A team of physicists from the U.S., Russia and Sweden have demonstrated a highly unusual optical effect: They managed to “virtually” absorb light employing a material that has no light-absorbing capacity. Published in the journal Optica, the research findings break new ground for the development of memory elements for light.
The absorption of electromagnetic radiation — light, among other things — 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 (for instance, during electron excitation). Coal, black paint, and carbon nanotube arrays — also known as Vantablack — look black because they absorb the energy of the incident light almost completely. Other materials, such as glass or quartz, have no absorbing properties and therefore look transparent.
In their theoretical research, the physicists managed to dispel that simple and intuitive concept by making a fully transparent material appear perfectly absorbing. In order to achieve this, the physicists used particular mathematical properties of the scattering matrix — a function that connects an incident electromagnetic field with the one scattered by the system. When a light beam of time-independent intensity strikes a transparent object, the light does not get absorbed instead it is scattered by the material, a phenomenon attributable to the unitary property of the scattering matrix. However, if the intensity of the incident beam is altered with time in a particular fashion, the unitary property can be disrupted for some time. Particularly, if the intensity growth becomes more rapid, the total incident light energy will become accumulated in the transparent material without leaving it (Figure 1). If this is the case, the system will seem perfectly absorbing from the outside.
Figure 2. 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.
In order to demonstrate the effect, the researchers studied a thin layer of a transparent dielectric and then calculated the intensity profile needed for absorbing the incident light. The calculations confirmed that when the intensity of the incident wave grows exponentially (the dotted line shown in Figure 2), the light is neither reflected nor transmitted (the solid curve shown in Figure 2). That is, the layer appears perfectly absorbing in spite of the fact that it lacks the real absorption capacity. However,
However, the energy locked in the layer is released when the exponential growth of the incident wave amplitude comes to a halt (at t = 0).
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, however, it was the mathematics that led us to the effect. Who knows, electrodynamics may well harbor other fascinating phenomena.
Denis Baranov, a doctoral student at MIPT and one of the authors of the study.
The study not only deepens the general understanding of how light acts when it interacts with common transparent materials, but also have a broad range of practical applications. For instance, the light accumulated in a transparent material can help design optical memory devices that could store optical information without any losses and release it when required.