Beam deflection and negative drag in a moving nonlinear medium

TL;DR

This paper investigates how nonlinear refractive index effects in moving media can cause beam deflections that mimic or obscure light drag, emphasizing the importance of measuring both beam shift and angle for accurate interpretation.

Contribution

It demonstrates experimentally that nonlinear refractive index can induce beam deflections in moving media, complicating the observation of light drag effects and highlighting the need for comprehensive measurement methods.

Findings

Negative group index causes light to be dragged opposite to medium motion

Positive nonlinear refractive index deflects the beam towards the medium's motion

Proper measurement of both beam shift and angle is essential to distinguish effects

Abstract

Light propagating in a moving medium with refractive index other than unity is subject to light drag. While the light drag effect due to the linear refractive index is often negligibly small, it can be enhanced in materials with a large group index. Here we show that the nonlinear refractive index can also play a crucial role in propagation of light in moving media and results in a beam deflection that might be confused with the transverse drag effect. We perform an experiment with a rotating ruby crystal which exhibits a very large negative group index and a positive nonlinear refractive index. The negative group index drags the light opposite to the motion of the medium. However, the positive nonlinear refractive index deflects the beam towards the motion of the medium and hinders the observation of the negative drag effect. Hence, we show that it is necessary to measure not only the transverse shift of the beam, but also its output angle to discriminate the light-drag effect from beam deflection -- a crucial step missing in earlier experiments.