Transparent Materials Absorb Light

Researchers have demonstrated an optical paradox — they have made a completely transparent material appear perfectly light-absorbing. The results of their research contradict the idea that materials that look transparent, such as glass, appear that way because they have no light-absorbing qualities. Based on their results, researchers introduce the concept of coherent virtual absorption in a lossless electromagnetic system, a phenomenon that arises when the incident electromagnetic field matches the spatiotemporal distribution of a complex scattering zero of a lossless system.

This is a schematic of a virtual light absorption process: A layer of a transparent material is exposed to light beams from both sides, with the light intensity increasing in time. Image courtesy of the MIPT researchers. Courtesy of MIPT Press Office.

Researchers from the Moscow Institute of Physics and Technology (MIPT) used special mathematical properties of the scattering matrix to achieve results.

When a light beam of time-independent intensity hits a transparent object, the light does not get absorbed but is scattered by the material. However, researchers found that if the intensity of the incident beam was grown exponentially, the total incident light energy accumulated in the transparent material and did not leave it, thus making the material appear perfectly absorbing from the outside. Interruption of the exponential driving gave rise to the release of energy stored in the lossless system through radiation in the background.

To illustrate the effect, researchers examined a thin layer of a transparent dielectric and calculated the intensity profile required for the absorption of the incident light. The calculations confirmed that when the incident wave intensity grew exponentially, the light was neither transmitted nor reflected — the layer looked perfectly absorbing despite the fact that it lacked the actual absorption capacity. However, when the exponential growth of the incident wave amplitude came to a halt (at t = 0), the energy that was locked in the layer was released.

This is virtual absorption effect in a thin layer of a transparent material. The dotted line indicates the amplitude of a time-dependent incident wave; the solid line is the amplitude of a scattered signal that comprises both incident and transmitted waves. The scattered signal is absent up to t = 0, suggesting that the incident wave energy is perfectly "locked" in the layer. Image courtesy of the researchers. Courtesy of MIPT Press Office.

Researchers found that this effect was robust against frequency dispersion of the system material, possible dissipation, and the finite geometry of the structure. The observed effect could have implications for flexible control of light propagation and storage, low-energy memory, and optical modulation, and could broaden understanding of how light behaves when it interacts with common transparent materials.

“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 researcher Denis Baranov. “However, it was the mathematics that led us to the effect. Who knows, electrodynamics may well harbor other fascinating phenomena.”

The research was published in Optica, a publication of OSA, The Optical Society (doi: 10.1364/OPTICA.4.001457).