Non-perturbative four-wave mixing in InSb with intense off-resonant multi-THz pulses

Abstract

Abstract. High-field multi-THz pulses are employed to analyze the coherent nonlinear response of the narrow-gap semiconductor InSb which is driven off-resonantly. Fieldresolved four-wave mixing signals manifest the onset of a non-perturbative regime of Rabi flopping at external amplitudes above 5 MV/cm per pulse. Simulations based on a two-level quantum system confirm these experimental results.Two-dimensional spectroscopy has been a powerful tool to examine spectral correlations and ultrafast dynamics in condensed-matter systems. For example, studies in the near-(NIR) and midinfrared (MIR) spectral region [1] have provided insight into molecular structures of proteins [2]. At yet lower THz frequencies two-dimensional spectroscopy has been anticipated to provide access to fundamental low-energy excitations such as inter-molecular motion in water [3] or gap dynamics in superconductors [4].Recently, a promising THz multi-wave mixing has been suggested [5]. These lead-off experiments exploiting field amplitudes of the order of 1 MV/cm have been limited to model systems with especially large dipole moments. It has been shown that Rabi oscillations can be driven resonantly in semiconductors with intense optical pulses [6,7]. The latest development of an ultraintense laser source for MIR pulses with peak fields exceeding 100 MV/cm paves the way towards a systematic study of extreme THz nonlinearities [8,9]. Based on the electro-optic detection scheme, the time trace of the carrier field can be retrieved with absolute amplitude and phase.Here, a high-field THz laser system is employed to study the nonlinear response of bulk semiconductors using a two-dimensional spectroscopy scheme [5]. We show that in spite of the excitation far below the interband resonance extreme THz electric fields are able to coherently drive a system close to population inversion.Our THz multi-wave mixing experiment is performed in the narrow-gap semiconductor indium antimonide InSb (E g /h = 41.1 THz) at room temperature. An undoped (100)-oriented single crystal has been mechanically polished to a thickness of 30 µm facilitating THz transmission. Intense THz transients centered at 25 THz are obtained by difference frequency mixing of near-infrared pulse trains in a GaSe emitter. All relevant frequency components are located well below the fundamental band gap of InSb. In contrast to previous studies [5], the generation of double pulses is implemented in our setup by splitting the THz beam after the emitter crystal to preclude nonlinear mixing effects of two beams within the emitter itself. Two equally intense transients are obtained via a Au-coated EPJ Web of Conferences