Abstract
Nonlinear two-dimensional terahertz (2D-THz) spectroscopy at frequencies of the emitted THz signal different from the driving frequencies allows for exploring the regime of (off-)resonant even-order nonlinearities in condensed matter. To demonstrate the potential of this method, we study two phenomena in the nonlinear THz response of bulk GaAs: (i) The nonlinear THz response to a pair of femtosecond near-infrared pulses unravels novel fourth- and sixth-order contributions involving interband shift currents, Raman-like excitations of transverse-optical phonon and intervalence-band coherences. (ii) Transient interband tunneling of electrons driven by ultrashort mid-infrared pulses can be effectively controlled by a low-frequency THz field with amplitudes below 50 kV/cm. The THz field controls the electron-hole separation modifying decoherence and the irreversibility of carrier generation.
Highlights
Nonlinear two-dimensional infrared (2D-IR) spectroscopy of molecular ensembles in liquids and solids is currently dominated by studies of the third-order or χ(3) response on femto- to picosecond time scales
Phase-resolved 2D-THz spectroscopy has first focused on electronic excitations of solids, which are characterized by transition dipole moments orders of magnitude larger than those of vibrational transitions
Moderate THz electric fields with amplitudes on the order of 1–50 kV/cm allow for driving light–matter interactions into the nonperturbative regime, well beyond the χ(3) limit
Summary
Nonlinear two-dimensional infrared (2D-IR) spectroscopy of molecular ensembles in liquids and solids is currently dominated by studies of the third-order or χ(3) response on femto- to picosecond time scales This approach is based on three—in most cases resonant—interactions with the electric field of a two- or threepulse sequence, generating a nonlinear signal. Scitation.org/journal/jcp with the latter three typically under nonresonant conditions.13–18 In this range of interaction strengths, each of the applied driving pulses can interact several times with the sample before the nonlinear THz signal field is emitted. Extending this study to the interaction of bulk GaAs with a pair of near-infrared pulses gives insight into the fourth-order THz response, which includes a frequency upshift of the TO phonon resonance due to local-field effects in the semiconductor crystal.. Novel fourthand sixth-order contributions to the nonlinear THz response are identified, in particular a component due to field-driven interband tunneling
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