Laura Kim1 Freddie Page2 Seyoon Kim3 Victor Brar4 Joachim Hamm2 Ortwin Hess2 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States
2, Imperial College London, London, , United Kingdom
3, ICFO–The Institute of Photonic Sciences, Barcelona, , Spain
4, University of Wisconsin-Madison, Madison, Wisconsin, United States

The decay dynamics of excited carriers in graphene have attracted wide scientific attention, owing to the much lower relaxation rate of excited ‘hot’ carriers than that seen in many three-dimensional materials, owing to the Dirac electronic dispersion of graphene. Plasmons in graphene can significantly reduce the lifetime of photoexcited charge carriers, and this plasmon effect on excited state decay increases with increasing carrier density, as indicated by theoretical calculations and ARPES experiments [1,2,3,4]. In a recent theoretical study, it was also shown that plasmons can be amplified in an inverted graphene and be spontaneously emitted on ultrafast time scale[5].
We report experimental demonstration of gate-tunable mid-infrared plasmon-coupled radiation from graphene under ultrafast optical pumping, and our experimental results suggest that graphene plasmons excited by ‘hot’ carriers affect the radiative emission rate. We have measured emission for several sample geometries: planar graphene, and non-resonant and resonant gold nanodisks(NDs) on graphene. In infrared emission spectroscopy measurements taken under optical excitation with a Ti:sapphire laser operating at 850nm with 100fs pulse duration, we observe broad radiative emission with features across an energy range of 150meV to 430meV (2.8-8um). The randomly distributed gold NDs on graphene facilitate out-coupling of plasmon excitations to free space light by accommodating the momentum mismatch. In addition, when NDs are resonant with the incoming laser frequency, ultrafast plasmon emission is enhanced in portion to the field intensity concentrated at the location of the graphene sheet. With a surface coverage of 1% for the resonant NDs and less than 3% for the non-resonant NDs, the collected plasmon-coupled light emission intensity is at least a factor of 8 and 4 larger, respectively, than that collected from planar graphene. In all cases, the emission intensity increases for higher graphene carrier density, which is controlled via the changes in applied gate voltage. For a given, moderate laser power, the emission intensity is approximately 1%, 6%, and 70% larger when the graphene Fermi level is at 0.4eV compared to the charge neutral point for planar graphene, and non-resonant and resonant NDs on graphene, respectively. This work has important implications for achieving ultrafast optical control of mid-infrared light emission. Our results are indicative of a spectral modification of plasmon-mediated emission arising from ‘hot’ plasmons in graphene created by ultrafast optical excitation.
1. A. Bostwick et al., Nature Phys., 2007, 3(1), pp.36-40.
2. A. Bostwick et al., Science, 2010, 328(5981), pp.999-1002.
3. F. Rana et al., Phys. Rev. B, 2011, 84(4), pp.045437
4. J. M. Hamm et al., Phys. Rev. B, 2016, 93(4), pp. 041408
5. A. F. Page et al., Phys. Rev. B, 2015, 91(7), pp. 075404