Magnetic mirrors reflect light more efficiently

Could lead to more powerful solar cells, lasers, and other optoelectronic devices
October 24, 2014

Artist’s impression of a comparison between a magnetic mirror with cube-shaped resonators (left) and a standard metallic mirror (right). The incoming and outgoing electric field of light (shown as alternating red and white bands) illustrates that the magnetic mirror retains light’s original electrical signature while a standard metallic mirror reverses it upon reflection. (Credit: Authors)

Sandia National Laboratories scientists have created a new type of mirror that reflects infrared light by using an unusual magnetic property of a non-metallic metamaterial, instead of a reflective material.

By placing nanoscale antennas at or very near the surface of these “magnetic mirrors,” scientists are able to capture and harness electromagnetic radiation in ways that have potential in new classes of chemical sensors, solar cells, lasers, and other optoelectronic devices.

These nanoscale cube-shaped resonators, based on the element tellurium, are smaller than the wavelengths of infrared light, which is essential to achieve magnetic-mirror behavior at these wavelengths.

“The size and shape of the resonators are critical, 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,” explained Michael Sinclair, co-author on an Optica paper on this finding and a scientist at Sandia National Laboratories who co-led a research team with fellow author and Sandia scientist Igal Brener.

Conventional mirrors reflect light by interacting with the electrical component of electromagnetic radiation. Because of this, however, they do more than reverse the image; they also reverse light’s electrical field. Though this has no impact on the human eye, it does have major implications in physics, especially at the point of reflection where the opposite incoming and outgoing electrical fields produce a canceling effect. This temporary squelching of light’s electrical properties prevents components like nanoscale antennas and quantum dots from interacting with light at the mirror’s surface.

How a magnetic mirror works

A magnetic mirror, in contrast, reflects light by interacting with its magnetic field, preserving its original electrical properties. “A magnetic mirror, therefore, produces a very strong electric field at the mirror surface, enabling maximum absorption of the electromagnetic wave energy and paving the way for exciting new applications,” said Brener.

Unlike silver and other metals, however, there is no natural material that reflects light magnetically. So the team developed a specially engineered two-dimensional array of non-metallic dielectric resonators — nanoscale structures that strongly interact with the magnetic component of incoming light. These resonators have a number of important advantages over the earlier designs:

  • The dielectric material used, tellurium, has much lower signal loss than do metals, making the new design much more reflective at infrared wavelengths and creating a much stronger electrical field at the mirror’s surface.
  • The nanoscale resonators can be manufactured using standard deposition-lithography and etching processes, which are already widely used in industry.

The reflective properties of the resonators emerge because they behave, in some respects, like artificial atoms, absorbing and then re-emitting photons. Atoms naturally do this by absorbing photons with their outer electrons and then re-emitting the photons in random directions. This is how molecules in the atmosphere scatter specific wavelengths of light, causing the sky to appear blue during the day and red at sunrise and sunset.

The metamaterials in the resonators achieve a similar effect, but absorb and re-emit photons without reversing their electric fields.

Looking to the future, the researchers will investigate other materials to demonstrate magnetic mirror behavior at even shorter (visible) wavelengths, which could enable smaller photodetectors, solar cells, and possibly lasers.

Abstract of Optical Magnetic Mirrors without Metals

The reflection of an optical wave from a metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieve high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a “magnetic mirror” which does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can only be achieved using artificially tailored materials. Here we experimentally demonstrate, for the first time, the magnetic mirror behavior of a low-loss, all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse electric dipoles placed close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.