News Resource: Advanced Research Institute of Multidisciplinary Science

Editor: News Agency of BIT

Translator: Shao Yikang, News Agency of BIT

Second harmonic generation, also known as Frequency-doubling Effects, are nonlinear optical processes that originate from the higher-order term of the electromagnetic field polarization under the excitation field. Their response is usually weak, thus requiring strict phase matching and sufficient acting length to enhance the nonlinear effect, which results in large nonlinear devices that are difficult to integrate. Monolayer 2D transition metal sulphides (TMDs) have very high second-order nonlinear polarization tensors, making them ideal for miniaturized on-chip frequency converter devices, all-optical interconnects, and optoelectronic integration. However, due to the sub-nanometer size thickness of single-layer TMDs (~0.8 nm), the efficiency of second harmonic conversion is still very low. Therefore, the use of micro-nano structure to enhance the interaction between light and matter in single-layer TMD and improve the efficiency of second harmonic conversion is particularly important for its practical application. The realization of the second harmonic directional emission can improve the utilization rate of the far-field beam, enabling it to be more used in silicon-based ultra-thin frequency multiplier devices, integrated photonics and other fields.

Recently, Professor Huang Yuan from the Advanced Research Institute of Multidisciplinary Science, together with Professor Liu Xinfeng of the National Center for Nanoscience and Qing Zhang of Peking University, published the research on achieving giant enhancement and directional second harmonic emission from monolayer WS2 on Silicon Substrate at ACS Nano. The team prepared a circular hole array on a SiO2/Si silicon substrate using UV lithography and reactive ion etching techniques, obtained a monolayer WS2 by mechanical cleavage and transferred it to a silicon substrate with a pore array. A maximum of three orders of magnitude of second harmonic enhancement were obtained in this suspended WS2 (Figure 1).

Fig.1: SHG enhancement of WS2 monolayer on the silicon substrate with holes

To verify the reliability of this method, the authors used SHG surface scanning to find that the second harmonic generation of the sample at the pore location were generally enhanced, as shown in Figure 2. The angular resolution results showed that there was a significant dispersion pattern at the porous sample, which indicated that the pores and the covered monolayer WS2 formed a Fabry-Pérot microcavity structure. Further numerical simulations showed that about 272-fold electric field enhancement was achieved in the microcavity structure compared to the structureless situation.

Fig. 2: Mechanism of SHG enhancement caused by the F-P cavity

The fact that single-layer WS2 has such a large second harmonic enhancement at the microcavity position is also related to the change in the density of the exciton state. By fitting the absorption spectra of single-layer WS2 on and without microcavities, the authors found that the increase in exciton state density contributed approximately 3.1 times to SHG enhancement, as shown in Figure 3.

Fig. 3: Contribution of the exciton state density to the SHG enhancement

By varying the excitation wavelength, it was found that the enhancement factor obtained a maximum enhancement of 1580 times at an excitation wavelength of 820 nm, as shown in Figure 4. Wavelength in Figure 4a, 4b corresponds to the mode position of the microcavity.

Fig. 4: Changing trends of enhancement factors of the WS2 monolayer on cavity under varied excitation wavelengths

Through far-field imaging, it has been found that due to the resonance coupling with the FP microcavity, the far-field divergence angle of WS2 on the microcavity is only 5°, indicating that the microcavity structure can achieve the regulation of the directionality of the second harmonic outlet, as shown in Figure 5.

Fig. 5: Directional divergence of SH emission on cavity

This work uses the suspended WS2 monolayer to demonstrate the great enhancement and low divergence of SHG emission. Through angular resolution spectroscopy, numerical simulation and absorption spectroscopy analysis, it is confirmed that the mechanism of SHG enhancement stems from the F-P microcavity-induced electric field enhancement and the stronger oscillation intensity of excitons. In addition, a low divergence angle (~5°) of the second harmonic emission is obtained, which is attributed to the resonance of the F-P microcavity. The result shows that by optimizing the preparation process of two-dimensional semiconductors, it is possible to achieve directional second harmonic enhancement characteristics, which opens the way for on-chip ultra-thin frequency multiplication and integrated photonics. The results were published in the top international journal ACS Nano under the title "Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS2 on Silicon Substrate via Fabry-Pérot Micro-Cavity" ( doi.org/10.1021/acsnano.2c03033, Impact Factor: 18.027), Prof. Yuan Huang and Xinfeng Liu, National Center for Nanoscience, and Qing Zhang, Peking University, are co-corresponding authors. The work has been supported by the National Natural Science Foundation of China, the Beijing Municipal Natural Science Foundation, the National Key Research and Development Program, the Strategic Key Research Program of the Chinese Academy of Sciences and the Chongqing Outstanding Youth Fund.

Paper link：https://pubs.acs.org/doi/full/10.1021/acsnano.2c03033

Author’s brief introduction attached:

Huang Yuan, the professor, doctoral supervisor from Advanced Research Institute of Multidisciplinary Science, BIT, majoring in research on the preparation, characterization, device processing, and physical property measurement or regulation of two-dimensional materials. More than 80 SCI papers has been published in Nature Physics, Nature Communications, Physical Review Letters, ACS Nano, etc. by him. He has published more than 40 papers as the first author and corresponding author. All the papers have been cited more than 4500 times. He presided over the National Key R&D Program (Youth Project), the National Fund Commission Outstanding Youth Fund and Noodle Project, and the Chongqing Outstanding Youth Fund Project. In 2019, he was selected as a member of the Youth Promotion Association of the Chinese Academy of Sciences. In 2020, he was awarded the honorary title of "China's Top Ten Science and Technology Emerging Figures" by the China Association for Science and Technology. The next year witnessed him being awarded the second prize of the Innovation Award of the China Invention Association Invention and Entrepreneurship Award (ranked first). He was awarded the second prize of the Entrepreneurship Award of the China Invention Association (ranked second) in 2022. Served as a reviewer for Physical Review Letters, Nature Communications, Advanced Functional Materials, ACS Nano, Nano Letters, Acta Physica Sinica, and other well-known journals at home and abroad, he is also the youth editorial board member for Physics, Chinese Physics Letters, InfoMat, and Materials.