Charge-transfer induced large nonlinear optical properties of small Al clusters:: Al4M4 (M = Li, Na, and K)

We investigate the linear and nonlinear electric polarizabilities of small Al4M4 (M = Li, Na, and K) clusters. Quantum chemical calculations reveal that these compounds exhibit an exceptionally high magnitude of linear and nonlinear optical (NLO) coefficients which are orders of magnitude higher than the conventional pi-conjugated systems of similar sizes. We attribute such phenomenal increase to noncentrosymmetricity incorporated in the systems by the alkali atoms surrounding the ring leading to charge transfer with small optical gap and low bond length alternation (BLA). Such a low magnitude of the BLA from a different origin suggests the possibility that these clusters are aromatic in character, and along with the large NLO coefficients, they appear to be better candidates for next generation NLO fabrication devices.