声子极化激元的电子能量损失谱研究进展

Recent progress on phonon polaritons probed by electron energy loss spectroscopy

声子极化激元是晶体中的光学支声子与电磁波耦合形成的元激发,具有独特的中远红外介电响应、低损耗等特性,在波导、超透镜、增强的能量感应与传输等方面展现了巨大的应用潜力,因此近年来发展迅速.声子极化激元的主要实验研究手段是扫描近场光学显微镜技术,但由于缺乏合适的光源,目前在远红外波段的研究严重受限.此外,光子与声子之间大的动量失配问题也限制了光学显微镜激发大动量的声子极化激元.本文介绍了近年来扫描透射电子显微镜电子能量损失谱技术在研究声子极化激元方面的主要进展.相比于近场光学方法,电子能量损失谱具有大动量转移、宽频激发与探测、高空间分辨能力、高激发探测效率、多模式激发等优点,并且能在自支撑样品上测量,避免衬底影响.因此,基于此,研究人员取得了一系列重要研究成果,该方法已成为近场光学方法很好的补充.本文简要总结以上研究进展,并对下一步的研究前景进行了展望.

Phonon polaritons are the elementary excitations formed by the strong coupling between optical phonons and electromagnetic waves in polar materials.Dr.Huang Kun first proposed the concept of phonon polaritons when he studied the interaction between infrared light and lattice optical vibration modes of polar crystals more than 70 years ago.Huang’s results revealed the infrared dispersion properties of ionic crystals,and also perfectly explained the physical origin of the so-called restrahlen band in which electromagnetic radiation cannot propagate.Since then,the concept of polariton has been extended to other fields of physics,including exciton polariton,magnon exciton,plasmon polariton,etc.Phonon polariton has now become a basic concept for understanding lattice vibrations in crystals,and the research object has also been expanded from isotropic dielectric materials to anisotropic materials.Various new physical properties of them are gradually revealed.Due to their unique mid-and far-infrared dielectric response and low loss characteristic,phonon polaritons show great application potential in waveguides,superlensing,enhanced energy transfer and sensing,and thus have attracted extensive attention in recent years.Most recent research focuses on material systems with lower symmetry or more complex structures,in which there are rich and novel phenomena possessing broad application prospects.The widely used research method to stimulate and probe phonon polaritons is scanning near-field optical microscopy.However,due to the lack of suitable light sources,the research in the far-infrared is severely limited.In addition,the large momentum mismatch between photons and phonons also limits the stimulation and detection of large-momentum phonon polaritons by optical spectroscopy.This paper reviews the recent progress on phonon polaritons using the electron energy loss spectroscopy in a scanning transmission electron microscope(STEM-EELS).Compared with near-field optical methods,STEM-EELS has the advantages of large momentum transfer,broadband excitation and detection,high spatial resolution,high excitation and detection efficiency,flexible operation mode,etc.,and can measure freestanding samples without the influence from substrates.Therefore,a series of important research results have been achieved based on this,which has become a good supplement to the near-field optical method.At present,the main disadvantage of STEM-EELS is that the energy resolution is not as high as the optical spectroscopy,but it can be expected that with the further development of the electron microscope such as monochromator hardware,electron gun and low temperature technology,the energy resolution of STEM-EELS can be further improved,making it as widely used in the field of phonon polaritons as the near-field optical method.With the rapid development of technology,it can be foreseen that in the future,STEM-EELS will play an increasingly important role in finding new polar materials,exciting new modes,detecting polar properties,and deeply understanding the interaction between light and matter at nanoscale.