Swiss researchers on the way to petahertz electronics
Today, switching speeds in the multi gigahertz range and even up to a terahertz are technologically mastered. The next generation of electronic components will push forward towards the petahertz range. It is however unknown how the movement of electrons can be controlled at such frequencies. A research team around ETH professor Ursula Keller has investigated how electrons react to petahertz fields.
In their experiment, the Swiss scientists exposed a tiny diamond with a thickness of just 50 nanometers to the pulse of an infrared laser. This pulse had a duration of a few femtoseconds. The electric field of this light has a frequency of about 0.5 petahertz; during the laser pulse it performed five oscillations that stimulated the electrons. The effect of electric fields to electrons within transparent materials can be measured indirectly by sending light through the material and observing its absorption on its way through the material.
While such measurements are simple as long as they are performed with constant electric fields, the high-frequency oscillations of a laser beam pose an enormous challenge to the researchers. In principle, the intention was to switch on the light only for a fracture of the oscillation period of the electric field. Hence, the measuring pulse needed to be shorter than a femtosecond. In addition, the exact phase position of the laser pulse’s electric field must be known at the moment when the pulse is activated. While this technique is basically known since the nineties, it was necessary to refine it significantly. In addition, a certain amount of detective work was required: First, fellow researchers from the Tsukuba University in Japan simulated the reaction of the electrons in the diamond on a supercomputer. They found the same absorption behavior as measured by the Zurich team. The simulation also enabled the scientists to switch on and off certain effects that occur in the real diamond. Therefore, the Zurich scientists were able to ascribe the characteristic absorption behavior of the diamond to only two energy levels. The researchers therefore concluded that the so-called dynamic Franz Keldysh effect was responsible for the absorption in the diamond under the influence of the laser pulse. While this effect is well understood for static fields, it has never before observed in its dynamic form for extremely fast oscillating electric fields. According to the researchers this shows that electrons can be controlled even by extremely high oscillation frequencies as associated to infrared laser beams.
Though from these findings to practical petahertz electronics there is still a long way to go and other physical effects can reduce the performance, the findings of the Swiss group nevertheless are relevant for next-generation electronics, because they prove the potential of influencing electron movement through electric fields even at extremely high frequencies. “Diamond is a crucial material that is applied in a host of technologies from optmechanics to biosensors,” comments Matteo Luccchini, a postdoc in Keller’s team. “The exact understanding of the interaction with light fields we achieved is therefore fundamental.”
Lucchini M, Sato SA, Ludwig A, Herrmann J, Volkov M, Kasmi L, Shinohara Y, Yabana K, Gallmann L, Keller U. Attosecond dynamical Franz-Keldysh effect in polycrystalline diamond. Science 26 Aug 2016: Vol. 353, Issue 6302, pp. 916-919, DOI: 10.1126/science.aag1268