[Paper Review] Time- and angle-resolved photoelectron spectroscopy of strong-field light-dressed solids: Prevalence of the adiabatic band picture

Ofer Neufeld, Wenwen Mao+5 more|arXiv (Cornell University)|Apr 29, 2022

Laser-Matter Interactions and Applications118 references19 citations

TL;DR

This study uses ab initio time- and angle-resolved photoelectron spectroscopy (Tr-ARPES) simulations to investigate the electronic band structure of a strong-field laser-dressed solid, finding that the adiabatic (instantaneous) band picture remains robust even at high intensities (~10¹² W/cm²) and across a broad range of wavelengths (800–4000 nm), with Keldysh parameters up to ~7. Non-adiabatic effects are identified as minor but detectable, especially when probed with bi-chromatic fields.

ABSTRACT

In recent years, strong-field physics in condensed-matter was pioneered as a novel approach for controlling material properties through laser-dressing, as well as for ultrafast spectroscopy via nonlinear light-matter interactions (e.g. harmonic generation). A potential controversy arising from these advancements is that it is sometimes vague which band-picture should be used to interpret strong-field experiments: the field-free bands, the adiabatic (instantaneous) field-dressed bands, Floquet bands, or some other intermediate picture. We here try to resolve this issue by performing 'theoretical experiments' of time- and angle-resolved photoelectron spectroscopy (Tr-ARPES) for a strong-field laser-pumped solid, which should give access to the actual observable bands of the irradiated material. To our surprise, we find that the adiabatic band-picture survives quite well, up to high field intensities (~10^12 W/cm^2), and in a wide frequency range (driving wavelengths of 4000 to 800nm, with Keldysh parameters ranging up to ~7). We conclude that to first order, the adiabatic instantaneous bands should be the standard blueprint for interpreting ultrafast electron dynamics in solids when the field is highly off-resonant with characteristic energy scales of the material. We then discuss weaker effects of modifications of the bands beyond this picture that are non-adiabatic, showing that by using bi-chromatic fields the deviations from the standard picture can be probed with enhanced sensitivity. Our work outlines a clear band picture for the physics of strong-field interactions in solids, which should be useful for designing and analyzing strong-field experimental observables and also to formulate simpler semi-empirical models.

Motivation & Objective

• To resolve the ambiguity in band-picture interpretation for strong-field light-dressed solids.
• To determine under which conditions the adiabatic (instantaneous) band picture remains valid.
• To identify and characterize non-adiabatic deviations from the adiabatic picture.
• To demonstrate that Tr-ARPES can serve as a direct probe of observable electronic bands in strongly driven systems.
• To enable the design of simpler semi-empirical models by establishing a reliable band framework.

Proposed method

• Performed ab initio time-dependent density functional theory (TDDFT) simulations of a monolayer hexagonal boron nitride (hBN) under strong-field laser irradiation.
• Used the time-dependent surface flux (T-SURFF) method to compute photoemission spectra with high momentum resolution.
• Employed a complex absorbing boundary condition to isolate photoelectron signals and minimize continuum propagation artifacts.
• Simulated Tr-ARPES with pump-probe delays to access both instantaneous (adiabatic) and time-averaged (Floquet) band structures.
• Compared full TDDFT results with an independent particle approximation (IPA) to isolate the role of electron-electron correlations.
• Used bi-chromatic laser fields to enhance sensitivity to non-adiabatic effects beyond the adiabatic picture.

Experimental results

Research questions

• RQ1 Does the adiabatic band picture remain valid under strong-field laser dressing of solids, even at intensities up to ~10¹² W/cm²?
• RQ2 How do the observed Tr-ARPES spectra compare to predictions from the adiabatic, Floquet, and intermediate band pictures?
• RQ3 To what extent do non-adiabatic effects modify the band structure beyond the instantaneous picture?
• RQ4 Can bi-chromatic fields enhance the detection of non-adiabatic deviations in Tr-ARPES?
• RQ5 Are dynamical electron-electron correlations a significant source of deviation from the adiabatic band description?

Key findings

• The adiabatic band picture remains highly accurate up to laser intensities of ~10¹² W/cm² and Keldysh parameters up to ~7, indicating robustness far beyond the traditional adiabatic regime.
• No observable breakdown of band structure or strong modification of energy scales was found, even when high harmonics up to ~25 eV were generated.
• The primary effect of the laser field is to shift the band origin via the instantaneous vector potential and to tune electronic phase factors, consistent with adiabatic theory.
• Non-adiabatic corrections to the band structure exist but are weak, with deviations detectable only through enhanced sensitivity using bi-chromatic fields.
• The independent particle approximation (IPA) reproduces the full TDDFT results extremely well, indicating that electron-electron correlations do not significantly alter the Tr-ARPES spectra or cause major deviations from adiabaticity.
• Tr-ARPES in the instantaneous regime enables precise measurement of photoemission time delays by mapping ionization timing to vector potential-induced band shifts.