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21.05.2013

Two fundamentally new types of lasers are presented

Normal laser stimulates emission of radiation by the transfer of electrons being in the working chamber to the excited state, and when they reach a high enough energy, they can give up a part of it by emitting photons. It was they who form a coherent beam of light that we call laser radiation.

The new laser does not operate with electrons but with polaritons. These component quasiparticles result from the interaction of photons and elementary excitations of the medium, and therefore their energy consists partly of electromagnetic energy, and partly of energy of excitation of their own medium. Polaritons differ depending on which exactly oscillations of the medium are "partners" of photons. In this concrete case excitons were used, i.e. quasiparticles of electron and hole. This means that exciton polaritons are used in a new laser.

When the energy is pumped into the working (crystal) medium of laser, exciton polaritons absorb it, and then quickly, almost instantaneously emit as photons. The difference is that in a normal laser most of electrons should be in a high energy state - or emission simply will not start. In contrast, the exciton-polariton laser can operate without this condition.

The concept of the device is relatively simple; it was first described in 1996. Something like this was even made - however, the pumping source was a normal laser, which severely limited the main advantage of a polariton system: in fact it does not need to spend for the start of radiation as much energy as standard analogues, but in conditions of a pumping by "normal" laser it was still not possible to save energy.

Now, independently prototypes are developed, which use for pumping only electricity and do not need a normal laser systems. Thus, we have the first suitable for practical application examples of such devices.

The threshold of start of radiation of the first prototypes is equal only to 12 amps per square centimeter. Since these are pioneering devices, the researchers hope to improve dramatically this result. Even the best today's lasers (quantum dots lasers), finalization of which required many years of hard work, have the threshold for the start of radiation exactly the same as the very first experimental samples of polariton lasers with electrical pumping.

Another advantage polariton lasers is much faster turning on and off, one cycle which lasts for a total just several picoseconds (quintillion of a second). This means that the signals sent by such devices over a fiber optic line can be sent more frequently than with the standard today’s lasers, and in fact more often than today's electronics can handle efficiently. But as an alternative the signals can be sent with the same speed, but with much less energy costs.

Besides that polariton lasers are able to operate using very perspective terahertz frequencies where current lasers suffer from a lack of compactness. This kind of devices can greatly complement the current X-ray analysis, while being much safer radiologically.

Now the samples for application lack a lot: both developments are based on gallium arsenide and require cooling to -243º C. As a next step, the researchers aim to develop a polariton lasers with electric pumping operating at room temperature, and in principle, there are no theoretical obstacles to this. The analogues with optical pumping already operate in such conditions.

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