Light beams for ultrafast control of electrical currents
Controlled electrical currents are the fundamental means of carrying information and energy in today’s world, and they also serve scientists in exploring material properties, as well as in testing new concepts of physics. Light beams can drive electrical currents at unprecedented switching speeds and great adaptability, far superior to possibilities offered by conventional voltage-driven systems. However, optical generation and manipulation of currents at nanometre spatial scales remains a challenge and a crucial step towards functional optoelectronic devices for microelectronics and information technology. Now, scientists have succeeded in generating directional electrical currents driven by pulses of light, on a surface of graphene patterned with metal nanoparticles. The work was published in the renowned scientific journal Nature.
Scientists from the USA and Germany worked together to perform these challenging experiments and prove the concept of ultrafast photocurrent generation and control with light beams. Very short laser pulses, with durations on the order of 100 femtoseconds, were directed at gold nanostructures patterned on top of graphene. The nanostructures serve to direct the generated photocurrent into a specific spatial direction across the surface of graphene. The ultrafast photocurrent is detected via emitted radiation in the terahertz (THz) part of the spectrum.
Image: Illustration of the generation of ultrafast electrical current by optical pulses on a structured surface. Image obtained from Pettine et al, Nature 626, 984-989 (2024), under Creative Commons 4.0.
The research makes use of the plasmonic effect, whereby light couples to charge motion on a surface. Charge flow can then be controlled by controlling properties of the light. In this case, light drives charge carriers in the metal nanoantenna, and currents flow in the direction determined by the shape of the nanoantenna. Since these currents are propelled by light pulses that are extremely short, the charge carriers are rapidly accelerated by the pulses. The acceleration of charge generates THz radiation that can be detected as a sign of the presence of the currents.
As an immediate application, the researchers demonstrate that these types of surfaces can serve as efficient sources of THz radiation, including broadband THz vector beams. Deeper considerations reveal that the physical mechanism involves a delicate interplay of light, heat, and electricity that dictates the behaviour of the currents. The intersection of femtosecond excitation and nanoscale localization plays an important role in fusing these three physical mechanisms, which the scientists clarify in detail, with experiments supported by calculations.
Graphene for this work was provided by Graphenea.
