Abstract
Light interacting with high-electron-density materials elicits an ultrafast coherent many-body screening response on sub- to few-femtosecond timescales, which makes its experimental observation challenging. Here, we describe the coherent two-dimensional (2D) multiphoton photoemission (mPP; ) study of the Shockley surface (SS) state of Ag(111) as a benchmark for spectroscopy of the coherent nonlinear responses of metals to intense optical fields in the perturbative regime; similar 2D signatures can be expected for coherent responses in other materials like low-dimensional semiconductors and strongly correlated materials. Employing interferometric time-resolved multiphoton photoemission spectroscopy (ITR-mPP), we correlate the coherent polarizations excited in the sample with photoelectron energy distributions where the interaction terminates and photoelectrons carry away the information on their excitation. By measuring the nonresonant three- and four-photon photoemission of the SS state, as well as its replicas in above-threshold photoemission (ATP), we record the coherent response of the Ag(111) surface by 2D photoemission spectroscopy and relate it to its band structure. A 2D analysis of the SS state and its ATP replicas shows similar behavior, indicating that they are th- and -order coherent processes in a contradiction of the common attribution of ATP as a sequential process where a photoelectron excited above the vacuum level absorbs one or more additional photons. We interpret the mPP process by an optical Bloch equation model, which reproduces the main features of the surface state coherent polarization dynamics in ITR-mPP experiments: The distributions of spectroscopic components in 2D photoelectron spectra of coherent mPP are shown to follow systematically the ratio, where and are orders of the induced coherence and the photoemission process contributing to the signal.
- Received 18 July 2018
- Revised 11 January 2019
DOI:https://doi.org/10.1103/PhysRevX.9.011044
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
When light strikes a metal surface, it causes free electrons to flow on timescales faster than a femtosecond, ultimately causing the light to be reflected. That makes it very challenging for researchers to investigate the details of this electron behavior—insight that is critical, for example, in some forms of quantum information processing and in the development of ultrashort laser pulses for studying the atomic realm with attosecond resolution. Here, we describe experiments that provide a benchmark for a new way of studying how metals respond to intense light.
We zero in on electrons in the metal that absorb enough photons for photoemission. We aim ultrafast pulses of laser light at a single-crystal silver surface to excite electrons in the metal, focusing on the simplest case of the minimum number of photons being absorbed to cause photoemission. We then record the energy and momentum of the emitted electrons using multidimensional photoemission spectroscopy, which allows us to keep track of how electrons gain energy from light to cause photoemission. For more than 20 years, researchers have thought that a small number of electrons with sufficient energy to escape a metal may still absorb another photon; our results contradict that supposition, finding that electron populations are determined by a distribution of photons that are absorbed.
The nature of how electrons are photoemitted has an important impact on the nonlinear optics of metals. For example, when considering attosecond pulse generation, our experiments describe a limit that serves as the baseline for processes where the field strengths start to modify the electronic structures of solids.
