I work in optics, so when I'm in grant-writing mode, optics and lasers seem to be the best technology choice for every problem, including powering coffee machines. But in the part of the multiverse called reality, lasers aren't always ideal. This becomes particularly true when we move to longer wavelengths; terahertz radiation (basically, heat), in particular, which has wavelengths at around 0.1mm, is best described as painful to work with.

Part of the reason for this is that the sources for THz radiation are, well, unfriendly. They are either really bulky, or so small that they emit their radiation everywhere, making it impossible to collect efficiently. Fortunately, a recent paper on combining plasmonics and THz radiation has given me just the excuse I need to introduce you all to the wonderful world of... coherent heat.

Why are researchers interested in making coherent heat? Many non-conducting materials, with the notable exception of water, are nearly transparent to THz radiation, making it useful for imaging and spectroscopy.

For instance, a drug tablet is mostly transparent, but each of the different layers generates a small reflection, so the internal structure of the tablet can be imaged. The very bonds between molecules in a crystal vibrate very slowly compared to the vibrations between atoms within a molecule, so you need THz radiation to study these vibrational modes. And, of course, security personnel go a bit dreamy whenever they contemplate imaging systems that can see through clothes.

But all of these require either a large amount of radiation, or that the radiation is coherent. So how do we make THz radiation? Most light sources use a natural material to provide the right wavelength of light—then we optimize it and arrange it so that we get as much light as possible from a modified version of that material.

In the THz range, that strategy just doesn't work. That isn't to say that nothing emits THz radiation—you just need to heat stuff up to get some. But the emission from these sorts of devices is too weak to be of much use, and there's not much we can do with the process to optimize it. As a result, researchers have had to be quite inventive to generate useful THz radiation.

Straining semiconductors

THz sources come in two varieties (there are others, actually, but for our purposes here I am going to ignore them), both of which can be classified as almost useless. The traditional generation route is via lasers; take a pulsed laser and focus the light onto a very stressed piece of semiconductor material and it will excite a whole lot of electrons. Normally, these electrons would meander around until they ran into an atom that has a vacancy for an electron (called a hole), whereupon they recombine. If you apply a voltage while this is happening, the electrons and holes will accelerate, and emit radiation as they do so.

To get THz radiation from this, the semiconductor is "strained," meaning that the atoms don't quite sit at evenly spaced intervals. The electrons will bounce off these atoms and recombine with holes very quickly—the time is actually around a picosecond, meaning that the frequency of the emitted light has frequencies of around a THz.

To optimize the process, the electrodes that apply the voltage are also THz frequency antennas. The laser light is focused on the gap between the antenna poles, and the coherent heat is radiated rather like radio waves from the antenna. The end result is a pulse of THz radiation emitted from the antenna, which can be collected for use.

The coherent heat produced by such an apparatus is very much like laser light: easily focused and emitted as a relatively narrow (for the wavelength) beam.

Quantum cascades

If you don't need quite so much fanciness, you go for the alternative, called a quantum cascade laser, which looks a bit like a diode laser. In this case, a very large sandwich of different semiconductors provides a series of little pots, each of which can hold an electron. Once a voltage is applied, the pots at one end of the stack drop down, so that each of the pots sits at a slightly lower energy than the one before it.

An electron enters the top pot, and, as with all excited objects, it wants to lose some energy. It can do this in two ways: it can recombine with a hole and vanish from our pot—that would be bad—or it can emit a photon and decay to the lower energy level within the pot—which would be good. We engineer the spacing of these energy levels by choosing materials and spacings so that the energy levels in the pots correspond to THz frequency photon energies.

So, assuming everything goes right, a THz photon is emitted and the electron ends up sitting in the lowest possible energy state in the top pot. But sitting right next to that is another pot, and its lowest energy level is even lower, and an excited state is nearly at the same energy as the pot the electron is in. The wave functions of these two states overlap and interfere to create one wave function that encompasses both pots. As a result, the electron tunnels to the next pot, whereupon it can emit another THz photon, and then it cascades down the second series of pots.

This can be helped by having lots of THz light flying around, because these light fields can stimulate emission and increase the probability of the electron doing the right thing (though the probability has to be high even in the absence of the light field). But THz radiation has a rather long wavelength, and this means that as it radiates, it spreads out rapidly, reducing its intensity quite severely.

To get around this, researchers often flank the semiconductor sandwich with metal plates, creating a waveguide that confines the THz radiation and increases the intensity of the light within the region of the pots. But the effect of this is disastrous in terms of utility, since the light emitted sprays out in every direction, making it impossible to collect most of it for use.

Playing tricks on nature

Ultimately, diode lasers are the way to go—teams of researchers celebrate every time some clever engineer figures out two things: how to replace a bench laser with a diode laser and how to replace a photomultiplier tube with a photodiode. So it is worth investing the time to figure out how to improve the emission properties from the quantum cascade, which is where the paper that launched this discussion comes in.

The quantum cascade laser contains a light field that is bound to the surface of a metal plate—this would typically be known as a surface plasmon polariton. Except it isn't; the wavelength is too long and, in a real surface plasmon polariton, the electrons in the metal are pushed around together by the light field. The electrons carry the light field with them, shortening the wavelength, compressing the light fields, and keeping the light bound to the surface. At long wavelengths, this kind of falls apart and the light field doesn't stay very attached to the metallic surface.

Why do we care? If the light were traveling as a surface plasmon polariton, then the researcher could engineer the exit of the quantum cascade laser to ensure that the light was emitted as a directed beam, rather than spraying everywhere. So, the researchers asked themselves whether we could fool nature and get it to believe there's a surface plasmon polariton in these quantum cascades.

The answer is to carve grooves into the surface of the metal. The grooves each reflect a tiny amount of radiation. Since the reflected radiation is itself reflected, the end result is that the entire wave slows down, allowing the electrons to respond and confining the wave to the surface, just like a surface plasmon.

Now that the researchers had something that looked like a surface plasmon, they could design a set of grooved patterns that resulted in the wave emitting in a narrower beam. The end result was a quantum cascade laser that emitted over a narrow range of angles, much more in keeping with regular laser diodes.

THz applications have not yet had the impact that researchers thought they would. But it's still early for the technology, and it certainly does have its uses. At the very least, it would obviously be a pretty damn cool way to make coffee.

Nature Materials, 2010, DOI: 10.1038/NMAT2822

Listing image by Sandia National Labs