In Lewis Caroll’s Through the Looking-Glass, and What Alice Found There (1871), the sequel to the classic Alice’s Adventures in Wonderland, Alice again enters a fantastical world, this time by climbing through a mirror into the world that she can see beyond it. Though far from Alice’s spectacular feat, scientists at the Sandia National Laboratories in Albuquerque, New Mexico demonstrated a new type of mirror that behaves like no other.
A mirror without metals
A conventional, metal coated mirror not only reverses the image, but also the light’s electric fields as well. This is because the mirror interacts with the electrical component of electromagnetic radiation. Of course, for those of us who use mirrors casually this physical alteration makes no difference, but it can become a real nuisance for physicists working on various optical and light absorbing/reflecting materials like solar cells, lasers and such. This becomes most intruding at the mirror’s surface, at the point of reflection where the opposite incoming and outgoing electrical fields produce a canceling effect. Yet, scientists in the US have made a breakthrough after they placed nanoscale antennas at or very near the surface of so-called “magnetic mirrors.”
“We have achieved a new milestone in magnetic mirror technology by experimentally demonstrating this remarkable behavior of light at infrared wavelengths. Our breakthrough comes from using a specially engineered, non-metallic surface studded with nanoscale resonators,” said Michael Sinclair, co-author on the Optica paper and a scientist at Sandia National Laboratories in Albuquerque, New Mexico, USA who co-led a research team with fellow author and Sandia scientist Igal Brener.
Unlike silver and other metals, however, there is no natural material that reflects light magnetically. Magnetic fields can reflect and even bottle-up charged particles like electrons and protons. But photons, which have no charge, pass through freely. To overcome this predicament, the researchers devised a matematerial – a material that can’t be found in nature, artificially created to suit certain needs – made up of nanoscale cube-shaped resonators, based on the element tellurium, each considerably smaller than the width of a human hair and even tinier than the wavelengths of infrared light, which is essential to achieve magnetic-mirror behavior at these incredibly short wavelengths.
“The size and shape of the resonators are critical,” explained Sinclair “as are their magnetic and electrical properties, all of which allow them to interact uniquely with light, scattering it across a specific range of wavelengths to produce a magnetic mirror effect.”
Typically, this sort of solution is practical only for long microwave frequencies, which limits the scope of applications to microwave antennas, mainly. The two-dimensional array of non-metallic dielectric resonators, however, overcomes these limitations – all without loss of signal. To prove their design actually works like a magnetic mirror, the Sandia scientists used a technique called time-domain spectroscopy.
“Our results clearly indicated that there was no phase reversal of the light,” remarked Sheng Liu, Sandia postdoctoral associate and lead author on the Optica paper. “This was the ultimate demonstration that this patterned surface behaves like an optical magnetic mirror.”
Next, the researchers plan on demonstrating magnetic mirrors at even shorter wavelengths. where extremely broad applications can be found.
“If efficient magnetic mirrors could be scaled to even shorter wavelengths, then they could enable smaller photodetectors, solar cells, and possibly lasers,” Liu concluded.