A short laser pulse travels through a diamond (black spheres) and excites electrons inside it. The strength of the excitation is measured using an attosecond ultraviolet pulse (violet). Credit: Matteo Lucchini, Copyright ETH Zurich
These insights are important for petahertz electronics of the future. Today’s electronic circuits already routinely work at frequencies of several gigahertz (a billion oscillations per second) up to terahertz (a thousand billion oscillations). Modern computers are as powerful as they are because tiny switches inside them steer electric currents in fractions of a billionth of a second. The incredible data flows of the internet are only possible because extremely fast electro-optic modulators can send information through fibre-optic cables as very short light pulses.
In their experiment, Keller et al exposed a tiny piece of diamond just 50nm thickness to an infrared laser pulse lasting a few femtoseconds. The electric field of that laser light, having a frequency of about half a petahertz, oscillated back and forth 5X in that short time and thus excited the electrons. Generally, the effect of electric fields on electrons in transparent materials can be measured indirectly by sending light through the material and then observing how strongly the material absorbs it. Whereas such measurements are easy for constant electric fields, the extremely rapidly oscillating fields of a laser beam pose a difficult challenges. In principle, the light used for measuring the absorption should only be switched on for a fraction of the oscillation period of the electric field. That, in turn, means that a probe pulse may only last less than a femtosecond. Moreover, the oscillation phase of the electric field of the laser pulse has to be known exactly when the probe pulse is switched on.
Keller’s team performed the groundwork for the solution of these problems already in the late 1990’s. “At the time we were the first to show how the oscillatory phase of a femtosecond laser pulse can be precisely stabilized,” Keller explains, “which, in turn, is a prerequisite for producing attosecond laser pulses.” That technique has since been refined and today allows the researchers to realize light pulses in the extreme ultraviolet, with wavelengths around 30 nanometres, that only last a fraction of a femtosecond and are also synchronized with the oscillatory phase of an infrared pulse. In recent experiments ETH researchers used such a harnessed team of laser pulses to excite the electrons in the diamond with the electric field of the infrared pulse and, at the same time, to measure the resulting absorption changes with the ultraviolet attosecond pulse. They observed, the absorption varied characteristically following the rhythm of the oscillating electric field of the infrared pulse.
In order to understand the details of what went on inside the diamond, however, some more detective work was necessary. They simulated the reaction of the electrons in diamond to the infrared pulse using a supercomputer, finding the same behaviour of the absorption that was measured in Zurich. Those simulations included the complex interplay between the electrons and the crystal lattice of diamond, which results in a large number of so-called energy bands which the electrons can occupy. “The advantage of the simulations compared to the experiment, however, is that several of the effects that occur in real diamond can be switched on or off,” says Matteo Lucchini, a postdoc in Keller’s group, “so that eventually we were able to ascribe the characteristic absorption behaviour of diamond to just two such energy bands.”
It was that realization that, in the end, was crucial for the interpretation of the experimental data. They concluded 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, Keller’s lab. 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 has shown the reaction of the material to the optical field is dominated by the motion of electrons in a single energy band rather than by transitions between different bands.
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.” https://www.ethz.ch/en/news-and-events/eth-news/news/2016/08/electrons-at-the-speed-limit.html
Recent Comments