摘要
散射式扫描近场光学显微是重要的近场光学显微技术之一,能够实现样品表面nm尺度的光学成像,在多个研究领域中有着广泛的应用。散射式扫描近场光学显微镜通过对针尖-样品处进行光激发,产生包含样品近场信息的近场散射信号以及包含近场信息的背景散射信号。尽管近场散射信号强度相比背景散射信号微弱,但可以通过轻敲模式的针尖周期性调制,利用一系列近场光学信号提取技术对其进行有效提取。针对散射式扫描近场光学显微术,利用针尖-样品物理模型分析了近场散射信号的产生机制,并详细介绍自零差检测、零差检测、外差检测、伪外差检测等主要近场信号提取技术,最后根据该领域的近期研究进展对未来研究方向进行了展望。
Significance Scattering-type-scanning near-field optical microscopy(s-SNOM) is one of the most widely used near-field optical microscopy techniques. It enables nanoscale spatial resolution of the surface of a sample or at a specific depth below the surface without causing damage to the sample. Consequently, its application spans across diverse scientific disciplines, such as physics,chemistry, and biology. Through the photoexcitation at the tip-sample boundary, s-SNOM simultaneously generates weak near-fieldscattering signals containing crucial near-field information and intense background scattering signals that can obscure this information.The challenge lies in suppressing the background noise, and various near-field signal extraction techniques have been proposed to address this issue. These techniques play a pivotal role in extracting genuine near-field information, enabling a more precise clarification of the sub-wavelength characteristics of samples in multiple research areas.Such potency in revealing the intricate nanoscale underscores the importance of mastering these techniques. In the pursuit of this objective, it is imperative to extensively investigate the mechanism for generating near-field-scattering signals and techniques for extracting near-field optical signals and their underlying principles. Such detailed investigations help refine near-field extraction techniques and are significant in observing the physical changes that occur during s-SNOM experiments. This knowledge facilitates the thorough analysis of near-field optical research findings and their practical applications, promoting advancements in various scientific disciplines.Progress To advance the techniques for extracting near-field optical signals, researchers have examined the generation mechanism of near-field-scattering signals inherent in the near-field detection process. They established several physical models, including the point-dipole and finite-dipole models, to analyze tip-sample interactions. These models enable a more accurate analysis of near-field experimental results, even without resorting to computationally tasking simulation methods. In addition, research on these models provides theoretical support for the development of near-field extraction techniques: the near-field-scattering signal exhibits a more pronounced variation in higher-order harmonics with tip vibration, which distinguishes it from the background scattering signal. This dissimilarity in response to periodic modulation of the tip allows for effective demodulation at higher harmonics of the vibration frequency of the tip using a lock-in amplifier, achieving signal suppression against background noise. Building upon demodulation at higher harmonics, researchers have proposed techniques for extracting near-field optical signals, including intensity and electric-field detection techniques.The intensity detection techniques, which were earlier and more extensively adopted, include prevalent techniques, such as the self-homodyne,(quadrature) homodyne, and pseudoheterodyne detection techniques. In addition, less-used techniques, such as phaseshifting interferometry, synthetic optical holography, and self-heterodyne detection, were developed. These intensity detection techniques often involve the addition of components, such as plane mirrors and acousto-optic modulators, to the interferometer system and generate reference signals with a specific phase or frequency. Such modifications effectively suppress the background noise,enabling the acquisition of a high signal-to-noise ratio and pristine near-field information. Some of these methods also facilitate the separation of near-field information into amplitude and phase components, which is crucial for the advancement of s-SNOM applications.Electric-field detection techniques, including electro-optic sampling and photoconductive antennas, are applied in radio frequency and terahertz detection bands because of their simple experimental setups and electric-field detection characteristics. Although electricfield detection techniques may not achieve the same extent of background noise suppression as the intensity detection techniques,these techniques contribute to diversifying technological options for experiments employing s-SNOM, further enriching the potential applications of this innovative microscopy.Conclusions and Prospects In this review, we first clarify the mechanism of near-field-scattering signals using tip-sample physical models and present an overview of the research development in tip-sample interactions. Subsequently, various techniques for extracting near-field optical signals developed to date are systematically introduced, with emphasis on crucial techniques, such as selfhomodyne,(quadrature) homodyne, heterodyne, and pseudoheterodyne detection. Through comparative experiments, the distinctions between several widely used techniques, including self-homodyne, quadrature homodyne, and pseudoheterodyne detection, are intuitively demonstrated, consistent with the theoretical expectations. Finally, the state-of-the-art advancements in near-field optical signal extraction are presented, along with a prospective outlook on their future development.Currently, several mainstream techniques for extracting near-field optical signals have distinct advantages in terms of the signalto-noise ratio, experimental efficiency, and complexity, helping researchers to make selections based on the specific requirements of their experiments. Consequently, research efforts in near-field extraction techniques predominantly concentrate on the advancement of existing techniques. This can be achieved by incorporating additional computation or experimental components. A typical example is the pseudoheterodyne interferometry for multicolor near-field imaging, which achieves simultaneous imaging of multiple spectra in a single experiment by calculating coefficient variations corresponding to the separation of the amplitude and phase at different detection wavelengths. Enhancements such as this maintain the original near-field optical signal extraction capabilities and address a broader range of experimental requirements, such as the improvement of experimental efficiency and the increase in imaging resolution. These advancements contribute to the improvement of experimental efficiency and the richness of information acquisition in s-SNOM experiments, encapsulating the future trajectory of near-field optical signal extraction techniques.
作者
江涛
王泽瑞
周雷
周州
赵之琛
程鑫彬
王占山
Jiang Tao;Wang Zerui;Zhou Lei;Zhou Zhou;Zhao Zhichen;Cheng Xinbin;Wang Zhanshan(School of Physics Science and Engineering,Tongji University,Shanghai Professional Technical Service Platform for FullSpectrum and HighPerformance Optical Thin Film Devices and Applications,Shanghai Frontiers Science Center of Digital Optics,MOE Key Laboratory of Advanced MicroStructured Materials,Institute of Precision Optical Engineering,Shanghai 200092,China;Shanghai Research Institute of Intelligent Autonomous Systems,Tongji University,Shanghai 201210,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2024年第14期1-17,共17页
Chinese Journal of Lasers
基金
国家自然科学基金(62175188,61925504,6201101335,62020106009,62192770,62192772,61621001)
上海市科学与技术委员会项目(23190712300,23ZR1465800)。
关键词
显微
散射式扫描近场光学
零差
伪外差
microscopy
scatteringtype scanning nearfield optics
homodyne
pseudoheterodyne
