Olaf Schubert

Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

Terahertz waveforms with peak fields of 72 MV cm−1 and a central frequency of 30 THz drive interband polarization in bulk GaSe off-resonantly and accelerate excited electron–hole pairs, inducing dynamical Bloch oscillations. This results in the emission of phase-stable, high-harmonic transients over the whole frequency range of 0.1–675 THz.

Real-time observation of interfering crystal electrons in high-harmonic generation

Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.

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.

Photo-Dember terahertz emitter excited with an Er:fiber laser

A terahertz emitter based on the lateral photo-Dember effect is shown to efficiently generateterahertz radiation with a peak frequency of 0.7 THz and an electric field amplitude up to 5 V/cm when excited by 90 fs pulses centered at 1.55 μ m . A thin layer of In 0.53 Ga 0.47 As grown on InP provides the substrate material in which unidirectional lateral photo-Dember currents are excited. Since photo-Dember terahertz emitters do not require an external bias, they do not suffer from high dark currents limiting the application of biased InGaAs photoconductive terahertz emitters.

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.

Rapid-scan acousto-optical delay line with 34 kHz scan rate and 15 as precision

An optical fast scan delay exploiting the near-collinear interaction between a train of ultrashort optical pulses and an acoustic wave propagating in a birefringent crystal is introduced. In combination with a femtosecond Er:fiber laser, the scheme is shown to delay few femtosecond pulses by up to 6 ps with a precision of 15 as. A resolution of 5 fs is obtained for a single sweep at a repetition rate of 34 kHz. This value can be improved to 39 as for multiple scans at a total rate of 0.3 kHz.

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.

High-harmonic generation in solids

A microscopic theory is presented to describe high-harmonic generation in solids with the semiconductor-Bloch equations. The approach includes the relevant interband polarizations and intraband currents. The appearance of even harmonic orders is shown to require at least three electronic bands and a mutual interband coupling between them. In experimental and theoretical time-resolved studies, this also manifests as a unipolar emission signature of the high-harmonic radiation.

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.