You start by focusing two infrared lasers in a device called a photomixer, with the lasers tuned so that the difference between their frequencies is a frequency corresponding to one of the terahertz transmission windows. The photomixer combines the lasers so that the resulting light “beats” at this terahertz- frequency difference. The beating laser drives a similar photoconductor-antenna structure to the one used to generate pulses, causing current to flow through it at the terahertz-beat frequency, thereby generating many microwatts of T-rays.
The method was demonstrated over a decade ago but became practical only a few years ago, thanks to pioneering work by researchers at the imaging start-up TeraView. The key was in a new type of photomixer, made of indium-gallium-arsenide, which could efficiently mix lasers of a wavelength easily carried on optical fibers. Channeling the lasers on optical fibers instead of having to carefully align laser beams with expensive optics has greatly simplified terahertz imagers and has also had the added benefit of driving down their cost.
These optoelectronic methods work well enough, but they are of limited brightness and are still quite cumbersome. What terahertz researchers really want is to replace these technologies with a bright, completely solid-state terahertz laser. It’s their best hope of getting imagers smaller, lighter, and cheap enough to mass-produce, not only because the light source is smaller but also because its higher brightness would allow for less expensive and more compact detector arrays. Unfortunately, the wavelength of semiconductor lasers is largely determined by the materials that are used to make them, and none naturally produce T-rays.
A device called a quantum cascade laser, invented in 1994 by Federico Capasso, among others, at Bell Labs, could be a big part of the answer. Unlike other semiconductor lasers, QC lasers can be engineered to emit any of a range of micrometer-wavelength light, including terahertz wavelengths. The secret to the QC laser is that its wavelength is determined by the thicknesses of the layers of semiconductor that make it up—something that can be carefully controlled.
Here’s how it works: lasers emit light when electrons that have been excited to a particular energy level fall to a lower energy level. A key difference between a laser and an ordinary light emitter is that there are always more electrons in the excited state than in the lower energy level. In a QC laser that aspect is guaranteed by sweeping fallen electrons from the lower, unexcited state into a third state, at a still lower energy level. In the QC laser these energy levels exist in three layers called quantum wells, each nanometers thick . Quantum wells are structures so thin that, from an electron’s perspective, they are two-dimensional. Confinement in a quantum well makes the electrons behave as though they were bound to an atom, with their energy constrained to certain specific levels.
An electron injected into the highest energy level falls to the lower one, emitting radiation (photons) of a wavelength that is determined by the thicknesses of the quantum wells. The electron then immediately falls into the still lower third state, emitting a quantum of heat called a phonon. What’s really remarkable is that this same three-layer structure can be repeated more than two dozen times. At each structure the electron goes through exactly the same dance, emitting the same color photon. So a single electron can emit 24 or more photons on its journey through the QC laser, as if it were falling down a set of stairs and emitting a photon at each step.
Last year, researchers at Sandia National Laboratories, in Albuquerque, used QC lasers to produce 138 milliwatts of terahertz laser power—a record. The one catch, and it’s a pretty big one, is that QC lasers must be cooled to within 10 degrees of absolute zero to perform at that level. At liquid nitrogen temperature, 77 K, QC lasers can’t even crack 10 mW. So the aim of much of the research into terahertz QC lasers is in improving the power output at higher and higher temperatures. The dream is a terahertz QC laser that operates at room temperature, or at least at 250 K, which is in the range of compact, inexpensive thermoelectric coolers.
T-rays can be detected in a number of ways. But one of the more common detector types is merely an extension of T-ray generation technology. Recall the picosecond-pulse generators and the continuous-wave generators. You can easily take the laser beam, split it, and feed it to another photoconductive antenna structure. But instead of applying a voltage across the antennas to push current through them and generate terahertz radiation, you measure the current through the antennas. As in the pulse-generation scheme, when the laser pulse hits a photoconductor, it creates short-lived pairs of electrons and holes. These then flow through the antenna under the influence of the electric field of incoming terahertz waves. So the current in the antenna, which is amplified, acts as a measure of terahertz radiation.
Because the detector is sensing T-rays only during the picosecond or so that the laser pulse allows, it takes several pulses to get the full waveform of the incoming T-rays. To get the full waveform, small increments of delay in the form of a longer path for the laser are added to the detector’s optical fiber line. Measuring the electric field at a number of increments produces slices of the terahertz wave that can be pasted together in a computer.
The same scheme works when pairs of tuned lasers are used instead of pulses. Recall that the lasers are tuned so that the difference in their frequencies is equal to a terahertz frequency. When T-rays hit the antenna, they mix with the terahertz frequency of the combined lasers to produce a dc signal. These two schemes are how detectors work in systems built by TeraView, Picometrix, and others.
Of course, detection is only the start of image making. The simplest way of producing an image is to scan a single transmitter and detector over an object and record the phase and amplitude of the T-rays that reflect back at each point. State-of-the-art terahertz-imaging security systems are capable of such raster scanning at a rate of 100 pixels per second, certainly not fast enough for video and only marginal for scanning a bag on a conveyor belt. A briefcase containing a gun, a glass bottle, and a knife would take half an hour to scan at a resolution of 1.5 millimeters per pixel using a T-ray pulse-based imager from Picometrix.
Although there are no terahertz camera chips, there are infrared camera chips, and you can tweak those so they can pick up T-rays. Such chips detect infrared radiation at each pixel because the radiation reduces the resistance of a minuscule patch of semiconductor there. By themselves, some of these chips are slightly sensitive to terahertz radiation, but to get a decent image you need a bright source such as the QC lasers under development.
Another infrared detector concept is electro-optic terahertz imaging. In this scheme, T-rays striking certain types of crystal—such as zinc telluride—will cause the crystal’s index of refraction to change. The result is that the polarization of infrared light passing through the crystal will rotate. Place a polarization filter between the crystal and a camera chip so that it blocks out any infrared light that hasn’t been rotated and you get an infrared facsimile of the terahertz image. In a sense, the terahertz radiation has been shifted up the electromagnetic spectrum into the infrared. Such cameras can produce pictures in less than one-sixtieth of a second, far quicker than raster scanners and fast enough to produce video. Unfortunately, they also erase the spectral information that lets you chemically fingerprint objects. All that information is wrapped up in the mix of T-ray wavelengths that strike the crystal, but the crystal’s change in refractive index, which produces the image, is relatively insensitive to color.
In an effort to get both speed and spectroscopy at a reasonable price, our team at New Jersey Institute of Technology, in Newark, has been developing an imager that, with only a dozen detectors, can produce complete images quickly enough for video and at a resolution comparable to what you’d expect from a kilopixel camera chip. The method, called interferometric imaging, relies on a common mathematical concept used in image processing, the spatial Fourier transform. According to Fourier theory, any signal can be broken down into the sum of many sine waves of different frequencies, phases, and amplitudes. Though it is less intuitive, the same can be said of any image. To get a grasp of spatial frequency in an image, imagine a photo of an American football referee in the traditional black-and-white vertical-stripe jersey. The spatial Fourier transform of that image would be dominated by the frequency that matches the jersey’s stripes.
