创新背景

极化子是极性晶体和离子晶体中导带的电子和与其结伴而行的晶格畸变的复合体。导带中的电子使晶格离子位移而伴生极化，其电场又反作用于电子，电子总是带着它所引起的晶格畸变一起运动。声子是晶格振动的简正模能量量子。在固体物理学概念中，结晶态固体中的原子或分子按一定的规律排列在晶格上。在晶体中，原子间有相互作用，原子总是围绕着其平衡位置在做不断的振动，同时这些原子又通过其间的相互作用力而联系在一起。
声子极化子是耦合到光的电磁波的极性电介质中离子的集体震荡，与光波长相比，其电磁场的压缩程度要高得多。当材料放置在高导电金属的顶部时，薄范德华晶体中的声子-极化子可以进一步压缩。在这样的构型中，极化晶体中的电荷在金属中“反射”，它们与光的耦合导致一种新型的偏振子波，称为图像声子极化子。高度压缩的图像模式提供强烈的光物质相互作用，但对基板粗糙度非常敏感，这阻碍了它们的实际应用。

创新过程

针对声子极化子的局限性，韩国科学技术学院电气工程学院Min Seok Jang教授领导研究小组与南丹麦大学纳米光学中心的N. Asger Mortensen教授的团队以及其他国内外研究人员合作，使用先进的制造和测量方法，开发了一个引导非常薄的范德华晶体中压缩光波的独特的新实验平台。研究结果《金晶体上六方氮化硼中图像声子极化子的近场探测》于2022年7月13日发表在《科学进展》杂志上。

研究人员使用灵敏度极高的扫描近场光学显微镜（SNOM），直接测量在单晶金基板上63nm厚的六方氮化硼（h-BN）板中传播的双曲图像声子极化子（HIP）的光场，显示介电晶体中压缩了一百倍的中红外光波。
研究实现了在常规h-BN晶体中检测到来自超压缩高阶HIP的信号，并获得HIP波在许多波长上传播的直接图像。研究使用南丹麦大学纳米光学中心制作的超光滑单晶金片，将其用作h-BN基底的自产金晶体的原子光滑表面，为范德华晶体中的声子极化子在不影响寿命的情况下被显著压缩提供可能。

在中红外频率下，金中几乎为零的表面散射和极小的欧姆损耗为HIP传播提供了低损耗环境。研究探测到的HIP模式的压缩率比以往高出2.4倍，但具有与低损耗介质衬底的声子极化相似的寿命，因此在归一化传播长度方面具有两倍的品质因数。
许多重要的有机分子在中红外中具有吸收线，传感应用中中红外光谱具有重要地位。传统的监测方法需要大量的分子进行操作，被超压缩的声子极化子场可以在微观水平上提供强光物质相互作用，从而显着提高检测明确性，限到单个分子。HIP在单晶金上的长寿命会进一步增强检测性能，有助于以最小损耗引导中红外光的方法将为超薄介电晶体在下一代光电器件中基于纳米级强光物质相互作用的实际应用提供突破。

此外，研究表明HIP和图像石墨烯等离子体之间存在较高的相似性，两种图像模式的电磁场都更有限，且寿命不受较短的偏振子波长的影响。研究结果为图像极化子提供了更广阔的视角，与介质基板上的范德华晶体中的传统低维极化子相比，它们在纳米光波导导方面更具有优越性。

创新关键点

使用高灵敏度光学显微镜测量图像声子极化子的光场，观察到中红外光波；将超光滑单晶金片用作h-BN基底的自产金晶体的原子光滑表面，为压缩声子极化子且不减损其寿命提供可能。

创新价值

研究证明了图像极化子的优势，可用于未来的光电器件，满足低损耗传播和强光物质相互作用。研究结果将为实现更高效的纳米光子器件铺平道路，例如超表面、光学开关、传感器和其他在红外频率下工作的应用。

The atom smooth surface of the gold crystal helps to compress the phonon polarizers

In view of the limitations of phonon polarization, Professor Min Seok Jang of the School of Electrical Engineering, Korea Institute of Science and Technology, led the research team with N. N. Kelly of the Center for Nanooptics at the University of Southern Denmark. Professor Asger Mortensen's team, along with other researchers at home and abroad, collaborated to develop a unique new experimental platform for guiding compressed light waves in very thin van der Waals crystals, using advanced manufacturing and measurement methods. The results of the study, "Near-field Detection of Image Phonon Polarons in Hexagonal Boron Nitride on Gold Crystals," were published in the journal Science Advances on July 13, 2022.

Using extremely sensitive scanning near-field light microscopy (SNOM), the researchers directly measured the light field of hyperbolic image phonon polarons (HEPs) propagating in a 63 nm thick hexagonal boron nitride (h-BN) plate on a single crystal gold substrate, showing that a hundred-fold compression of mid-infrared light waves in the dielectric crystal.

The study enabled the detection of signals from ultracompressed high-order HIP in conventional h-BN crystals and the acquisition of direct images of HIP waves propagating across many wavelengths. The study used ultra-smooth single-crystal gold sheets made at the University of Southern Denmark's Centre for Nano-Optics to be used as the atomic smooth surface of the h-BN substrate of homegrown gold crystals, providing the possibility that the phonon polarizers in van der Waals crystals could be significantly compressed without affecting lifespan.

At mid-infrared frequencies, almost zero surface scattering and extremely small ohmic losses in gold provide a low-loss environment for HIP propagation. The HIP pattern detected in the study has a compression rate of 2.4 times higher than before, but has a similar lifetime to the phonon polarization of low-loss dielectric substrates, so it has twice the quality factor in terms of normalized propagation length.

Many important organic molecules have absorption lines in the mid-infrared, and the mid-infrared spectrum in sensing applications has an important position. Traditional monitoring methods require a large number of molecules to operate, and ultracompressed phonon polarization subfields can provide strong light matter interactions at the microscopic level, thereby significantly improving detection certainty to a single molecule. The long life of HIP on single crystal gold will further enhance the detection performance, helping to guide mid-infrared light with minimal loss will provide a breakthrough for the practical application of ultra-thin dielectric crystals based on nanoscale strong light material interactions in next-generation optoelectronic devices.

In addition, studies have shown high similarities between HIP and image graphene plasmas, with both image modes having more limited electromagnetic fields and lifespans independent of shorter polaron wavelengths. The results provide a broader perspective on image polarizers, which are superior in nano-optical waveguides compared to traditional low-dimensional polarizers in van der Waals crystals on a dielectric substrate.