Alexander Holleitner and Reinhard Kienberger at the Technical University of Munich succeeded in generating electric pulses in the frequency range up to 10 terahertz using plasmonic antennas on a sapphire substrate test chip. Previous on-chip connections have run up to 1THz.
The shape of the nanometre-sized asymmetrical antennas is key so that 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 by the laser.
"In photoemission, the light pulse causes electrons to be emitted from the metal into the vacuum," said Christoph Karnetzky, lead author of the paper published in Nature today. "All the lighting effects are stronger on the sharp side, including the photoemission that we use to generate a small amount of current." The work is funded by the European Research Council (ERC) as part of the NanoREAL project and the "Nanosystems Initiative Munich" (NIM).
The light pulses lasted only a few femtoseconds, producing electricl pulses of a similar length, so 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, says Karnetzky.
The researchers used sapphire as the chip material because it cannot be stimulated optically and, thus, causes no interference with standard 1500nm wavelength lasers that are used in fibre-optic cables.
One key discovery was that 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 before," said Alexander Holleitner. Using this effect he hopes to discover even faster tunnel emission effects in the antennas and to use them for chip applications. The structure is particularly interesting because the nano-antennas