A Layered Crystal Learns to Catch Light express
The crystal bismuth selenide (Bi₂Se₃) is symmetric by itself, like a neatly stacked sandwich: flip it over, and nothing changes. But twist one layer slightly relative to the other, and the symmetry breaks. The material begins to 'feel' which side the light comes from and responds with an electric current.
The authors tested three ways to break the symmetry: rotating the layers by an angle of 21.78°, adding a point defect (removing a selenium atom), and applying an external electric field. In all cases, a so-called shift current emerged — a direct current that flows while the light is on. Its magnitude turned out to be comparable to the best two-dimensional materials, such as GeS.
Such a current is especially interesting for the terahertz range — between microwaves and infrared light. There, simple sources and detectors are scarce, but here the material itself can act as an antenna, converting light into current. In the long run, this leads to new solar cells and fast optoelectronics.
Original abstract
as written by the authors arXiv:2603.24834
Paucity of naturally occurring noncentrosymmetric materials is stimulating growing interest in engineered two-dimensional systems for nonlinear optical applications. Here, we show that breaking inversion symmetry in centrosymmetric bilayer Bi$$_2$$Se$$_3$$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer Bi$$_2$$Se$$_3$$ at the first commensurate angle of 21.78$$^\circ$$, we find peak shift and injection current conductivities of -14 $$ nm.\mu AV^{-2}$$ and 104 $$\times 10^8$$ $$nm.A V^{-2}s^{-1}$$, respectively, which lie in the visible spectrum and enable efficient THz applications. The external electric field and point-defect insertion both transform the bilayer into C$$_ {3v}$$ symmetry, with the selenium vacancy (V$$_{Se}$$) achieving peak shift and injection current conductivities of -190 nm.$$\mu AV^{-2}$$ and -170 $$\times 10^8$$ $$nm.A V^{-2}s^{-1}$$. In all three cases, the peak nonlinear optical responses are found to be comparable to those of benchmark 2D materials such as GeS, and the broadband responses, including helicity-dependent current generation, make these engineered bilayers viable candidates for next-generation 2D photovoltaics.
The angle of 21.78° is not random: at this angle, the two layers first form a periodic superlattice, like two fences with different spacings that align only after several spans.