G band technology
According to Institute of Electrical and Electronic Engineers (IEEE) there are 13 frequency ranges, each of which has its own letter. Letter G designates sub-terahertz frequency range between 110 and 300 GHz. This frequency range is also called millimeter band.
IEEE G band radiation has its specific properties; one of them is high atmospheric attenuation: atmospheric gases (oxygen and water vapor) absorb these waves. Therefore, special frequency ranges called “transparency windows” are used to create applications. Good example is Atacama Large Millimeter Array (ALMA) radio telescope build in desert to avoid influence of water vapor.
Traditional applications of G band waves count for radio astronomy, remote security sensing and telecommunications. G band is ideal for NDT (non-destructive testing) and for personnel security body scanners as well, the most interesting and important applications. NDT control works with materials like wood, plastics, grains, dry food, paper, ceramic and many more.
THz technology grows rapidly in this frequency range since G band has many advantages over other ranges:
- Many dielectric materials are translucent for G band waves;
- Spatial resolution in G band is reasonably acceptable for many industrial applications and lays in the range of 1-3 mm;
- G band sources could be built taking advantage of IMPATT diode technology that gives high output power;
- And last, but not least, price of THz wave sources and THz cameras could be very affordable that boosts popularity of G band devices.
G band generators
IMPATT (IMPact ionization Avalanche Transit Time) diode technology is a widespread solution for G band generators. Having negative resistance, IMPATT diodes are naturally used as oscillators in high-frequency signal generation, up to several hundred GHz.
Beside IMPATT diode technology, generators for G band can be built on InP double heterojunction bipolar transistor oscillators. Indium Phosphide is used in high-power and high-frequency electronics on grounds of its superior electron velocity with respect to the more common semiconductors like Si and GaAs.
Another solution for G band generators can be resonant-tunneling diodes (RTD) technology. RTDs can be very compact and are also capable of ultra-high-speed operation because the quantum tunneling effect through the very thin layers is a very fast process.
G band detectors
All THz detectors could be divided into two groups: frequency selective and non-selective detectors. Non-selective (thermal) detectors, such as bolometers and Golay cells, measure integral power of signal without detailing the THz radiation spectrum. Big disadvantage of bolometers is their long response time. Bolometers need tens of milliseconds to measure the signal.
Terasense offers a plasmonic-based room-temperature operating zero-bias detectors. In a TeraSense detector terahertz radiation is converted into the alternating potential of a relativistic plasma wave via a broadband antenna structure deposited onto the crystal surface. Then the alternating potential of the plasma wave is rectified to yield the measured photoresponse signal owing to the asymmetry of the plasmonic waveguide. These detectors could have response time as short as 150 picoseconds, spatial resolution of 1-3 mm and affordable price.</p