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Scientists Make Transparent Materials Absorb Light

29 November 2017

A group of physicists from Russia, Sweden and the U.S. have demonstrated a highly unusual optical effect. They managed to “virtually” absorb light using a material that has no light-absorbing capacity.

The absorption of electromagnetic radiation is one of the main effects of electromagnetism. This process takes place when electromagnetic energy is converted to heat or another kind of energy within an absorbing material, like electron excitation. Coal, black paint and carbon nanotube arrays, also known as Vantablack, appear black because they absorb the energy of the incident light almost completely. Other materials, like quartz or glass, have no absorbing properties, so they appear transparent.

 Schematic geometry of the problem: a planar dielectric slab of thickness L is illuminated by two normally incident waves of increasing amplitudes from both sides. (b) ??S-matrix eigenvalue zeros of a nonmagnetic planar slab with refractive index ??=3n=3 for normal incidence. The plus and minus signs refer to zeros of symmetric and antisymmetric modes of the slab, respectively. (Denis G. Baranov,) Schematic geometry of the problem: a planar dielectric slab of thickness L is illuminated by two normally incident waves of increasing amplitudes from both sides. (b) ??S-matrix eigenvalue zeros of a nonmagnetic planar slab with refractive index ??=3n=3 for normal incidence. The plus and minus signs refer to zeros of symmetric and antisymmetric modes of the slab, respectively. (Denis G. Baranov,)

Through the theoretical research, the physicists managed to dispel the simple and intuitive notion by making a completely transparent material appear to be perfectly absorbing. In order to achieve that, the researchers employed special mathematical properties of the scattering matrix — a function that relates an incident electromagnetic field with the one scattered by the system. When a light beam of time-independent intensity hits a transparent object, the light does not get absorbed but is scattered by the material — a phenomenon caused by the unitary property of the scattering matrix.

It turned out, however, that if the intensity of the incident beam is varied with time in a certain fashion, the unitary property can be disrupted for at least some time. In particular, if the intensity growth is exponential, the total incident light energy will accumulate in the transparent material without leaving it. That being the case, the system will appear perfectly absorbing from the outside.

To illustrate the effect, the researchers examined a thin layer of a transparent dielectric and calculated the intensity profile required for the absorption of the incident light. The calculation confirmed that, when the incident wave grows exponentially, the light is neither transmitted nor reflected. The layer looks perfectly absorbent despite the fact that it lacks the actual absorption capacity. But, when the exponential growth of the incident wave amplitude comes to a half, the energy locked in the layer is released.

“Our theoretical findings appear to be rather counterintuitive. Up until we started our research, we couldn’t even imagine that it would be possible to ‘pull off such a trick’ with a transparent structure,” said Denis Baranov, a doctoral student at MIPT and one of the authors of the study. “However, it was the mathematics that led us to the effect. Who knows, electrodynamics may well harbor other fascinating phenomena.”

The results of the study broadens general understanding of how light behaves when it interacts with common transparent materials, but it also has a wide range of practical applications. For example, the accumulation of light in a transparent material may help design optical memory devices that would store optical information without any losses and release it when needed.

The paper on this research was published in Optica.

To contact the author of this article, email Siobhan.Treacy@ieeeglobalspec.com


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