Stacking order and layer thickness exert significant influence on the physical properties of two-dimensional materials. Here, using first-principles calculations and symmetry analysis, we theoretically investigate the stacking- and layer-dependent Raman responses of InSe and infer the corresponding symmetry-based second-harmonic generation (SHG) selection rules. For bulklike InSe, the calculated low-frequency Raman spectra (below $30 {\mathrm{cm}}^{-1}$) enable direct phase-sensitive spectral fingerprints: no peak is found for $\gamma$-InSe, whereas $\epsilon$- and $\beta$-InSe exhibit peaks near 20.2 and $17.8 {\mathrm{cm}}^{-1}$, respectively. Point-group analysis further indicates that SHG response is symmetry-allowed for bulk $\gamma$- and $\epsilon$-InSe, whereas it is symmetry-forbidden for bulk $\beta$-InSe. For few-layer InSe, the frequencies of the strongest shear modes exhibit a blueshift for the AB-stacked $\epsilon$ and $\beta$ phases, but a redshift for the ABC-stacked $\gamma$ phase, as the layer number increases. SHG response is symmetry-allowed for both odd- and even-layer $\gamma$- and $\epsilon$-InSe, whereas it is allowed for odd layers and forbidden for even layers in $\beta$ phase. This layer-dependent evolution of the low-frequency Raman modes, combined with symmetry-based SHG selection rules, provide an effective route for distinguishing the three InSe polytypes in the few-layer regime. In the high-frequency region, the ${{\mathrm{A}}^{\prime }}_1^1(\sim 110$ ${\mathrm{cm}}^{-1}$) and ${{\mathrm{A}}^{\prime }}_1^2(\sim 235$ ${\mathrm{cm}}^{-1}$) modes display a blueshift and a redshift, respectively, for all three phases with increasing layer thickness. The low-frequency shear modes and the frequency difference between ${{\mathrm{A}}^{\prime }}_1^1$ and ${{\mathrm{A}}^{\prime }}_1^2$ can serve as indicators of layer number in ultrathin films. Combined with analytical lattice-dynamical models, we reveal that the competition between the surface effect and the thickness effect governs the overall evolution of the high-frequency Raman trends. This work clarifies the microscopic origin of Raman responses in InSe and provides theoretical guidance for phase identification, thickness estimation, and elastic modulus characterization of layered materials.

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• Phonons
• 2-dimensional systems
• Semiconductors
• Surfaces
• Raman spectroscopy