Optical Properties of Boron Nitride Nanotubes

Optical properties of single-walled boron nitride nanotubes have been studied theoretically. The dielectric functions calculated from the gradient approximation and the random-phase approximation are consistent with each other. The imaginary and the real parts of the dielectric function, respectively, exihibit the special peaks and dips. The strong e-h absorption peaks at ω < 4 γ₀ comes from the π band and the others from the π + σ bands (γ₀ is the nearest-neighbor interaction of 2p_z orbitals). Such single-particle excitations also induce the peak structures in the reflectance spectrum. On the other hand, the loss function shows the prominent π and π + σ-plasmon peaks. The π and π + σ plasmons (collective excitations) reveal themselves in the reflectance spectrum as strong and abrupt edges. The optical properties are affected by the polarization direction and the nanotube radius, but not the chiral angle. The calculated results could be experimentally checked with optical spectroscopies or EELS.