Skip to main content

Attosecond observation and control of collective electron dynamics in nanoparticles

Final Activity Report Summary - KLING-CED (Attosecond observation and control of collective electron dynamics in nanoparticles)

By drawing on the newly developed tools of attosecond metrology, we were able to observe electrons as they tunnel through the potential binding them to the atomic core under the influence of laser light. The key to this feat has been an intense pulse of red laser light comprising merely a few, well controlled oscillations of its electric field and an attosecond pulse of extreme ultraviolet light coming in perfect synchronism with the few-cycle laser wave. Acting on the atoms, the electric field of this red laser wave exerts a strong force on an electron at the periphery of the atom. As a consequence, as the few-cycle pulse passes through the atom, the probability of the electron being set free is expected to increase stepwise: within a period of several hundred attoseconds (1 attosecond = one thousandth of a femtosecond) the probability rises each time the laser wave crest hits the atom. To observe this process, we have used a gas of neon atoms, in which the electrons reside in closed shells and are very tightly bound and resist the attempt of the laser field to free them. Only electrons hit by an attosecond ultraviolet light pulse are promoted to the periphery of the atom and can be detached from the atom via tunnelling. The experiments provide the first direct insight into the dynamics of electron tunnelling and reveal low light-field-induced tunnelling can be exploited for real-time observation of intra-atomic or intramolecular motion of electrons. Gaining increasing insight into and control over the atomic-scale motion of electrons will be instrumental in developing compact sources of X-ray light, pushing the frontiers of microelectronics into the multi-THz regime and advancing biological imaging and radiation therapies.

Metallic nanostructures, consisting of a few thousand atoms, exhibit optical and electronic properties which are not present in extended solid state systems. The interaction of electromagnetic radiation (light) with nanoparticles leads to collective, coherent oscillations of electrons (so called surface plasmons). We have proposed a new microscope that allows for the first time to resolve the ultrafast dynamics of plasmonic fields with high spatial and temporal resolution. We simulated a geometric assembly of silver nanoparticles on a surface, which are then excited by an (extremely short) few femtosecond pulse (a femtosecond is a millionth of a billionth of a second). The interaction with the light-pulse -consisting of only a few oscillation periods- leads to the formation of plasmonic fields, whose amplitudes and frequencies (between the near infrared and near ultraviolet) depend on the size, shape, and environment of the nanoparticles. The plasmon dynamics are probed by a 170 attosecond, extreme ultraviolet laser pulse incident on the nanosystem that is synchronised with the excitation pulse and releases electrons. The plasmonic fields are monitored by the energy and spatial distribution of these so called photoelectrons as they were - prior to their detection - accelerated by these fields.

Generally the nanoplasmonic ultramicroscope would allow for the first direct observation of ultrafast processes in nanosystems, such as the conversion of sunlight into electrical energy. Future applications of the technique lie particularly in the development of novel devices, in which localised nanoplasmonic fields replace electrons in conventional electronics, i.e. are used for information transfer, processing, and storage. The advantage would be that plasmons in these nanosystems allow for information processing and transfer at much higher frequencies (ca. 100,000 times) as compared to electrons in solid state systems. This way, extremely fast optoelectronic and optical devices for computations and information processing may be realised.