N*

We study the shift-current optical response in a noncentrosymmetric polytype of the quasi-2D layered semiconductor BC₂N [1]. Employing a recently developed first-principles Wannier-interpolation technique [2] implemented in the latest release of Wannier90 [3], we find that the photoconductivity exhibits two distinctive features at the band edge. First, it ranks among the largest bulk nonlinear responses reported to date, and peaks in an energy range suitable for optical manipulation. Secondly, it is strongly anisotropic, due to the vanishing of particular tensor components not foretold by phenomenological symmetry arguments; this is a consequence of dipole selection rules imposed by mirror symmetry, which imply that the relative parities between valence and conduction bands are key for determining the directionality of the band-edge response. The implications of the dipole selection rules should apply to a broad class of nonlinear responses.

[1] J. Ibañez-Azpiroz, I. Souza, and F. de Juan, arXiv:1906.07627
[2] J. Ibañez-Azpiroz, S. S. Tsirkin, and I. Souza, PRB 97, 245143 (2018)
[3] G. Pizzi et. al., arXiv:1907.09788