Session Z41: Strong-field Physics and Ultrafast Spectroscopy in Solids

Strong-field physics and HHG in solids: topology, magnetism, Floquet

In recent years strong-field laser-driven phenomena in condensed matter have been employed for novel ultrafast spectroscopies, e.g. all-optical band-structure reconstruction, probing exciton formation and recombination in real-time, and tracking ultrafast phase transitions. In this approach, intense laser pulses irradiate a sample, triggering non-perturbative responses such as high harmonic generation (HHG), photoionization, etc.; which can be analyzed to obtain information about the sample and ultrafast (potentially attosecond) out-of-equilibrium dynamics occurring within it. However, in order to faithfully extract information from spectra a deep physical understanding of the mechanisms generating the response, and a proper comparison between theory and experiment, are required. I will discuss this fundamental issue in the context of ab-initio calculations, and present our recent work attempting to probe material topology with HHG. I will show that topology is not necessarily imprinted onto spectra in a straightforward manner, and that its typical contribution to HHG can be rather weak, contrary to the common understanding in the field. I will also discuss our work on other emerging topics in strong-field physics in solids such as the role of Floquet light-driven phases of matter in the electronic response, and connections to ultrafast magnetism. Specifically, I will show that Floquet physics plays a major role in the tunneling process, and that strong-field laser irradiation allows inducing, and probing, attosecond magnetic responses, which are the fastest magnetic responses predicted to date.   

Characterizing Anomalous High-Harmonic Generation in Solids

High-harmonic generation (HHG) in condensed-phase systems has gained intense interest in the last decade, for its dual promise as a compact source of ultrafast extreme ultraviolet laser pulses, and as a spectroscopic, sub-femtosecond-resolution tool of driven electron dynamics. In this work we characterize the nonlinear anomalous current that arises when a material with Berry curvature is exposed to an intense, infrared laser pulse, through ab-initio calculations of HHG in MoS2. We resolve the nonlinear current that gives rise to high-order harmonics into its four different components and discuss their relative importance. We identify two unique properties of the anomalous harmonic yield that could be exploited to disentangle the anomalous harmonics from competing HHG mechanisms: an overall yield increase with laser wavelength; and pronounced minima at certain laser wavelengths and laser intensities. Finally, we show how the Berry curvature can be reconstructed from the angular and spectral variation of the harmonic yield.   

High-harmonic spectroscopy of quantum materials

High-order harmonic generation (HHG)  is a nonlinear optical phenomena that has led to generation of attosecond pulses as well as to spectroscopic probing of the source material. So far HHG has been used extensively in atomic and molecular systems in the gas phase. In this talk, I will discuss our initiatives in extending these capabilities to solid-state materials such as 2D crystals, heterostructures, and three-dimensional topological insulators. Our recent results show that HHG can be developed as a novel all-optical probe for the structure and dynamics including correlated dynamics in quantum materials.    

High-harmonic generation in strongly correlated systems

The recent development of strong lasers enables us to study various nonlinear optical phenomena. One fundamental example is high-harmonic generation (HHG). HHG was observed and intensively studied first in gases, and later in semiconductors. More recently, the scope is further extended to strongly correlated electron systems (SCESs). SCESs are a potentially interesting playground for the HHG research since i) the excitation structures cannot be described by independent electrons and holes unlike conventional semiconductors, ii) various degrees of freedom (charge, spin and orbitals) can be strongly intertwined in SCESs and iii) SCESs host intriguing phases. Due to these properties, HHG in SCESs may show peculiar properties absent in HHG in semiconductors.

 

Here, we discuss the basic feature and origin of HHG in Mott insulators described by the single-band Hubbard model, a standard model for SCESs [1-3]. In the Mott phase, each site is occupied by a single electron, and it cannot move due to the strong on-site Coulomb interaction. Photo-excitation creates doubly occupied sites (doublons) and no occupied sites (holons). They are charge carriers of this system and their dynamics leads to the light emission. Firstly, we reveal a generic HHG feature of the Mott insulator using the nonequilibrium dynamical-mean field theory [1]. We show the HHG spectrum qualitatively changes depending on the field strength, reflecting the itinerant or localized nature of the doublon-holon pairs. We also show that the HHG feature is not directly related to the dispersions observed in the single-particle spectrum unlike in semiconductors. Secondly, to reveal the relation between HHG and many body elemental excitations, we study the one-dimensional system [2]. We show that the semi-classical three step model combined with the elemental excitations works well to describe the HHG process. Finally, we study the effects of strong spin-charge couplings on HHG, which becomes important in higher dimensions than one [3]. We show that the spin-charge coupling leads to intriguing temperature (or band-gap) dependence of HHG as has been reported in the experiment on a Mott insulator Ca₂RuO₄ [4]. Our results suggest the potential application of HHG to detect many-body excitations, and possibility to find intriguing HHG behavior in SCESs.

    

Lightwave-driven currents in graphene

Control of electrons in 2D materials has been at the forefront of attosecond and condensed matter physics resulting in the control of physical observables such as ballistic currents dependent on the sub-cycle properties of the driving lightwaves. This light–matter interaction regime forms the basis for lightwave electronics, suggesting a clear path toward the processing of information at the femtosecond time scale by interaction of strong coherent fields with solids.

I first will discuss how we can generate logic gates on a graphene structure capable of performing Boolean operations driven by few-cycle optical pulses. By disentangling the roles of real and virtual charge carriers in the graphene structure, we can gain a deeper understanding of how to manipulate electrons on the attosecond timescale for lightwave electronics. In the second half, I will discuss our recent work of band structure engineering of graphene using complex two-color waveforms. By tailoring the driving laser’s waveform, we can reshape graphene’s band structure to Floquet-Bloch bands that allow for valley-selective physics. We observe currents that are sensitive to the two-color phase and polarization which correspond to valley selective currents and an absence of current corresponding to valley polarization.