Scientists have developed a laser that emits in the terahertz range and operates without a cryogenic cooler, allowing it to be used outside the laboratory for the first time. The operating temperature of the laser, equal to -23 degrees Celsius, can be maintained with an ordinary inexpensive cooler. The study is published in the journal Nature Photonics.
The frequency spectrum of terahertz radiation is located between the infrared and microwave bands. The most modern devices emitting in this range are quantum cascade solid-state lasers based on semiconductors made of gallium arsenide doped with aluminum-gallium arsenide.
Radiation in quantum cascade lasers is based on intraband transitions, and it is facilitated by numerous discrete energy levels arising from the features of the quantum heterostructure, which due to its small size (less than 200 angstroms per layer) literally restricts the movement of charge carriers.
Since the radiation of quantum cascade lasers is unipolar, the structure is a repeating alternating layers of semiconductors, the interface of which is a kind of barrier. An electron passing from one core to another through these barriers gives off energy in the form of radiation.
It is not possible to apply terahertz lasers in all areas of science where they are required. One of the most significant obstacles is the low operating temperature (about 200 Kelvin), which is easy to achieve only with laboratory equipment. On the contrary, most of the tasks for the application of terahertz radiation requires portability emitters. To successfully solve these problems, you need to make the laser work at room temperature or with the help of portable coolers.
The problem partly lies in the leakage of charge carriers due to the tunneling effect, when electrons with energy less than the band gap "escape" from the isolated area of the device. As the temperature rises, the leakage increases, and the efficiency of the laser decreases. In previous studies, researchers have foundthat leakage with increasing temperature is particularly active in the barrier region, between different layers of the heterostructure.
Ali Khalatpour from the Massachusetts Institute of technology and colleagues designed a quantum cascade solid-state laser that emits waves with a frequency of four terahertz and operates at the maximum possible temperature of 250 Kelvin. Temperatures of 250 Kelvin or -23 degrees Celsius can be achieved using an inexpensive portable cooler. The previous temperature record was 210 Kelvin.
The laser consists of alternating layers of gallium arsenide and aluminum-gallium arsenide, just like its predecessors, but scientists compensated for the leakage of charge carriers by increasing the width of the band gap of the barrier by increasing the content of aluminum in the alloying impurity aluminum-gallium arsenide from 15 to 30 percent.
The researchers tested four structures with different layer thicknesses to achieve an optimal ratio of low leakage and high injection force of charge carriers through the barrier. The layers were grown by molecular beam epitaxy. Scientists concluded that the quality of its implementation also depends on the effectiveness of the future device, but how exactly — is not specified.
Terahertz radiation can be used to study the bioactivity of chemical compounds, for spectral analysis of the atmosphere of planets in astrophysics, as well as, for example, to find explosives (C-4, TNT) that are specifically displayed during such irradiation. The data obtained by the scientists will allow the laser to be used in portable devices outside the laboratory and make possible rapid spectral analysis in real time.
The portability of lasers plays a key role in modern applications, for example, the Americans plan to use a three-kilowatt laser installation based on an armored vehicle to clear minefields.