atomic-scale electron transport in solids: timed by an attosecond stopwatch
The motion of electrons in ever smaller (at present: tens-of-nanometres) solids-state structures is the basis of modern electronics. Its advancement toward its ultimate limit: atomic-scale structures and speeds requires detailed insight into electron transport on this scale. The extension of attosecond metrology from isolated atoms
, to solids has permitted the first real-time observation of the travel of electrons across atomic layers, i.e., on the length scale of future (atomic-scale) electronics. In this proof-of-concept experiment, two different types of electrons are knocked out from a crystal, from a depth smaller than 1 nm below its surface, by an attosecond extreme ultraviolet pulse. Because the electrons have different kinetic energy and effective mass, the “fast” electron arrives slightly earlier than the “slow” one at the crystal surface. Upon arrival, they are exposed to the electric field of a few-cycle laser wave impinging simultaneously with the attosecond pulse. Because the strength of the laser electric field, E(t), is different at the arrival moments of the electrons, t_1 and t_2, it causes a different change, \Delta v_1 and \Delta v_2, of the electrons’ initial velocity.
, to solids has permitted the first real-time observation of the travel of electrons across atomic layers, i.e., on the length scale of future (atomic-scale) electronics. In this proof-of-concept experiment, two different types of electrons are knocked out from a crystal, from a depth smaller than 1 nm below its surface, by an attosecond extreme ultraviolet pulse. Because the electrons have different kinetic energy and effective mass, the “fast” electron arrives slightly earlier than the “slow” one at the crystal surface. Upon arrival, they are exposed to the electric field of a few-cycle laser wave impinging simultaneously with the attosecond pulse. Because the strength of the laser electric field, E(t), is different at the arrival moments of the electrons, t_1 and t_2, it causes a different change, \Delta v_1 and \Delta v_2, of the electrons’ initial velocity.Measuring these velocity changes allows one to determine the difference in arrival time, \Delta t=t_2-t_1, with attosecond resolution: This resolution stems from the variation of the laser field on this time scale: the oscillating field plays the role of an attosecond stopwatch. The experiment has yielded a difference in arrival, i.e. transport, time of ~ 110 attoseconds and demonstrates the technical capability of measuring atomic-scale electronic charge transport in condensed matter: The measuring tool for advancing electronics towards light wave frequencies is now available.
is now available.More information:
The first time-resolved observation of the quantum mechanical process of tunnelling has been reported by:
Attosecond spectroscopy in condensed matter, A. Cavalieri et al., Nature 449, 1029 (2007)
Commented in the same issue by:
At a glance D. M. Villeneuve, Nature 449, 997 (2007)
And highlighted by the Editors of Nature among the most important papers published by the Nature Publishing Group in 2007:
Attosecond laser pulses illuminate fleeting dance of electrons, The Editors of Science, Nature 450, ix (12/27/2007)
Further reports include:
Solid-state attosecond metrology , Physics Today, 12, 20 (2007).
Fig. 1. Illustration of the first real-time measurement of sub-nanometer travel times of electrons in solid matter. An attosecond extreme ultraviolet pulse releases electrons of different types, from loosely- and deeply-bound states from the solid. The former are faster and reach the surface first, the latter are slower, and hence are lagging behind. Upon their arrival on the surface the laser field is different, changing their speed differently. This difference can be readily measured, from which the shift in the instant of electron arrival on the surface can be inferred.