English Translation

Second‑Harmonic Generation (SHG) arises when two photons of identical frequency interact with a nonlinear material and merge into a new photon with twice the frequency. The SHG signal can only be observed in materials with a non‑centrosymmetric chemical structure (absence of inversion symmetry).

The MStarter 100 SHG Second‑Harmonic‑Generation Spectroscopy System integrates picosecond or femtosecond ultrafast laser into the optical microscope path. The laser beam is focused onto the sample surface via a 100× objective lens to excite micro‑region second‑harmonic signals of specimens. It enables non‑destructive characterization of lattice orientation, grain‑boundary distribution, atomic‑layer number and atomic‑stacking configuration of samples.

Testable Items

a) Dependence of SHG intensity on laser power: Fast‑acquisition of SHG data under varied laser power to investigate the correlation between SHG intensity and laser power.

b) Dependence of SHG intensity on polarization angle (Polar Plot): Equipped with an electrically‑driven half‑wave‑plate rotation stage. The laser polarization direction is software‑controlled for automatic measurement of SHG intensity at different angles and generation of polar plots.

c) High‑resolution fast scanning for SHG mapping: Combined with a piezo‑electric translation stage, an ultra‑low‑dark‑noise (7 count/s), high‑sensitivity photon counter delivers high‑speed scanning‑imaging performance of over 500 pixels per second, with more than 10‑fold enhancement in detection sensitivity*.

d) Micro‑nano dimensional measurement: The focused laser spot on the sample surface is smaller than 1 μm, and the minimum scanning step size can reach 50 nm.

Application Fields

a) Precise characterization and localization of material grain boundariesb) SHG signal characterization for‑two‑dimensional material heterostructuresc) Investigation of phase structure and stacking configuration of 2D materialsd) Nonlinear‑optical‑property research on micro‑nano materials and structurese) Study on polarization domains and crystallographic orientation of ferroelectric materialsf) Research of organic‑based micro‑/nano‑scale low‑dimensional structures

Ø Technical Parameters*：

*Please consult the staff for detailed parameters.

Ø Application Cases of Second‑Harmonic Generation Testing：

1. Determine the lattice orientation

Figure 1. (A) Bright‑field image; (B) Angular dependence of second‑harmonic signal

Figure 1 shows the variation of the second‑harmonic generation signal of the monolayer WSe₂ sample with the polarization angle. The lattice orientation of the monolayer WSe₂ sample was determined from the polar curve of its second‑harmonic generation signal, and the direction with the maximum intensity of the second‑harmonic generation signal points to the mirror‑symmetry direction of the WSe₂ lattice.

Figure 2. (A) Crystal structure. Top‑down view along the c‑axis, side views along the b‑axis and a‑axis; (B) Optical image of a CrOCl flake suspended over a SiO₂/Si hole. Inset: the corresponding AFM image and Raman mapping of the pattern, scale bar = 10 μm; (C) Schematic diagram of the micro‑Raman thermometry experimental setup, Inset: the corresponding cross‑section; (D) Polar plot of SHG intensity for the suspended CrOCl flake.

Layered CrOCl is an antiferromagnetic insulator with a low‑symmetry crystal structure and atomic‑level flatness. Figure 2 (D) shows the polar plot of SHG intensity for this material.

Figure 3. Thickness‑dependent second‑harmonic generation (SHG). (a) Polar plot of SHG intensity in the H and V directions (laboratory coordinates) as a function of the linear polarization of the excitation laser for a 100‑nm‑thick CIPS flake. (b) Temperature dependence of SHG intensity for CIPS flakes with thicknesses of 100, 50, 30 and 10 nm, respectively. The SHG intensity for each thickness is normalized to its intensity at 300 K.

Figure 3 (A) shows the dependence of second‑harmonic‑generation (SHG) intensity on excitation polarization for the two‑dimensional ferroelectric material CuInP₂S₆. The non‑zero SHG signal directly reveals the broken inversion symmetry that gives rise to ferroelectricity. The authors further investigated the temperature‑dependent SHG intensity of CIPS flakes with thicknesses ranging from 100 down to ~10 nm. Figure 4b plots the normalized intensity as a function of temperature. All flakes follow the same trend: a prominent SHG signal exists below Tc, yet the SHG intensity gradually decreases with rising temperature and nearly vanishes at high temperatures. This provides strong evidence for a ferroelectric‑to‑paraelectric phase transition around Tc, which involves a structural change from the non‑centrosymmetric (m) to the centrosymmetric (2/m) phase.

Figure 4. (A) Bright‑field image; SHG mapping: (B) no polarizer, (C) polarization angle 0°, (D) polarization angle 90°

Figure 4 shows the second‑harmonic‑generation (SHG) signal images of the monolayer WSe₂ sample on a sapphire substrate under different polarization detection conditions. From the SHG mapping image in Figure 2(B) obtained without a polarizer, samples in three regions can be clearly observed with comparable SHG signal intensities. In contrast, when the polarization angles of the polarizer are set to 0° and 90°, the three domains of the sample exhibit distinct SHG signals. The differences in SHG signal intensity originate from the varied crystallographic orientations of each domain.

2. Observe the grain boundaries

Figure 5. (A) SHG mapping; (B) Count values for the region indicated by the arrow (measured using APD photon‑counting)

Figure 5 uses an APD photon‑counting probe to acquire SHG mapping data. It can be clearly observed that the sample contains multiple grain boundaries, indicating a polycrystalline structure. Our software can be used to select a specified region and examine the intensity‑distribution relationship of the SHG signal.

3. Determine the number of layers of two‑dimensional materials

Figure 6. (A) Bright‑field image; SHG mapping: (B) Analyzer angle 0°, (C) Analyzer angle 90°

Figure 6 shows the second‑harmonic generation (SHG) imaging images of the multilayer WSe₂ sample under parallel and perpendicular conditions of the polarizer and analyzer, respectively. Grain boundaries of the material can be clearly observed in the SHG images. Meanwhile, since the second‑harmonic generation signal is related to the layer number of two‑dimensional materials, the layer number of different regions can be determined by observing the intensity of the second‑harmonic generation signal of the multilayer sample.

4. Analysis of SHG Power Dependence

Figure 7. (A) Schematic diagram of the SHG response in a nonlinear‑material system.

Variation of SHG intensity with wavelength as a function of excitation power. (C) Linear fitting of SHG intensity on a logarithmic scale.

The author investigated homogeneous stacked polycrystalline WSe₂ with branched patterns and the correlation between SHG intensity and excitation power. At the identical excitation wavelength of 1064 nm, the variation of SHG intensity was measured under different incident laser powers ranging from 2.8 to 12.5 mW. As expected, there was a strong interdependence between SHG intensity and excitation power. It can be seen that the SHG peak intensity, located at ~532 nm, increases significantly with rising laser power (Figure 7b). The peak values of SHG intensity were extracted and plotted against the corresponding excitation powers. Consequently, these two variables exhibit an obvious linear relationship in the double‑logarithmic coordinate system.