Sebastian Baierl

Coherent cyclotron motion beyond Kohn’s theorem

Kohn’s theorem states that the electron cyclotron resonance is unaffected by many-body interactions in a static magnetic field. Yet, intense terahertz pulses do introduce Coulomb effects between electrons—holding promise for quantum control of electrons.

Femtosecond terahertz time-domain spectroscopy at 36 kHz scan rate using an acousto-optic delay

We present a rapid-scan, time-domain terahertz spectrometer employing femtosecond Er:fiber technology and an acousto-optic delay with attosecond precision, enabling scanning of terahertz transients over a 12.4-ps time window at a waveform refresh rate of 36 kHz, and a signal-to-noise ratio of 1.7 × 105/Hz. Our approach enables real-time monitoring of dynamic THz processes at unprecedented speeds, which we demonstrate through rapid 2D thickness mapping of a spinning teflon disc at a precision of 10 nm/Hz. The compact, all-optical design ensures alignment-free operation even in harsh environments.

Efficient nonlinear control of spins by ultrashort THz-fields

Ultrashort pulses of intense THz radiation have been shown to represent a powerful and versatile tool for spin control [1–6].

Terahertz subcycle control: From high-harmonic generation to molecular snapshots

Atomically strong THz fields accelerate electrons in bulk semiconductors, new 2D materials, and atomically sharp tunneling junctions. By tracking this lightwave-driven charge transport with subcycle resolution, we explore dynamical Bloch oscillations as well as quasiparticle collisions and record the first single-molecule femtosecond movie.

Extending nonlinear coherent quantum control with intense terahertz pulses

Dynamics in solid-state systems are governed by many-body interactions that are inherently tied to the high density of electrons and ions. For most elementary excitations, however, Coulomb (i.e., elastic) scattering leads to dephasing within a few to a fewhundreds of femtoseconds. Coherent quantum control (the precise manipulation of the phases of quantum states) is therefore usually considered to be a daunting challenge in many-body systems. In 1961, however, Walter Kohn found that the cyclotron resonance (CR) of Landau electrons (i.e., the harmonic motion of electrons in a magnetic field) is immune to electron–electron Coulomb interactions. Kohn’s theorem thus shows that the CR is one of the most robust manifestations of the quantum harmonic oscillator, and excludes the possibility of any nonlinear light– matter interactions.1 Although the CR has given rise to a number of sophisticated quantum phenomena, such as ultrastrong light–matter coupling,2 superradiance,3 coherent control,4 and superfluorescence,5 the complete absence of nonlinearities suggests that many intriguing possibilities (e.g., quantum logic operations) are excluded. In this work,6 we show how strong terahertz (THz) pulses can be used to create non-perturbative THz excitations of a magnetically biased, 2D electron gas (2DEG). The pulses induce strong, coherent nonlinearities and facilitate coherent quantum control of multiple Landau levels, leading to population inversion. In our approach, the 2DEG is contained within two 30nm-wide gallium arsenide quantum wells (each n-doped at 1.6 1011cm 2) Figure 1. (a) The transmitted terahertz (THz) field is used to monitor the coherent inter-Landau level polarization. Results are shown for different amplitudes of the driving field (between 0.7 and 8.7kVcm 1), as a function of the electro-optic sampling (EOS) delay time (t). (b) The decay constant ( c) that is extracted from the data in (a), as a function of the initial field (E0). The Landau-level population, for fields of 4.3 and 8.7kVcm 1 (blue and red bars, respectively), is shown in the inset. f: Population density. h̄!LO: Longitudinal optical phonon energy.

Nonperturbative THz nonlinearities for many-body quantum control in semiconductors

Quantum computing and ultrafast quantum electronics constitute pivotal technologies of the 21st century and revolutionize the way we process information. Successful implementations require controlling superpositions of states and coherence in matter, and exploit nonlinear effects for elementary logic operations. In the THz frequency range between optics and electronics, solid state systems offer a rich spectrum of collective excitations such as excitons, phonons, magnons, or Landau electrons. Here, single-cycle THz transients of 8.7 kV/cm amplitude centered at 1 THz strongly excite inter-Landau-level transitions of magnetically biased GaAs quantum wells, facilitating coherent Landau ladder climbing by more than six rungs, population inversion, and coherent polarization control. Strong, highly nonlinear pump-probe and four- and six-wave mixing signals, entirely unexpected for this paragon of the harmonic oscillator, are revealed through two-time THz spectroscopy. In this scenario of nonperturbative polarization dynamics, our microscopic theory shows how the protective limits of Kohn’s theorem are ultimately surpassed by dynamically enhanced Coulomb interactions, opening the door to exploiting many-body dynamics for nonlinear quantum control.

Sub-cycle control of multi-THz high-harmonic generation and all-coherent charge transport in bulk semiconductors

Ultrafast transport of electrons in semiconductors lies at the heart of high-speed electronics, electro-optics and fundamental solid-state physics. Intense phase-locked terahertz (THz) pulses at photon energies far below electronic interband resonances may serve as a precisely adjustable alternating bias, strongly exceeding d.c. breakdown voltages. Here, we exploit the near-field enhancement in gold metamaterial structures on undoped bulk GaAs, driven by few-cycle THz transients centered at 1 THz, to bias the semiconductor substrate with field amplitudes exceeding 12 MV/cm. Such fields correspond to a potential drop of the bandgap energy over a distance of only two unit cells. In this extremely off-resonant scenario characterized by a Keldysh parameter of γK ≈ 0.02, massive interband Zener tunneling injects a sizeable carrier density exceeding 1019 cm-3, and strong photoluminescence results. At a center frequency of 30 THz, THz transients with peak fields of 72 MV/cm analogously excite carriers in a bulk, semiconducting GaSe crystal, without metamaterial. Here, in contrast, we are able to drive coherent interband polarization and furthermore dynamical Bloch oscillations of electrons in the conduction band, on femtosecond time scales. The dynamics entail the generation of absolutely phase-stable high-harmonic transients containing spectral components up to the 22nd order of the fundamental frequency, spanning 12.7 optical octaves throughout the entire terahertz-to-visible domain between 0.1 and 675 THz. Our experiments establish a new field of light-wave electronics exploring coherent charge transport at optical clock rates and bring picosecond-scale electric circuitry at the interface of THz optics and electronics into reach.

Extreme Terahertz Nonlinearities in Undoped GaAs Driven by Ultrahigh Near-Fields in Metamaterials

Local terahertz fields of multiple 10 MV/cm tailored in gold metamaterials drive electronic interband transitions in intrinsic GaAs. The bandgap exceeds the THz photon energy 400-fold. Photoluminescence microscopy maps the THz near-field distribution.