Terahertz (THz) fields are often resonant with internal transitions of many-body states, and they also can “overclock” GHz electronics via optical means (1 THz = 1000 GHz). In particular, one can nowadays generate THz pulses with 100MV/cm fields strengths, and still avoid crystal damage by making the pulse only a few-cycles long. Such fields create more than a 1 eV gradient over a 1Å distance such that one can create massive distortions of potential landscape of solid-state devices, which can steer electronics on an ultrafast time scale. By ultrafast, we mean down to the scale of femtoseconds.

We develop a general many-body theory to describe all aspects of THz excitations in solids. Over the decades, we have studied effects ranging from detecting exciton formation to high-harmonic and harmonic sideband generation. Recently, we have shown how ultrafast experiments can access delicate quantum processes such as dynamical Bloch oscillations, electronic quantum interferences, nonlinear Coulomb effects among Landau electrons, and pure correlation transport across a semiconductor interface. All these developments offer interesting experimental possibilities to realize THz-field driven optics, or light-source development for next-generation applications.

### Selected references:

M. Kira, W. Hoyer, T. Stroucken, and S.W. Koch,* Exciton Formation in Semiconductors and Influence of a Photonic Environment*, Phys. Rev. Lett. **87**, 176401 (2001).

J.R. Danielson, Y.-S. Lee, J.P. Prineas, J.T. Steiner, M. Kira, and S.W. Koch,* Interaction of Strong Single-Cycle Terahertz Pulses with Semiconductor Quantum Wells*, Phys. Rev. Lett. **99**, 237401 (2007).

O. Vänskä, I. Tittonen, S.W. Koch, and M. Kira, *Coherent terahertz control of vertical transport in semiconductor heterostructures*, Phys. Rev. Lett. **114**, 116802 (2015).

M. Hohenleutner, F. Langer, O. Schubert, M. Knorr, U. Huttner, S.W. Koch, M. Kira, and R. Huber, *Real-time observation of interfering crystal electrons in high-harmonic generation*, Nature **523**, 572 (2015).

T. Maag, A. Bayer, S. Baierl, M. Hohenleutner, T. Korn, C. Schüller, D. Schuh, D. Bougeard, C. Lange, R. Huber, M. Mootz, J. E. Sipe, S. W. Koch, and M. Kira, *Coherent cyclotron motion beyond Kohn’s theorem*, Nat. Phys. **12**, 119 (2016).

F. Langer, M. Hohenleutner, C. Schmid, C. Poellmann, P. Nagler, T. Korn, C. Schüller, M. S. Sherwin, U. Huttner, J. T. Steiner, S. W. Koch, M. Kira, and R. Huber, *Lightwave-driven quasiparticle collisions on a sub-cycle timescale*, Nature **533**, 225 (2016).