Floquet Engineering

Floquet Engineering: Shaping Matter with Light

Light can do more than simply probe a material,  sufficiently strong and precisely controlled lightwaves can temporarily change its electronic properties. Floquet engineering uses this idea to create new states of matter that exist only while the material is driven by light. By controlling the intensity, color, polarization, and waveform of the light, we can reshape electronic energy bands, break symmetries, and modify how electrons move through a material.

This provides a powerful way to create properties that a material does not possess under normal conditions. For example, we have studied light-dressed excitons in two-dimensional semiconductors and demonstrated how graphene can be driven into a Floquet topological state with unusual electronic transport properties.

In graphene, circularly polarized light can reshape the electronic band structure and generate topological properties on ultrafast timescales. In recent work, we combined two precisely synchronized colors of light to not only create this transient state, but also control the motion of electrons within it. By changing the relative phase and polarization of the light fields, we can steer electrical currents, reverse their direction, and selectively control electrons associated with different regions of the electronic structure.

Our goal is to push this concept further: Can we use the electric field of light itself as a control knob for quantum materials? Ultimately, we aim to switch electronic, topological, and quantum properties within only a few femtoseconds — and eventually within a fraction of a single cycle of light.

To investigate these light-induced states, we combine ultrafast spectroscopy with electrical measurements, including transient absorption, light-induced Hall currents, polarization-dependent photocurrents, and valley-selective current detection. Together, these approaches allow us to watch and control how electronic states evolve while the lightwave is present.

These developments form the basis for lightwave-driven Floquet electronics, in which electronic and quantum properties are created, manipulated, and read out directly with the oscillating electric field of light.

What is a Floquet topological state?

A Floquet topological state is a temporary electronic state created by periodically driving a material with light. The oscillating electric field of the light interacts with the electrons and can reshape the material’s energy bands while the light is present. In graphene, for example, circularly polarized light can break symmetries and open light-induced energy gaps. This changes the geometry and topology of the electronic states and can produce effects such as Berry curvature, Hall-like currents, and valley-selective transport. The important point is that these properties are not permanently built into the material. They are created and controlled by the light field itself. By changing the light’s intensity, polarization, frequency, or phase, we can in principle switch between different electronic and topological states on femtosecond timescales.