Recently, Weigao Xu’s group, in collaboration with Daiqian Xie’s group at Nanjing University, Yi Luo’s group at University of Science and Technology of China, and Sai Duan’s group at Fudan University, developed a mechano-Raman spectroscopy (MRS) method, based on the optomechanical coupling effects between vibrators and plasmonic cavities. The theoretical model and experimental protocol for MRS are established. The results have been published on March 30, 2023 in Nature Photonics entitled “Direct characterization of shear phonons in layered materials by mechano-Raman spectroscopy” (DOI: 10.1038/s41566-023-01181-5), which received high evaluations from two reviewers: “I also feel excited to see this milestone achievement in the Raman spectroscopy field”; “it is a rare piece of work that represents a landmark in the field of Raman spectroscopy”.
Mechanical interactions at the nanoscale interface carry critical information about atomic-level interface structure, heat transport and optoelectronic properties. However, due to their limited electron-phonon coupling effect, such interactions cannot be directly measured by classical vibrational spectroscopic methods. Taking the ultralow-frequency shear phonons in layered graphite crystals as an example, phonon branches with co-directional atomic-layer motions carry unique information about the global structure and hidden interfaces in layered crystals and heterostructures. However, the polarizability changes for adjacent layers cancel each other out, which prevents the generation of detectable dipole radiation. Up to now, there have been no well-developed spectroscopic methods to effectively obtain such kind of information, and to apply it in the characterization of global crystal structure, surface/interface interaction and micromechanical vibrators.
Faced with the above challenges, Xu et al. proposed a mechano-Raman spectroscopy (MRS) technique (Figure 1). Under incident light (hν0) excitation, the localized plasmon are synchronously modulated by a mechanical vibrator with frequency νmech, which generates a Stokes Raman signal with frequency of ν0 − νmech and an anti-Stokes Raman signal with frequency of ν0 + νmech. In MRS investigations of layered crystals, Xu et al. found that the collective motion of atomic layers in the lattice can drive periodic motions of plasmonic metals and generate inelastic scattering signals.
Figure 1: MRS for direct frequency-domain detection of mechanical vibrations
Figure 2a presents MRS investigations on a series of graphite flakes (from 3 to 12 layers) and the quantitative analyses on the mechanical coupling effect. The energy transfer between the lattice vibrators and the plasmonic metal determines the effective displacement of the plasmonic metal and thus the MRS intensity. According to the MRS theory, the MRS intensity is proportional to the square of the effective displacement of the plasmonic metal, which is confirmed in more accurate MRS investigations on a 16LG lattice vibrator.
Figure 2: MRS for dark phonon observation and quantitative mechanical
investigations.
In optical Raman spectroscopy, the statistical distribution of phonon occupations determines the ratio of anti-Stokes and Stokes emissions (IaS/IS), which follows Bose–Einstein distribution. Compared with optical Raman process, MRS has a remarkable thermal-noise-free characteristic: (1) IaS/IS equals to ~1 for the entire temperature range of 77–477 K; (2) The full-width at half-maximum (FWHM) of the MRS signals does not show thermal broadening. This feature guarantees MRS unique advantages in vibration measurement (Figure 3). Furthermore, Xu et al. carried out MRS studies on a series of multi-component vibrators, showing the long-range propagating behavior of MRS and the capacity for the detection of the hidden interfaces.
Figure 3: The thermal-noise-free feature of MRS.
As a new type of Raman spectroscopy, MRS is expected to be applied in global crystal structure characterization, mechanical vibration sensing, and mechanical modulation of light. The study also provides new ideas for the realization of quantized energy transfer from lattice vibrators to nanomaterials in the field of quantum optics.
Ph.D. candidate Susu Fang at Nanjing University and Prof. Sai Duan are co-first authors of the paper. Prof. Weigao Xu, Prof. Daiqian Xie, and Prof. Yi Luo are co-corresponding authors. The research groups of Jun Zhang at Institute of Semiconductors (CAS), Xinglin Wen at Huazhong University of Science and Technology, Lei Zhang and Yagang Yao at Nanjing University, have provided supports in data analysis, electromagnetic field simulation, and sample preparation or characterization, respectively. This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province and the National Key R&D Program of China.
Link to article: https://www.nature.com/articles/s41566-023-01181-5