Time-of-flight imaging of invisibility cloaks

TL;DR

This paper demonstrates how time-of-flight imaging can be used to reveal imperfections in invisibility cloaks, providing a direct method to assess cloaking performance through calculated images based on Fermat's principle.

Contribution

It introduces a method to generate time-of-flight images for various cloaks using a ray-velocity equation derived from Fermat's principle, aiding in cloaking assessment.

Findings

Time-of-flight images reveal cloaking imperfections.

Calculated images for multiple cloak types demonstrate the method's applicability.

Ray-velocity equations effectively model light propagation in cloaking scenarios.

Abstract

As invisibility cloaking has recently become experimental reality, it is interesting to explore ways to reveal remaining imperfections. In essence, the idea of most invisibility cloaks is to recover the optical path lengths without an object (to be made invisible) by a suitable arrangement around that object. Optical path length is proportional to the time of flight of a light ray or to the optical phase accumulated by a light wave. Thus, time-of-flight images provide a direct and intuitive tool for probing imperfections. Indeed, recent phase-sensitive experiments on the carpet cloak have already made early steps in this direction. In the macroscopic world, time-of-flight images could be measured directly by light detection and ranging (LIDAR). Here, we show calculated time-of-flight images of the conformal Gaussian carpet cloak, the conformal grating cloak, the cylindrical free-space cloak, and of the invisible sphere. All results are obtained by using a ray-velocity equation of motion derived from Fermat's principle.