Transparent Materials Can Absorb Light | An Unusual Optical Effect

  • Physicists have made transparent material ‘virtually’ absorb light under certain conditions. 
  • They studied a thin layer of transparent dielectric and measured the sufficient intensity required for absorbing incident beam of light.
  • When the intensity of incident light increases exponentially, the beam of light is neither reflected nor transmitted. 

In school, we’ve learned that when visible light waves, consisting of continuous range of wavelengths, strike an object, a number of things could happen – it could be absorbed and converted to heat, reflected or transmitted by the object.

It mostly depends on the frequency of light and nature of the object – one object might reflect blue light while absorbing all other frequencies of the visible light, whereas another object might selectively transmit green light while doing the same.

When the object is transparent, the electrons’ vibrations are transmitted to neighboring atoms through bulk of the material and re-emitted on the opposite side of the object. One of the main effects of the electromagnetism is absorption of electromagnetic radiation. It happens if electromagnetic energy is converted to heat (or some other type of energy) within an absorbing substance.

Carbon nanotube arrays and coal appear black in color because they completely absorb all of the incident light energy. Quartz and glass, on the other hand, have no absorbing powers, thus they look transparent.

However, a team of physicists from the United States, Sweden and Russia has demonstrated a quite odd behavior of light – they made transparent material ‘virtually’ absorb light. The research and experiment are fascinating because, as we’ve mentioned, transparent materials are known to have no light-absorbing capacity at all. So how did they do that?

Theoretical Research

Artist’s conception of virtual light absorption process in transparent material

To make transparent material appear absorbing light, the physicists used some special mathematical properties of scattering matrix. They employed an incident electromagnetic field function with the one scattered by the system.

A unitary property of the scattering matrix shows that the light gets scattered when a beam of time-independent intensity hits any transparent object. However, this unitary property can be tweaked (at least for a certain amount of time) if the intensity of the incident beam is varied with respect to time in a specific way.

If the intensity increases exponentially, the total energy of incident light accumulates inside transparent material without leaving it, making the material appear perfectly absorbing from the outside.

Reference: OSApublishing | doi:10.1364/OPTICA.4.001457

Experiment And Calculations

Virtual absorption effect in a thin transparent material layer

In order to demonstrate this effect, the team studied a thin layer of transparent dielectric and measured the sufficient intensity required for absorbing incident beam of light. Their measurements confirmed that when intensity of incident light increases exponentially (dotted curve), the light beam is neither reflected nor transmitted (solid curve).

Although it has zero actual absorption capacity, the sheet of transparent object seems perfectly absorbing. When you stop increasing the amplitude of incident wave, the energy trapped in the layer is released (at time t =0).

The effect is not exclusive to lossless systems. The simulations carried out by the team show that the effect is robust to the degree of loss of material, given that the incident light beam has an accurate time dependence matching the scattering zero’s position in the complicated frequency plane.

The ideal light storage won’t occur in lossy systems, since only a tiny amount of the incident beam is re-emitted, while a finite amount of energy is absorbed by the object when a beam of light comes in contact. The same response can be observed in canonical structures like dielectric rods and slab.

The effect is not only limited to optics. The scattering matrix formalism is equally applied to lower frequencies, for which slower variations are anticipated, to acoustics and to single-particle quantum mechanical issues.

Read: Graphene Nanoribbons Emit 10 Million Photons Per Second | New Study

Applications

The research not only expands our understanding of behavior of light when it comes in contact with transparent materials, but also has numerous useful applications. For instance, the light trapped inside a transparent material may help physicists develop optical memory instruments for storing optical data without any losses. The possibilities are countless, who know, electrodynamics may harbor other interesting phenomena.

Written by
Varun Kumar

Varun Kumar is a professional science and technology journalist and a big fan of AI, machines, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out about his latest projects, feel free to directly email him at [email protected] 

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