基于硫属化物相变材料的可重构太赫兹超表面器件的研究进展
1 引言
太赫兹波,频谱在0.1~10 THz范围,位于微波和红外波段之间。由于其具有的独特性质,包括非电离性、穿透性、独特的光谱指纹性以及高的可用带宽,太赫兹波在光谱、传感、成像、安检和高速通信等领域有着大量的应用潜力[1-5]。这些应用的发展和技术的进步,不仅需要高效的太赫兹波源及探测器,也需要高质量的太赫兹波功能器件,包括用于调制太赫兹波振幅、相位和偏振的波片、分束器、透镜和吸收器等。然而目前在实验室使用的传统光学器件体积庞大且种类匮乏,无法满足光学系统集成化和多功能的发展需求[6-7]。
超表面,是由亚波长人工微结构组成的超薄平面,克服了超材料复杂的设计和大体积的缺点,一经提出便受到了广泛的关注。超表面在控制电磁波的透射/反射振幅[8-9]、相位[10-11]、偏振[12-14]以及实现复杂的光场分布[14-19]等方面表现出了杰出的能力。亚波长的厚度使其适用于发展集成化器件。目前,大量的太赫兹波超表面器件已经被提出,包括平面超透镜[20-21]、异常偏折器件[22-23]、全息板[24]等。然而尽管这些器件在操纵太赫兹波方面发挥了巨大的作用,但他们的性质是静态的,光学性能难以改变,这限制了它们的进一步应用。因此有必要发展可调谐的动态超表面器件来实现对电磁波的主动操纵。近几年,各种各样的主动调控方式已经被提出,包括利用相变材料VO2[25-26]、液晶[27-28]、石墨烯[29-30]、半导体材料[31-32]和MEMS[33-34]等,其光学性质可通过光、电和热等外界激励来改变。这些主动调制方法已经被用于实现多种动态功能器件,例如太赫兹波空间光调制器、波片、全息和电磁诱导透明等。然而,以上提及的大多数调制手段都有着易失性的属性,即当外界激励撤去后其功能也会随之发生改变,需要恒定的能量去维持性能,不利于实际的使用和节能环保。因此,有必要发展一种具有非易失属性的主动调制方法。
硫属化物相变材料通常是由锗(Ge)、碲(Te)和锑(Sb)三种原子按照不同比例掺杂而成的合金,在不同的掺杂比例下,相变材料在相变速度、相变温度、稳定性等方面表现出不同的性质[35]。Ge2Sb2Te5 (GST)由于具有的快的相变速度、长期的稳定性和良好的可重复性,已发展成为目前应用最广泛的硫属化物相变材料之一,很早便被应用于商业化可擦写光盘以及电子存储器件等[36-37]。同时,近些年来,研究者们发现GST在非晶态和结晶态下表现出的巨大的光学对比度在光子学方面也有着应用潜力。在可见光和红外波段,GST已经被用于实现集成全光记忆器件[38-39]、片上光子突触[40-41]、颜色显示器件[42]、热发射器[43]、以及各种金属和介质超表面器件等[44-48],通过利用光、电和热激励等实现了GST在非晶态、结晶态以及中间态间的反复切换。GST 在可见光和红外波段已经开展了很多研究,相关综述也已经对此做了介绍[35, 49-50],然而在占据重要光谱位置的太赫兹波段,最近几年才得以发展和应用[51-52],本文将重点介绍GST在太赫兹波段的研究进展。
本文首先介绍了GST在太赫兹波段的光谱特性,利用光、热等激励实现了大面积GST的多级可逆相变,演示了非易失的太赫兹多级记忆器件,为实现可重构太赫兹波器件奠定了基础[53]。其次,我们从GST对太赫兹波的不同操控维度对太赫兹波调制器件展开介绍,包括对太赫兹波的振幅、相位和偏振中的一维或多维的非易失调控[54-62],以及实现器件功能的切换[58, 63]。除了利用GST的非易失属性外,其超快易失性也受到了关注[64],我们也回顾了相关工作。最后我们讨论了目前的挑战以及对未来更多的应用进行了展望。
2 GST的太赫兹光谱特性
GST存在三种稳定状态,包括非晶态、亚稳态面心立方态(face-centered cubic phase, FCC)以及六角密堆积态(hexagonal closed packed phase, HCP),通过利用合适的外界激励可实现三种状态之间的可逆切换。如
图 1. GST的太赫兹光谱特性和可逆相变[57]
Fig. 1. Terahertz spectral properties and reversible phase transition of GST[57]
由于GST的多级相变特性、非易失性、可重复擦写特性以及长期稳定性等优良的性质,可被用于实现太赫兹记忆器件[53],如
3 太赫兹波调制器件
在过去的十年中,利用超材料/超表面实现对太赫兹波的振幅、相位和偏振的一维或多维调制是一个非常重要且基础的研究领域。在上一章节中我们介绍了GST的太赫兹光谱特性以及热退火和光脉冲激励诱导GST可逆相变的条件,由于其具有的可逆相变特性和对太赫兹波的调制能力,可将GST薄膜结合到超表面结构的设计中实现对太赫兹波的一维或多维的非易失可重构操纵。在这一章节中,我们对最近几年的研究进展进行了总结。
3.1 非易失可重构的太赫兹波振幅调制器件
对太赫兹波透过率振幅的调制是最基本的光学应用,GST薄膜在相变前后对太赫兹波强度有大的调制深度,通过与金属等离激元谐振结构结合,具有调制谐振响应的潜力。不对称开口环谐振器(Asymmetric split ring resonator, ASRR)在太赫兹波段具有强的谐振响应,能够激发Fano谐振和偶极子谐振,高Q的Fano谐振对外界环境的变化具有高的灵敏度[54]。如
图 3. 非易失可重构的太赫兹波振幅调制器件。(a-c) Fano调制器件[54];(d-g) EIT器件[55];(h-k) EOT器件[56];(l-o) 二聚体器件[57]
Fig. 3. Nonvolatile and reconfigurable terahertz wave amplitude modulation devices. (a-c) Fano modulation devices[54]; (d-g) EIT devices[55]; (h-k) EOT devices[56]; (l-o) Dimer devices[57]
电磁诱导透明 (Electromagnetically induced transparency, EIT)是一种量子现象,描述了在相干驱动的三能级原子系统中对窄光谱上光吸收的相干相消。由于实现传统的EIT现象条件苛刻,近年来,利用超表面实现EIT效应引起了广泛的关注[55]。Liu等人通过在超表面EIT结构中引入相变材料GST实现了对透过振幅的可重构调制。单元结构如
超表面异常光透射 (Extraordinary optical transmission, EOT)是控制太赫兹波振幅的一个重要研究领域。如
Chen等通过结合GST提出了可调谐二聚体结构[57],如
3.2 非易失可重构的太赫兹波偏振调制器件
利用超表面实现对太赫兹波偏振的调制具有重要的应用前景。通过控制太赫兹波在两个垂直方向上电和磁分量的相位和强度,可以改变其偏振态。手性,指的是没有任何镜像对称面的结构,手性超材料可被用于调整手性响应,应用于波片和圆偏振器件中。此外,具有极性或离子元素的大分子由于集体振动模式和生物聚合物的存在会对太赫兹波产生强烈的吸收,即由手性结构组成的DNA、蛋白质和RNA在太赫兹波段会选择性地吸收圆偏振光,因此在太赫兹波段,实现对手性的动态调控具有重要的应用前景[59]。Bao等人利用GST实现了对手性的可重构调制,如
图 4. 非易失可重构的太赫兹波偏振调制器件。(a-d)手性调制器件[59];(e-h)偏振转换双功能器件[58];(i-k)柔性线偏振转换器件[61]
Fig. 4. Nonvolatile reconfigurable terahertz wave polarization modulation devices. (a-d) Chiral modulation devices[59]; (e-h) Polarization conversion bifunctional devices[58]; (i-k) Flexible linear polarization conversion devices[61]
3.3 非易失可重构的太赫兹波前调制器件
利用超表面结构实现对太赫兹波前的调制是实现太赫兹波异常偏折器、聚焦透镜、和涡旋器件等必不可少的。金属等离子体结构可实现对太赫兹波相位的调制,结合GST的相变特性可实现对太赫兹波的可重构波前调制,包括强度和相位的两维调制[60]。C型开口环谐振器(C-shaped split-ring resonators, CSRR),对太赫兹波辐射有强的谐振响应。根据巴比涅原理,互补C环也具有同样的谐振响应,如
图 5. 非易失可重构的太赫兹波前调制器件。(a-d)太赫兹波多级开关调制器件[60];(e-h)太赫兹波功能切换器件[63];(i-l)太赫兹波无光刻调制器件[62]
Fig. 5. Nonvolatile reconfigurable terahertz wavefront modulation devices. (a-d) Terahertz wave multi-level switching modulation devices[60]; (e-h) Terahertz wave function switching devices[63]; (i-l) Terahertz wave non-lithographic modulation devices[62]
3.4 易失性太赫兹波调制器件
以上总结的相关研究工作都是利用GST的非易失性来实现各种非易失太赫兹波调制器件,器件同时具备可重构性和多级调制等特性。在GST相变过程中,利用光激励可达到纳秒量级的切换速度,但同时注意到利用热和电激励诱导GST的相态切换仍然需要分钟量级的时间尺度,这对于实现太赫兹波超快调制器件来说是远远不够的。Pitchappa等人利用不同相态下的GST的半导体特性结合光激励实现了超快易失性切换器件[54]。GST在非晶态和结晶态下带隙分别为0.8 eV和0.5 eV,当使用1.55 eV的光子能量泵浦GST薄膜时,光激发载流子会提高GST电导率,降低其太赫兹波透过率。如
图 6. 易失性太赫兹波调制器件。(a-c)光泵浦Fano调制器件[54];(d-f)柔性超快太赫兹波调制器件[64]
Fig. 6. Volatile terahertz wave modulation devices. (a-c) Optically pumped Fano modulation devices[54]; (d-f) Flexible ultrafast terahertz wave modulation devices[64]
4 总结
本文系统回顾了近年来基于硫属化物相变材料的可重构太赫兹超表面器件的研究进展。首先介绍了GST在太赫兹波段的光谱特性以及利用光脉冲和热退火实现GST的可逆相变条件。GST在非晶态和结晶态下表现出不同的电导率,在非晶态下,GST的电导率接近0,在结晶态下,GST的电导率在3×105 S/m量级。当GST以薄膜形式存在于器件中时会对器件的整体透过率以及入射的太赫兹波和器件的耦合作用产生调制;当GST以连接岛的形式存在于结构间隙位置时会对器件的谐振响应产生调制。因此通过将GST与超表面设计相结合可实现多种非易失可重构的太赫兹波调制器件。本文详细阐述了基于GST的超表面器件用于实现对太赫兹波振幅、偏振和波前调制的原理和应用,利用光、热和电激励实现了器件的开关、多级调制以及功能的切换,相比于VO2等相变材料,无需外界激励来维持器件的光学性能,更有利于实际应用。此外,本文也介绍了利用GST的半导体特性来实现超快易失性太赫兹波调制的相关工作,实现了ps量级的调制速度。
基于GST的非易失和超快易失性调制器件进一步丰富和发展了太赫兹波调制器件,有望应用于太赫兹波成像、传感和通信等领域。但同时注意到目前仍然存在一些亟待解决的问题。首先,在实现GST的可逆相变调控方式上,在太赫兹波段,目前主要应用激光脉冲来诱导GST的非晶化,以及热退火诱导GST的结晶化。尽管诱导GST实现非晶态达到了ns量级,但结晶化至少需要两分钟以上的热退火,这不利于实际的应用。在红外波段,全光激励和全电激励的可逆相变已经实现,光激励可诱导非晶态GST相变为FCC态,对于红外波段,FCC态与非晶态GST的光学对比度足够大,但对于太赫兹波段仍然较小,需要进一步诱导GST相变到HCP态。受限于GST材料本身性质,其相变温度随着加热速率的上升而升高,在ns量级的激光脉冲激励下其HCP相变温度超过了非晶化临界点温度,因此限制了超短激光脉冲诱导GST相变到HCP态。在红外波段,利用电激励焦耳加热的方式实现了GST的可逆相变,但扩展到太赫兹波段其器件尺寸也相应扩大到了厘米量级,这对电压源提出了更高的要求。因此在太赫兹波段实现GST的全光/全电可逆相变是目前亟待解决的问题,通过改变GST的掺杂比、更加巧妙的结构设计以及使用更长脉宽的脉冲等有望解决上述难题。其次,目前大部分的器件仍然是基于金属结构的等离激元谐振效应,由于固有的金属损耗以及低的偏振转换效率等,导致器件整体效率较低,如何实现更加高效的动态可调谐器件也是目前需要解决的,金属反射式结构设计以及利用介质单元结构是较为可行的途径。此外,目前实现的对动态器件的调制仍然局限于整体效应的调控,实现的功能仍然受限,如何实现可编程调制即对不同像素施加不同激励值得进一步的深入研究。总之,在近几年,基于硫属化物相变材料的可重构太赫兹超表面器件得到了长足的发展,但目前仍然面临着诸多挑战;同时 6G技术的推进以及人工智能的发展也给这一领域带来了更多机遇。
Overview: We review the process on reconfigurable terahertz metasurface devices based on sulfide phase-change materials. Currently, most existing reconfigurable metasurfaces are limited by their volatile properties and single functionality, which hinder their applications in advanced photonics. The chalcogenide phase-change material Ge2Sb2Te5 (GST) exhibits non-volatility, reconfigurability, and large optical contrast, which can be used to realize tunable metasurface devices.
Firstly, the reversible phase transition of GST was realized in the terahertz band, its terahertz spectral characteristics were tested, and a multi-level memory device was realized.
One-dimensional or multi-dimensional dynamic modulation of the amplitude, phase, and polarization of terahertz waves can be achieved by combining GST with metasurfaces. Multilevel modulation of Fano resonances can be achieved by combining GST with asymmetric split-ring resonators and inducing phase transitions of GST. Using electrical excitation, a spatial light modulator with 2×2 pixels can be realized. The use of metasurfaces to achieve electromagnetically induced transparency (EIT) has attracted widespread attention, and placing GST at the openings can achieve multi-level modulation of the transmission amplitude. Extraordinary optical transmission (EOT) on metasurfaces is an important research area for controlling the amplitude of terahertz waves. The subwavelength gold hole array plays an important role in the coupling of surface plasmons on the gold surface, and the resonant coupling of EOT can be controlled by placing the GST under the gold hole. In the amorphous state, the conductivity is low, which has little effect on EOT. In the crystalline state, the conductivity is high, which reduces the transmission. By incorporating GST, tunable plasmonic dimers are proposed. The structure consists of two trapezoidal metal rings connected by GST islands. Near-field coupling occurs between the two metal rings, and the active modulation of the resonant mode can be achieved by changing the conductivity of the GST islands.
The use of metasurfaces to realize the modulation of the polarization of terahertz waves has important application fields. Chiral switching can be achieved by combining GST with a bilayer structure. Realizing the polarization conversion of linear polarization is of great significance for the realization of applications such as terahertz polarizers. Combining the phase-change characteristics of GST can further realize the switching of dual functions. Combined with flexible substrates, flexible polarization conversion devices can also be realized.
The modulation of terahertz wavefront by metasurface structures is of great significance for the realization of terahertz wave anomalous deflectors, focusing lenses, and vortex devices. The phase modulation of the terahertz wave can be realized by using the metal structure, and the wavefront modulation of the terahertz wave can be realized by combining the phase-change characteristics of GST, including two-dimensional modulation of intensity and phase.
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Article Outline
张寿俊, 曹暾, 田震. 基于硫属化物相变材料的可重构太赫兹超表面器件的研究进展[J]. 光电工程, 2023, 50(9): 230142. Shoujun Zhang, Tun Cao, Zhen Tian. Progress on reconfigurable terahertz metasurface devices based on sulfide phase change materials[J]. Opto-Electronic Engineering, 2023, 50(9): 230142.