This theoretical study investigates the linear and nonlinear optical properties of zigzag carbon nanotubes (CNTs) and silicene nanotubes (SiNTs) with varying radii, focusing on their behavior in the infrared and ultraviolet energy ranges. In the infrared region, absorption spectra exhibit several peaks resulting from allowed transitions between valence and conduction bands. The number of absorption peaks increases with radius for both nanotube types, with SiNTs showing peaks at lower energy ranges and higher intensities compared to CNTs. Conversely, CNTs display markedly higher absorption intensities in the ultraviolet region. The quadratic electronic optic (DC Kerr) effect reveals sharp peaks near the band gap with multiple sign changes, attributed to allowed optical transitions at band edges. The third-order optical susceptibility for both CNT and SiNT structures show several peaks below the band gap energy due to multiphoton resonance absorption. In the infrared region, two highest and lowest subbands near to the Fermi level play a dominant role in the peaks formation. The position and intensity of peaks demonstrate a strong dependence on nanotube radius and type with higher intensity for the SiNTs. The tunable nature of the optical properties of CNTs and SiNTs by their radius and the enhanced nonlinear optical response of SiNTs, characterized by lower energy peaks and higher intensities, show their significant potential for advanced applications in nonlinear optics, optical detection, and high-energy optical systems.
