Demonstrating backflow in classical two beams' interference
ABSTRACT :
The well-known interference pattern of bright and dark fringes was first observed for light beams in 1801 by Thomas Young. An interesting outcome that might not be entirely intuitive, however, is that the wavefront of the superposition of two beams with unequal amplitudes will exhibit a wavy behavior. The first successful attempt to observe ‘backflow’ was made recently in an assembly of optical fields [1], by synthesizing their wavefront in a complex manner. In our work [2], we experimentally observe, by measuring the transverse local wave-vectors using a Shack-Hartmann wavefront sensor [3], that the phenomenon of backflow arises naturally from the interference of two unequal beams. This phenomenon effectively occurs when the local momentum of a state is not present in its global Fourier spectrum, an outcome also referred to as super (sub) oscillations. Until recently, this subject was largely a fundamental phenomenon under study. However, there has been a newfound interest in its potential applications in super-resolution imaging [4] and in manipulating a particle’s path. Our study corroborates the theoretical observation [5,6] that backflow is not necessarily the purely quantum phenomenon that it is often thought to be but arises from wave interference. With our demonstration, the effect has been made accessible and easy to achieve in the lab , thus opening pathways for future applications. Additionally, we discuss several practical aspects of observing backflow, which have hitherto been little explored, including the finite sampling in position due the size of the lenslets of the Shack-Hartmann sensor and how it limits the maximum observable backflow for any given set of parameters.