Max Planck Institute for Polymer Research, Mainz
Tuesday, October 2nd, 2018
5:15 p.m. — room Jean Jaures (29 rue d’Ulm)
Tea and cookies will be served at 4:45 p.m. in the Hall (24 rue Lhomond)
Graphene is an attractive candidate for many optoelectronic applications because of its vanishing bandgap and high carrier mobility. An essential process for such applications is the dissipation of the energy of photo-excited charge carriers in graphene. Two competing energy relaxation mechanisms for optically excited carriers exist [1]: They can (i) thermalize with intrinsic carriers near the Fermi level, heating them to higher energy states through the process of ‘hot carrier multiplication’; or (ii) the excess energy of the optically excited carrier can be lost via emission of phonons. With ultrafast terahertz spectroscopy, an optical method for probing photoconductivities on ultrafast timescales, we quantify relaxation pathways, and show how branching ratios can be tuned [2]. Carrier heating is also highly efficient when intrinsic carriers are accelerated by a strong electric field [3], which can be used to uniquely efficiently generate harmonics of terahertz radiation [4].
[1] K. J. Tielrooij, et al. Photoexcitation cascade and multiple hot-carrier generation in graphene, Nature Phys. 2013, 9, 248.
[2] A. Tomadin, et al., The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies, Science Advances 2018, 4 (5), eaar5313
[3] Z. Mics, et al., Thermodynamic picture of ultrafast charge transport in graphene, Nature Comm. 2015, 6, 7655.
[4] H.A. Hafez, et al., Room-temperature THz High Harmonics Generation in Graphene, Nature 2018, in print.