arXiv subjects

Dieter Bauer

Publications and source records attributed to Dieter Bauer.

28 records · Page 2Linked to original sources

arXiv2020

Nonlinearity-induced photonic topological insulator

The hallmark feature of topological insulators renders edge transport virtually impervious to scattering at defects and lattice disorder. In our work, we experimentally demonstrate a topological system, using a photonic platform, in which the very existence of the topological phase is brought about by nonlinearity. Whereas in the linear regime, the lattice structure remains topologically trivial, light beams launched above a certain power threshold drive the system into its transient topological regime, and thereby define a nonlinear unidirectional channel along its edge. Our work studies topological properties of matter in the nonlinear regime, and may pave the way towards compact devices that harness topological features in an on-demand fashion.

physics.optics↗

arXiv2020

Helicity flip of high-harmonic photons in Haldane nanoribbons

Recent studies in high-harmonic spectroscopy of condensed matter mainly focused on the bulk of the system under consideration. In this work, we investigate the response of thin, hexagonal nanoribbons to an intense laser pulse that is linearly polarized along the ribbon. Such nanoribbons are prime examples of two-dimensional systems that are bulk-like in one direction and finite in the other direction. Despite the atomically thin scale in the direction perpendicular to the linearly polarized driving laser field, the emitted harmonics are elliptically polarized if an alternating onsite potential and Haldane hopping is taken into account. For given hoppings, we find a sudden change of the helicity for a certain harmonic order. The origin of this flip is traced back to phase differences between the components of Bloch states.

cond-mat.mes-hall↗

arXiv2019

QPROP with faster calculation of photoelectron spectra

The calculation of accurate photoelectron spectra (PES) for strong-field laser-atom experiments is a demanding computational task, even in single-active-electron approximation. The QPROP code, published in 2006, has been extended in 2016 in order to provide the possibility to calculate PES using the so-called t-SURFF approach [L. Tao, A. Scrinzi, New J. Phys. 14, 013021 (2012)]. In t-SURFF, the flux through a surface while the laser is on is monitored. Calculating PES from this flux through a surface enclosing a relatively small computational grid is much more efficient than calculating it from the widely spread wavefunction at the end of the laser pulse on a much larger grid. However, the smaller the minimum photoelectron energy of interest is, the more post-propagation after the actual laser pulse is necessary. This drawback of t-SURFF has been overcome by Morales et al. [F. Morales, T. Bredtmann, S. Patchkovskii, J. Phys. B: At. Mol. Opt. Phys. 49, 245001 (2016)] by noticing that the propagation of the wavefunction from the end of the laser pulse to infinity can be performed very efficiently in a single step. In this work, we introduce QPROP 3.0, in which this single-step post-propagation (dubbed i-SURFV) is added. Examples, illustrating the new feature, are discussed. A few other improvements, concerning mainly the parameter files, are also explained.

physics.comp-ph↗

arXiv2019

High-harmonic generation in Su-Schrieffer-Heeger chains

Su-Schrieffer-Heeger (SSH) chains are the simplest model systems that display topological edge states. We calculate high-harmonic spectra of SSH chains that are coupled to an external laser field of a frequency much smaller than the band gap. We find huge differences between the harmonic yield for the two topological phases, similar to recent results obtained with more demanding time-dependent density functional calculations [D. Bauer, K.K. Hansen, Phys. Rev. Lett. 120, 177401 (2018)]. This shows that the tight-binding SSH model captures the essential topological aspects of the laser-chain interaction (while higher harmonics involving higher bands or screening in the metal phase are absent). We study the robustness of the topological difference with respect to disorder, a continuous phase transition in position space, and on-site potentials. Further, we address the question whether the edges need to be illuminated by the laser for the huge difference in the harmonic spectra to be present.

physics.optics↗

arXiv2019

Robustness of topologically sensitive harmonic generation in laser-driven linear chains

A huge difference in the harmonic yield from the two topological phases of finite, dimerizing linear chains in laser fields has recently been observed in all-electron time-dependent density functional simulations [D. Bauer, K.K. Hansen, Phys. Rev. Lett. 120, 177401 (2018)]. In this work, we explore the robustness of the effect concerning the size of the chains, a continuous transition between the two topological phases, and disorder. A high robustness of both the degeneracy of the edge states in the topologically non-trivial phase and of the pronounced destructive interference causing a dip in the harmonic spectra in the topologically trivial phase is observed.

physics.atom-ph↗

arXiv2018

Finite-system effects on high-harmonic generation: from atoms to solids

Using time-dependent density functional theory, high-harmonic generation (HHG) is studied in one-dimensional structures of sizes from a single nucleus up to hundreds of nuclei. The HHG cutoff is observed to extent linearly with the system size from the well known atomic HHG cutoff and is found to converge into the previously observed cutoffs for bulk solids only for large systems. A change in the response from that of single atoms or small molecules is observed from system sizes of $N$ $\approx$ 6 nuclei and becomes independent of system size at $N \gtrsim 60$. The system-size dependence of the observed HHG cutoffs is found to follow the limitations, set by the finite size solid, on the classical motion of electron-hole pairs. Because of the relation between recombination energy and electron-hole propagation length in the system, high-energy recombination events are not possible in small systems but become accessible for larger systems resulting in the change of the cutoff energies with system size. When varying the field intensity we observe that the cutoffs move linearly with the intensity even for small systems of $N\gtrsim 6$ that are far from the limit of a bulk solid.

physics.atom-ph↗

arXiv2018

High-harmonic generation in solids with and without topological edge states

High-harmonic generation (HHG) in the two topological phases of a finite, one-dimensional, periodic structure is investigated using a self-consistent time-dependent density functional theory (TDDFT) approach. For harmonic photon energies smaller than the band gap, the harmonic yield is found to differ up to fourteen orders of magnitude for the two topological phases. This giant topological effect is explained by the degree of destructive interference in the harmonic emission of all valence-band (and edge-state) electrons, which strongly depends on whether topological edge states are present or not. The combination of strong-field laser physics with topological condensed matter opens up new possibilities to electronically control strong-field-based light or particle sources or---vice versa---to steer by all optical means topological electronics.

physics.optics↗

arXiv2017

High-order harmonic generation in solid slabs beyond the single active electron

High-harmonic generation by a laser-driven solid slab is simulated using time-dependent density functional theory. Multiple harmonic plateaus up to very high harmonic orders are observed already at surprisingly low field strengths. The full all-electron harmonic spectra are, in general, very different from those of any individual Kohn-Sham orbital. Freezing the Kohn-Sham potential instead is found to be a good approximation for the laser intensities and harmonic orders considered. The origins of the plateau cutoffs are explained in terms of band gaps that can be reached by Kohn-Sham electrons and holes moving through the band structure.

physics.atom-ph↗

arXiv2016

Photoelectron spectra with Qprop and t-SURFF

Calculating strong-field, momentum-resolved photoelectron spectra (PES) from numerical solutions of the time-dependent Schrödinger equation (TDSE) is a very demanding task due to the large spatial excursions and drifts of electrons in intense laser fields. The time-dependent surface flux (t-SURFF) method for the calculation of PES [L. Tao, A. Scrinzi, New Journal of Physics 14, 013021 (2012)] allows to keep the numerical grid much smaller than the space over which the wavefunction would be spread at the end of the laser pulse. We present an implementation of the t-SURFF method in the well established TDSE-solver Qprop [D. Bauer, P. Koval, Comput. Phys. Commun. 174, 396 (2006)]. Qprop efficiently propagates wavefunctions for single-active electron systems with spherically symmetric binding potentials in classical, linearly (along $z$) or elliptically (in the $xy$-plane) polarized laser fields in dipole approximation. Its combination with t-SURFF makes the simulation of PES feasible in cases where it is just too expensive to keep the entire wavefunction on the numerical grid, e.g., in the long-wavelength or long-pulse regime.

physics.atom-ph↗

arXiv2015

Selective Strong-Field Enhancement and Suppression of Ionization with Short Laser Pulses

We experimentally demonstrate robust selective excitation and attenuation of atomic Rydberg level populations in sodium vapor (Na I) using intense laser pulses in the strong field limit ($> 10^{12} W/cm^{2}$). The coherent control of the atomic population and related ionization channels is realized for intensities above the over-the-barrier ionization intensity. A qualitative model predicts that this strong field coherent control arises through the manifestation of a Freeman resonance.

physics.optics↗