Closing the Terahertz Radiation Gap by Way of Superconductors
Stacks of Josephson junctions help form oscillations that produce terahertz frequencies.
Lynn M. Savage
Much progress is being made in using electromagnetic radiation at terahertz frequencies for noninvasive detection and pharmaceutical testing applications, among others. Current technologies, however, limit generation and detection of the frequencies to less than 0.5 and more than 2.0 THz. This leaves a rather large gap to fill wherein advances in medical and security technologies presumably reside.
Electromagnetic radiation between 0.5 and 2.0 THz can be generated using a stack of superconducting layers of Bi2Sr2CaCu2O8 (BSCCO) sandwiched around nonsuperconducting insulators. Reprinted with permission of Science.
Now a multinational team of investigators has developed a novel method of generating radiation at a frequency of up to 0.85 THz using superconducting materials.
Led by Ulrich Welp of Argonne National Laboratory in Illinois, the researchers made a radiation source using crystals of Bi2Sr2CaCu2O8 (BSCCO), a high-temperature superconductor. The crystal consists of stacks of so-called intrinsic Josephson junctions — superconducting CuO2 layers that are separated by insulating Bi-Sr-O layers. Josephson junctions exhibit a unique electrical property: When an external voltage is applied, an alternating current flows back and forth across the junctions at a frequency proportional to the strength of the voltage. The currents produce the electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage (~2 mV per junction) can induce frequencies in the terahertz range. Each stack of junctions contained approximately 670 layers, forming a “mesa” about 1 μm tall.
Much like a laser — in which photons are oscillated between two mirrors until the light becomes coherent — the sidewalls of the BSCCO mesa reflect the electromagnetic waves back and forth until all the junctions oscillate coherently in phase. The researchers found that the emission power increased with the square of the number of coherent junctions; however, having too many junctions leads to thermal overload that cancels the superconductivity of the cuprate layers. At under 50 K, the system generated coherent radiation at up to ~0.5 μW at temperatures low enough to maintain superconductivity.
The scientists anticipate that the output power can be increased, making the technique a viable alternative as a source for midrange terahertz radiation.
Science, Nov. 23, 2007, pp. 1291-1293.
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