Scientists Propose New Type Of Terahertz Solid-State Laser

Terahertz radiation is a type of electromagnetic radiation with a frequency spectrum between IR and microwave bands. One of the most interesting things about it is that it can penetrate most dielectrics. For one, wood, plastic, and ceramic are transparent to it. Also, as opposed to x-radiation, it is totally harmless to the human organism.

These characteristics make terahertz radiation sources most attractive for a whole range of practical applications. For instance, terahertz radiation can be used in security systems for scanning people and luggage. It can be used to detect metal, ceramic, plastic and other objects hidden under one’s clothes from at least ten meters away. In medicine, terahertz imaging systems can be used to study the outer layers of the body (like skin and vessels) and get precise images of tumors. Also, terahertz technologies can be used to get images of surfaces covered by layers of paint or plaster, which is most important in restoration of art objects.

Intensive research in this field has already been going on for the last twenty years. During this time, scientists have proposed different concepts of terahertz lasers, both atom and solid-state. For instance, there are graphene-based gas lasers, lasers based on photonic crystals, quantum-cascade lasers, etc.

To be applied in the aforementioned fields, a terahertz laser has to be compact, portable, and have high efficiency. As of now, most existing devices have a problem with the latter. What’s more, tuning is an issue, as well: the radiation’s properties - most importantly, their frequency - directly depend on the systems’ parameters. This is why tuning becomes almost impossible.

Recent research by a team of ITMO scientists - Vanik Shahnazaryan, Ivan Shelykh, Alexander Alodjants, and their colleague Igor Chestnov from Vladimir State University was dedicated to solving these problems. They proposed a new concept of a solid-state terahertz laser based on asymmetrical gallium-nitride quantum dots.

Vanik Shahnazaryan

“We aimed to create a terahertz laser that would be highly tunable. Our model allows to freely control the radiation frequency by simply changing the frequency or amplitude of the outer “dressing” field,” comments Vanik Shahnazaryan.

Electromagnetic “dressing” here means the mode of a strong interaction between the substance (the simplest method to modeling it is by using a two-state quantum system) and external laser radiation. In this case, the presence of laser radiation has a considerable effect on the properties of the material medium. Thus, the hybrid “dressed” states emerge; terahertz emission originates from the transitions at Rabi energy between the neighboring dressed states.

“In symmetrical two-state structures, such a transition is impossible, and most two-state quantum systems (most atoms in their ground states, for instance) are symmetrical,” adds  Vanik Shahnazaryan.

This is the reason why the scientists had to look for a two-state asymmetrical system (there are examples of those in the field of superconducting qubits). Thus, the authors chose gallium-nitride quantum dots. One of their remarkable characteristics is piezomagnetism due to the peculiarities of the material’s crystalline framework.

ITMO scientists have been researching similar structures since 2012, when Ivan Shelykh and his team started researching the emission spectrum of a separate quantum dot under the effect of a dressing electromagnetic field. Their results gave evidence to the emergence of the transition between neighboring dressed states, where the frequency lies in terahertz band.

“We’ve developed this idea in our current research. More specifically, we decided to study the case when we have a multitude of such dots, amplify the signal by putting them into a resonator, and thus get a terahertz laser. Still, one has to understand that before one decides to put this concept into practice, there’s a whole range of issues to be solved,” explains Vanik Shahnazaryan.

The first problem is that as opposed to atomic systems, quantum dots differ in size. The size of a quantum dot defines its energy spectrum, thus, each quantum dot has its own energy of a dressed state. As a result, the quantum dots can’t emit synchronously, and an emission from a great number of them would have a wide spectrum. This means that as opposed to optical band (in common lasers, the width of the line is a lot less than the emission frequency), in this case, the broadening of the frequency is so high that it can completely “kill” the laser emission.

Another problem is the necessity to arrange the quantum dots, namely ensure that the growth axes of the crystalline framework are codirectional. In other words, to get laser emission, one would have to grow multiple quantum dots that would not just be identical in size, but also have parallel asymmetry axes.

As of now, the scientists succeeded in creating a promising theoretical model of the new type of a terahertz laser. They calculated the system’s parameters at which it would generate terahertz emission, and also computed its quantum efficiency. The research’s results were published in the ACS Photonics journal; also, a review by Dr. Simone De Liberato from the University of Southampton was published in Nature’s “News and Views".

In future, so as to test their theoretical model experimentally, the scientists will have to solve issues related to broadening due to the inequality of the quantum dots, as well as the high intensities of the dressing field. According to Vanik Shahnazaryan, that would be no easy task at the current level of experimental technology. Still, Simone De Liberato states in his article that despite this fact, the concept proposed by the Russian scientists opens up a new trend for future research.

“Technological advances in quantum dot fabrication could in fact pave the way for both the reduction of the inhomogeneous broadening and an increase in the surface density. This, together with improvements in the design of broadband, ultrahigh-quality-factor terahertz resonators, could turn this proposal into a ground-breaking viable device. Moreover, the theory of the asymmetric and inhomogeneously broadened dressed-state laser developed here for quantum dots can now be applied to investigate other non-centrosymmetric systems,” states Simone De Liberato.

Reference: Terahertz Lasing in Ensemble of Asymmetric Quantum Dots, Chestnov, I. Yu., Shahnazaryan, V. A., Alodjants, A. P. & Shelykh, I. A. ACS Photonics. 4, 2726–2737 (2017)