The optical properties of tri-group (B,Al,Ga,In) doped (6,6) SiC nanotubes (SiCNTs) are studied from first principles.The results show that the main absorption and dispersion of SiCNTs caused by the intrinsic transition appear in the ultraviolet-visible region (below 500 nm),and the tri-group doping increases the minimum dielectric constant value resulting in enhanced transmittance.In addition,the tri-group doping can introduce a weak absorption and dispersion region in the near-mid-infrared region,and the response peak blue shifts as the diameter of the doping atom increases.Comparative studies of reflectance,absorptivity,and transmittance show that the key factors affecting the transmittance of SiCNTs are reflectance (or refractive index) rather than absorption coefficient.