In the world of physics, there is nothing with higher geek credibility than making a normally opaque object appear transparent. One of the first examples of this was something called electromagnetically induced transparency (EIT). This basically involves shining one light beam on a substance to modify it in a way that allows a different light beam to pass through unhindered.

Now, the cool thing about this is that it depends on the details of the atomic or molecular structure of the substance. Which means that, aside from letting physicists do EIT party tricks, it can be used as a sensitive probe that can separate nearly identical substances from one another.

Show me how to turn invisible EIT is a very strange phenomenon, and it takes a bit of explaining. First let's create an atom with just three energetic states. Even though the absolute energy levels of these states don't matter, I will refer to one as the excited state, and the other two as ground states, which I will call "home" and "away." To make EIT work, home and away must be states that cannot be reached directly from each other. In other words, even if we shine a light beam with the right frequency, no light will be absorbed and no atoms will be excited from home to away or visa versa. This can occur when two states have different angular momentum requirements. In this case, the light beam and the electron that makes the transition cannot exchange the right amount of angular momentum with each other to allow the transition to take place. The excited state, however, has no such restriction. An atom in the excited state can decay into either ground state, emitting a photon. Conversely, a light field with the appropriate frequency will excite atoms from one of the ground states to the excited state. So, we'll take this as a description of a gas of atoms, which all start out in the home state. If we shine a light field, called the probe, with the right color to excite atoms from home to the excited state, it will be quickly absorbed and excite some of the atoms. If we instead shined light so that it would excite atoms from the away state to the excited state, well, there are no atoms in either state, so the light is not absorbed at all. This second light field is called the control. But, even though no atoms are in the excited state or the away state, the control light field can still have an effect on the atoms. The energetic states still exist, even when there are no atoms populating them. If there were no home state, then the control light would drive all the atoms into the excited state and then back down into the away state again. This sequence of collective excitation and relaxation, called Rabi oscillations, will continue at a characteristic rate, called the Rabi frequency. The Rabi oscillations involve shuffling of electrons, which generates a very strong internal electric field oscillation within the atoms. This splits the excited state into two different energy levels, separated by exactly the Rabi frequency. Amazingly enough, this splitting still occurs when there are no atoms in either the excited state or the away state. Simply the possibility that it could happen—due to the presence of the probe and control light fields—is enough to set things in motion. And this change has consequences. Once the excited state energy level has been split into two levels that have a slightly different energy, then the probe light field no longer has the right frequency to excite atoms out of the home state and into the excited state. So the light is no longer absorbed, and passes straight through—the material has become transparent thanks to the probe's electromagnetic field. Hence EIT.

When an atom has a light shining on it, it's not the same as an atom in the dark (see side bar), since the light field can shift the exact colors that an atom likes to absorb. By hitting a gas of atoms with a very strong light field at one color (called the control field), we can shift its electronic states. If we shift them far enough, then a second light field that would normally be absorbed, called the probe, does not get absorbed at all. This is called electromagnetically induces transparency.

The usual description of EIT, involving just three energy states of an atom, works really well for things like simple gases of alkali atoms (think sodium). But molecules have a much more complicated set of energy levels that combine electronic states with vibrational and rotational motion. The result is that many of the states have very similar absorption features that simply cannot be separated by normal spectroscopic techniques. This makes the identification of molecules very difficult, and the identification of mixtures even more challenging.

So, a trio of researchers from Vancouver have calculated how the energy level shifts that occur during EIT would effect spectroscopic measurements of molecules. To picture this, imagine you have a molecule with two excited states that are so close in energy that you cannot see the difference with your spectrometer—all you see is the average of the two.

Now, we can tune the color of our control laser across this transition. As we tune, we are going to sweep back and forth with our probe beam to see what happens. As the control laser frequency approaches the right color for the first excited state, that state splits into two energy levels, one much higher than its natural position and one much lower. The second excited state, which requires a slightly different color from the control laser is not split.

The result is that the probe beam is only absorbed by the second excited state and not the first. We will observe that the peak of the absorption will shift to center on the second excited state. As we continue to tune the control laser, the splitting of the first excited state reduces to nothing and the second excited state starts to split. We can then measure the exact location of that state as well.

With that, we have individually detected two different excited states when previously we would have only suspected one.

I have to say that I love the idea in this paper. It connects in all sorts of ways to things that we have been working on in the last few years. There is even a diagram in there that looks suspiciously similar to something we are about to submit for publication (though luckily it was an afterthought in our paper, so we can still publish).

On the other hand, I think they are a little too optimistic in some respects. That control light field has to be strong enough to drive Rabi oscillations and the molecules have to be in a state where they can support Rabi oscillations. Unfortunately, every time a molecule collides with another molecule, it forgets what the hell it was doing and has to leave the room and come back again. This stops Rabi oscillations dead in their tracks.

Then there is the fact that it takes a certain amount of light to get the whole thing rolling. You need quite fast Rabi oscillations, meaning that you need a strong light source—for many vibrational transitions, we are talking about really, really strong light sources. If it's strong enough, you end up with other physical processes going on that disturb your measurement, and there is a good chance that these will be a problem.

Even so, I still love it, and can't wait to see some experimental results.

Journal of Chemical Physics, 2012, DOI: 10.1063/1.3683159