Terahertz frequency generator (THz) uses high temperature superconducting materials.

Electromagnetic radiation in the THz frequency band (1012 Hz) can yield enormous applications, from detecting explosives to cancer diagnosis and treatment. But the obstacle between the microwave and infrared radiation (THz radiation) is not easily overcome by THz radiation, which is not easily produced because their frequencies are too high for spirits. The package is based on semiconductor materials, but is too low to be made by solid state laser machines. And recently, US, Turkish and Japanese researchers have shown that this problem can be solved by exploiting the Josephson contact layer in high-temperature superconducting materials ( Science 318, 1291 ).

The Josephson junction is made up of two layers of superconducting material separated by a long known dielectric layer, as one of the typical quantum tunneling effects. If we place a voltage across this contact layer to create an alternating superconducting current, it will lead to the emission of photons at frequencies consistent with the superconducting energy gap. In other words, the Josephson contact layer can produce electromagnetic radiation .

Unfortunately, the energy gap in the Josephson contacts created in the laboratory based on traditional low-temperature superconductors (such as Niobium, Nb) is too small to produce radiation in the frequency range. THz and worse, the output capacity is also quite low. The researchers tried to use the trick to create contact layer arrays to increase the transmit power, but the synchronization of the contact layers was very difficult so the generation of electromagnetic radiation combined more become more difficult.

Figure 1. Component structure of Welp group (Science 318, 1291).

Ulrich Welp (from Argone National Laboratory, USA) and his colleagues confirmed that the two problems can be solved by using high-temperature superconducting materials. Unlike low-temperature superconductors, high-temperature superconductors do not need to be created in the Josephson junction because they naturally contain quantities everywhere in single-layer structures. And at the same time, they also have relatively large energy slots that can radiate radiation in the THz wave range. And more importantly, Welp's team discovered a very simple way to synchronize radiation (the phase of intrinsic waves from Josephson junctions in high-temperature superconductors) to have can generate transmit power at miliwatts (mW). "We can see a lot of applications such as exploration, image recording . using THz radiation for this range of capacity" - Welp said.

The team used Bi ₂ Sr ₂ CaCu ₂ O ₈ high-temperature superconducting materials, known as BSCCO acronyms with intrinsic Josephson layers created and sequentially arranged between CuO superconducting layers. ₂ rubbish strips and BiO and SrO dielectric layers. When placing a voltage across the BSCCO sample, these layers will cause electromagnetic radiation to emit a certain frequency but not phase. Just like with lasers, the trick to creating synchronous radiation is to change the voltage until the emitted frequency corresponds to the cavity resonance frequency. At that frequency, the electric field will compensate for each other in phase and help the radiation to synchronize. At first there were only a few layers of synchronous contact, but then this effect was intensified more intensely, thanks to the type of feedback that resulted in the entire wave being emitted.

Figure 2. Result of radiated frequency ( Science 318, 1291 ).

Welp's team made BSCCO samples with a height of 300 µm, and created a system with 200,000 intrinsic Josephson junctions, and emitted 0.5 µW of power for frequencies up to 0.85 THz .

Welp said he expected that due to technical optimization, the output power could reach 1 mW. This level of power can be used, for example, at the airport to find the markings of explosives, although he admits that there are some difficulties with using this device at the commercial level. " In general, the higher the capacity, the better the signal-to-noise ratio will be and will be able to proceed faster and more accurately in image-recording applications " - Welp added.

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According to Science & Physicsworld.com, Vietnam Physics