A team of researchers have detailed a range of techniques which they suggest could one day lead to a genuine cloak of invisibility – although, thanks to Toyota’s involvement, they’re looking to start by making the windscreen pillars disappear from your next car.
“We are always looking for ways to keep drivers and passengers safe while driving,” explained author Debasish Banerjee of the work, which was led by the Toyota Research Institute of North America in partnership with South Korea’s Inha University and the University of Michigan. “We started exploring whether we could make the light go around the pillar so it appeared transparent.”
While the stanchions framing a car’s windscreen provide a key aspect of its structural integrity, they’re annoyingly located right where you would want to look in order to see other road users. Removing them would weaken the car, so Toyota’s looking into the next best thing: rendering them invisible.
The result of its research into that somewhat tricky problem: a paper which reviews the various methods by which optical cloaking may be achieved. Should the company’s efforts succeed, it’s likely to find itself buried under hefty bags of money from a wide range of intelligence and military agencies around the world, who are impressed enough when you can hide tanks from radar or turn a jet into a flock of birds.
“Perfect optical cloaking requires the total scattering of electromagnetic waves around an object at all angles, all polarisations, over a wide frequency range, irrespective of the medium,” the researchers wrote in the paper’s abstract. “Such a device is still far-fetched, requiring the transformation of space around a cloaked region such that the phase velocity is faster than other areas to preserve the phase relationships.
“However, by simplifying the invisibility requirements, pioneering work on spherical transformation cloaks, carpet cloaks, plasmonic cloaks, and mantle cloaks has been realised in narrowband microwave, infrared, and even optical wavelengths.”
The paper ran through a wide range of experiments, theoretical and otherwise, which have been undertaken for cloaking, ranging from metamaterial-based physical cloaks and an “intelligent cloak” made from a reconfigurable metasurface to a “unidirectional invisible cloak,” geometrical optics-based cloaking – including the surprisingly simple approach of, effectively, putting mirrors around the object to be hidden – and digital cloaks which combine cameras and an “output surface” display.
“Despite the many challenges outlined herein,” the team’s paper concluded, “it is undeniable that in recent years, optical invisibility has become a fact rather than ideas from fiction books. Useful cloaking devices have been successfully realised beyond electromagnetics including acoustics, thermal, and fluidic devices, owing to their low-loss physical characteristics compared to optical devices.
“While most of the studies focus on fundamental physics and cloaking device proof of concept designs, it is worth pointing out that the need for invisibility in modern society will remain high in the future. Starting from obvious applications in areas such as modern warfare, surveillance, blind-spot removal in vehicles, spacecraft, and highly efficient solar cells, applications in sensing and display devices are virtually endless.”
The team concluded that we may one day “enjoy optical cloaking technology in everyday life” – but stopped short of offering a timescale to mail-order invisibility cloaks being advertised in the backs of grubby magazines next to the X-ray specs.
The paper is available under open access terms in the Journal of Applied Physics. ®