In Astro Space Center of Lebedev Physical Institute (FIAN ASC), together with colleagues from Moscow State Pedagogical University (Moscow State Pedagogical University) have developed a new class of mixers for terahertz radiation detectors. In its basic characteristics — noise temperature and a range of intermediate frequency band received mixers significantly outperform the existing counterparts. Its application can find it in the device space telescope operating in the THz range.
Simple terms, our scientists have come up with antennas for domestic space telescope project Millimetron and security scanners at airports considerably higher than on the characteristics of their foreign counterparts.
The first telescope, receiving terahertz radiation was established in 2005 on a high plateau in the Chilean Andes, the Atacama. And in 2009 launched the space observatory "Herschel" also analyzes the cosmic radiation terahertz range. Russia plans to launch its own satellite project with similar features "Millimetron" in 2018, its development is done in ASC LPI. The reason for the increasing attention to submillimeter wavelengths is simple enough — more than half of the cosmic radiation lies in this range.
Commented one of the developers, the candidate of physical and mathematical sciences Matvei Finkel: "In the terahertz range are a few key for astronomy and astrophysics, molecular and ionic lines. Knowing with sufficient accuracy, their intensity and Doppler frequency shift, we can study a variety of chemical and physical characteristics. For example, the education of new stars — protostellar discs hidden dense gas and dust clouds. Only Terahertz radiation can penetrate through the clouds, bringing, for example, information about the density, temperature and velocity of the substance involved in star formation. "
For the detection of terahertz radiation using a detector based on mixers — superconducting bolometers with electronic heating. In the most basic case, the bolometer — this is a very thin metal plate. Upon irradiation with light is slightly heated, which leads to higher electrical resistance, which is fixed measuring system. The usual bolometer — a slow device, so when you use it as a mixer take special measures to accelerate and expand its intermediate frequency band. So research group FIAN-MPGU develops and explores the low-temperature superconducting bolometers. Operation they similar — the temperature is maintained at a plate of several degrees Kelvin, and its typical dimensions are tens of nanometers. Reduced heating a plate leads to the emergence of resistance and a sharp transition from the superconducting to the normal state. As a result, the bolometers have increased sensitivity and less prone to thermal noise.
The main characteristics of the mixers can be called intermediate frequency band and noise temperature. Ultimately, they determine both the effectiveness of the telescope. For example, the wider the band of intermediate frequencies, the greater the amount of information can be examined — because of the Doppler effect, scientists do not always know exactly at what frequency will be based on the spectral lines of interest to them. Noise at the same temperature receiver designs, however, trying to reduce.
"Given the sensitivity of the accumulation time the square of the noise temperature. Therefore, the lower the noise temperature — the better the telescope observing time is spent and the more faint signals from distant objects can be observed. To reduce the noise temperature is not important to reduce the thermal noise — they are already quite low, on the order of 5-10 K, and reduce the loss of the useful signal, which largely depend on the quality contacts between the antenna and the superconductor. "
Mixer disclosed in work, has unique characteristics — a record low noise temperature of 600 K in a wide range of intermediate frequency 7.1 GHz. For comparison, similar characteristics heterodyne receiver on board "Herschel" are 900 K and 3.5 GHz. Achieve such unique results have been achieved through several innovative solutions. Thus, the researchers had chosen the optimum thickness of a superconducting plate NbN (mixer basis) of 3.5 nm. This thickness with submicron dimensions of the bolometer helps combine the two mechanisms of cooling of hot electrons — due to the electron-phonon interaction in the bulk of the plate and the diffusion of electrons in contact with the antenna plate. And in the cooling rate depends mixer intermediate frequency band and the noise band of the receiver as a whole. In addition, the researchers developed a unique technique for creating mixers.
"Impressive results have been made possible thanks to the development of technology for the manufacture of mixers with the application of the gold coating in situ. Immediately after the deposition of niobium nitride, which occurs in the initial conditions of deep vacuum and inert gas atmosphere is deposited gold layer, which covers an ultra-thin film of niobium nitride from external influences. But Guide to get the mixer should be the active region of the film release of gold. We do this through successive processes of ion and selective chemical etching. At that contacts gold antenna with an active bridge of NbN obtained almost ideal, not letting any flow of electric current or heat transfer electrons from the bridge to the gold antenna NbN ", — Matthew says Finkel.
Bolometers on the effect of electron heating is also used to generate terahertz imagers, imaging systems, which compares favorably to the safety of X-ray and infrared thermal imagers from — power. Exit the article with a detailed examination of the results is scheduled for the coming time. Summary of the papers can be found in Applied Physics Letters.