Making Waves

Electromagnetic radiation can be divided into many frequency bands, such as x-ray, visible light, and radio, and technologies have steadily emerged to make use of most of these frequencies—except for radiation having an oscillation frequency in the terahertz range. With wavelengths of a fraction of a millimeter, shorter than microwaves and longer than infrared, terahertz waves are comparatively difficult to create, manipulate, and detect.

Los Alamos scientists Nathan Moody, Lev Bulaevskii, and Vitaly Pavlenko are pursuing new methods that may make this type of electromagnetic radiation more accessible. If their research into a practical terahertz source continues to prove successful, it could open up a wide range of important new applications, owing to the novel properties of these waves. Terahertz waves can pass through many materials (such as paper, wood, plastic, cloth, and others), yet they are harmless to living tissue. They can also interact with everyday molecules to "see" what an object is made of. Thus, they could potentially improve security and medical scans by imaging something hidden, for example, inside a container or beneath the skin, and identifying that hidden object's chemical composition. Other potential applications abound, including improvements in high-speed terrestrial and satellite communications.

Unfortunately, existing electronic components can't keep up with terahertz frequencies' trillions of oscillations per second because of the finite electron mobility in conventional materials, so new materials and new techniques are needed. Moody and his team are exploiting a quantum mechanical phenomenon called the Josephson effect as the basis of their approach. When a steady voltage difference is applied across a Josephson junction—a thin electrical insulator sandwiched between two superconducting slabs—an electrical oscillation is produced: pairs of electrons repeatedly jump through the insulator in a quantum mechanical process called tunneling. As it turns out, generating terahertz-frequency oscillations capable of producing the desired waves requires only a small applied voltage, about one-thousandth of the voltage provided by an ordinary AA battery. Additionally, the effect is tunable: a different applied voltage yields a different terahertz frequency.

Schematic

Schematic for a Josephson junction stack, with insulating layers (gray) sandwiched between superconducting layers (blue). An external, applied voltage difference generates an alternating current in the stack to produce terahertz waves.

The problem, however, is scale. A single Josephson junction generates only about a trillionth of a watt of terahertz power, which is far too weak to be useful. A billionfold improvement is needed, but trying to manufacture a very large array of junctions would result in insurmountable engineering challenges. Instead, Moody's team is leveraging an existing effect in a crystalline film called BSCCO (pronounced "bisco") to act as an assembly of about 10,000 atomic-scale Josephson junctions, packed within the size of a single sub-millimeter wavelength—close enough that their individual oscillations synchronize and give rise to coherent, laserlike emission.

Within the complex atomic structure of the BSCCO ceramic, a thin layer of bismuth, calcium, and lanthanum oxides serves as the Josephson junction's insulator, while a copper layer serves as its high-temperature superconductor. ("High temperature" in this context is about 200 degrees below zero Celsius, which is attained with relatively inexpensive, off-the-shelf cryocoolers.) Because the atomic layers must be oriented the right way for the superconducting current to flow across the insulating layers, Moody's team is pioneering new ways to either grow or modify the material, essentially laying down a row of nanometer-sized layers across the surface of a substrate (like books on a bookshelf), rather than the more typical approach which yields a vertical stack (like books stacked in a pile). They use either pulsed-laser deposition to grow the crystal on the substrate or pregrown crystals that are mechanically bonded and then etched to achieve the desired row geometry. Use of established manufacturing techniques, such as argon ion milling, might ultimately pave the way toward large-scale production at a reasonable cost.

Already, the team has produced several device designs based on Bulaevskii's foundational work, and is now engaging Los Alamos materials scientists to evaluate and improve their sideways BSCCO orientation process. Within a year, they expect to have a working demonstration of the fundamentals of a tunable terahertz source. The design of this source may also inspire or complement other functional components—filters, modulators, mixers, amplifiers, detectors, and so on—needed to assemble an entire terahertz system for a specific mission or application.

—Craig Tyler

 

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