As a branch of modern optics, nonlinear optics has developed into a key technology, which proved to have many advantages in characterizing the crystal, electronic, and spins structures of low-dimensional materials, which are characterized by their ultrathin nature, high surface area, and exceptional mechanical, electronic, and spin properties. It is convenient to combine nonlinear optics and microscope techniques to image 2D materials. The focused laser spots ranging from microns to hundreds of nanometers are very suitable for most 2D materials and are highly sensitive to its weak signals.

Among the optical techniques, second harmonic generation (SHG) is one of the simplest nonlinear optical process where two photons of the same frequency combine to generate a new photon with twice the frequency. SHG can occur when a material with a non-centrosymmetric crystal structure is excited by a laser beam. In the context of 2D materials, SHG arises from the asymmetric distribution of electron charge and spin density, which leads to a non-zero second-order susceptibility tensor. Thus, it is very promising to engage SHG technique to research the structural-changing related physical process, as well as to research potential applications in areas such as ultrafast photonics, nonlinear optics, and quantum computing.

Photoluminescence is a phenomenon that occurs when a material absorbs light energy and then re-emits the energy in the form of light. In 2D materials, the electronic structure is strongly influenced by quantum confinement effects, resulting in unique optical properties. The photoluminescence emission spectrum provides valuable information about the electronic band structure and excitonic properties of 2D materials. The intensity and peak position of the photoluminescence emission can be used to extract important parameters such as the bandgap, exciton binding energy, and doping concentration.

Raman spectroscopy is a non-destructive and non-invasive technique that is widely used to study the structural, electronic, and vibrational properties of materials. In 2D materials, the Raman spectrum is highly sensitive to the number of layers, doping, strain, defects, and other physical and chemical properties. The ratio of the intensities of these peaks can provide information about the number of layers, while the position and width of the 2D peak can be used to determine the degree of disorder and the electronic band structure.

After nearly 4 years of development, the optical Lab of our team is operating normally. We have shock-isolation optical platforms, wavelength tunable femtosecond laser amplifier, OptiCool multi-functional sample cavity, grating spectrometer, etc. Based on this Lab, we have successfully built many nonlinear optical measurement systems, including far-field and microscopic SHG systems, horizontal and vertical microscopic PL systems, and a far-field SHG system based on OptiCool, and many scientific research results have been produced.