Femtosecond Optics for Faster Chips

Pulses of femtosecond length from the pump laser (left) generate on-chip electric pulses in the terahertz frequency range. With the right laser, the information is read out again. (Source: C. Hohmann, NIM / A. Holleitner, TUM)

A team headed by the physi­cists Alexander Holleitner and Reinhard Kien­berger of the Technical Univer­sity Munich TUM has succeeded for the first time in gene­rating ultrashort electric pulses on a chip using metal antennas only a few nano­meters in size, then running the signals a few milli­meters above the surface and reading them in again a controlled manner. Classical elec­tronics allows frequen­cies up to around 100 gigahertz. Opto­electronics uses electro­magnetic phenomena starting at 10 terahertz. This range in between is referred to as the terahertz gap, since components for signal gene­ration, conversion and detection have been extremely difficult to implement.

The reseachers succeeded in gene­rating electric pulses in the frequency range up to 10 terahertz using tiny, plasmonic antennas and run them over a chip. Researchers call antennas plasmonic if, because of their shape, they amplify the light intensity at the metal surfaces. The asymme­trical shape of the antennas is important: One side of the nano­meter-sized metal structures is more pointed than the other. When a lens-focused laser pulse excites the antennas, they emit more electrons on their pointed side than on the opposite flat ones. An electric current flows between the contacts but only as long as the antennas are excited with the laser light.

“In photo­emission, the light pulse causes electrons to be emitted from the metal into the vacuum,” explains Christoph Kar­netzky. “All the lighting effects are stronger on the sharp side, including the photo­emission that we use to generate a small amount of current.” The light pulses lasted only a few femto­seconds. Correspon­dingly short were the electrical pulses in the antennas. Techni­cally, the structure is parti­cularly interes­ting because the nano-antennas can be inte­grated into terahertz circuits a mere several milli­meters across.

In this way, a femto­second laser pulse with a frequency of 200 terahertz could generate an ultra-short terahertz signal with a frequency of up to 10 terahertz in the circuits on the chip, according to Karnetzky. The researchers used sapphire as the chip material because it cannot be stimu­lated opti­cally and, thus, causes no inter­ference. With an eye on future appli­cations, they used 1.5-micron wave­length lasers deployed in tradi­tional internet fiber-optic cables.

Holleitner and his colleagues made yet another amazing disco­very: Both the elec­trical and the tera­hertz pulses were non-linearly dependent on the exci­tation power of the laser used. This indicates that the photo­emission in the antennas is triggered by the absorp­tion of multiple photons per light pulse. “Such fast, nonlinear on-chip pulses did not exist hitherto,” says Holleitner. Uti­lizing this effect he hopes to discover even faster tunnel emission effects in the antennas and to use them for chip appli­cations. (Source: TUM)

Reference: C. Karnetzky et al.: Towards femtosecond on-chip electronics based on plasmonic hot electron nano-emitters, Nat. Commun. 92471 (2018); DOI: 10.1038/s41467-018-04666-y

Link: Walter Schottky Institute (A. Holleitner), Technical University of Munich, Garching, Germany

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