Friederike Junginger

Single-cycle multiterahertz transients with peak fields above 10 MV/cm

Phase-locked single-cycle transients with frequency components between 1 and 60THz and peak fields of up to 12MV/cm are generated as the idler wave of a parametric amplifier. To achieve broadband conversion in GaSe nonlinear crystals, we match the group velocities of signal and idler components. The influence of group-velocity dispersion is minimized by long-wavelength pumping at 1.18mum. Free-space electro-optic sampling monitors the multiterahertz waveforms with direct field resolution.

Nonperturbative Interband Response of a Bulk InSb Semiconductor Driven Off Resonantly by Terahertz Electromagnetic Few-Cycle Pulses

Intense multi-THz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field pulses drive the interband resonances into a non-perturbative regime of Rabi flopping.

Quantum Physics With Ultrabroadband and Intense Terahertz Pulses

We present recent advances in the generation of highly intense multiterahertz transients and their application to nonlinear spectroscopy of bulk semiconductors. An optimized scheme of parametric amplification results in broadband single- or few-cycle terahertz transients with peak electric fields up to 10 or 25 MV/cm, respectively. Time-resolved four-wave mixing terahertz spectroscopy of InSb far away from the interband resonance demonstrates clear signatures of a nonperturbative regime of Rabi flopping. We qualitatively explain the observed behavior within a model of a quantum two-level system. In addition, we demonstrate the dynamical Franz-Keldysh effect in InP resolved on a subcycle timescale. The field-induced modulation of the interband optical absorption at the second harmonic of the driving terahertz field is observed in full agreement with theoretical predictions.

Ultrashort pulse characterization with a terahertz streak camera

A phase-locked terahertz transient is exploited as an ultrafast phase gate for femtosecond optical pulses. We directly map out the group delay dispersion of a low-power near-infrared pulse by measuring the electro-optically induced polarization rotation as a function of wavelength. Our experiment covers the spectral window from 1.0 to 1.4 μm and reaches a temporal precision better than 1 fs. A quantitative analysis of the detector response confirms that this streaking technique requires no reconstruction algorithm and is also well suited for the characterization of pulses spanning more than one optical octave.

Nonlinear response of semiconductors driven by intense THz pulses

We present a review of our recent nonlinear spectroscopy experiments on bulk semiconductors performed using a novel source of ultra-intense multi-THz transients. The field-induced interband optical absorption in InP is studied on subcycle timescales. Our simulations corroborate the Franz-Keldysh effect as the main reason for the observed optical anomalies. The time-resolved four-wave mixing signals generated in InSb demonstrate clear signatures of a nonperturbative excitation regime and can be qualitatively reproduced by a simplified model of a two-level system driven far from the resonance.

Characterization of ultrashort laser pulses with a terahertz streak camera

Precise characterization techniques for ultrashort optical pulses have become indispensible with the emergence of ever shorter waveforms. Typically the pulse under study is overlapped with an optical gate and analyzed after a nonlinear interaction with the latter. If the duration of the pulse to be characterized is much shorter or longer than available gate sources, streak cameras [1, 2] or electro-optic sampling [3] are used, respectively. If the duration of both pulses is similar, auto- and cross-correlation techniques [4] are required, which need complex deconvolution algorithms and are often limited to bandwidths below one optical octave.

Multi-THz nonlinear optics and sub-cycle control of charge and spin

Strong THz electric and magnetic fields are harnessed to coherently control charge and spin in solids with sub-cycle resolution. Exploiting coherent THz phonons we transiently induce and destroy spin density wave order in pnictides.

Field-resolved four-wave mixing with multi-THz transients in InSb

Two-dimensional-spectroscopy with near-infrared (NIR) or mid-infrared pulses has given insight into ultrafast dynamics and spectral correlations in many systems [1]. Terahertz (THz) four-wave-mixing (FWM) has attracted growing interest as a potential way to access important low-energy excitations such as inter- and intra-molecular vibrations or energy gaps in superconductors. However, lead-off experiments [2] have been limited to model systems due to moderate THz fields. Here we employ a recently developed ultraintense multi-THz source [3,4] to pursue THz four-wave mixing in a bulk semiconductor even under two-photon resonant conditions. The temporal profile of the strong FWM signal shows first indications of a THz photon echo. A non-collinear geometry allows for spatial selection of nonlinear signals and background-free measurements.

Interband transitions in InP biased with THz fields of 4 MV/cm

The influence of strong electric fields on electronic properties of semiconductors is of particular interest both for fundamental science and applications in high speed electronics. Investigations using large quasi-static [1] and THz [2] fields have been performed. However, the accessible field amplitudes have been limited to values typically below 1 MV/cm due to the dielectric breakdown under stationary bias and the availability of intense phase-stable THz sources, respectively.

Ultrafast dynamics of semiconductor interband transitions in THz fields up to 4 MV/cm

Phase-locked multi-THz transients bias semiconductors far above the usual threshold for dielectric breakdown. Few-cycle NIR pulses synchronized to the THz transients on an as-timescale, probe interband transitions under electric fields of 4 MV/cm.