Hidden Portals

Platform 9¾ from the Potter books may still elude us Muggles, but scientists are working on a circuitous route that may yet get us there.

Invisibility cloaks, shielding devices and hidden portals have long been a staple of science fiction and fantasy, but the development of new materials has given rise to transformation optics—a field of optical and material engineering—which is bringing them closer to reality.

In a recent breakthrough, a team from Hong Kong University of Science and Technology has conceptualised a gateway that blocks electromagnetic waves but allows the passage of other entities. The concept is based on photonic crystals that allow the passage of electro-magnetic waves—visible light, radio waves, X-rays—within a select range.

The gateway conceived by the team uses a magnetic photonic crystal made from an array of iron oxide ceramic rods. If a material of this type can be so engineered that it keeps out all visible light, then a true portal becomes a reality. It would, to all purposes, appear like a mirror that you could actually walk through.

Since their method allows the material to be tuned by magnetic fields, the portal could be controlled remotely—causing it to appear or disappear at will.

Huanyang Chen, one of the researchers, says, “In the frequency range in which the metamaterial possesses a negative refraction index, people standing outside the gateway would see something like a mirror. Whether it can block all visible light depends on whether one can make a metamaterial that has a negative refractive index from 300 to 800 nanometres.”

As the team has said in an article published in the New Journal of Physics, their work shows that “such gateway-type devices can actually be realised with simple parameters and they can have wider bandwidths such that the concept is closer to reality than previously thought. The structure can be implemented by using magnetic photonic crystal structures that are field tunable, resulting in an invisible electromagnetic gateway that can be open or shut using magnetic fields… it would be reasonably feasible for the present gateway to be realised in the near future.”

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