objects with temperatures above absolute zero due to the thermal motion of particles. Most thermal radiators lack directionality, resulting in energy loss in unnecessary directions, which reduces the efficiency of many thermal devices and applications. In practical applications, thermal radiators are usually required to exhibit different thermal radiation capabilities in different directions, therefore, controlling the directionality of the thermal emission is crucial in efficient heat transfer. The study of directional thermal radiation is of great significance in thermal imaging and sensing, radiative cooling, infrared encryption, and energy utilization. The review first describes the difference between traditional and directional thermal radiation, as well as the potential value of the latter at the frontiers of science. Subsequently, based on the characteristics of thermal radiation, it systematically organizes the research progress of domestic and foreign scholars in directional thermal radiation in terms of both narrowband directional thermal radiation and broadband directional thermal radiation. Finally, future research trends in this field are envisioned, and major challenges are analyzed, aiming to provide theoretical guidance and practical insights for the further development of directional thermal radiation technology.
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Overview: Thermal radiation is a fundamental physical process that refers to the spontaneous emission of electromagnetic energy from objects with temperatures above absolute zero due to the thermal motion of particles. Most thermal radiators lack directionality, resulting in energy loss in unnecessary directions, which reduces the efficiency of many thermal devices and applications. In practical applications, thermal radiators are usually required to exhibit different thermal radiation capabilities in different directions, therefore, controlling the directionality of the thermal emission is crucial in efficient heat transfer. The study of directional thermal radiation is of great significance in thermal imaging and sensing, radiative cooling, infrared encryption, and energy utilization. In this context, we classify directional thermal radiation modulation into two categories, broadband and narrowband, and summarize the relevant studies in recent years. The earliest proposed directional modulation was based on grating structures, which realized narrowband modulation. Based on this research, non-reciprocal narrowband directional modulation based on grating structure and narrowband directional modulation based on multilayer film structure were further developed. Due to the theoretical broadband nature of thermal radiation, it is of great significance to realize broadband directional thermal radiation. Broadband directional thermal radiation was first designed through gradient ENZ materials, based on which some researchers have proposed to realize non-reciprocal broadband directional thermal radiation using magnetic gradient ENZ materials. In addition, there are studies on broadband directional thermal radiation based on other materials. Finally, we summarize and analyze the possible future directions and main challenges.
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1. 引 言
辐射是通过电磁波传播能量，而热辐射则是物体由于其热能产生的电磁波辐射。当物体的温度高于绝对零度(0 K)时，其分子和原子的热运动会导致物体不断将热能转化为辐射能，并向外发射电磁波[1-3]。同时，物体也会吸收来自周围环境的热辐射，并将吸收的辐射能重新转化为热能。热能传递的基本方式包括热传导、热对流和热辐射。然而，与热传导和热对流不同的是，热辐射是通过电磁波传递能量，因此不需要介质的存在，并且在真空中其辐射效率最高[4-6]。这也使得热辐射在能源利用、隐身技术、生物医学应用以及红外成像等民用和军用领域中被广泛应用[7-10]。
目前，人们已经对热辐射进行了广泛研究。随着微纳光学的迅速发展，研究人员在法布里-珀罗腔、光栅、光子晶体和超表面等微纳结构的研究方面取得了显著成果[10-13]。这些微纳结构在热辐射领域得到了成功应用，不仅提高了器件的集成度，还为热辐射的光谱和空间调控提供了更多自由度和灵活性。精确设计和利用这些结构，对控制和操纵热辐射开辟了更广阔的前景。
传统的热辐射通常是全向的，在不需要的方向上，过剩的热辐射会导致能量浪费，并降低热辐射器件的效率。此外，由于实际环境温度的不均匀性，需要使用具有角度选择性的热辐射材料来匹配这种不均匀性[14]。例如，由于太阳热量增加，夏季的地面温度显著上升，因而，为提高能源效率，建筑墙体需要具有定向红外发射率的材料，能够同时向冷源辐射(如向天空具有高发射率)，并反射来自热环境的热量(如向地面具有低发射率)。综上所述，研究热辐射的方向性调控对于提高能源利用效率、改善热成像和传感器性能具有重要意义。本文分类总结了定向热辐射的最新研究进展，并对其未来可能的发展方向和主要挑战提出了见解。
2. 窄带定向热辐射控制
研究表明，某些平面光源在平面内的光谱相干长度可能远远超过波长，甚至在近场可以表现出准单色性[15-16]，这为构建具有窄角波瓣内辐射光的热源提供了可能。光栅结构能够在表面波与传播波之间产生耦合，从而提高热源的发射率和方向性。通过在极性材料上精心设计周期性微结构，可以构建红外天线，这些天线在加热时能够以较窄的立体角辐射红外光。
2.1 基于光栅微纳米结构的互易性定向热辐射控制
基于Wolf等人[17-18]以及Le Gall等人[19]的理论研究，Greffet等人[20]通过在SiC衬底上构建一维(1D)波长尺度光栅(6.25 μm间距)，首次报道了在p偏振下单一热波长(11.36 μm)的窄角发射(图1(a))。他们通过干预光栅与传播表面波的耦合，使局部空间的光源具有相干性，从而产生小角宽度的热辐射(图1(b))。他们验证了通过调整光栅的周期可以调控定向辐射的波长。Greffet等人的研究成果引发了该领域的持续关注与研究[21-23]。Park等人[21]制备了由金属表面上等间距同心圆形槽组成的热辐射器件(图1(c))。研究发现，这些槽可以作为表面等离激元的衍射光栅，在高达900 ℃的高温下产生高度定向的单色辐射源。他们还利用HfO2层防止金属结构的变形和氧化，从而提高了热稳定性。
Figure 1. Reciprocal directional thermal radiation design based on micro- and nano-structures. (a) First directional thermal radiation structure based on SiC grating; (b) Emissivity of SiC grating at p-polarization for different wavelengths and angles of incidence [20]; (c) Directional heat radiation devices consisting of equally spaced concentric circular grooves on a W or Mo metal surface [21]; (d) Directional and frequency-selective thermal emission using Au-SiN-Au metasurface [14]; (e) Thermal emission of phonons controlled by magnetic resonance modes based on Ag/SiO2/Al-based metasurface [22]; (f) Control of thermal emission via a metasurface of Al/SiN/Al nanosandwich photoabsorber using modulation of parity-symmetric Fano resonance [23]另一方面，随着对不同超表面的研究，MIM (金属/绝缘体/金属)或MIS (金属-绝缘体-半导体)等三明治结构因其出色的灵活性、高效性以及紧凑性等优点，被广泛运用于各种光学设计中。Costantini等人[14]制备了由Au/SiN/Au组成的超表面(图1(d))。该等离激元超表面可以在600 ℃下可靠工作，并且在较窄的频带和较窄的立体角内的发射率高于0.85。这些研究为开发紧凑、高效、廉价的红外光源和气体检测系统铺平了道路。Zhang等人[22]在硅片上制备的Ag/SiO2/Al(MIM)磁性超材料，通过超表面的磁共振模式控制声子的热发射角度(图1(e))。此外，他们还设计制备了基于Al/SiN/Si(MIS)的三明治结构[23]，通过调节结构参数实现了定向热辐射(图1(f))。他们利用超表面上的暗共振模式、明共振模式和表面晶格模式相互耦合，同时引入了Fano共振，从而实现了在不同结构参数下的定向热辐射。Costantini等人和Zhang等人的研究为各种热管理应用提供了一种新的热发射结构。
2.2 基于薄膜结构的互易性定向热辐射控制
尽管微纳米结构可以通过等离激元激发灵活地调控发射率，但以往的定向热辐射设计大多仅限于对p极化光的定向控制，而直接调控非极化光源相对困难。此外，等离激元超材料的制备通常需要高成本的高精度纳米加工技术。相比之下，多层薄膜结构也能通过膜层设计实现光谱调控。例如，光子晶体因其出色的光子操纵能力，显示出良好的辐射调控潜力。具有独特光-物质相互作用特性的二维材料，如石墨烯和过渡金属二卤化物，在实现角度调控方面也表现出巨大潜力。
在早期的研究中，De Zoysa等人[24]提出了将多量子阱(MQW)中的电子子带间跃迁与光子晶体共振效应相结合的方法，实现了热辐射的窄带调控(图2(a))。他们获得了峰值发射率为80%的窄带热辐射，并且主峰可以保持在±10°左右，减少了对其他角度的散射。结果表明，在相同输入功率和热管理条件下，器件的发射峰值强度比黑体样品高出4倍以上。与之相比，Liu等人[25]提出了更优化的设计方案，利用一维光子晶体、介质层和金属薄膜组成的结构，实现了光谱选择性、高定向性和易制备性等优点(图2(b))。他们设计的器件在特定波长处实现了窄带热辐射峰，并且将热发射的能量引导到特定的角度范围内。此外，该结构在s和p偏振下均可实现良好的定向热辐射。另外，Qu等人[26]通过在垂直空腔增强型谐振热发射器上引入角度选择器，实现了定向窄带热辐射器，只有角度选择器透明窗口内的热发射才能向外辐射能量，其他角度的热发射会被反射和抑制。谐振热发射器由7层硫化锌薄膜(低折射率)和硅薄膜(高折射率)组成，并覆盖在金薄膜上，如图2(d)所示。通过设计不同堆层数的角度选择器，可以实现不同性能的方向选择性热辐射，如图2(e)。
Figure 2. Reciprocal directional thermal radiation design based on multilayer membrane structure. (a) Small-angle directional thermal radiation modulation achieved by a MQW layer and a two-dimensional photonic crystal (2D PC) with a lattice constant of 6.5 mm [24]; (b) A directional thermal emitter consisting of a one-dimensional photonic crystal (1D PC) film, a dielectric spacer layer and a continuous metal film [25]; (c) Narrowband radiation directional modulation by separating an Au layer from a 2D material graphene by a dielectric spacer layer [27];(d) Directional thermal radiation modulation by photonic crystals designed with thermal emitters and angle selectors, respectively; (e) Demonstration of the emissivity of a directional narrow-band thermal emitter by the combination of an angle selector with 20 and 70 layers, respectively, and a thermal emitter [26]除了使用光子晶体外，二维材料也具有通过外加电场、机械应变或其他方式灵活调控其光学性质的潜力，在辐射调控中前景广阔。Zhu等人[27]成功将石墨烯与特定光子结构相结合，实现了角度选择性的完美光吸收(图2(c))。他们将石墨烯转移到以Au层作为镜面支撑的Ge23Sb7S70硫系化合物间隔层上，在波长约为13 μm的波段实现了77.6%的总吸收率。
2.3 非互易性定向窄带热辐射控制
已知黑体的定向行为遵循朗伯余弦定律，即黑体的发射通量与极角呈余弦关系。类似地，金属和掺杂半导体等体热发射体也在大范围的角度上辐射。这限制了依赖辐射热传递的装置(如热光电和红外光谱)或光源(如LED和白炽光源)的设计。其角度选择性取决于具体设计和材料。为了提高这些装置的效率和寿命，需要对热辐射方向进行优化。传统热辐射通常表现为双向性，即在表面法线两侧的角度范围内对称分布。如果打破互易性，可以单独控制吸收率和发射率，从而提高热光伏系统、中红外光源和红外光谱等领域的效率[28-32]。非互易热发射器在能量收集[33-34]和热管理方面具有重大前景。
为了满足这一要求，许多研究致力于设计具有高度不对称定向性的热源。例如，等离子体带隙结构、耦合共振空穴和多量子阱光子晶体热发射体等技术。其中，通过浅光栅上非定域表面模产生的高方向性远场热发射是一种很有前途的方法，因为只需改变表面上的空间周期，就可以调整空间角度依赖性。然而，这些发射在法向上是对称的。在中红外范围内，先前研究报道了利用置于SiC上方空气中的双槽Si光栅的倏逝耦合实现的非对称热发射。
Inampudi等人设计了SiC超薄超表面[35]，该超表面依靠两个反向传播的表面光学声子(SPhP)耦合到单一发射方向，如图3(b)所示。通过严格的耦合波分析优化，他们在设计波长为11.36 μm、发射角度为−45°处实现了一个窄的发射峰，其他波长下的热则散射到较宽的角度范围内(图3(a))。Zhao等人设计了基于Weyl半金属的宽角度、宽频带非互易热发射器[36]，具有多个衍射通道的超表面来扩大非互易效应的角度范围，并打破吸收率和发射率之间的对称性(图3(d))。通过耦合多个衍射通道，发射器的吸收率和发射率之间的对称关系被打破，并且非互易效应较强的角度范围也被显著拓宽。Yu等人制备了基于Au/Ge/ZnS/Au的超表面[37]，提出了一种在互易系统中实现角度非对称控制热辐射的通用策略(图3(e))。通过周期性扰动超表面并打破镜面对称性，他们实现了不同入射角下的吸收率不对称性。该结构利用金属-绝缘体-金属组成，周期性扰动导致导模的形成，从而使得各导模在对称输出角方向的辐射衰减率不同，实现了角度不对称的定向热辐射。
Figure 3. Design of non-reciprocal directional thermal radiation based on micro-nanostructures. (a) Schematic of single- and multi-channel non-reciprocal emitters; (b) Schematic of unidirectional nonreciprocal thermal emission from SiC grating; (c) Design optimization of SiC grating in the SPhP region with respect to wavelength and emission angle and emissivity [35]; (d) Schematic of a nonreciprocal multiport emitter and the corresponding angular distributions of emissivity and absorptivity [36]; (e) Schematic of a variable gate structure prepared from Au/Ge/ZnS/Au to realize an asymmetric directional thermal emission structure [37]3. 宽带定向热辐射控制
尽管过去的研究已经证明了在狭窄频带内实现热辐射的反常角度控制，但这些方法通常局限于有限的波长范围，且辐射方向会随波长变化。然而，热辐射本质上是宽带的。如果能将宽光谱热辐射限制在一组固定、狭窄的方向上，将显著增强其应用能力，这不仅可以在带宽方面提高高效辐射和热传导，还能将热传导限制在特定角度范围内。这种能力对于一系列传感和能源应用至关重要，包括热成像[38]、热光电[39-40]和辐射冷却[41-43]。在这些应用中，避免方向性噪声和寄生热增益或损失可以大幅提高设备效率。因此，需要将研究重点转向宽带定向热辐射调控技术，以满足在不同波长范围内的热辐射传输需求，这是迈向更广泛领域的必然选择。通过深入研究宽带调控技术，探索更加灵活和高效的热辐射控制手段，将为能源利用、热管理和光学应用等领域带来深远影响。
3.1 基于ENZ材料的互易性宽带定向热辐射控制
吸收和发射角度控制的另一种方法是使用具有epsilon(ε)-near-zero (ENZ)特性的材料亚波长薄膜。这些材料在材料共振极附近支持一种泄漏的p极化电磁模式，即Berreman模式。该模式可以在一定波矢范围内与传播的自由空间模式耦合，从而形成一定的入射角。如图4(a)所示，Johns等人尝试利用ENZ介质中的超薄膜干涉效应调控红外吸收和热发射[44]。他们以Au薄膜衬底上掺杂氧化镉(CdO)和SiO2的超薄ENZ层为模型系统，在1.8~1.9 μm波长范围内对发射率实现了角度控制。通过控制ENZ材料的掺杂浓度和膜厚，可以裁剪模式色散，从而实现对光谱范围(宽度0.1~1 μm)的精确定向控制和宽调谐。尽管这种角度响应引人注目，但其本质上仍是窄带的。
Figure 4. Realization of broadband directional thermal radiation modulation based on ENZ material. (a) ENZ material consisting of doped CdO and SiO2 with Au substrate realizes narrowband directional thermal radiation modulation [44]; (b) Broadband directional thermal radiation modulation was achieved in the 7-12 μm and 10-14 μm wavelengths by separately designed gradient ENZ materials [45]; (c) A dielectric gap (Ge) is used to connect and combine two gradient ENZ materials with different wavelengths to realize broadband directional thermal emission modulation covering the full long-wave infrared (LWIR) range [46]; (d) Gradient ENZ metamaterials made of high-temperature-resistant oxides MgO, BZHO, and NiO, demonstrating the temperature insensitivity of broadband directional thermal emission modulation [47]; (e) Comparative emissivity plots of this gradient ENZ metamaterials at different temperatures (25 ℃ and 1000 ℃) [47]为了实现宽带定向热发射，Xu等人首次将梯度ENZ材料的概念引入定向宽带辐射调控设计中[45]。这种材料的介电常数在一定频率范围内过零，而这些频率在空间梯度上各不相同。研究表明，梯度ENZ薄膜沿深度维度可支持宽谱泄漏模式，这些漏模以几乎相同的入射角度在该带宽上与自由空间传播模耦合，从而起到宽带定向热发射器的作用，其热光束的角度范围可由薄膜总厚度控制。Xu等人基于以上理论设计并制备了两个深亚波长光子结构，作为梯度ENZ薄膜概念的实验实现，每种结构都由三种极性氧化物材料组成(图4(b))。第一种结构在70°~85°的角度范围内，在7.7~11.5 μm的p偏振范围内显示了宽带热光束；第二种结构在60°~75°的角度范围内，在10~14.3 μm的范围内显示了宽带热光束。
在Xu等人的工作的启发下，许多基于梯度ENZ材料的宽带定向热辐射调控的工作被提出。Ying等人指出尽管宽带定向热发射器可以通过利用沉积在金属上的梯度ENZ材料实现的，但其工作波长范围未能覆盖整个8~14 μm大气窗口。于是在Xu等人的工作基础上，Ying等人设计并制备了一种包含整个长波红外(LWIR)的定向热辐射结构[46]。如图4(c)所示，其结构由金属表面的上ENZ薄膜(SiO2/SiO/Al2O3)、介电间隙(Ge)和下ENZ薄膜(TiO2/Ta2O5)组成，其中介质间隙(Ge)用于形成连接多层ENZ薄膜的法布里-珀罗(FP)腔。通过优化Ge膜层，他们实现了在整个LWIR波段内的高发射率定向控制(ε > 0.9)。此外，McSherry等人提出了一种由难熔氧化物(MgO、BZHO和NiO)制成的温度不敏感的宽带定向辐射调控的工作[47]。他们的耐火ENZ超材料由五层Mg1-xNixO层组成(图4(d))。图4(e)展示了他们所制备的结构分别在常温下(25 ℃)以及1000 ℃下的发射率，可以看到定向发射的光谱带宽在高温下仍然保持良好的定向宽带调节性能。
Hwang等人设计并实验验证了一种利用掺杂III–V半导体同时调整红外发射率的光谱峰值、带宽和方向性的方法[48]，如图5(a)所示。其通过在GaAs基底上生长具有不同掺杂浓度分布和厚度的基于InAs表征出梯度ENZ。研究表明，通过控制梯度ENZ结构的总厚度，可以独立调节其角度响应。此外，基于半导体梯度ENZ光子结构的光学响应是由自由载流子浓度驱动的，因此这些特性可以在宽带工作波长范围内动态控制，实现宽带非互易热发射控制。与之前的工作相比，Bae等人制备了一种透明的梯度ENZ材料宽带定向热辐射结构[49]，如图5(b)所示，旨在应用于安装在垂直表面上的散热器。该透明材料在可见光波段的透射率超过84%。其结构由ITO涂层玻璃上含有Si3N4和Al2O3的双层ENZ材料组成。这种创新设计可以有效地将城市区域内的滞留热量耗散到外部散热器，从而降低冷却能耗。
Figure 5. Realization of broadband directional thermal radiation modulation based on ENZ materials. (a) Gradient ENZ was characterized by growing InAs with different doping concentrations to achieve dynamic modulation of directional broadband thermal radiation [48]; (b) Transparent directional broadband thermal radiation structure based on Al2O3/Si3N4/ITO material designed for vertical surface radiative cooling [49]3.2 基于其他结构的互易性定向宽带热辐射控制
除了使用ENZ材料进行宽带互易性定向辐射调控外，其他结构也被广泛研究。Qu等人提出了一种基于经典热发射器和角度选择热发射器的设计方法，实现了定向宽带热发射，实现了定向宽带热发射器[26]，如图6(a)所示。他们利用Au/SiO2/Si3N4薄膜组成经典的宽带热发射器，并通过结合各向异性材料和各向同性材料的光子晶体，实现了宽带调控。这种方法不同于仅使用两种各向同性光子晶体(如BaF2和ZnS)的单一角度调控，扩展了其应用范围。为了减少上述设计原理中的膜层数量和厚度，Chamoli等人提出了一种新的设计，如图6(b)所示[50]。他们在选择性热发射器(Si3N4)上覆盖了角度依赖透射滤光片的薄膜堆栈，该结构包括Si3N4吸收层，以及由BaF2和LiF组成的高折射率镜面，用于实现角度依赖透射滤光。这一设计的总厚度约为16 μm，比先前的角度选择性薄膜薄了两个数量级，且层数减少了三个数量级。此外，该结构在s和p偏振光下均能工作，并能实现对小于50°的角度具有高辐射率，而对大于50°的角度具有显著低的辐射率。该设计在提供广泛的角度和光谱带宽控制的同时，简化了结构，降低了制造难度和成本。Ying等人提出了一种更加简单的多层膜结构，实现了覆盖两个大气窗口的定向热辐射调控，如图6(c)所示[51]。他们通过辐射基板实现超宽带定向热发射匹配大气传输窗口，四分之一波长电介质膜形成允许特定波长选择性反射或透射的FP谐振腔。此外，布鲁斯特效应产生特定角度的p偏振光的无反射透射窗口。通过对不同材料的研究，Ying等人最终将结构定为在柔性PEI基底上沉积750 nm的Ge膜层，实现了在特定方向(76°~84°)具有高p极化发射率(> 0.9)，如图6(d)所示。
图 6. (a) 通过设计光子晶体分别作为热发射器和角度选择器，实现定向宽带热辐射调控[26]；(b) 由Si3N4热发射器和由BaF2及LiF组成的顶部透射滤光器构成的结构，实现定向宽带热辐射[50]；(c) 与大气窗口匹配的超宽带定向热辐射结构示意图，系统由辐射基底上的薄介质膜组成；(d) 由柔性PEI基底和Ge膜层组成的结构，实现特定方向的宽带热辐射调控[51]；(e) 由交替的三相超材料组成的结构，包括椭球形核壳结构阵列和由PMMA覆盖ITO共形涂层的圆柱体阵列，用于实现超宽带定向热辐射[52]；(f) 基于SU-8光刻胶基底、聚合物结构和银反射层组成的PDME结构，实现高指向性和宽带特性的热辐射调控[53]Figure 6. (a) Directional broadband thermal radiation modulation by designing photonic crystals as thermal emitter and angle selector, respectively [26]; (b) Structure consisting of a Si3N4 thermal emitter and a top transmission filter composed of BaF2 and LiF for directional broadband thermal radiation [50]; (c) Schematic of an ultra-broadband directional thermal radiation structure matched to an atmospheric window, with the system consisting of a thin dielectric film on a radiating substrate; (d) Structure consisting of a flexible PEI substrate and a Ge film layer for direction-specific broadband thermal radiation modulation [51]; (e) Structure consisting of alternating three-phase metamaterials, including an array of ellipsoidal core-shell structures and a cylindrical array of PMMA covered with an ITO conformal coating, for realizing ultra-broadband directional thermal radiation [52]; (f) Thermal radiation modulation for high directivity and broadband characteristics based on a PDME structure consisting of SU-8 photoresist substrate, polymer structure, and silver reflective layer [53]不同于上述常用的多层膜结构对热辐射进行调控，Wang等人基于有效介质理论建立了一种不受材料限制实现超宽带定向热发射器的普适方法[52]，基于该方法分别设计了多个交替式三相超宽带定向热发射器的结构用于实现超宽带定向热辐射调控(图6(e))。他们使用ITO、PMMA、空气以及作为完美电导体Ag分别设计了椭球形核壳结构阵列和亚波长介质柱阵列。这两种结构都是以PMMA为芯，外面覆盖一层ITO保形涂层，而其余部分由空气填充。全波仿真结果表明，这些结构在5~30 μm光谱范围和76°~89°方向上具有高发射率(> 0.8)。另外，在最新的研究中，Fan等学者[53]创新性地开发了一种PDME结构，该结构融合了SU-8光刻胶基底、聚合物组件与银质反射层，实现了在5~20 μm宽广波段内对定向热辐射的精确调控，展现出高指向性和显著的发射率动态范围。具体而言，如图6(f)所呈现，当角度在0~15°区间时，该结构展现出平均0.8的高发射率；而当角度超过15°后，其平均发射率骤降至0.2以下，彰显出卓越的角度依赖性能。
3.3 非互易性定向宽带热辐射控制
实现非互易发射和吸收是改进一系列能源技术的基本途径，如太阳能收集[54-56]、热光伏[57]、外层辐射能量收集以及同时收集太阳能和外层空间能量[58]。为了在这些应用中达到最终的热力学极限，非互易发射和吸收需要在宽带上进行定向调控。磁化纳米结构中的非互易吸收在非互易热光学领域获得了越来越多的关注[59-63]，有望在热辐射调控中实现若干革命性技术，如非互易能量收集装置[64]、非互易热光伏电池[65]和非互易辐射热管理和通信。然而，由于热辐射的宽带特性，如何在这些磁光(MO)器件中实现宽带非互易也非常重要。
为了实现非互易的定向宽带热辐射调控，Liu等人设计并制备了一种磁化梯度ENZ材料[66]。如图7(a)所示，该结构由梯度掺杂的InAs薄膜组成，掺杂浓度从底部到顶部逐渐降低，确保在宽波段范围内实现梯度ENZ特性。在适度的外部磁场作用下，该结构实现了宽带非互易吸收。通过设计具有不同掺杂浓度的磁化InAs薄膜，可以灵活控制工作频率和非互易带宽。Liu等人所提出的概念和机制在其他材料和结构中的潜在应用，如III-V族半导体、磁化金属和拓扑Wely半金属。Zhang等人介绍了一种梯度ε近零(ENZ)磁光超材料，实现了宽带非互易热发射[67]。他们首先分析了基于ε-近零磁光材料薄层在不同外部磁场中的非互易热吸收率(图7(b))。为了突破磁场对这种磁性材料实现非互易热辐射调控的限制，他们尝试使用梯度掺杂半导体多层和磁性Weyl半金属直接实现非互易热辐射定向调控(图7(c))。他们证明了磁性Weyl半金属在不使用任何磁场的情况下具有支持宽带非互易发射的潜力。此外，Shi等人提出了一种无需外加磁场的基于多层Weyl半金属的超宽带宽角非互易热发射器[68]。这种多层结构由交替排列的Weyl半金属(WSM)和电介质膜组成，其中WSM1和WSM2是两个具有不同费米能级的WSM层(图7(d))。他们通过ENZ诱导的布鲁斯特模和两种不同费米能级的WSM的本征非互易性，解释了所构建的鲁棒的超宽带非对称辐射系统。
Figure 7. Directed broadband non-reciprocal thermal radiation modulation. (a) Gradient magnetized ENZ multilayers composed of InAs films with different doping concentrations are utilized for non-reciprocal thermal radiation modulation [66]; (b) Variation of emissivity and absorptivity of nonreciprocal emitters of single-layer magneto-optical materials in the presence of different external magnetic fields; (c) A structure consisting of a magnetic Wely metal (MWS) layer with a chemical potential gradient above Au enables directional broadband non-reciprocal thermal emission in the absence of an external magnetic field [67]; (d) A structure consisting of two WSM layers with different Fermi energy levels and a dielectric film placed alternately on the Au reflector, which enables directional broadband nonreciprocal thermal emission in the absence of an external magnetic field [68]4. 总结与展望
当前研究定向热辐射主要集中在宽带定向调控方面。由于热辐射的宽带性质，这种技术在辐射冷却、红外伪装、热成像和传感等领域具有更高的效率应用前景。特别是实现非互易性的宽带热发射调控，可以提高热光伏系统、中红外光源和红外光谱等领域的性能和效率。尽管已经提出了许多解决方案，但在定向宽带热辐射调控的广泛应用方面，还有很多问题亟待解决：
1)发射率定向调控的性能提升。当前很多研究的平均发射率集中在0.6~0.8之间，且对角度的调控尚不够精细。更高的发射率意味着更好的性能，能够更高效地实现辐射散热、红外伪装等功能。高品质因子的定向热发射器对于红外传感器和探测应用至关重要，因此提高热发射器的Q因子可以更好地满足商业需求。
2)材料和结构的大规模制备。很多定向热辐射调控结构依赖于微纳加工技术，这限制了其大规模、低成本应用。因此，需要寻找更加简单有效的制备方法，以降低成本并实现大规模生产。此外，多层膜结构在提高性能方面可能会增加制备的复杂性，因此需要在性能和成本之间寻找平衡。
3)材料和结构的应用性。热辐射调控的应用环境复杂多样，因此还需考虑材料的耐腐蚀、耐高温、导热性、柔性等应用性问题。此外，电控制是实现高热辐射调制速度的关键方法。未来，结合超快激光技术的定向热辐射器件设计可能会将角度可调热发射的调制速度提升到新的水平，推动各种动态热开关器件的发展。
4)实现动态调控。实际应用中，环境条件的变化可能影响热辐射调控的效率。为了提高结构的适应性，需要实现定向热辐射材料的动态调控，使其可以在不同的波段、角度下实现热辐射调控。可以开发基于石墨烯、碳纳米管等电致变发射率材料或者氧化钒等热致变发射率材料，实现动态可调结构的发射率和角度调节，以适应不同的应用场景。
总而言之，随着定向热辐射技术的研究深入和实验技术的成熟，在能源利用、热成像、红外探测和辐射冷却等领域将会有更广泛的应用和发展。因此，未来的研究方向应重点解决上述问题，推动定向热辐射技术在实际应用中的广泛应用和进一步发展。
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图 1 基于微纳结构的互易性定向热辐射设计。(a) 基于SiC光栅的首个定向热辐射结构；(b) SiC光栅在p偏振下不同波长和入射角度的发射率[20]；(c) 由W或Mo金属表面上等间距同心圆形槽组成的定向热辐射器件[21]；(d) 利用Au/SiN/Au超表面结构实现定向和频率选择性热发射[14]；(e) 基于Ag/SiO2/Al的超表面，通过磁共振模式控制声子的热发射[22]；(f) 由Al/SiN/Al纳米三明治光吸收体的超表面，利用对奇偶对称Fano共振的调控实现热发射的控制[23]
Figure 1. Reciprocal directional thermal radiation design based on micro- and nano-structures. (a) First directional thermal radiation structure based on SiC grating; (b) Emissivity of SiC grating at p-polarization for different wavelengths and angles of incidence [20]; (c) Directional heat radiation devices consisting of equally spaced concentric circular grooves on a W or Mo metal surface [21]; (d) Directional and frequency-selective thermal emission using Au-SiN-Au metasurface [14]; (e) Thermal emission of phonons controlled by magnetic resonance modes based on Ag/SiO2/Al-based metasurface [22]; (f) Control of thermal emission via a metasurface of Al/SiN/Al nanosandwich photoabsorber using modulation of parity-symmetric Fano resonance [23]
图 2 基于多层膜结构的互易性定向热辐射设计。(a) 由MQW层和晶格常数为6.5 mm的二维光子晶体(2D PC)实现小角度的定向热辐射调控[24]；(b) 由一维光子晶体(1D PC)薄膜、介质间隔层和连续金属薄膜组成的定向热发射器[25]；(c) 由介质间隔层分离Au层与二维材料石墨烯，实现窄带辐射定向调控[27]；(d) 通过光子晶体分别设计的热发射器和角度选择器，实现定向热辐射调控；(e)由分别具有20层和70层的角度选择器与热发射器组合，展示定向窄带热发射器的发射率[26]
Figure 2. Reciprocal directional thermal radiation design based on multilayer membrane structure. (a) Small-angle directional thermal radiation modulation achieved by a MQW layer and a two-dimensional photonic crystal (2D PC) with a lattice constant of 6.5 mm [24]; (b) A directional thermal emitter consisting of a one-dimensional photonic crystal (1D PC) film, a dielectric spacer layer and a continuous metal film [25]; (c) Narrowband radiation directional modulation by separating an Au layer from a 2D material graphene by a dielectric spacer layer [27];(d) Directional thermal radiation modulation by photonic crystals designed with thermal emitters and angle selectors, respectively; (e) Demonstration of the emissivity of a directional narrow-band thermal emitter by the combination of an angle selector with 20 and 70 layers, respectively, and a thermal emitter [26]
图 3 基于微纳结构的非互易定向热辐射设计。(a) 单通道和多通道非互易发射器示意图；(b) SiC光栅单向非互易热发射示意图；(c) 设计优化SiC光栅在SPhP 区域中波长和发射角和发射率的关系[35]；(d) 非互易多端口发射器示意图以及对应的发射率和吸收率的角分布[36]；(e) 由Au/Ge/ZnS/Au制备的变栅结构实现非对称定向热发射结构示意图[37]
Figure 3. Design of non-reciprocal directional thermal radiation based on micro-nanostructures. (a) Schematic of single- and multi-channel non-reciprocal emitters; (b) Schematic of unidirectional nonreciprocal thermal emission from SiC grating; (c) Design optimization of SiC grating in the SPhP region with respect to wavelength and emission angle and emissivity [35]; (d) Schematic of a nonreciprocal multiport emitter and the corresponding angular distributions of emissivity and absorptivity [36]; (e) Schematic of a variable gate structure prepared from Au/Ge/ZnS/Au to realize an asymmetric directional thermal emission structure [37]
图 4 基于ENZ材料实现宽带定向热辐射调控。(a) 由掺杂CdO和SiO2构成的ENZ材料与Au基底结构，实现了窄带定向热辐射调控[44]；(b) 分别设计的梯度ENZ材料在7~12 μm和10~14 μm波段内实现了宽带定向热辐射调控[45]；(c) 通过介质间隙(Ge)来连接组合两个不同波段实现宽带调控的梯度ENZ材料，覆盖全长波红外(LWIR)范围，实现宽带定向热发射[46]；(d) 由耐高温氧化物MgO、BZHO和NiO制成的梯度ENZ超材料，展示了宽带定向热辐射调控的温度不敏感性[47]；(e) 不同温度下(25 ℃和1000 ℃)该梯度ENZ超材料的发射率对比图[47]
Figure 4. Realization of broadband directional thermal radiation modulation based on ENZ material. (a) ENZ material consisting of doped CdO and SiO2 with Au substrate realizes narrowband directional thermal radiation modulation [44]; (b) Broadband directional thermal radiation modulation was achieved in the 7-12 μm and 10-14 μm wavelengths by separately designed gradient ENZ materials [45]; (c) A dielectric gap (Ge) is used to connect and combine two gradient ENZ materials with different wavelengths to realize broadband directional thermal emission modulation covering the full long-wave infrared (LWIR) range [46]; (d) Gradient ENZ metamaterials made of high-temperature-resistant oxides MgO, BZHO, and NiO, demonstrating the temperature insensitivity of broadband directional thermal emission modulation [47]; (e) Comparative emissivity plots of this gradient ENZ metamaterials at different temperatures (25 ℃ and 1000 ℃) [47]
图 5 基于ENZ材料实现宽带定向热辐射调控。(a) 通过生长不同掺杂浓度的InAs表征出梯度ENZ，实现定向宽带热辐射的动态调控[48]；(b) 基于Al2O3/Si3N4/ITO材料设计了用于垂直表面辐射冷却的透明定向宽带热辐射结构[49]
Figure 5. Realization of broadband directional thermal radiation modulation based on ENZ materials. (a) Gradient ENZ was characterized by growing InAs with different doping concentrations to achieve dynamic modulation of directional broadband thermal radiation [48]; (b) Transparent directional broadband thermal radiation structure based on Al2O3/Si3N4/ITO material designed for vertical surface radiative cooling [49]
图 6 (a) 通过设计光子晶体分别作为热发射器和角度选择器，实现定向宽带热辐射调控[26]；(b) 由Si3N4热发射器和由BaF2及LiF组成的顶部透射滤光器构成的结构，实现定向宽带热辐射[50]；(c) 与大气窗口匹配的超宽带定向热辐射结构示意图，系统由辐射基底上的薄介质膜组成；(d) 由柔性PEI基底和Ge膜层组成的结构，实现特定方向的宽带热辐射调控[51]；(e) 由交替的三相超材料组成的结构，包括椭球形核壳结构阵列和由PMMA覆盖ITO共形涂层的圆柱体阵列，用于实现超宽带定向热辐射[52]；(f) 基于SU-8光刻胶基底、聚合物结构和银反射层组成的PDME结构，实现高指向性和宽带特性的热辐射调控[53]
Figure 6. (a) Directional broadband thermal radiation modulation by designing photonic crystals as thermal emitter and angle selector, respectively [26]; (b) Structure consisting of a Si3N4 thermal emitter and a top transmission filter composed of BaF2 and LiF for directional broadband thermal radiation [50]; (c) Schematic of an ultra-broadband directional thermal radiation structure matched to an atmospheric window, with the system consisting of a thin dielectric film on a radiating substrate; (d) Structure consisting of a flexible PEI substrate and a Ge film layer for direction-specific broadband thermal radiation modulation [51]; (e) Structure consisting of alternating three-phase metamaterials, including an array of ellipsoidal core-shell structures and a cylindrical array of PMMA covered with an ITO conformal coating, for realizing ultra-broadband directional thermal radiation [52]; (f) Thermal radiation modulation for high directivity and broadband characteristics based on a PDME structure consisting of SU-8 photoresist substrate, polymer structure, and silver reflective layer [53]
图 7 定向宽带非互易热辐射调控。(a) 利用不同掺杂浓度的InAs薄膜组成梯度磁化ENZ多层膜，实现非互易热辐射调控[66]；(b) 单层磁光材料非互易发射器在不同外部磁场作用下的发射率和吸收率的变化；(c) 由Au上方化学势梯度的磁性Wely金属(MWS)层构成的结构，在无外部磁场的作用下可实现定向宽带非互易性热发射[67]；(d)由两个具有不同费米能级的WSM层和交替放置在Au反射镜上的介质薄膜组成，可在无外部磁场的作用下实现定向宽带非互易性热发射[68]
Figure 7. Directed broadband non-reciprocal thermal radiation modulation. (a) Gradient magnetized ENZ multilayers composed of InAs films with different doping concentrations are utilized for non-reciprocal thermal radiation modulation [66]; (b) Variation of emissivity and absorptivity of nonreciprocal emitters of single-layer magneto-optical materials in the presence of different external magnetic fields; (c) A structure consisting of a magnetic Wely metal (MWS) layer with a chemical potential gradient above Au enables directional broadband non-reciprocal thermal emission in the absence of an external magnetic field [67]; (d) A structure consisting of two WSM layers with different Fermi energy levels and a dielectric film placed alternately on the Au reflector, which enables directional broadband nonreciprocal thermal emission in the absence of an external magnetic field [68]
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[1] Inoue T, de Zoysa M, Asano T, et al. Realization of dynamic thermal emission control[J]. Nat Mater, 2014, 13(10): 928−931. doi: 10.1038/nmat4043
[2] Coppens Z J, Valentine J G. Spatial and temporal modulation of thermal emission[J]. Adv Mater, 2017, 29(39): 1701275. doi: 10.1002/adma.201701275
[3] Beenakker C W J. Thermal radiation and amplified spontaneous emission from a random medium[J]. Phys Rev Lett, 1998, 81(9): 1829−1832. doi: 10.1103/PhysRevLett.81.1829
[4] Mason J A, Smith S, Wasserman D. Strong absorption and selective thermal emission from a midinfrared metamaterial[J]. Appl Phys Lett, 2011, 98(24): 241105. doi: 10.1063/1.3600779
[5] Jung D, Bank S, Lee M L, et al. Next-generation mid-infrared sources[J]. J Opt, 2017, 19(12): 123001. doi: 10.1088/2040-8986/aa939b
[6] Palchetti L, Di Natale G, Bianchini G. Remote sensing of cirrus cloud microphysical properties using spectral measurements over the full range of their thermal emission[J]. J Geophys Res Atmos, 2016, 121(18): 10804−10819. doi: 10.1002/2016JD025162
[7] Fu Z, Zhong D, et al. Scalable asymmetric fabric evaporator for solar desalination and thermoelectricity generation[J]. Adv Sci, 2024, 20: e2406474. doi: 10.1002/advs.202406474
[8] Zhang R, Song Z, et al. Multispectral smart window: Dynamic light modulation and electromagnetic microwave shielding[J]. Light Sci Appl, 2024, 13: 223. doi: 10.1038/s41377-024-01541-y
[9] Xu G, Kang Q, et al. Inverse-design laser-infrared compatible stealth with thermal management enabled by wavelength-selective thermal emitter[J]. Appl Therm Eng, 2024, 255: 124063. doi: 10.1016/j.applthermaleng.2024.124063
[10] Xu Z Q, Luo H, Zhu H Z, et al. Nonvolatile optically reconfigurable radiative metasurface with visible tunability for anticounterfeiting[J]. Nano Lett, 2021, 21(12): 5269−5276. doi: 10.1021/acs.nanolett.1c01396
[11] Pralle M U, Moelders N, McNeal M P, et al. Photonic crystal enhanced narrow-band infrared emitters[J]. Appl Phys Lett, 2002, 81(25): 4685−4687. doi: 10.1063/1.1526919
[12] Liu X L, Tyler T, Starr T, et al. Taming the blackbody with infrared metamaterials as selective thermal emitters[J]. Phys Rev Lett, 2011, 107(4): 045901. doi: 10.1103/PhysRevLett.107.045901
[13] Morsy A M, Barako M T, Jankovic V, et al. Experimental demonstration of dynamic thermal regulation using vanadium dioxide thin films[J]. Sci Rep, 2020, 10(1): 13964. doi: 10.1038/s41598-020-70931-0
[14] Costantini D, Lefebvre A, Coutrot A L, et al. Plasmonic metasurface for directional and frequency-selective thermal emission[J]. Phys Rev Appl, 2015, 4(1): 014023. doi: 10.1103/PhysRevApplied.4.014023
[15] Shchegrov A V, Joulain K, Carminati R, et al. Near-field spectral effects due to electromagnetic surface excitations[J]. Phys Rev Lett, 2000, 85(7): 1548−1551. doi: 10.1103/PhysRevLett.85.1548
[16] Carminati R, Greffet J J. Near-field effects in spatial coherence of thermal sources[J]. Phys Rev Lett, 1999, 82(8): 1660−1663. doi: 10.1103/PhysRevLett.82.1660
[17] Wolf E. Non-cosmological redshifts of spectral lines[J]. Nature, 1987, 326(6111): 363−365. doi: 10.1038/326363a0
[18] Wolf E, James D F V. Correlation-induced spectral changes[J]. Rep Prog Phys, 1996, 59(6): 771−818. doi: 10.1088/0034-4885/59/6/002
[19] Le Gall J, Olivier M, Greffet J J. Experimental and theoretical study of reflection and coherent thermal emissionby a SiC grating supporting a surface-phonon polariton[J]. Phys Rev B, 1997, 55(15): 10105−10114. doi: 10.1103/PhysRevB.55.10105
[20] Greffet J J, Carminati R, Joulain K, et al. Coherent emission of light by thermal sources[J]. Nature, 2002, 416(6876): 61−64. doi: 10.1038/416061a
[21] Park J H, Han S E, Nagpal P, et al. Observation of thermal beaming from tungsten and molybdenum bull’s eyes[J]. ACS Photon, 2016, 3(3): 494−500. doi: 10.1021/acsphotonics.6b00022
[22] Zhang X, Liu H, Zhang Z G, et al. Controlling thermal emission of phonon by magnetic metasurfaces[J]. Sci Rep, 2017, 7: 41858. doi: 10.1038/srep41858
[23] Zhang X, Zhang Z G, Wang Q, et al. Controlling thermal emission by parity-symmetric fano resonance of optical absorbers in metasurfaces[J]. ACS Photon, 2019, 6(11): 2671−2676. doi: 10.1021/acsphotonics.9b00024
[24] De Zoysa M, Asano T, Mochizuki K, et al. Conversion of broadband to narrowband thermal emission through energy recycling[J]. Nat Photon, 2012, 6(8): 535−539. doi: 10.1038/nphoton.2012.146
[25] Liu X X, Li Z W, Wen Z J, et al. Large-area, lithography-free, narrow-band and highly directional thermal emitter[J]. Nanoscale, 2019, 11(42): 19742−19750. doi: 10.1039/C9NR06181A
[26] Qu Y R, Pan M Y, Qiu M. Directional and spectral control of thermal emission and its application in radiative cooling and infrared light sources[J]. Phys Rev Appl, 2020, 13(6): 064052. doi: 10.1103/PhysRevApplied.13.064052
[27] Zhu L X, Liu F Y, Lin H T, et al. Angle-selective perfect absorption with two-dimensional materials[J]. Light Sci Appl, 2016, 5(3): e16052. doi: 10.1038/lsa.2016.52
[28] Miller D A B, Zhu L X, Fan S H. Universal modal radiation laws for all thermal emitters[J]. Proc Natl Acad Sci USA, 2017, 114(17): 4336−4341. doi: 10.1073/pnas.1701606114
[29] Zhu L X, Fan S H. Near-complete violation of detailed balance in thermal radiation[J]. Phys Rev B, 2014, 90(22): 220301(R). doi: 10.1103/PhysRevB.90.220301
[30] Zhao B, Shi Y, Wang J H, et al. Near-complete violation of Kirchhoff’s law of thermal radiation with a 0.3 T magnetic field[J]. Opt Lett, 2019, 44(17): 4203−4206. doi: 10.1364/OL.44.004203
[31] Zhao B, Guo C, Garcia C A C, et al. Axion-field-enabled nonreciprocal thermal radiation in Weyl semimetals[J]. Nano Lett, 2020, 20(3): 1923−1927. doi: 10.1021/acs.nanolett.9b05179
[32] Tsurimaki Y, Qian X, Pajovic S, et al. Large nonreciprocal absorption and emission of radiation in type-I Weyl semimetals with time reversal symmetry breaking[J]. Phys Rev B, 2020, 101(16): 165426. doi: 10.1103/PhysRevB.101.165426
[33] Landsberg P T, Tonge G. Thermodynamic energy conversion efficiencies[J]. J Appl Phys, 1980, 51(7): R1−R20. doi: 10.1063/1.328187
[34] Green M A. Time-asymmetric photovoltaics[J]. Nano Lett, 2012, 12(11): 5985−5988. doi: 10.1021/nl3034784
[35] Inampudi S, Cheng J R, Salary M M, et al. Unidirectional thermal radiation from a SiC metasurface[J]. J Opt Soc Am B, 2018, 35(1): 39−46. doi: 10.1364/JOSAB.35.000039
[36] Zhao B, Wang J H, Zhao Z X, et al. Nonreciprocal thermal emitters using metasurfaces with multiple diffraction channels[J]. Phys Rev Appl, 2021, 16(6): 064001. doi: 10.1103/PhysRevApplied.16.064001
[37] Yu J B, Qin R, Ying Y B, et al. Asymmetric directional control of thermal emission[J]. Adv Mater, 2023, 35(45): 2302478. doi: 10.1002/adma.202302478
[38] Lucchi E. Applications of the infrared thermography in the energy audit of buildings: a review[J]. Renew Sustain Energy Rev, 2018, 82: 3077−3090. doi: 10.1016/j.rser.2017.10.031
[39] Sakr E, Bermel P. Thermophotovoltaics with spectral and angular selective doped-oxide thermal emitters[J]. Opt Express, 2017, 25(20): A880−A895. doi: 10.1364/OE.25.00A880
[40] Sakakibara R, Stelmakh V, Chan W R, et al. Practical emitters for thermophotovoltaics: a review[J]. J Photon Energy, 2019, 9(3): 32713. doi: 10.1117/1.JPE.9.032713
[41] Raman A P, Li W, Fan S H. Generating light from darkness[J]. Joule, 2019, 3(11): 2679−2686. doi: 10.1016/j.joule.2019.08.009
[42] Raman A P, Anoma M A, Zhu L X, et al. Passive radiative cooling below ambient air temperature under direct sunlight[J]. Nature, 2014, 515(7528): 540−544. doi: 10.1038/nature13883
[43] Yin X B, Yang R G, Tan G, et al. Terrestrial radiative cooling: using the cold universe as a renewable and sustainable energy source[J]. Science, 2020, 370(6518): 786−791. doi: 10.1126/science.abb0971
[44] Johns B, Chattopadhyay S, Mitra J. Tailoring infrared absorption and thermal emission with ultrathin film interferences in Epsilon-Near-Zero media[J]. Adv Photon Res, 2022, 3(1): 2100153. doi: 10.1002/adpr.202100153
[45] Xu J, Mandal J, Raman A P. Broadband directional control of thermal emission[J]. Science, 2021, 372(6540): 393−397. doi: 10.1126/science.abc5381
[46] Ying Y B, Ma B Z, Yu J B, et al. Whole LWIR directional thermal emission based on ENZ thin films[J]. Laser Photon Rev, 2022, 16(8): 2200018. doi: 10.1002/lpor.202200018
[47] McSherry S, Lenert A. Design of a gradient epsilon-near-zero refractory metamaterial with temperature-insensitive broadband directional emission[J]. Appl Phys Lett, 2022, 121(19): 191702. doi: 10.1063/5.0122535
[48] Hwang J S, Xu J, Raman A P. Simultaneous control of spectral and directional emissivity with gradient Epsilon-Near-Zero InAs photonic structures[J]. Adv Mater, 2023, 35(39): 2302956. doi: 10.1002/adma.202302956
[49] Bae M, Kim D H, Kim S K, et al. Transparent energy-saving windows based on broadband directional thermal emission[J]. Nanophotonics, 2024, 13(5): 749−761. doi: 10.1515/nanoph-2023-0580
[50] Chamoli S K, Li W, Guo C L, et al. Angularly selective thermal emitters for deep subfreezing daytime radiative cooling[J]. Nanophotonics, 2022, 11(16): 3709−3717. doi: 10.1515/nanoph-2022-0032
[51] Ying Y B, Yu J B, Qin B, et al. Directional thermal emission covering two atmospheric windows[J]. Laser Photon Rev, 2023, 17(11): 2300407. doi: 10.1002/lpor.202300407
[52] Sarkar M, Giteau M, Enders M T, et al. Lithography-free directional control of thermal emission[J]. Nanophotonics, 2024, 13(5): 763−771. doi: 10.1515/nanoph-2023-0595
[53] Wang Q Y, Liu T J, Li L N, et al. Ultra-broadband directional thermal emission[J]. Nanophotonics, 2024, 13(5): 793−801. doi: 10.1515/nanoph-2023-0742
[54] Fan Z W, Hwang T, Lin S, et al. Directional thermal emission and display using pixelated non-imaging micro-optics[J]. Nat Commun, 2024, 15(1): 4544. doi: 10.1038/s41467-024-48826-9
[55] Brongersma M L, Cui Y, Fan S H. Light management for photovoltaics using high-index nanostructures[J]. Nat Mater, 2014, 13(5): 451−460. doi: 10.1038/nmat3921
[56] Park Y, Zhao B, Fan S H. Reaching the ultimate efficiency of solar energy harvesting with a nonreciprocal multijunction solar cell[J]. Nano Lett, 2022, 22(1): 448−452. doi: 10.1021/acs.nanolett.1c04288
[57] Buddhiraju S, Santhanam P, Fan S H. Thermodynamic limits of energy harvesting from outgoing thermal radiation[J]. Proc Natl Acad Sci USA, 2018, 115(6): E3609−E3615. doi: 10.1073/pnas.1717595115
[58] Li W, Buddhiraju S, Fan S H. Thermodynamic limits for simultaneous energy harvesting from the hot sun and cold outer space[J]. Light Sci Appl, 2020, 9: 68. doi: 10.1038/s41377-020-0296-x
[59] Zhang Z N, Zhu L X. Nonreciprocal thermal photonics for energy conversion and radiative heat transfer[J]. Phys Rev Appl, 2022, 18(2): 027001. doi: 10.1103/PhysRevApplied.18.027001
[60] Pajovic S, Tsurimaki Y, Qian X, et al. Intrinsic nonreciprocal reflection and violation of Kirchhoff’s law of radiation in planar type-I magnetic Weyl semimetal surfaces[J]. Phys Rev B, 2020, 102(16): 165417. doi: 10.1103/PhysRevB.102.165417
[61] Park Y, Asadchy V S, Zhao B, et al. Violating Kirchhoff’s law of thermal radiation in semitransparent structures[J]. ACS Photon, 2021, 8(8): 2417−2424. doi: 10.1021/acsphotonics.1c00612
[62] Hadad Y, Soric J C, Alu A. Breaking temporal symmetries for emission and absorption[J]. Proc Natl Acad Sci USA, 2016, 113(13): 3471−3475. doi: 10.1073/pnas.1517363113
[63] Liu M Q, Zhao C Y. Near-infrared nonreciprocal thermal emitters induced by asymmetric embedded eigenstates[J]. Int J Heat Mass Transf, 2022, 186: 122435. doi: 10.1016/j.ijheatmasstransfer.2021.122435
[64] Ghalekohneh S J, Zhao B. Nonreciprocal solar thermophotovoltaics[J]. Phys Rev Appl, 2022, 18(3): 034083. doi: 10.1103/PhysRevApplied.18.034083
[65] Park Y, Omair Z, Fan S H. Nonreciprocal thermophotovoltaic systems[J]. ACS Photon, 2022, 9(12): 3943−3949. doi: 10.1021/acsphotonics.2c01308
[66] Liu M Q, Xia S, Wan W J, et al. Broadband mid-infrared non-reciprocal absorption using magnetized gradient epsilon-near-zero thin films[J]. Nat Mater, 2023, 22(10): 1196−1202. doi: 10.1038/s41563-023-01635-9
[67] Zhang Z N, Zhu L X. Broadband nonreciprocal thermal emission[J]. Phys Rev Appl, 2023, 19(1): 014013. doi: 10.1103/PhysRevApplied.19.014013
[68] Shi K Z, Sun Y W, Hu R, et al. Ultra-broadband and wide-angle nonreciprocal thermal emitter based on Weyl semimetal metamaterials[J]. Nanophotonics, 2024, 13(5): 737−747 doi: 10.1515/nanoph-2023-0520
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