Clean CARS Spectroscopy Enables Fast Graphene Imaging

​By analysing the interactions between infra-red light and graphene, researchers at Graphene Flagship partners the Italian Institute of Technology (IIT), the University of Rome, Politecnico di Milano, Italy, and the Cambridge Graphene Centre, UK, have produced fast images of both single- and multi-layer graphene using coherent anti-Stokes Raman scattering (CARS).¹

Raman spectroscopy is one of the most used techniques in graphene science and technology, thanks to the pioneering work done by Graphene Flagship Partner University of Cambridge. Raman can be used to provide information on the number of graphene layers, doping, defect density and strain and many other crucial parameters.

Graphene Flagship researchers have now applied coherent anti-Stokes Raman spectroscopy (CARS) to graphene for the first time. CARS is a type of Raman spectroscopy that uses two synchronized laser pulses to induce strong, coherent atomic oscillations in samples under investigation. This results in exponentially higher acquisition speed, leading to fast imaging. However, the laser pulses also cause materials to emit a background signal, which overlaps the useful part of the spectrum. Because graphene is electronically resonant at every wavelength, the background signal behaves in an unusual way, resulting in CARS spectra that look very different to standard Raman spectra and difficult to interpret.

Now, by understanding the interaction of the two CARS laser pulses with the electronic structure of graphene, Graphene Flagship researchers discovered a way to reduce the overlap and obtain a clear, easily interpretable CARS spectrum. "Typically, the pulses are synchronised – they impinge on the sample at the same time. By modifying the delay between the pulses, we can change the relative weight of the vibrational signal of interest with respect to the background, enhancing the former with respect to the latter," says Tullio Scopigno from Graphene Flagship partner IIT. "This enables us to get CARS spectra that look like conventional Raman ones, and allows us to use CARS for the vibrational imaging of graphene. We can now record CARS spectra of graphene with an equivalent contrast to spontaneous Raman, but with a much higher speed," he continues.

Raman spectroscopy is normally done on samples of graphene where the electrons and holes are in thermodynamic equilibrium. To take this technology even further, the researchers successfully performed Raman spectroscopy on graphene with a strongly out-of-equilibrium population of hot electrons and holes. By carefully selecting the duration of an ultrashort excitation pulse, they found a pulse duration short enough to create a non-equilibrium carrier distribution, but at the same time long enough to provide the frequency resolution necessary for Raman spectroscopy. Their findings are published in Nature Communications.⁴ "This is important because most optoelectronic and photonic devices based on graphene, such as photodectectors, modulators and saturable absorbers, work out-of-equilibrium on ultrashort timescales," explains Giulio Cerullo, from Graphene Flagship partner Politecnico di Milano. "Therefore, for the design and modelling of these devices it is vital to understand the non-equilibrium interactions of hot electrons with phonons in graphene – and this is precisely the information provided by our experiments in this paper."

Frank Koppens, Graphene Flagship's Work Package Leader for Photonics and Optoelectronics, is also optimistic about the future applications of their research: "They have demonstrated a very powerful technique for imaging graphene. This can be used for many more applications, including in the medical field, as currently developed in by the Graphene Flagship," he comments.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "Raman spectroscopy is the most-used non-destructive technique to characterize graphene. This result in principle allows high speed mapping, and could be developed for in-process monitoring."

Indeed, the applications do not stop here – CARS microscopy is commonly used to image biomolecules and tissues, and now it could potentially be used in tumour identification. These Graphene Flagship researchers have now devised a new laser architecture for CARS microscopy that incorporates a graphene saturable absorber, and they first reported this at the Conference on Lasers and Electro-Optics, USA.⁵ Graphene Flagship partners Politecnico di Milano, IIT, Italy, and the Cambridge Graphene Centre, UK, have patented this technology, and expect to find commercial applications within the bioimaging industry soon. "This is drastically simplified technology: it promises turn-key operation, and costs one order of magnitude less than the state-of-the-art. A company which aims to commercialise this technique, Cambridge Raman Imaging Limited, has been founded", comments Cerullo.

References

1. Coherent anti-Stokes Raman Spectroscopy of single and multi-layer graphene, Nature Communications, 10, 3658 (2019). A. Virga, C. Ferrante, G. Batignani, D. De Fazio, A. D. G. Nunn, A. C. Ferrari, G. Cerullo and T. Scopigno.
2. Raman Spectrum of Graphene and Graphene Layers, Phys. Rev. Lett., 97, 187401 (2006). A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim.
3. Raman spectroscopy as a versatile tool for studying the properties of graphene, Nature Nanotechnology, 8, 235-246 (2013). A. C. Ferrari and D. M. Basko.
4. Raman spectroscopy of graphene under ultrafast laser excitation, Nature Communications, 9, 308 (2018). C. Ferrante, A. Virga, L. Benfatto, M. Martinati, D. De Fazio, U. Sassi, C. Fasolato, A. K. Ott, P. Postorino, D. Yoon, G. Cerullo, F. Mauri, A. C. Ferrari and T. Scopigno.
5. Coherent Raman spectroscopy with a graphene-synchronized all-fiber laser, Conference on Lasers and Electro-Optics, OSA Technical Digest (online, 2017). D. Popa, D. Viola, G. Soavi, B. Fu, L. Lombardi, S. Hodge, D. Polli, T. Scopigno, G. Cerullo and A. C. Ferrari.