New Transparent Materials Absorb Light

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. (Source: MIPT)

A group of physicists from Russia, Sweden, and the U.S. has demon­strated a highly unusual optical effect: They managed to virtually absorb light using a material that has no light-absorbing capacity. The research findings break new ground for the creation of memory elements for light. 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.

In their theo­retical research the physicists managed to dispel that simple and intui­tive notion by making a completely transparent material appear perfectly absorbing. 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, at least for 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 calcu­lations confirmed that when the incident wave intensity grows exponen­tially, the light is neither trans­mitted nor reflected. That is, the layer looks perfectly absorbing despite the fact that it lacks the actual absorp­tion capacity. However, when the expo­nential growth of the incident wave ampli­tude comes to a halt, 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,” says Denis Baranov, a doctoral student at MIPT. „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 not only broaden our general understanding of how light behaves when it interacts with common transparent materials, but also have a wide range of practical appli­cations. To give an 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. (Source: MIPT)

Reference: D. G. Baranov et al.: Coherent virtual absorption based on complex zero excitation for ideal light capturing, Optica 4, 1457 (2017); DOI: 10.1364/OPTICA.4.001457

Link: Moscow Inst. of Physics and Tech. MIPT, Dolgoprudny, Russia

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