  One-dimensional selenium and tellurium crystalize in helical chainlike
structures and thus exhibit fascinating properties. By performing
first-principles calculations, we have researched the linear and nonlinear
optical (NLO) properties of 1D Se and Te, and find that both systems exhibit
pronounced NLO responses. In particular, 1D Se is found to possess large
second-harmonic generation coefficient being up to 7 times larger than that of
GaN, and even is several times larger than that of the bulk counterpart. On the
other hand, 1D Te also produces significant NLO susceptibility which exceeds
the bulk GaN by 5 times. Furthermore, 1D Te is shown to possess prominent
linear electro-optic coefficient rxxx(0). Particularly, Te chain exhibits large
shift current response and the maximum is more than the maximal photovoltaic
responses obtained from BaTiO3 by 2 times. Therefore, 1D Se and Te may find
potential applications in solar energy conversion, electro-optical switches,
and so on. Finally, much stronger NLO effects of 1D Se and Te are attributed to
their one-dimensional structures with high anisotropy, strong covalent bonding
and lone-pair electrons. These findings will contribute to the further study in
experiments and search for excellent materials with large NLO effects.
