Fig. 3. Photon energy scaling for intraband heating.

(A) The THz photoconductivity Δσ_THz as a function of pump pulse fluence, parametrized by the photoexcited carrier density n_exc, for two pump photon energies E_ph = 1.0 eV (purple circles) and 2.5 eV (green circles). The solid lines serve as guides to the eye based on saturation curves. These measurements are done at V_g = 0 V, where we measure a DC conductivity of σ₀ ≈ 8 e²/h. Using the mobility of ~1000 cm²/Vs, we extract an equilibrium Fermi energy of |E_F| ≃ 0.25 eV, corresponding to graphene gated away from the charge neutrality point. The THz photoconductivity saturates for n_ctexc ≳ 0.05 × 10¹²/cm², corresponding to a relatively low-incident fluence < 1 μJ/cm², as observed earlier (27, 30). The signal (and therefore the hot-carrier density) is larger for the larger photon energy. Thus, larger photon energies are responsible for larger hot-carrier densities. This is a key signature of efficient intraband heating (25). (B) The THz photoconductivity at n_exc = 0.2 × 10¹²/cm² [vertical dashed line in (A)], as a function of the pump photon energy. The solid circles are experimental data (right vertical axis) and the solid line the result of the theoretical calculation (left vertical axis) for E_F = 0.25 eV. Experiment and theory show good qualitative agreement, with a larger absolute value of the THz photoconductivity for larger photon energy. (C) The calculated hot-CM factor HCM, defined in the main text, as a function of the pump photon energy. The HCM-factor increases with photon energy. (D) The electron distributions and the state occupation in the Dirac cone, compare with Fig. 2, before and after photoexcitation with pump photon energy E_ph = 2.5 eV (left, green wavy line) and 1.0 eV (right, purple wavy line). The broadening of the electron distribution is larger in the E_ph = 2.5 eV case, indicating more intraband heating for larger photon energy.