Is stealth possible? How to make it a reality? What would you use this technology for if you mastered it?

From fantasy, mythology to science fiction, "stealth" has become the dream of some science fiction fans. While Star Trek brought the idea of invisibility into the public consciousness, the invisibility cloak in Harry Potter brought "invisibility" to the screen as we imagined it would. So, with our existing technology, can "stealth" be achieved? In fact, the most likely way we can achieve invisibility is through the development of "technology" that can only provide effective invisibility at wavelengths longer than what the human eye perceives.

The ability to bend light around an object and reveal the background, incoming light from any angle and distance may become a reality thanks to a combination of advances in metamaterials, nanolenses, and conversion optics.

Radar invisibility, or electromagnetic radiation from microwaves to radio wavelengths, could be a first step. And recent developments in metamaterials extend this further, bending light around objects and rendering them undetectable. On January 1, 2018, "Nature" published an article titled "A Broadband Achromatic Metalens for Focusing and Imaging in Visible Light", in which new materials called "Broadband Achromatic Metalens" An object can be rendered undetectable across the entire visible spectrum. Through it we can realize a visible light cloaking device.

Over the past two decades, special multilayer coatings of substances called metamaterials have been developed that allow certain wavelengths of electromagnetic radiation to pass freely around objects. This is different from light transmission, which is through the material. The structure of the metamaterial guides light around the object so that it is emitted undisturbed in the direction it entered.

Transformation optics is an emerging theory of artificially transforming electromagnetic fields. Traditional optics studies how changes in refractive index affect the path of light. Transformation optics realizes specific light paths by engineering the properties of various materials. To hide objects using an "invisibility cloak," one can design a specific light path to guide the light, and then use transformation optics to design the metamaterials needed to form this light path. That is, one can create a switchable electromagnetic field on demand, and if the structure of the magnetic field is properly designed and the bending of incoming light is controlled, objects within the magnetic field can become invisible at specific wavelengths.

▲ More than ten years ago, the first 2D cloak was developed, which can hide objects when viewed from a certain angle. Today, we're hard at work creating a true 3D cape.

According to the authors, the key to the "broadband achromatic metalens" is the application of titanium-based nanofins. Depending on the wavelength of the incident light, these nanofins will guide light through different parts of the material, allowing the light to bend precisely, focusing the entire visible spectrum, including white light (the combination of all spectral colors) to the same point and eliminating chromatic aberration.

▲ Focuses the entire spectrum of light on a single point, eliminating chromatic aberration

Currently, the biggest challenge in building a realistic invisibility cloak is combining multiple wavelengths, because the material of the cloak must vary from point to point to properly bend (and then not bend) light, we can't yet penetrate the visible part of the spectrum with a cloak . However, recent advances in metalens seem to suggest that if this can be done at a single, narrow wavelength, then this nanofin technology can be applied to greatly expand the wavelengths covered, allowing objects around objects from any angle and distance. The incident light is bent and the object background is revealed, which means that visible light invisibility, 3D invisibility cloaks may become a reality.

▲ If the latest progress in metalensing is successfully applied, the cloak can be extended to the visible part of the spectrum, and an "invisibility cloak" can be realized

Today, advances in metalens technology herald the arrival of true cloaking devices. Stealth technology, as originally envisioned in Star Trek, will take centuries to perfect. On Earth, it might only take ten or two years. If this latest metalensing technique can be quickly applied to metamaterial cloaks, optical cloaks ("invisibility cloaks") may finally become a reality in the near future for humans.

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