The new T-ray sources created at Argonne use high-temperature superconducting crystals grown in Japan. The crystals are composed of stacks of so-called Josephson junctions, exhibiting a unique electrical property: when an external voltage is applied to the crystal, an alternating current flows back and forth across the junctions at a frequency proportional to the strength of the voltage. This phenomenon is known as the Josephson Effect. The alternating currents produce electromagnetic fields whose frequency is tuned by the applied voltage. It’s enough to apply a voltage of 2 millivolts per junction to the crystals to induce electromagnetic fields of frequencies in the terahertz range.
Since the junctions within the crystals are so small in size (1/10,000 the width of a human hair), the scientists stacked approximately 1,000 of them in order to generate a sufficiently powerful signal. The real challenge was finding a way to make them all radiate in phase, so that they do not cancel each other out. In order to synchronize the signal, Argonne physicist Alexei Koshelev suggested that the stacks of junctions be shaped into resonant cavities. When the width of the cavities was precisely tuned to the frequencies set by the voltage, the natural resonance of the structure synchronized the oscillations and amplified the T-ray output, in a method similar to the production of light in a laser.
The researchers are currently working on improving the extraction efficiency. Getting the signal up to 1 milliwatt will be considered a great success that will enable the creation of a new generation of scanners. In the meantime, T-rays capable of penetrating the human body by almost half a centimeter are already helping doctors to better detect and treat certain types of cancer, such as skin and breast cancer.
More information about T-rays and their potential applications can be found on the Argonne National Laboratory website.