Although graphitic C 3 N 4 (g-C 3 N 4 ) has been demonstrated to be a potential candidate for solar cell absorber and photovoltaic materials, the application has been limited by the low photoconversion efficiency in the visible range. Here, we explored that a g-C 3 N 4 bilayer has much better visible-light adsorption than a single layer via first-principles calculations, and the calculated optical adsorption threshold of bilayer significantly shifts downward by 0.8 eV, which is induced by the interlayer coupling. Additionally, we also found that the optical energy gap of bilayer can be engineered by the external electric field. The insights obtained in this study are general and will be helpful for future studies of twodimensional solar cell absorber and photovoltaic materials.

In this paper, two typo errors are found in equations (7) and (13). The corrected versions are displayed in the PDF file.

We propose a class of invisible slab cloaks based on the embedded optical transformation. The designed slab structures will guide the incident electromagnetic (EM) waves to propagate around the cloaked objects inside the slabs. Such EM waves will return to their original propagation directions without any distortions. We show that there are no singular values in the EM parameters of the slab cloaks and they could be realized using metamaterials. General formulations of the inhomogeneous and anisotropic permittivity and permeability tensors are derived for the slab cloaks using linear and nonlinear optical transformations. Numerical simulations based on the finite element method have been performed to confirm the design and analysis. The potential application of the slab cloak is a new type substrate for integrated circuits, which will protect the devices inside from the interference of EM waves but not disturb their EM properties.