Session Y49: Strong Light-Field on Topological Matter

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An appealing and challenging route towards engineering materials with specific properties is to find ways of designing or selectively manipulate materials, especially at the quantum level. We will discuss new states of matter that are optically induced and have no equilibrium counterparts, and we will identify the fingerprints of these novel states that will be probed with pump-probe spectroscopies. A particular appeal of light dressing is the possibility to engineer symmetry breaking which can lead to novel properties of materials, e.g coupling to circularly polarized photons leads to local breaking of time-reversal symmetry enabling the control over a large variety of materials properties (e.g.topology). We show that the new quantum electrodynamics density-functional formalism (QEDFT) can acocunt for those effects. For example, the concept of symmetry breaking with light to induce topological phases with linearly and circularly polarized light will be presented to show the transition to topologically non-trivial states in 2D materials. By controlling the Berry curvature in 2D layered materials (metal/insulator transition metal dichalcogenides, or TMD), a new class of quantum Hall states can be induced. In these states, the valley degree of freedom can be tuned with light. The theory for such states requires the treatment of strong electric fields; that is, low driving frequencies and the inclusion of dissipation and lattice degree of freedom.   

Live

We investigate theoretically how the harmonic emission changes when the illuminated condensed-matter system undergoes a topological phase transition. We start with two-band bulk systems (examples being the Su-Schrieffer-Heeger chain, the Haldane or the Qi-Wu-Zhang models) and derive an explicit equation for the velocity of the laser-driven electrons, including all topological terms [1] that are often swept under the rug by approximations in treatments based on the semi-conductor Bloch equations. We find that the helicity of the harmonics changes through a topological phase transition; however, not all harmonics change their helicity in the same way. We continue with thin, hexagonal nanoribbons where four bands play a role and a flip of the helicity is observed at a certain harmonic order that depends on the Haldane-like next-nearest neighbor hopping [2]. Finally, we discuss the difference between finite and bulk systems. Despite the ubiquitous “bulk-boundary correspondence” the presence of topologically protected edge currents in finite systems affect the laser-driven electron dynamics and thus harmonic generation.

[1] Daniel Moos, Christoph Jürß, Dieter Bauer, Phys. Rev. A (accepted) [arXiv:2007.13434]
[2] Christoph Jürß, Dieter Bauer, Phys. Rev. A 102, 043105 (2020) [arXiv:2006.16037]   

Live

Optical driving has been proposed as a means of engineering topological properties in topologically trivial systems. One proposal for such a “Floquet topological insulator” is based on breaking time-reversal symmetry in graphene through a coherent interaction with circularly polarized light [1]. This was predicted to lift the degeneracy of the Dirac point, opening a topological band gap in the resulting photon-dressed band structure accompanied by dressed chiral edge states [2]. While quantum simulation experiments have validated aspects of this proposal [3,4], and Floquet–Bloch bands have been observed in a topological insulator [5], the transport properties of such a light-induced topological state have remained elusive in a real material.

In this talk, I will report on our recent observation of a light-induced anomalous Hall effect in graphene driven by a femtosecond pulse of circularly polarized light [6]. Electrical transport was probed using an ultrafast device architecture based on photoconductive switches. The dependence of the anomalous Hall effect on a gate potential used to tune the equilibrium Fermi level revealed multiple features that reflect a Floquet-engineered topological band structure, similar to the band structure originally proposed by Haldane [7]. This included an approximately 60 meV wide conductance plateau centered at the Dirac point, where a gap of equal magnitude was predicted to open. We found that when the Fermi level was tuned within this plateau, the estimated anomalous Hall conductance saturated around 1.8+/-0.4 e^2/h.

[1] T. Oka & H. Aoki. Phys. Rev. B 79, 081406 (2009)
[2] T. Kitagawa et al. Phys. Rev. B 84, 235108 (2011)
[3] M.C. Rechtsman et al., Nature 496, 196 (2013)
[4] G. Jotzu et al., Nature 515, 237 (2014)
[5] Y.H. Wang et al., Science 342, 453 (2013)
[6] J.W. McIver et al. Nat. Phys. 16, 38 (2020)
[7] F.D.M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)   

Live

Strong ultrafast optical pulses with an amplitude of up to 1 V/A and a duration of a few femtoseconds provide an extra tool to probe topological properties of solids. In relation to ultrafast electron dynamics under the field of the pulse, the topology manifests itself in unique properties of interband dipole couplings, which are related to the non-Abelian Berry connections, and in large accumulation of topological phase during a long electron excursion induced by a laser pulse in the reciprocal space. We have studied theoretically ultrafast electron dynamics in different types of topological materials, such as graphene, transition metal dichalcogenides (TMDCs), 3D topological insulators, Weyl semimetals, and others. Our extensive numerical analysis has shown that the topologically-related features in interband and intraband electron dynamics result in experimentally observable effects, such as interference patterns in the conduction band population distributions in the reciprocal space of graphene, large ultrafast valley polarization in TMDCs, an ultrafast anomalous Hall effect in gapped graphene-like materials, Berry phase-induced singularities in the conduction band population distribution in specially designed graphene nanosystems, a highly nonlinear optical absorbance in graphene-like materials. Some of these processes are related to the effect of ultrafast topological resonance, which is due to a mutual compensation of the dynamic phase and the topological phase during the pulse.   

Live

High-order harmonic generation (HHG) has been studied extensively in gaseous media. One of the interesting applications of HHG is the use of underlying strong-field-driven electron dynamics to probe the orbital structures and dynamics in isolated small molecules. Recently, the HHG process has also been realized in solid materials, which has sparked interest in scrutinizing its prospects as a novel ultrafast probe of the solid materials in an all-optical setting [1]. The ongoing activities involve the use of underlying inter- and intra-band dynamics to probe valence charge densities in the real space and the band structure in the momentum space including topological properties. The latest theoretical results predict the possibility of probing topological phase transitions in the Haldane system using circular dichroism in HHG [2]. In this talk, I will give a brief overview of the field and present our new experimental and theoretical results on the generation of high-order harmonics from a prototypical three-dimensional topological insulator Bi₂Se₃ subjected to strong mid-infrared laser fields [3]. Our study scrutinizes HHG as a novel spectroscopic nonlinear optical probe of topological insulators. Its unique features include the unprecedented access to ultrafast dynamics occurring in the sub-cycle time scale of the driving laser field, with implications for ultrafast metrology and light-field driven electronics.
[1] “Review: High-harmonic generation from solids” Shambhu Ghimire and David Reis, Nature Physics, 15, 10-16, 2019 [2] “Circular dichroism in higher-order harmonic generation: Heralding topological phases and transitions in Chern insulators” Alexis Chacón et al., Phys. Rev. B 102, 134115 (2020) [3] “Strong-field physics in three-dimensional topological insulators” Denitsa Baykusheva et al., arXiv:2008.01265