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An effective solution to the lack of directionality of some lasers

Terahertz cascade lasers (1) are a new family of semi-conductor lasers which emit in the frequency range of the terahertz, or 1012 hertz. Because of their potential applications, they currently raise considerable interest, despite one major drawback: their output beam is markedly divergent.

To solve this problem, a French-English collaboration (2) led by Raffaele Colombelli, researcher at CNRS, has used special microscopic components called photonic crystals. In combination with the laser, the team was able to control the laser beam and considerably restrict its divergence. Published on January 8 in the journal Nature, this finding opens the way to a large number of promising applications, for instance in the field of terahertz medical imaging.

Situated in the far infrared range of the electromagnetic spectrum, between mid-infrared and microwaves, terahertz waves have some major advantages: they can penetrate through skin, clothing, paper, wood, card and plastic. These properties offer applications in medical imaging, spectroscopy, and environmental detection (detection of biological agents, pollutants etc.). .

Terahertz cascade laser systems raise considerable interest due to their numerous advantages: they are compact (3), they use electrical energy -- reference is made to electrically "pumped" lasers (4) - and they operate in the terahertz range of the electromagnetic spectrum (THz). Indeed, the generation of radiation in the frequency range between 1 and 10 THz (also called the THz "gap") with a compact device has proven extremely challenging. This explains the considerable interest raised by terahertz cascade lasers, which are the only compact sources (smaller than a millimeter) operating within this range of frequencies. However, these promising lasers have one weakness: the marked divergence of their output beam, which prevents their widespread use.

The scientists used very small structures, photonic crystals, to influence the optical properties of the material and thus enable control over the light trajectory. By combining these components with the terahertz laser, they managed to design an ingenious system that emits terahertz waves but also, and above all, enables precise control of the laser beam. Thanks to this effective technology, this beam now diverges very little.

This novel system opens up numerous fundamental and applied perspectives. It is now necessary for the researchers to maximize the output power of these lasers. Furthermore, better control of the photonic crystal technology may enable the design of new terahertz lasers of an even smaller size. The technique thus developed could be generalized to other lasers operating in different ranges of wavelengths. Finally, these results may give rise to several applications, notably in the fields of spectroscopy and THz imaging.

This work was made possible by the EURYI award given in 2004 to Raffaele Colombelli by the European Science Foundation. This allowed him to set up a research team within the Institut d'électronique fondamentale at the Faculty of Sciences in Orsay, where he is supervising the doctoral thesis of Yannick Chassagneux, the lead author of this publication.

(1) The first terahertz quantum cascade laser was invented in 2002.

(2) Belonging to two units: the Institut d'électronique fondamentale
(CNRS / Université Paris-Sud 11) and the laboratory "Matériaux et
phénomènes quantiques" (CNRS / Université Paris Diderot -- Paris 7) and
to the Universities of Cambridge and Leeds.
(3) This is a characteristic of semi-conductor lasers, which take up
little space (unlike gas lasers).
(4) The alternative is a laser with an optical pump, but a second laser
is necessary to supply energy.

Julien Guillaume | alfa
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