On the road to terahertz electronics

Asymmetric plasmonic antennas deliver femtosecond pulses for fast optoelectronics.

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. (Image: Christoph Hohmann / NIM, Holleitner / TUM)
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. (Image: Christoph Hohmann / NIM, Holleitner / TUM)

Classical electronics permits frequencies up to around 100 gigahertz. Optoelectronics utilizes electromagnetic phenomena beginning at 10 terahertz. This range in the middle of is alluded to as the terahertz gap, since segments for signal generation, transformation and recognition have been extremely difficult to execute.

Now, for the first time, TUM scientists have successfully generated ultrashort electric pulses on a chip using metal antennas only a few nanometers in size, at that point running the signs a couple of millimeters over the surface and understanding them in again a controlled way.

Scientists generated electric pulses in the frequency range up to 10 terahertz utilizing small plasmonic antennas and run them over a chip. They call this antenna plasmonic if, due to their shape, they intensify the light force at the metal surfaces.

Christoph Karnetzky, lead author of the Nature work said, “The shape of the antennas is important. They are asymmetrical: 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. 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 endured just a couple of femtoseconds. Correspondingly short were the electrical pulses in the antennas. In fact, the structure is especially intriguing in light of the fact that the nano-reception apparatuses can be incorporated into terahertz circuits an insignificant a few millimeters over.

Along these lines, a femtosecond laser pulse with a frequency of 200 terahertz could create a ultra-short terahertz signal with a frequency of up to 10 terahertz in the circuits on the chip.

The scientists utilized sapphire as the chip material since it can’t be empowered optically and, in this way, causes no impedance. With an eye on future applications, they utilized 1.5-micron wavelength lasers conveyed in customary web fiber-optic links.

Alexander Holleitner, TUM physicist said, “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. Utilizing this effect he hopes to discover even faster tunnel emission effects in the antennas and to use them for chip applications.”

The study is publshed in the journal Nature Communications.