Scientific Method / Science & Exploration

A laser that works better shaken, not stirred

Researchers show that giving a laser a good shaking can make it perform better …

I don't know about you, but when I think of lasers, I think of boxes on heavy, stabilized tables. Inside the boxes, the optical elements are mounted on stabilized mounts and everything is generally held as solidly in place as possible. The one thing that you generally don't do is give a laser a good shaking. Unless it has already stopped working, in which case, have at it... preferably with a hammer.

Finding a paper that demonstrated a laser with better performance when it was being shaken compared to when it was held came as a bit of a shock.

The reason why the idea of shaking a laser is so shocking to me is that the lasers I am used to working with have optical elements that need to maintain a precise alignment with respect to one another. Temperature changes, vibrations, shaking, and "thumping the box to fix it" are all really bad ideas. But of course not all lasers are like this.

The laser in your laser pointer, CD, DVD, and Blu-Ray players are all monolithic devices. That is, they are made from a single piece of material, or materials, that are deposited on one another. You can obviously shake a laser pointer (much to the delight of cats), but this capability doesn't scale. If you were to shake a laser pointer with acoustic waves that had a wavelength about the same size as the device (in the GHz range), then I would expect that things would probably go wrong rather quickly.

In most cases, at least. A group of researchers from Germany and Russia have now made a laser that works better when it is shaken. The reason why this occurs lies in the peculiar nature of the laser used by the researchers.

The laser that the researchers worked with was made from quantum dots (see side bar) embedded in a semiconductor material that had mirrors deposited on either side of it. This means that the distance between the two mirrors was extremely short. The researchers don't state how big the distance was, but from the figures, I estimate that it wasn't much longer than eight micrometers. For comparison, the wavelength emitted by the quantum dots was around 900nm.

How do quantum dots work as lasers? Lasers rely on light building up between the two mirrors, and only particular frequencies of light will do that. Consider it like this: a quantum dot emits a photon, which travels—or rather expands, since it takes time to emit a photon—toward a mirror. It reflects off that mirror, travels to the second mirror, and returns to the quantum dot, where the photon interferes with itself. If the interference is constructive—constructive interference is when all the peaks line up—light will build up in the cavity. If the interference is destructive, the light will die out. 

Constructive interference only occurs when light has to travel a complete number of half-wavelengths. In other words, we should be able to divide the length of the cavity by the wavelength of the light and get an integer result.

As the distance between two mirrors gets shorter, the gaps between wavelengths that will fit into the cavity get larger. In the laser that the researchers were using, the cavity is short enough that, within the range of wavelengths that the quantum dots can emit, only one satisfies that condition (or rather a small range of wavelengths around where the interference is perfectly constructive).

This presents something of a problem, because the inconsistencies among quantum dots means that many of them don't have the right energy levels to contribute photons that make up the laser light. Instead, they emit light in different directions and with the wrong wavelength.

To shake things up, the researchers sent sound waves into the laser. The sound waves repetitively strain the quantum dots, which temporarily modifies their energy levels. As a result, more of the quantum dots spend some time with energy levels that correspond to the wavelength of light that the laser is able to emit.

Even this might not help, because, in some materials, electrons don't like to stay in the excited state, and emit a photon to decay to a lower energy level almost immediately. In this case, the sound wave wouldn't have enough time to shift the energy levels before the electron was gone from the excited state. 

Luckily, left to themselves, the electrons in quantum dots are quite happy in the excited state, so they will hang around for a nanosecond or longer before decaying back to the ground state and emitting a photon. With the sound wave present, the quantum dots periodically enter into resonance with the cavity and are stimulated to emit. Under the influence of other photons, the electrons only take 20ps to decide that it's time to emit a photon. What that means is that the sound wave has plenty of time to move quantum dots into and out of resonance, so that more quantum dots can contribute to laser emission.

The result was a laser power that was 200 times higher than the laser without the sound waves, which is a pretty spectacular increase in efficiency. Normally, even if you got such an increase, you would expect that the spectral properties of the laser (the range of wavelengths that it emitted) would be messed up. In this case, though, the very short cavity, combined with the high reflectivity of the mirrors, prevents that from occurring. Overall, it means that there isn't really any serious downside to this at all.

Nature Photonics, 2011, DOI: 10.1038/NPHOTON.2011.269

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