First-principles prediction of optical second-order harmonic generation in the endohedralN@C60compound

Abstract

Non-linear-optical properties in ${\mathrm{C}}_{60}$ have attracted enormous attention for over two decades. The endohedral complex $\mathrm{N}@{\mathrm{C}}_{60}$, with its remarkable thermal stability and spin-quartet ground state, is a candidate for future room-temperature quantum computing, but there has been no investigation of its non-linear-optical properties. Here, a first-principles calculation shows that $\mathrm{N}@{\mathrm{C}}_{60}$ is a promising material for nanoscale and ultrafast modulations. Excitation by a pump laser pulse of the nitrogen-atom vibration inside the ${\mathrm{C}}_{60}$ cage transiently breaks inversion symmetry and can enable second-harmonic generation (SHG) from a probe pulse. Unlike the SHG observed in ${\mathrm{C}}_{60}$ thin films, this harmonic signal is switched on and off periodically every 345 fs. For an fcc crystal of $\mathrm{N}@{\mathrm{C}}_{60}$, the second-order susceptibility ${\ensuremath{\chi}}^{(2)}$ is on the order of ${10}^{\ensuremath{-}8}\phantom{\rule{0.28em}{0ex}}\mathrm{esu}$, similar to commercially used nonlinear materials.