It was that realization that, in the end, was crucial for the interpretation of the experimental data. The researchers were able to conclude that the dynamical Franz-Keldysh effect was responsible for the absorption in diamond under the influence of the infrared laser pulse. Whereas the Franz-Keldysh effect for static electric fields has been known and well understood for several years, its dynamical counterpart for extremely rapidly oscillating fields had not been observed until now. "The fact that we could still see that effect even at petahertz excitation frequencies confirmed that the electrons could, indeed, be influenced at the speed limit of the laser field", explains Lukas Gallmann, a Senior Scientist in Keller's lab. The dynamical interaction is also of fundamental interest as it appears in a regime that is neither dominated by quantum mechanical nor by classical light-matter interactions. This means that two kinds of physical effects simultaneously play a role: those in which light acts as energy quanta (photons), and those in which it is represented by a classical electromagnetic field. The work now published has shown that the reaction of the material to the optical field is dominated by the motion of the electrons in a single energy band rather than by transitions between different bands. In similar experiments it had been unclear up to now what exactly was going on, but the experiment at ETH has now settled that question.
It may still be a long way from this point to the realization of petahertz electronics, and other physical effects might still limit device performance. Gallmann points out, however, that the new results are relevant in several respects, showing, as they do, that at such high frequencies electrons can still be steered and switched with electric fields. "Diamond is an important material that is used in a variety of technologies ranging from opto-mechanics to biosensors", Lucchini adds. "A detailed understanding of the interaction with light fields, which we have demonstrated now, 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