Abstract
The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron–phonon interactions; however, their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3 ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700–3100 K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidths of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones’ broadening and electron–phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics.
Highlights
The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron–phonon interactions; their non-equilibrium properties in the presence of hot charge carriers are still not fully explored
The D peak is due to the breathing modes of six-atom rings, and requires a defect for its activation[19,20,21]. It originates from transverse optical (TO) phonons around the Brillouin Zone edge K[19], it is active by double resonance (DR)[20] and, due to a Kohn Anomaly at K[22], it is dispersive with excitation energy
The temperature dependence of the Raman spectrum of singlelayer graphene (SLG) is dominated by anharmonicity, which is responsible for mode softening, leading to a redshift of the Raman peaks[10,41,42]
Summary
The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron–phonon interactions; their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. We localize energy into hot carriers, generating nonequilibrium temperatures in the ~1700–3100 K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response The linewidths of both G and 2D peaks show an increase as function of the electronic temperature. The distribution of charge carriers has a pivotal role in determining fundamental features of condensed matter systems, such as mobility, electrical conductivity, spinrelated effects, transport, and optical properties Understanding how these proprieties can be affected and, manipulated by external perturbations is important for technological applications in diverse areas, ranging from electronics to spintronics, optoelectronics and photonics[1,2,3].
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