## 2 Materials and Methods
### Experiment System and Method  
The experiments in this work were carried out based on polarized micro-Raman system. Figure 1 shows the optical-path of different polarized micro-Raman system. In detail, Figure 1a gives the optical-path of general polarized micro-Raman system. When the optical axes of both the polarizer and the analyzer in Figure 1a are all 360\({}^{\circ}\) continuously adjustable, most of typical polarized configurations of backscattering Raman are realizable. For instance, the parallel-polarized (PP) configuration is realized by keeping the polarized direction of the scattering light paralleling to that of the incident laser.  
Figure 1b shows the optical-path of a standard micro-Raman system, which has no analyzer but is equipped with a continuously adjustable polarizer in front of the laser output. Using this system, the polarized direction of the incident laser can be adjusted continuously but that of the scattering light is un-adjustable, which means that the scattering data with all the polarized directions are received and detected by the spectrograph. This polarized configuration is named as the non-analyzer (NA) configuration.  
Figure 1c is an upgrade optical-path of micro-Raman system equipped with a continuously adjustable polarizer inside the microscope and an orthogonally switched analyzer inside the spectrograph. The polarizer is a continuously synergistic/co-variant polarized accessory (CS/CV-PA, Chinese patent CN102426163A) and it locates at the overlap path of the incident laser and scattering light. The orthogonally switched analyzer (OSA) is an optional accessory of most of the advanced micro-Raman system such as Renishaw InVia Reflex. When the OSA is switched to parallel, the parallel-polarized configuration is realized and the polarized direction of the scattering light, paralleling to that of the incident laser, is adjusted through the CS/CV-PA. At this time, the system in Figure 1c is similar with that in Figure 1a. When the OSA is switched to gap, the polarized direction of the incident laser can be adjusted continuously by the CS/CV-PA and the scattering data with all the polarized directions are received and detected by the spectrograph, which acts in a similar way to the system in Figure 1b.  
The polarized Raman experiments in this work adopted both the non-analyzer and the parallel-polarized configurations by using a Renishaw InVia Reflex confocal micro-Raman spectroscopy system whose optical-path is similar with that in Figure 1c. A Leica 100 \(\times\) Lens (numerical aperture  
Figure 1: Optical-path of (**a**) general polarized micro-Raman system, (**b**) standard micro-Raman system (without analyzer) equipped with a continuously adjustable polarizer, (**c**) standard micro-Raman system equipped with a continuously adjustable polarizer and an orthogonally switched analyzer.  
= 0.85), a 1200 l/mm grating, and a 633 nm laser (1.96 eV) were chosen. The exposure time of each Raman detection was 4 s, and the laser power reached the specimen surface was about 0.64 mW.  
During the experiments, the sampling spots were selected using the microscope of the Raman system, the polarized configuration was switched between NA and PP by the OSA, and the polarized direction of the incident laser was rotated from 0\({}^{\circ}\) to 360\({}^{\circ}\) with a step of 10\({}^{\circ}\) by CS/CV-PA. At each sampling spot, the Raman spectra were detected in situ at different polarized directions of the incident laser under both NA and PP configuration. The Raman intensities of both Ag\({}^{1}\) and Ag\({}^{2}\) modes were obtained by fitting the spectra using a pseudo-Voigt function.