![]() ![]() Furthermore, some new nonlinear techniques, such as coherent Raman spectroscopy, have recently attracted increasing attention due to their superior sensitivity to the chemical specificity of materials, which gradually becomes a promising candidate to replace the spontaneous Raman spectroscopy. The application of third-harmonic generation (THG) and four-wave mixing (FWM) techniques in determining the layer numbers and anisotropy of crystallinity are also explored. For example, second-harmonic generation (SHG) is one of the well-known NLO characterization techniques, which is widely used in the research of crystal axis orientation and other related directions. ![]() With the development of research in recent years, NLO has shown its potential as a powerful characterization tool. ![]() On the other hand, the NLO response is used as a means of 2D material characterization. On one hand, it aims to apply a nonlinear response for the fabrication of various functional devices. The current research on the NLO response of 2D materials can be divided into two aspects. High spatial resolution can be achieved by combining with a high numerical aperture objective lens further. Generally, strong nonlinear signals can be acquired through the excitation of tens of milliwatts of laser energy, which enables multi-modal fast scanning to be completed in a short exposure time. It is very convenient to combine NLO and microscope techniques to image 2D materials. Although the as-mentioned diverse characterization methods provide us with powerful research tools, the problem is that each method has its own advantages and disadvantages, and it requires multiple methods and cumbersome processes to achieve the comprehensive examination of the properties of 2D materials.Īs a branch of modern optics, nonlinear optics (NLO) has developed into a key technology, which has been widely used in material characterization, light generation, quantum optics, and other fields. Commonly used characterization techniques include Raman microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and fluorescence microscopy. The accurate characterization of various physical and chemical properties of 2D materials is crucial in the functional device applications, such as layer number, crystal structure, crystal phase, defects, strain, chemical specificity. These 2D materials have different band structures and exhibit unique optical, electrical, and mechanical properties, providing a material basis for subversive innovation in many fields such as optoelectronics, catalysis, energy storage conversion, sensors, and biomedicine. In addition to graphene, there are also representative transition metal chalcogenides (MX 2, M = molybdenum, tungsten X = S, Se), main group metal chalcogenides (GaS, InSe, SnS, SnS 2, etc.) and many other 2D materials, covering materials types such as superconductors, metals, semi-metals, semiconductors, insulators. In a short period of time, the 2D materials family has continued growing and developing. Graphene has attracted much attention because of its unique physical and chemical properties. In 2004, Konstantin Novoselov and Andre Geim used Scotch tape to strip graphene with monoatomic thickness from graphite, marking the creation of two-dimensional (2D) materials for the first time. In particular, we also discuss the machine learning and stimulated Raman scattering (SRS) technologies which are expected to provide promising opportunities for 2D material characterization. Finally, the future development trends, challenges of advanced equipment construction, and issues of signal modulation are discussed. Second, we introduce the recent research progress on the NLO characterization of several important properties of 2D materials, including the number of layers, crystal orientation, crystal phase, defects, chemical specificity, strain, chemical dynamics, and ultrafast dynamics of excitons and phonons, aiming to provide a comprehensive review on laser-based characterization for exploring 2D material properties. First, we introduce the principles of NLO and common detection methods. Here, we summarize the research progress of NLO in 2D materials characterization. As a powerful characterization method, nonlinear optics (NLO) spectroscopy has been widely used in the characterization of 2D materials. Characterizing the physical and chemical properties of two-dimensional (2D) materials is of great significance for performance analysis and functional device applications. ![]()
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