Observation of carrier relaxation and recombination dynamics in optically pumped epitaxial graphene heterostructures using terahertz emission spectroscopy

Linear dispersion relations for the electron and hole energy spectra with zero energy band gap in graphene [1] provide nontrivial features like negative-dynamic conductivity in far infrared and terahertz (THz) spectral ranges [2]. Due to a very short time of the optical phonon emission τ0 ≈ 10⁻¹²s, the photogeneration of electrons and holes leads to the emission of a cascade of optical phonons, so that the photoelectrons and photoholes occupy the states with the energies close to εN = ℏ(Ω / 2 - Nω0 ) ≪ ℏω0 , where ℏΩ the pumping photon energy, ℏω0 the optical phonon energy, and N the number of emitted optical phonons. As a consequence, photoelectrons/holes recombine to radiate THz photons with the energy ℏω = 2εN (inset in Fig. 1(a)) [1]. The incident photon spectra will reflect on the THz photoemission spectra as a proof of occurrence of such a process.