Abstract
We study how a liquid-based inferior mirage, obtained by layering distilled water and ethanol, transforms the phase structure of a light beam possessing a helical wave front. An inferior mirage amounts for one total internal reflection, which effectively reverses the handedness of the wave front. We show that this transformation is accompanied by smooth unidirectional astigmatic changes and variations of the non-canonical strength of the phase singularity nested in the beam. A skew in the beam intensity distribution is observed where the phase singularity is inverted and allows the direct measurement of the topological charge of the beam. Freely propagating, partially inverted beams possessing spatially varying orbital fluxes can be obtained at the exit plane of the solution by adjusting the incidence conditions of the beam. This work lays the foundations for phase engineering of light beams in liquid-based optical mirages.
Highlights
Optical mirages are well known effects appreciated in nature for centuries
We study how a liquid-based inferior mirage, obtained by layering distilled water and ethanol, transforms the phase structure of a light beam possessing a helical wave front
An inferior mirage amounts for one total internal reflection, which effectively reverses the handedness of the wave front
Summary
Optical mirages are well known effects appreciated in nature for centuries. At the origin of these effects is a vertical gradient of refractive index (VGRIN) modifying both the amplitude and phase of the optical beam propagating through it [1]. A mirage amounts for a total internal reflection (mirror inversion), upon which the whole phase structure of the beam is disrupted and angular momentum is reversed. Two media of different refractive index, in a VGRIN, total internal reflection occurs progressively, through several layers of different refractive index This allows a deconstruction and appreciation of the dynamics of the phase transformation in optical mirages. VGRIN describing a cylindrical lens or a locally compressed optical fibre have been shown to affect the phase structure of helical beams [8, 9]. These VGRIN progressively break the cylindrical symmetry of the helical beam in one direction, much like a liquid-based VGRIN.
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