Session M10: Focus Session: Strong-field Physics in Solids

Chip-based attosecond time-domain spectroscopy

Time-domain sampling of arbitrary electric fields with sub-cycle resolution enables complementary time-frequency analyses of a system's electromagnetic response. Time-domain spectroscopy has many applications from tracking physical phenomena to biology and medicine. Commercial optical-field-resolved time-domain spectroscopy systems operating in the THz regime, capable of sub-cycle field sampling, are now readily available and often used for industrial and scientific application with favorable outcome. Despite these compelling methodologies, scaling such techniques to the visible and near-IR spectral regimes has remained challenging, and seemingly demands high-energy optical sources and complicated optical apparatuses. We have explored chip-based strong-field tunneling emission from nanostructures as a new route for optical-field driven electronic systems. Our approach has developed and demonstrated an all-on-chip, optoelectronic device capable of sampling arbitrary, low-energy, waveforms under ambient conditions with attosecond time-resolution. This detector uses strong field driven photoemission from plasmonic nanoantennas to generate attosecond electron bursts to probe the electric field of weak optical waveforms, enabling a bandwidth greater than 1 PHz. Additionally, our measurements can directly reveal the localized plasmonic dynamics of the emitting nanoantennas in situ. Our work demonstrates a compact and integratable sampling technology with the requisite bandwidth and field sensitivity for real-world applications, enabling time-domain, optical-field-resolved spectroscopy of low-energy optical waveforms spanning visible to mid-infrared and even THz wavelengths.   

Coherently Controlled Terahertz Magnetic Impulses in GaAs

Wire-based magnetic field sources limit magnetic field metrology. Using ultrashort laser pulses to drive ultrafast currents in solids and gases paves a novel route towards generating spatially isolated, intense, THz magnetic impulses [1]. In semiconductors, ultrafast currents can be injected and controlled by exploiting quantum interference between different pathways linking the same initial and final states, popularly known as coherent control. We apply cylindrical vector beams to excite the ring currents and their accompanying magnetic fields [1,2].

 

As coherent control is sensitive to the relative phase between two laser pulses, modulating the phase of one pulse enables us to structure the generated currents. We used two femtosecond pulses, one circularly polarized fundamental (1480 nm) and the other a linearly polarized second harmonic (740 nm) that has been reflected from a spatial light modulator, to excite programmable current patterns [3,4] in the GaAs. These reconfigurable transient current structures serve as a source of spatially tailored ultrafast THz magnetic fields.

 

We have measured electric fields of the THz radiation emitted by ring currents using electro-optic sampling. We calculate the corresponding magnetic field using Maxwell’s equations. These all-optically driven magnetic fields find potential applications in magnetic materials, imaging and spintronics.

 

1.S. Sederberg et al. Phys. Rev. X 34, 011063 (2020).

2.S. Sederberg et al. Nat. Photonics 14,680(2020).

3. K. Jana et al. Nat. Photonics 15, 622(2021).

4. K. Jana et al. Nanophotonics (2021).   

Signatures of multi-band effects in high-harmonic generation in monolayer MoS2

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, in a combined theoretical and experimental effort, we illustrate that the polarization properties of high harmonics in monolayer MoS₂ encode the dynamical symmetry properties of the crystal and laser, as well as material-specific properties such as the vectorial character of the transition dipole moments from different valence-conduction-band pairs. Our findings and methodology can readily be generalized to other condensed-matter systems, and relevant for the materials community where this new spectroscopy is expected to find increasingly applications to uncover material properties.   

Probing topology and correlations of quantum materials in strong laser fields

The interaction of intense laser pulses with quantum materials gives rise to a variety of intriguing physical phenomena. One of them is high harmonic generation (HHG), microscopically originating from the interplay of tunneling currents, dynamical Bloch oscillations, and electron-hole recollisions on a sub-cycle time scale. Focusing on the paradigmatic 3D topological insulator Bi₂Se₃, I will first discuss the potential of HHG driven by circularly-polarized fields as a probe of topological band structures. I will then examine how light-induced changes to the band structure dynamically modify the HHG emission spectrum. HHG in solids has traditionally been interpreted within the single-particle picture, where the electronic bands are assumed to remain “frozen” during the light-matter interaction. However, recent theoretical work [1] has challenged this notion and revealed that the onsite Coulomb repulsion in strongly correlated systems (Hubbard U) can be substantially modified by strong non-resonant laser fields and lead to a dramatic reshaping of the HHG spectrum. By combining time-resolved x-ray absorption experiments and exact diagonalization calculations, I will show that intense femtosecond pulses selectively induce a transient renormalization of the Hubbard-U in the prototypical cuprate superconductor La_2-xBa_xCuO₄. This result has far-reaching implications for HHG, attosecond spectroscopy, and ultrafast magnetism.

[1] N. Tancogne-Dejean, M. A. Sentef, and A. Rubio, Phys. Rev. Lett. 121, 097402 (2018)   

Light dressing of excitonic states in monolayer WS2

Monolayer transition-metal dichalcogenides (TMDs) offer a novel platform of optoelectronic devices through their unique excitonic properties. Here, we perform optical transient absorption spectroscopy on monolayer tungsten disulfide (WS₂) under an intense mid-infrared field, and observe a significant (more than a hundred meV) transient tuning of excitonic absorption. Experimentally, mid-infrared pulses (4 um, 100 fs, 0.2 V/nm) induce virtual 1s-np intraexcitonic transitions, and the resulting light-dressing effect is manifested in the optical absorption spectra (500-900 nm), featuring an AC-Stark shift and truncated free-induction decay. Theoretical insights are obtained by analyzing a few-level system within Floquet theory, and also by numerically solving the time-dependent Schrödinger equation for the driven system. Our results demonstrate a new scheme for the ultrafast and enhanced control of the excitonic properties in monolayer TMDs.   

Withdrawn

We show that core-level x-ray absorption near edge structure (XANES) spectroscopy with attosecond soft x-ray (SXR) pulses can image the flow of energy inside a material in real time. Our attosecond-resolved measurement with a pump-probe delay step size of 0.6 fs reveals the buildup of coherent charge oscillations, i.e., polarization of the material. These oscillations occur at occupied states below and unoccupied states above the Fermi level predominantly at the pump carrier frequency. We identify the incoherent background due to the dephasing of coherent charge oscillation. This background rises within a few oscillations of the light field, signifying the ultrafast transfer of energy from the light field into the electron and hole excitation of the material. We find that ultrafast dephasing of the coherent carrier dynamics is governed by impact excitation (IE) for electrons, while holes exhibit a switch-over from impact excitation to Auger heating (AH) already during the 11-fs duration of the infrared light field. We further analyze the coherent phonon signal and find that the non-Raman active strongly coupled optical phonon contributes to 90% of the total dephasing, originating from very strong electron-SCOPs coupling, thus acting almost impulsively. In conclusion, we demonstrate the ability to track energy flow upon light absorption between electrons, holes, and phonons in real-time.    

Strong-field photoelectron (PE) emission of plasmonic nanoparticles (NPs)

We measured [1] and numerically simulated [2] velocity-map-imaging (VMI) maps resulting from the strong-field PE emission of metal NPs by intense infrared laser pulses. Our semi-classical model consists of two distinct steps: (i) PE release by tunneling induced by an intense IR laser pulse and (ii) classical PE propagation to the detector within a trajectory sampling approach [3], distinguishing the effects of PE correlation, PE - residual-charge interactions, PE rescattering and recombination, and transient laser-induced plasmonic fields. By comparing our experimental and numerical results for 5, 30, and 70 nm diameter gold nanospheres and peak laser-pulse intensities of 8×10¹² and 1.2×10¹³ W/cm², we showed how VMI maps are distinctly shaped by PE Coulomb repulsion, residual-charge accumulations, and plasmonic near fields. Compared to gaseous atomic targets [4], we find much larger PE cutoff energies. These exceed the incident laser-pulse ponderomotive energy by two orders of magnitude, for both directly emitted and rescattered PEs.