Controllable beam slicing through fractional phase shifts accumulated at an atomically-thin interface

The interface between different media has been one of the crucial sources of light–matter interactions where properties of light, such as intensity, phase, or polarization, may change. One previously unexplored regime is when light propagates parallel to and bisected by the interface. Here, we realized an optically suspended and atomically-thin interface based on wafer-scale monolayer MoS₂ on fused silica that is immersed inside index-matching liquid. With this system we observed that a beam propagating along the interface is sliced into two sub-beams leaving a node at the interface. We quantified the upper and lower bounds of absorption and concluded that the atomically-thin interface behaves like a partial mirror that reflects light with an additional phase shift, which results in self-interference with the incident beam. In addition, we found that the degree of interference can be tuned by using interfaces with different number of MoS₂ layers. Our results indicate that the value of phase shift by a monolayer is a fraction of π, not an integer of π, which is unexpected from classical interfaces.