Top: THz transmission image of a battery, circuit and speaker—which, together, form a device that is used in a musical greeting card to play music when the card is opened (bottom).
Of course, titanium:sapphire lasers, which emit femtosecond pulses between 700 and 1,000 nm, still have their price. Femtosecond fiber lasers with emission wavelengths around 1,064 and 1,550 nm are also available. They are somewhat less expensive but call for new semiconductor materials with a lower band gap because of the longer emission wavelength. Lately, good progress has been made using LT-GaAsSb, LT-InGaAs, LT-GaAsBi and Fe-implanted InGaAs. Alternatively, nonlinear crystals can be used.
A cost-effective alternative, which also falls in the second class, is photomixer-based systems. Here, the femtosecond laser is replaced by two single-mode diode lasers with slightly different emission frequency or a single laser operating on two lines simultaneously. The emission of these lasers is superimposed again on a photoconductive antenna. The resulting light beat is converted into an oscillating antenna current, which is the source for a monochromatic THz wave.
As in the pulsed case, the power level of this continuous-wave radiation is in the microwatt range. Again, the THz wave can be detected coherently and background-free by a second antenna. Although this scheme is sufficient for simple industrial applications, it is less powerful than time-domain spectroscopy, because images are typically obtained only at one fixed frequency. To derive tomographic information, lengthy frequency scans would become necessary.
The list of THz receivers is somewhat shorter. Coherent detection using photoconductive antennas is very powerful. In addition, one can use Schottky mixers, which embody classical microwave technology, or superconductor-insulator-superconductor mixers, which require cryogenic cooling and are often used as heterodyne detectors in radio astronomy. There are also detectors based on the heat generated by the incident THz radiation. This class includes Golay cells, pyrometers and a variety of bolometers. Golay cells, which cost several thousand dollars and use the heat-induced expansion of a gas, are currently not produced and out of stock.
Costing just a few dollars, pyrometers are much cheaper but are not very responsive. Very sensitive bolometers also need liquid helium temperatures to operate. Furthermore, there are several detector schemes based on more or less exotic semiconductor effects.
In addition to these single-pixel detectors, two-dimensional detector arrays are currently under development, predominantly for military and security applications. Most of them are based on highly developed microwave technology at a few hundred GHz. Higher THz frequencies can be detected with room-temperature micro-bolometer arrays for the mid-infrared. These cameras have been shown to hold a small residual sensitivity at THz frequencies. Hence, a powerful THz source such as a gas laser or QCL is required for illumination.
Finally, many optical components are being developed for the THz range. Traditionally, plane and curved metal surfaces have been used to reflect and focus THz waves. In recent years, however, several approaches have emerged for developing a variety of quasi-optical components from dielectric materials. These components include mirrors, filters, lenses and waveguides.
Applications for THz imaging systems
In a few years from now, microwave-based cameras will likely become fixtures in many airports. They will be used to search for concealed and suspicious objects on fully clothed airline passengers. This will become possible because low-frequency THz waves have the capability of penetrating most textiles, while water-containing objects—including the human body—are impenetrable and reflect the waves. At present, several companies are intensively developing this type of security camera and working to improve imaging speed and quality.
Microwave-based arrays are also being developed for military purposes. They are being used to detect hostile targets even under bad weather conditions. In the civilian sector, this technology may play a role in landing aids for aircraft. These microwave systems are not optical or optoelectronic systems in the classical sense.
A mature THz technology will take spectroscopic investigations to the next level, allowing explorations of rotational transitions of polar molecules in the gas phase and vibrational modes of crystals and biological macromolecules. Although almost all classes of materials have been spectroscopically investigated, including semiconductors, metals, superconductors, insulators, polymers, biomolecules, explosives, drugs, liquids, gases and metamaterials, material characterization is ongoing and the catalogue of substances to be studied is nearly endless.
Applications for imaging systems based on THz time-domain spectroscopy may be used for security checks for luggage or mail. The top image in the figure above shows the THz transmission image of a battery, circuit and speaker—which, together, form a device that is used in a musical greeting card to play music when the card is opened. Although the device is hidden inside an envelope, the THz waves render it clearly visible.
Although detecting metallic parts inside a paper envelope may seem trivial, it becomes very important when the object to be uncovered is not a music-playing device but a detonator for a mail bomb (which may look very similar). Researchers are also discussing if it is possible to identify drugs and explosives in an envelope or package using spectroscopic fingerprints. Although first experiments look very promising, this undertaking is challenging because the spectra may be masked by the absorption of other items in the beam patch and by interferences arising from the multitude of interfaces that a probing THz beam experiences.
THz waves could also play a role in diagnosing skin diseases. (Because THz waves cannot penetrate more than a few hundred micrometers of human tissue, their use is restricted to an investigation of the body surface.) Research has indicated that pulsed THz imaging may have the potential to distinguish between basal cell carcinoma and other skin irregularities. Dermatologists have not shown much interest in this technology, however, mainly due to the high price of pulsed THz systems.