According to the consultation of Mamms, the study of radiation in Tera Hertz (THz) band, including infrared and microwave frequencies, is expected to make new progress in Astrophysics and bioimaging. But for some applications, there are still troubling obstacles on the way to compact, sensitive terahertz light sources and detectors: most must operate at very low temperatures, which increases the size and complexity of devices. However, recently published research by two groups has provided some ways to mitigate low temperatures during detection and generation of terahertz radiation.

Astrophysical tools

Astronomers have long dreamed of observing distant galaxies carefully. But limited by technology, it is impossible to detect and analyze stars and space activities billions of miles away in detail. Researchers at the Samueli School of Engineering at the University of California, Los Angeles (UCLA) have developed an ultra-sensitive light detection system that allows astronomers to observe galaxies, stars and planetary systems carefully.

Unlike previous optical sensors, the system developed by UCLA can work at room temperature. By contrast, similar technologies can only work at temperatures close to -270 C (454 F). For a detailed introduction and progress of this technology, please refer to the papers published in Natural Astronomy.

This new sensor system can detect radiation in terahertz band of electromagnetic spectrum, including far infrared and microwave bands. It can produce ultra-high resolution images and detect terahertz waves in a wide spectral range. It is reported that the spectral range of this kind of wave is 10 times larger than that of the current technology which only detects such wave in narrow spectral range.

At present, the sensor system envisaged by scientists needs several different types of instruments. For example, the system can identify whether an element or a molecule exists in a spatial region by looking at its unique indicative spectral characteristics.

Mona Jarrahi, professor of electrical and computer engineering at UCLA, said in a statement that observing terahertz frequencies allows us to see details that are not visible through other bands of the spectrum. In astronomy, the advantage of terahertz detection is that, unlike infrared and visible light, terahertz waves are not obscured by interstellar gas and dust surrounding these astronomical structures.

Jarrahi adds that this technique is particularly effective in space-based observations because, unlike Earth's environment, terahertz waves can be detected without atmospheric interference.

Scientists believe that the system can further understand the composition of astronomical objects and their structures, as well as how they were born and died. The system can also reveal the details of gas, dust and radiation interactions between stars and galaxies, as well as clues to the origin of the molecular universe, which can be used to determine whether planets are suitable for life.

To solve the above problems, it is necessary to extract available signals from very few deep space terahertz photons reaching ground telescopes using devices operating close to the limit of quantum sensitivity. The problem is that in order to reach this limit in heterodyne terahertz detectors, superconductor-insulator-superconductor (SIS) mixers are usually required. They operate only at low temperatures and convert terahertz frequencies to radio frequency bands for signal processing. In addition, such terahertz detectors often have relatively limited spectral bandwidth, which means that multiple different devices must be used to detect a wider range of terahertz radiation of interest.

Plasma solution

Jarrahi and her team solved this problem by radically changing the detector architecture. Most importantly, they replaced superconducting mixers with optical mixers including plasma contacts. The contact end is composed of a Ti/Au grating with a thickness of 50 nm and a close interval, which is connected to the logarithmic helical antenna on the top of the optical absorption semiconductor substrate.

At terahertz frequency, the grating is pumped by a beam at terahertz beat frequency, and the incident radiation is converted into surface plasma wave, which strictly restricts the electronic oscillation at the metal-dielectric interface. The result is a well-performing local oscillator that can be mixed with input terahertz signals from (for example) astronomical telescopes to produce down-converted beat-frequency signals, which can be easily processed by standard RF signal processing electronic devices.

Broadband and room temperature operation

UCLA researchers tested their prototype and found that it worked effectively at room temperature, with sensitivity only about three times higher than the quantum noise limit. In addition, a single integrated device can pick up terahertz signals in the frequency range of 0.1-5 THz by scanning tunable optical pumping beams in a certain frequency range through the geometry of the whole line. In contrast, researchers point out that the current traditional terahertz detection technology requires "a large number of cryogenically cooled SIS mixers, HEB (hot electron bolometer) mixers and terahertz local oscillators" to achieve considerable sensitivity in similar spectral ranges.

At a press conference on the study, Jarrahi and her colleagues pointed out that the new device was particularly useful in space-based telescopes because of weight and after-sales problems that made it difficult to fit traditional cryogenic terahertz detection systems with cooling tanks needed for long-term operation. The team believes that in addition to astronomical applications, such compact room temperature detectors can also be used in the fields of atmospheric science, gas sensing and basic quantum optics. "When we look at terahertz frequencies, we can see details that are not visible in other bands of the spectrum," Jarrahi said.

Terahertz quantum cascade laser

Meanwhile, on the other side of the Atlantic, a team led by OSA associate and Professor J r? Me Faist of the Federal Institute of Technology in Zurich, Switzerland, has been working on how to make compact terahertz light sources that do not require cryogenic cooling. There is a candidate, at least in terms of compactness, for quantum cascade laser (QCL), an injection laser based on semiconductor heterostructure, first demonstrated by Faist and his colleagues at Bell Laboratory in 1994.

For example, QCL has been widely used as a light source to support compact mid-infrared sensors for environmental applications. The development of terahertz band QCL still has a long way to go. However, despite great efforts to increase their maximum operating temperature, terahertz QCL still needs to be cooled below 200 K (-73 degree C). This means that no matter how compact the structure of semiconductor lasers is, they must carry cryogenic cooling equipment, which makes the devices based on them more expensive, larger and less mobile.

Double-well design

Faist's team, including lead authors Lorenzo Bosco and Martin Franckie, focused on the structure of two quantum wells per cycle to overcome the 200 K barrier and eliminate the need for cryogenic cooling. Previous modeling and experiments have shown that this is a possible way to achieve higher operating temperatures at terahertz bands. But it is extremely difficult and sensitive to design and optimize such a structure. Therefore, the team used the non-equilibrium Green's function numerical modeling, which is computationally intensive but highly efficient, to optimize the dual-well structure for the device.

The Thermoelectric Cooling Terahertz QCL (top left) of the Zurich Federal Institute of Technology team consists of a laser chip (top right) consisting of a set of laser ridges (bottom left). Laser design is based on the double quantum well structure (lower right) designed by calculation.

Abandoning Low Temperature

In manufacturing the model device, the team found that it could effectively emit lasers at temperatures up to 210K (-63 degree C). Although not entirely stable, it increased the maximum operating temperature of terahertz QCL by a full 10K. Although the 10K improvement does not sound like much, it is enough for the team to completely abandon cryogenic cooling equipment and use standard small thermoelectric coolers to cool the laser. According to the team, this makes the study the first demonstration of terahertz QCL using thermoelectric refrigeration rather than cryogenic refrigeration.

The team believes that this advantage could "pave the way for a new generation of on-chip portable terahertz devices based on high-power terahertz coherent light sources". Researchers also saw potential applications of the results in noninvasive biomedicine and industrial imaging, safety screening and other fields.