Making an electrical insulator conduct within a femtosecond

State-of-the-art FETs can process electrical signals at rates up to 100 GHz, and all-optical methods have made it possible to control electric currents in semiconductors at terahertz frequencies. Researchers led by Ferenc Krausz of the Max Planck Institute of Quantum Optics have now increased that rate into the petahertz regime. Their demonstration uses visible light pulses intense enough (10¹⁴ W/cm²) to briefly transform a dielectric insulator into a conductor and yet short enough in duration (4 fs) to preserve the material’s solidity. They fashioned a circuit made from silica and two gold electrodes and measured a time-integrated current. By exciting carriers with a strong pulse and then driving them with the field of a time-delayed replica pulse, they were able to steer the current toward one electrode or the other via a change in delay of just half the field’s period—evidence that the rise in conductivity followed the field. But to show that the conductivity could be switched off just as rapidly, the researchers turned to a second experiment. They again exposed silica to a light pulse and simultaneously probed its absorption of extreme UV radiation using far shorter attosecond pulses, which took snapshots of the electronic structure. Modulations in absorption as a function of the probe’s delay time relative to the visible pulse confirmed the ultrafast rise and fall in conductivity. Krausz envisions that the effect can prove useful for metrology and pulse diagnostics. (A. Schiffrin et al., Nature 493, 70, 2013; M. Schultze et al., Nature 493, 75, 2013.)—R. Mark Wilson