As early as the Middle Ages, alchemists tried to turn lead into gold - admittedly with very dubious success. Over the past century, modern materials science has developed from these rudimentary beginnings thanks to new theoretical concepts and experimental methods. Today, it is possible, for example, to modify the electronic properties of materials by varying their chemical composition or by external parameters such as temperature, or pressure. All these methods are now well established, but they have one major drawback: they are slow.
We are exploring a new alternative method that is capable of changing the electronic properties of a material within a few femtoseconds (one millionth of a billionth of a second). The underlying idea can be explained quite simply using a children's toy: a spherical spinning top with a cylindrical hole surrounding the central rod. When the top is at rest, the rod points upward (see Fig. 1a). If one sets the top in rotation, it will turn upside down (see Fig. 1b). The properties of the rotating top are different from those of the top at rest.
In our research we replace the top with a solid and the child that sets the top in rotation with the extremely strong light fields of a femtosecond laser amplifier, which coherently accelerate electrons and atoms inside the solid. To this end, we are developing tailored laser pulses that selectively excite different degrees of freedom of the materials we study. Changes of the electronic properties then occur on the timescale of a single oscillation cycle of the driving laser field. There are no limits to the imagination: using periodic driving one can turn an insulator into a metal, a metal into a superconductor, a trivial semiconductor into a semiconductor with topologically protected edge states, and much more. There are many exciting theoretical predictions on this topic as well as first experimental successes.
To find out if periodic driving of the solid with strong laser pulses has the desired effect, one needs a method to study the electronic properties of driven solids. We are using time- and angle-resolved photoelectron spectroscopy (trARPES) which provides direct access to the transient band structure E(k) of a material.
If successful, our research will establish driven solids as a completely new class of materials with tunable functionalities. The ability to control band structure and electron spin with light will certainly find applications in novel ultrafast optoelectronic and optospintronic devices.
