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. 2015 Sep 24;6:8105. doi: 10.1038/ncomms9105

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(a) Schematic diagram of the ultrafast time-resolved THz spectroscopy set-up. (Inset: schematic diagram of a MEG sample with a gradient doping density profile). (b) Normalized differential THz transmission spectra Δt(ω)/t(ω) recorded at a pump fluence of 0.87 μJ cm⁻² and a substrate temperature of 40 K for a few different pump–probe delays for a MEG sample with ∼63 layers. The black dashed line indicates the experimental noise level. The THz spectra are remarkably dispersionless in the detectable frequency range under all experimental conditions. (c,d) Linear (c) and logarithmic (d) plots of normalized differential THz transmission at the peak of the THz probe pulse Δt/t as a function of pump–probe delay recorded at a pump fluence of 23.4 μJ cm⁻² for a few different substrate temperatures for a MEG sample with ∼63 layers. The THz carrier dynamics evolve from a faster mono-exponential relaxation at room temperature to a slower bi-exponential relaxation at cryogenic temperatures. Subfigures (c) and (d) share the same legend. (e,f) Short and long relaxation times as a function of substrate temperature for a few different pump fluences for a MEG sample with ∼63 (e) and ∼35 (f) layers. The values are extracted from phenomenological fits to normalized differential THz transmission Δt/t. The long relaxation times increase with the number of epitaxial graphene layers, which indicate the presence of interlayer interaction in MEG with HD and LD layers. (g) Normalized differential THz transmission at the peak of the THz probe pulse Δt/t as a function of pump–probe delay recorded at a pump fluence of 60.0 μJ cm⁻² for a few different substrate temperatures for a MEG sample with ∼3 layers. The THz carrier dynamics follow a fast mono-exponential relaxation at all temperatures. (h) Relaxation times as a function of substrate temperature for a few different pump fluences for a MEG sample with ∼3 layers. The relaxation times are completely independent of the substrate temperature, because there is practically very little or no interlayer energy transfer in MEG with all HD layers. The experimental error in all relaxation times is due primarily to long-term drift of the optomechanical components and the ultrafast Ti:sapphire laser system, and is estimated not to exceed ∼5%.