Achieving full control over the properties of quantum materials is one of the long-standing goals of science and technology. Historically, this task has been tackled in the static limit by altering materials in various ways, for instance with chemical substitution, heterostructuring and van der Waals stacking, or through the application of slowly varying external stimuli. More recently, there has been a growing interest in the use of light as a means to realize the dynamical manipulation of quantum materials. Through various light-matter interactions, ultrashort laser pulses have the potential to drive specific collective modes to large amplitudes, selectively break material symmetries, or coherently dress electronic states. If realized, this form of nonequilibrium control would lead to the selective tuning of microscopic interactions (e.g., electron-phonon, exchange, etc.), the precise steering of non-thermal phase transitions, or the ultrafast modulation of band topology. This, in turn, would open a new era in high-speed (i.e. sub-picosecond) photonics, data storage, and quantum devices. However, in practice, this quantum design of materials by light has yet to be achieved.
To tackle this challenge, in our group we develop novel platforms for tailored material control with coherent terahertz (THz) fields and realize their integration into sensitive spectroscopy and microscopy techniques. Our coherent THz fields are rapidly varying fields of different character (strain, photonic, polaritonic), with a frequency content within the 0.01-50 THz range. Unlike light pulses with high (eV) photon energy, our THz fields have direct access to the energy scale of the degrees of freedom that govern the quantum behavior of most materials and thus modify relevant properties in a highly selective manner.
For example, one of our protocols is based on the precise design of intense acoustic waves with tailored laser pulses. We then use these acoustic transients to excite target materials, moving their atoms far away from their equilibrium positions. Through this ultrafast strain engineering, we aim to establish protocols for high-speed modulation of electronic, optical, and magnetic properties and to effectively act on the structure-function relationship of materials. Another approach relies on the use of single- and multi-cycle photon fields in the THz range to induce complex transformations in solids. These transformations can involve the reorganization of nanoscopic domains into a macroscopically aligned state, or the transient emergence of entirely new material properties. We are especially interested in the time-periodic driving of quantum phases, realized either through the oscillating electric field of multi-cycle THz pulses or through the periodic potential of coherently excited collective modes. This strategy will enable us to attack important technological problems, such as the realization of ultrafast topological phase transitions.