A technique that controls the motion of electrons using very fast laser pulses, first demonstrated using gaseous atoms or molecules, has been shown for the first time to work for electrons emitted from a solid metal tip. The results could allow scientists to study electron dynamics in solid-state systems on subfemtosecond and subnanometer scales; the simple, compact and very sensitive system could lead to the development of ultrafast optical transistors. Typical time structure of the electric field of femtosecond laser pulses. The maximum of the oscillation of the carrier wave (blue) depends upon the phase relative to the maximum of “the envelope” (green). In the left pulse, the phase difference amounts to 180°, whereas in the right pulse, 0°. Courtesy of MPQ. Researchers at the Max Planck Institute of Quantum Optics (MPQ) showed that they can steer the emission of electrons from the metal tip with the phase of the optical cycle using relatively small laser intensities. The researchers used a tungsten tip that was irradiated with light pulses a few femtoseconds in duration and discovered that, if the intensity of the pulse is high enough, the electrons could absorb the amount of energy needed to be released from the metal tip. With a curvature radius of about 10 nm, the extremely sharp tip greatly amplifies the intensity of the laser light. Because of their short duration, the laser pulses contain only a few cycles. Energy spectra of electrons for different phase shifts (180° and 0°). At a phase shift of 180°, pronounced equidistant maxima are observed, whereas there is no interference structure for 0°. The insets explain the physical processes: On the left, electrons with high energies are emitted at two time intervals (red ellipse) during the pulse, leading to the quantum mechanical interference pattern of the spectrum. Instead, on the right, electron emission is possible only once per pulse; therefore, no interference can occur. In this case, however, the electrons gain on average more kinetic energy, and the number of electrons at higher energies is larger. The shallow slope of the curve between 5 and 10 eV (especially at 0°) indicates elastic re-scattering in the laser field. Courtesy of MPQ. During the experiment, the scientists measured the kinetic energy of the emitted electrons for different phase shifts. They noted that the structure of the electron spectrum was strongly influenced by the phase shift. “The higher the electron energy, the more we approach the situation that we are able to switch the current on or off by simply changing the phase shift by 180 degrees,” said Michael Krüger, a co-author of the paper published in the July 6 issue of Nature (doi: 10.1038/nature10196). The researchers also observed that the chosen phase shifts determined whether pronounced peaks in the spectra could be observed. These maxima are a consequence of the quantum mechanical wave nature of electrons. Electrons could be emitted during two time intervals of the light pulse at a phase shift of 180°, and the interference of the two matter wave packets at the metal tip lead to the observed interference in the spectrum. On the other hand, if the phase shift equals 0°, the electrons would be emitted only once per pulse, making the maxima disappear, and no interference would occur. Metal tip irradiated with a laser pulse. Courtesy of Thorsten Naeser, MPQ. The researchers concluded that the laser field would continue to influence the motion of the electrons even after emissions from the metal tip occurred. It is implied that the released electrons would be driven back into the tip by the laser field and scatter elastically off the tip before being detected. “As is demonstrated in the experiment, this scattering process does not destroy the interference of the electron wave packets, that means it takes place in a coherent way,” concluded co-author Markus Schenk.