Nonlinear optical response of zigzag carbon nanotubes under mechanical strain

This study investigates the strain-induced modifications in the linear and nonlinear optical properties of zigzag carbon nanotubes (CNTs). The analyzed optical functions include the linear absorption spectra, the electro-optic effect (QEO) and the two-photon absorption spectrum. A tight-binding model, combined with the density matrix formalism and perturbation theory, is employed to analyze these properties, considering all interband transitions across the entire Brillouin zone. The results demonstrate that both tensile and compressive strains significantly alter the optical responses of zigzag CNTs. In linear optical spectra, strain modifies the positions and intensities of absorption peaks, particularly those associated with E₁₁ and E₂₂ transitions. Similarly, strain also impacts the nonlinear optical responses, leading to noticeable peak shifts and intensity variations, particularly in the energy region below the band gap. The observed red- and blue-shifts of peaks vary depending on the specific category of zigzag CNTs, highlighting distinct strain-dependent behaviors. Furthermore, all strain-dependent optical characteristics demonstrate that semiconducting zigzag CNTs exhibit two distinct response patterns, related to their classification differences. The strain-induced shifts in nonlinear optical peaks suggest potential applications in strain-engineered photonic devices, such as ultrafast optical modulators and nonlinear optical switches.