Raman spectroscopy is a versatile tool widely used for comprehensive probing of crystal information. However, generally when applied in narrow-band-gap van der Waals crystals, it is liable to form a “bug,” especially in transition-metal-dichalcogenides (TMDs). That is, several resonant Raman-scattering (RS) modes will inevitably appear in the Raman spectra with strong intensity, interfering with the desired signal of optical-phonon modes. Here, we propose cross-sectional polarized Raman scattering capable of regulating the intensity of RS modes in accordance with quasi-sinusoidal rules. Typically, for MoS2 and WS2, when the polarization vector of excited light is along the c axis of the crystal, all RS modes are nearly completely “expunged” from the Raman spectra. The mechanism is that the absorption of most TMDs with a space group of R3m for the light polarized along the c axis is infinitesimal, thus forming a small coupling intensity of electronic states excited optically and acoustic-phonon modes at point M, which in turn restrain the appearance of RS modes. The regulating strategy proposed can be applied to other van der Waals crystals so as to obtain a high signal-to-noise ratio Raman spectrum.

The so-called “phase difference” is commonly introduced as a phenomenological parameter in Raman tensor theory, so as to fit the experimental data well. Although phase difference is widely recognized as an intrinsic property of crystals, its physics still remains ambiguous. Recently, Kranert et al. have presented a new formalism to explain the origin of phase difference theoretically. Here, we systematically conducted experimental research with polar phonons in wurtzite crystals, the results of which strongly suggest that the phase difference should be predetermined in a Raman tensor, rather than be treated as Raman tensor elements traditionally or as an intrinsic property. On the grounds of pinpointing existing logical flaws in Raman tensor study, we provide a logically clear paradigm.