Stationary reflectivity spectrum (IMAGE)
Caption
Fig. 2. (a) Stationary reflectivity spectrum (thick black line) of a stack of 8 to 9 hexagonal boron nitride layers in the range of the TO phonon resonance. The reflectivity is plotted as a function of frequency. The symbols display the change of reflectivity DR=(R-R0)/R0 observed after femtosecond excitation of the sample at the delay times given in the inset (R, R0: reflectivity with and without excitation). At delay times longer than 3 ps, the increase of reflectivity at low frequency and its decrease at high frequency correspond to a down-shift of the TO phonon resonance, induced by excitation of shear and breathing modes. (b) Change of reflectivity at 40.7 THz (red arrow in panel (a)) and at 41 THz (blue arrow in panel (a)) as a function of the delay time between pump and probe pulses (in picoseconds). The transients exhibit a fast decay with a time constant of 1.2 ps, the TO phonon lifetime, and a slow decay with 22 ps, the lifetime of shear and breathing modes.
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Stationary reflectivity spectrum
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