Imaging and Controlling Coherent Phonon Wave Packets in Single Graphene Nanoribbons

TL;DR

This paper demonstrates the use of femtosecond coherent anti-Stokes Raman spectroscopy within a scanning tunnelling microscope to track, control, and analyze quantum coherences of phonon wave packets in single graphene nanoribbons, revealing mode couplings.

Contribution

It introduces a novel method for locally probing and controlling vibrational coherences in a single graphene nanoribbon using ultrafast spectroscopy in an STM.

Findings

Determined phonon dephasing (~440 fs) and decay (~1.8 ps) times.

Tracked quantum coherence evolution on ~70 fs timescale.

Revealed mode couplings via 2D frequency correlation spectrum.

Abstract

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast time scale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~ 440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~ 70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.