Relaxation dynamics in Landau-quantized graphene probed in the mid-infrared range

In contrast to usual semiconductors in graphene the Landau levels (LL) are not equidistant. This provides the unique feature to investigate a single LL transition selectively. By applying magnetic fields of up to 7 T we could investigate three different transitions at a fixed wavelength of 16.5 µm via pump-probe measurements. By varying the magnetic field the photon energies got resonant to the different transitions. For the transition LL0(-1)->LL1(0) we could not only observe an increase of the pump-probe signal by a factor of 2.5 but also a decrease of the relaxation time from 20 ps to 5 ps. Interestingly, the reduced relaxation time is observed in a wider range of magnetic fields then the increase of the signal amplitude. Additionally the minimum of the relaxation time is shifted in respect to the maximum signal. For the transitions LL-1(-2)->LL2(1) and LL-2(-3)->LL3(2) we could only observe an slight increase of the pump-probe signal.