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Breakthrough in Terahertz Photonics

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Scientists from Regensburg, Pisa and Leeds developed a key photonic component. By strongly coupling electronic resonances with the light field of a microresonator, they were able to operate a saturable absorber even at extremely low intensities, which in future could enable ultra-short pulses from terahertz lasers. The international research team presented their results in Nature Communications.

Terahertz radiation is electromagnetic radiation in the inaccessible frequency window between microwave electronics and long-wave infrared. It opens up a diverse spectrum of applications, ranging from security scanners at airports and trace gas detection to ultra-fast communication technology and medical technology. Many other technologies could be added if ultrashort pulses could be generated directly from electrically pumped, compact terahertz lasers, so-called quantum cascade lasers. So far, however, these have only worked in continuous wave operation, i.e. without any variation in performance over time.

Using so-called saturable absorbers, inexpensive quantum cascade lasers can easily be used to elicit short terahertz pulses. The way a saturable absorber works can be compared to that of a fogged mirror, which becomes clear again as soon as sufficient intense light falls on it. If such an element is incorporated into a quantum cascade laser, the light intensity in the case of continuous light emission is not sufficient to make the mirror clear – the high losses mean that the laser emits only weak light or no light at all. If, on the other hand, the entire power of the laser is concentrated in a single, short light pulse, this is intense enough to saturate the absorber. Here the light experiences significantly lower losses, so that he develops a preference for this operating mode. Up to now, however, saturable absorbers for the terahertz spectral range have been difficult to implement and, moreover, required light intensities far beyond the capabilities of a quantum cascade laser.

In order to develop a new class of saturable absorbers, the working group of Professor Dr. Rupert Huber at the Institute for Experimental and Applied Physics at the University of Regensburg, together with Professor Miriam Vitiello, NEST Pisa, and Professor Edmund Linfield, University of Leeds, are inspired by music: where does the unique sound of a Steinway piano come from, for example? The secret is not in the strings, but rather in the sound box. There sound and dynamics arise after a keystroke. “Basically, we are adopting this idea in the terahertz optics,” says Jürgen Raab, the first author of the publication. Miriam Vitiello's group developed a micro-structured arrangement consisting of a gold mirror and a gold grid, which together act as a resonance body for terahertz radiation.

In a high-precision slow-motion camera developed in Regensburg, the scientists observed how the new structures react to a strong "keystroke", i.e. the stimulation with an intense terahertz pulse. On the time scale of femtoseconds – the millionth part of a billionth of a second – an astonishing result was shown: the absorber was already saturated at an intensity ten times less than the pure semiconductor structure alone. In addition, this reaction set in faster than a single light oscillation of the terahertz pulse and the "tone" changed during saturation in such a way that almost no absorption of the intense terahertz pulse took place. Vitiello is enthusiastic: "We now have all the necessary components in our hands,

Since terahertz radiation oscillates a thousand times faster than the clock rates of modern computers, ultra-short terahertz pulses could enable a new generation of telecommunications connections - far faster than 5G. Important advances in the field of chemical analysis and medical diagnostics are also conceivable. An important milestone on this path has now been reached.

Raab J, Mezzapesa FP, Vitiet L al. Ultrafast terahertz saturable absorbers using tailored intersubband polaritons. Nature Communications. 2020;11(4290). doi: 10.1038/s41467-020-18004-8

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