▎ 摘　　要

We theoretically study the orientation dependence of even-order harmonics in biased bilayer graphene (BLG) by numerically solving the semiconductor Bloch equations in the velocity gauge. It is shown that the yield of even-order harmonics has a perfect periodicity of 60 degrees with the rotation of the crystal orientation. The even-harmonic intensity reaches the maximum when the laser field is polarized along with the nearest-neighbor atom. By investigating the crystal orientation dependence for the parallel and perpendicular components of even harmonics, it is found that the Berry curvature plays an insignificant role in the even-harmonic generation of biased BLG. Further analysis reveals that the asymmetric electron density between in-plane adjacent atoms caused by polarization is responsible for the generation of even harmonics in biased BLG. Additionally, we study the ellipticity dependence of even harmonics in biased BLG when the elliptical principal axis of the laser pulse is along with the in-plane nearest-neighbor atoms. As expected, the yield of even harmonics decreases with the ellipticity. Our findings give access to understanding deeply the mechanism of the orientation dependence of even harmonics in biased BLG.