Coherent anti-Stokes Raman spectroscopy of single and multi-layer graphene

A. Virga,T. Scopigno,C. Ferrante,G. Batignani,A. C. Ferrari,G. Cerullo,D. De Fazio,A. D. G. Nunn

doi:10.1038/s41467-019-11165-1

Abstract

Spontaneous Raman spectroscopy is a powerful characterization tool for graphene research. Its extension to the coherent regime, despite the large nonlinear third-order susceptibility of graphene, has so far proven challenging. Due to its gapless nature, several interfering electronic and phononic transitions concur to generate its optical response, preventing to retrieve spectral profiles analogous to those of spontaneous Raman. Here we report stimulated Raman spectroscopy of the G-phonon in single and multi-layer graphene, through coherent anti-Stokes Raman Scattering. The nonlinear signal is dominated by a vibrationally non-resonant background, obscuring the Raman lineshape. We demonstrate that the vibrationally resonant coherent anti-Stokes Raman Scattering peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the vibrationally non-resonant background. We model the spectra, taking into account the electronically resonant nature of both. We show how coherent anti-Stokes Raman Scattering can be used for graphene imaging with vibrational sensitivity.

Highlights

Coherent anti-Stokes Raman scattering (CARS)[11,12,13,14] is a FWM process that exploits the nonlinear interaction of two laser beams, the pump field EP at frequency ωP and the Stokes field ES at frequency ωS < ωP, to access the vibrational properties of a material
We demonstrate that the vibrationally resonant Coherent anti-Stokes Raman Scattering (CARS) peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the non-vibrationally resonant background (NVRB)
While spontaneous Raman (SR) scattering is an incoherent signal[15], since the phases of the electromagnetic fields emitted by individual scatterers are uncorrelated[15], in CARS, atomic vibrations are coherently stimulated, i.e. atoms oscillate with the same phase[4], potentially leading to a signal enhancement of several orders of magnitude depending on incident power and scatterer density[16, 17]

Summary

Introduction

Coherent anti-Stokes Raman scattering (CARS)[11,12,13,14] is a FWM process that exploits the nonlinear interaction of two laser beams, the pump field EP at frequency ωP and the Stokes field ES at frequency ωS < ωP , to access the vibrational properties of a material. The same combination of optical fields used for CARS can generate another FWM signal, a non-vibrationally resonant background (NVRB)[2], Fig.1b.

Results

Conclusion