Coherent Two-Dimensional Multiphoton Photoelectron Spectroscopy of Metal Surfaces

Marcel Reutzel,Hrvoje Petek,Andi Li

doi:10.1103/physrevx.9.011044

Abstract

Light interacting with metals elicits an ultrafast coherent many-body screening response on sub- to few-femtosecond time-scales, which makes its experimental observation challenging. Here, we describe the coherent two-dimensional (2D) multi-photon photoemission study of the Shockley surface state (SS) of Ag(111) as a benchmark for spectroscopy of the coherent nonlinear response of metals to an optical field in the perturbative regime. Employing interferometrically time-resolved multi-photon photoemission spectroscopy (ITR-mPP), we correlate the coherent polarizations and populations excited in the sample with final photoelectron distributions where the interaction terminates. By measuring the non-resonant 3- and 4-photon photoemission of the SS state, as well as its replica structures in the 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. We interpret the mPP process by an optical Bloch equation (OBE) model, which reproduces the main features of the surface state coherent polarization dynamics recorded in ITR-mPP experiments: The spectroscopic components of the 2D photoelectron spectra are shown to depend on the nonlinear orders of the coherent photoemission process m as well as on the induced coherence n.

Highlights

The quantum optoelectronic response of metals is dominantly coherent, as manifested by specular reflection of light, yet large bandwidths and ultrafast phase relaxation of the many-body electronic system impede its time-resolved spectroscopic investigations [1,2,3,4,5]
We interpret the multiphoton photoemission (mPP) process by an optical Bloch equation model, which reproduces the main features of the surface state coherent polarization dynamics in interferometric time-resolved multiphoton photoemission spectroscopy (ITR-mPP) experiments: The distributions of spectroscopic components in 2D photoelectron spectra of coherent mPP are shown to follow systematically the n=m ratio, where n and m are orders of the induced coherence and the photoemission process contributing to the signal
The nonresonant energy level climbing of Shockley surface (SS) electrons in the perturbative regime is dominantly caused by oscillations of the nonlinear polarization at high orders of the driving field, as recorded by ITR-mPP measurements and their Fourier analysis; the coherence can dephase to populate nonresonant intermediate states, such as the image potential (IP) states, it can be rectified to produce the mPP signal or, in higher order, its above-threshold photoemission (ATP) replicas, or it can decay by producing harmonic emission [70]

Summary

Introduction

The quantum optoelectronic response of metals is dominantly coherent, as manifested by specular reflection of light, yet large bandwidths and ultrafast phase relaxation of the many-body electronic system impede its time-resolved spectroscopic investigations [1,2,3,4,5]. Atomic gas phase models for HHG [20,23] have been applied to ultrafast high-field optical responses of solids, such as photon dressing of FloquetBloch states in topological insulators [24], laser-assisted photoemission from metal surfaces [25] by attosecond streaking [26,27,28,29], high-order multiphoton photoemission (mPP) from sharp metal tips [15,18,30,31,32], and coherent microscopy of plasmonic fields [33,34,35,36,37,38]. The ability to probe and characterize how fields drive coherent nonlinear phenomena in the solid state [40,41] enables fundamental studies of coherent control [2,42], electronic processes and interactions [2,4], and quantum information processing, as well as applications such as the generation of ultrashort electron pulses [30,43,44] for time-resolved diffraction [45] and microscopy [46,47] with potentially attosecond time resolution [40,41]

Methods

Results

Conclusion