We study the electron dynamics in low-dimensional material systems when strong and short electromagnetic pulses drive the electrons into highly non-equilibrium states. High-field charge transport and many-body interactions in condensed matter are of fundamental scientific interest, yet the ultrafast electron dynamics driven by strong electric fields are largely unknown. Understanding and controlling the high-field electron dynamics is indispensable for next-generation high-speed electronic and photonic applications, for which the operating frequency of nanodevices goes beyond 100 GHz and the electric field inside the devices exceeds 100 kV/cm.

Recent advances in high-field terahertz (THz) spectroscopy provide unprecedented opportunities to explore the electron dynamics in condensed matter under the extreme conditions. THz spectroscopy is a powerful tool to study carrier dynamics in semiconductors, in which intraband transitions are directly accessible by THz excitations (the transition energies of ~1 meV correspond to THz photon energies, i.e., 4.1 meV at 1 THz). THz-matter interactions undergo qualitative changes in the high-field regime, in which the interaction energy (e. g., U=p*E for electric dipole interactions) is comparable to or even greater than the internal Coulomb energy of the matter system (e.g., [if gte msEquation 12]><m:oMath><m:sSub><m:sSubPr><span style='font-family:"Cambria Math","serif";font-style:italic'><m:ctrlPr></m:ctrlPr></span></m:sSubPr><m:e><i><span style='font-family:"Cambria Math","serif"'><m:r>E</m:r></span></i></m:e><m:sub><i><span style='font-family:"Cambria Math","serif"'><m:r>0</m:r></span></i></m:sub></m:sSub></m:oMath><![endif][if !msEquation][endif]~1 eV for a band gap). Furthermore, the relatively long acceleration time in a THz field allows another possibility of extreme light-matter interactions unique in the THz regime: the kinetic energy gained by charge acceleration becomes comparable to the unperturbed matter energy. In this strong interaction regime, intense THz pulses excite electrons far from equilibrium and give rise to qualitative changes in optical and electronic properties. We have demonstrated the strong-field driven electron dynamics in several nanoscale material systems:

[if !supportLists](i) [endif]Direct measurement of light–matter energy exchange inside a microcavity

[if !supportLists](ii) [endif]THz induced transparency in graphene

[if !supportLists](iii) [endif]Nonlinear THz absorption in intrinsic GaAs

[if !supportLists](iv) [endif]Field-induced insulator-to-metal transition in vanadium dioxide.