Classical electronics allows frequencies up to around 100 gigahertz. Optoelectronics uses electromagnetic phenomena starting at 10 terahertz. This intermediate range is known as the "terahertz gap" because the components for generating, converting, and detecting signals have been extremely difficult to implement.

TUM physicists Alexander Holleitner and Reinhard Kienberger succeeded in generating electrical pulses in the frequency range up to 10 terahertz using tiny plasmonic antennas and running them on a chip. The researchers call these antennas plasmonic because, due to their shape, they amplify the intensity of light on metallic surfaces.

Asymmetric Antennas
The shape of the antennas is important. They are asymmetric: one side of the nanometer-sized metallic structures is more pointed than the other. When a laser pulse focused on the lens excites the antennas, they emit more electrons from their pointed side than from the opposite planes. An electric current flows between the contacts, but only while the antennas are excited by the laser light. "In photoemission, the light pulse causes electrons to be emitted from the metal into the vacuum," explains Christoph Karnetzky, lead author of the Nature paper. "All the lighting effects are stronger on the pointed side, including the photoemission we use to generate a small amount of current."

Ultrashort Terahertz Signals:
The light pulses lasted only a few femtoseconds. As a result, the electrical pulses in the antennas were short. Technically, the structure is particularly interesting because the nano-antennas can be integrated into terahertz circuits just a few millimeters in diameter. In this way, a femtosecond laser pulse with a frequency of 200 terahertz could generate an ultrashort terahertz signal with a frequency of up to 10 terahertz in the chip's circuits, according to Karnetzky.

The researchers used sapphire as the chip material because it cannot be optically stimulated and therefore does not cause interference. Looking ahead to future applications, they used 1.5-micron wavelength lasers deployed in traditional fiber optic internet cables.

A surprising discovery.
Holleitner and his colleagues made another surprising discovery: both the electrical and terahertz pulses were not linearly dependent on the excitation power of the laser used. This indicates that photoemission in the antennas is triggered by the absorption of multiple photons per light pulse. "Such fast, nonlinear pulses on the chip did not exist until now," says Alexander Holleitner. Using this effect, he hopes to discover even faster tunneling emission effects in the antennas and use them for chip applications.