Fastest laser measures electron’s band-gap jump
By probing a silicon crystal with attosecond laser pulses, University of California Berkeley scientists have been able to measure the timing of an electron jumping from valence band into the conduction band and the resulting rebound of the atomic lattice.
The attosecond pulses – one billion billionth of a second – of soft x-ray light are the shortest time pulses yet achieved and an international team of physicists and chemists based at UCB to take snapshots of this event.
Earlier femtosecond lasers were unable to resolve the jump from the valence shell of the silicon atom across the band-gap into the conduction electron region, the latest experiments now show that this transition takes less than 450 attoseconds.
The excitation of a semiconductor with light is traditionally conceived as a two-stage process. First, electrons absorb light and get excited. Then, the lattice, composed of the individual atoms in the crystal, rearranges in response to this redistribution of electrons, turning part of the absorbed energy into heat carried by vibrational waves called phonons.
Snapshots of an electrons bandgap jump. Source: Stephen Leone & Daniel Neumark, Attosecond Physics Laboratory, UC Berkeley.
In analyzing their data, the team showed that initially, only the electrons react to the impinging light while the atomic lattice remains unaffected. Long after the excitation laser pulse has left the sample – some 60 femtoseconds later – they observed the onset of a collective movement of the atoms, that is, phonons. This is near the 64-femtosecond period of the fastest lattice vibrations.
Based on current theory, the researchers calculated that the lattice spacing rebounded about 6 picometers as a result of the electron jump, consistent with other estimates.
The work was performed at the Attosecond Physics Laboratory run by Stephen Leone, UC Berkeley professor of chemistry and physics and Daniel Neumark, UC Berkeley professor of chemistry. The results achieved with colleagues and collaborators from the Ludwig-Maximilians Universitaet in Munich, Germany, the University of Tsukuba, Japan, and the Molecular Foundry at the Department of Energy’s Lawrence Berkeley National Laboratory report their achievement in the Dec. 12 issue of the journal Science.
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
