Over the past three decades, scientists have been working on making light and matter interact strongly. This may come as a surprise, since nearly every bit of color we see around us is generally due to the interaction of light and matter. But this interaction is quite weak, and what we see is the result of light interacting billions and billions of times.
If you want to observe light interacting with a single atom or molecule, what must be done? The answer is to confine the light into a very small volume that just happens to contain the atom. But this is easier said than done. To make this job easier, researchers have shown us how to put light in a bottle.
Getting light to interact with matter isn't hard—light will always interact with a single atom, but it is, for all practical purposes, impossible to detect. To observe these interactions, what we need to do is take the same set of photons and bounce them off the same atom repeatedly until something happens.
Tuning an optical resonator
To do this, light needs to be confined in an optical cavity. Take two very highly reflecting mirrors—a typical silver mirror reflects between 70 and 95 percent of the light, while these mirrors have reflectivities of 99.99 percent—and place them facing each other, so that the photons between the two mirrors will bounce back and forth billions of times. An atom between the two mirrors has a billion chances to interact with the photon. To increase the chances further, the mirrors are curved so that they focus light to a very tiny volume at the center point between them. Placing the atom in the focus increases the chance of interaction.
These two properties—the number of times a photon can bounce back and forth through the volume where the atom lies, and the size of that volume—determine how strongly the light will interact with the atom. The larger the number of bounces and the smaller the volume, the better.
One of the best types of containers (called resonators) for this type of work is a perfect sphere of glass. Photons can travel around the inside of the glass sphere in a so-called whispering gallery mode, always hitting the surface at an angle such that the reflectivity of the surface is exactly 100 percent, called total internal reflection.
Despite being inside a solid glass sphere, the photons can still interact with atoms outside the sphere through what is called the evanescent field. Effectively, part of the photon is always outside the sphere, but, until it interacts with something, there is no energy in the bit that is outside—if this sounds counter-intuitive, it is. But let's face it, all quantum tunneling descriptions are. So we have one great property: the number of times that a photon circles inside the sphere is huge.
Better yet, these spheres are tiny, less than 100 micrometers in diameter, so the volume of space that the photons interact with is tiny. This gives microspheres the best of all possible worlds for measuring light-matter interactions at a single atom and single photon level. Except... they are really, really horrible to work with.
To get light into an optical cavity, it has to fit. That is, when the photon travels a complete circuit of the sphere, it must travel a whole number of half-wavelengths. The spheres are tiny, so the color difference between two wavelengths that fit is huge. This becomes a problem because we can't precisely control the size of the spheres during manufacture, and nature chooses which colors of light atoms will interact with—the two rarely end up matching.
The usual solution is to make a bajillion spheres and find one that is close to right. Then you can heat the sphere so that it expands until you get to exactly the right color. It would be much better to just have a resonator that could adapt itself to any color of light.
Rolling out the barrel
This is precisely what a group of German researchers have achieved. They created an optical cavity based on the same principles as a microsphere, but using something shaped like a barrel with both ends pinching off similar to the opening of a wine bottle. In these objects, the light travels in a spiral until it reaches the end. At the end point, the photon needs to change the orientation of its electric fields to keep moving. For this to happen, the photon needs some extra angular momentum. Since angular momentum doesn't just appear out of nowhere, the photon reflects off the end point and spirals back up the barrel to repeat the process at the other end.
The cool thing is that although the dimensions are still very small—the bottles are 35 micrometers in diameter and around 100 micrometers long—the different colors that can fit in this cavity are closely spaced because they take different spiral paths along the barrel. Furthermore, fine-tuning can be achieved by tugging on the ends of the resonator, changing its dimensions to fit the desired wavelength.
That sounds pretty good, but surely the volume goes up, making matters worse? Well, yes and no; the volume does increase, but not that much. And, better yet, the photons spend a lot more time at the turning points near the ends of the barrel than they do in the center, so there are optical sweet spots to play with.
So, is there a down side? Yes—getting light into these cavities is more difficult than with a spherical resonator. For a spherical resonator, one places a tapered optical fiber near the resonator, which allows the photons to leak from to the optical fiber into the whispering gallery mode of the resonator via evanescent fields.
Because the exact track of the whispering gallery mode of the barrel changes depending on the color, one must carefully position the fiber along the barrel so that it sits over one of the spirals. In addition, the light in the barrel and the light in the fiber must be traveling at the same speed—actually, the conditions are more complicated than that, but basically, momentum must be conserved in the transfer—so the geometry of the fiber must be carefully controlled and changed depending on the color of light to be coupled into the cavity.
If this is so difficult, is it worth it? Well, sometimes yes. Coupling into resonators is always going to be difficult, and the problems described in the previous paragraph are always present. It's just that, with a resonator that will only take one color, you don't notice them so much.
I have a friend who is doing quantum optics with a microtoroid and, because he cannot control the exact dimensions of his toroid, he is going to spend half his life trying to make a laser that can change color and yet be very, very stable once set. This is, of course, doable, but it's much easier to stabilize a laser with a fixed wavelength. So, I suspect he would rather have a more complicated coupling problem rather than two complicated laser problems.
I should also note that this not the first time that a bottle resonator has been demonstrated—it is simply the first one that compares nicely to microspheres and microtoroids.
Physical Review Letters, 2009, DOI: 10.1103/PhysRevLett.103.053901