Broadband and picosecond intraband absorption in lead based colloidal quantum dots

Using femtosecond transient absorption spectroscopy we demonstrate that lead chalcogenide nanocrystals show efficient, photoinduced absorption (PA) in a broad wavelength range starting just below the bandgap. The time-dependent decay of the PA signal correlates well with the recovery of the band gap absorption. Therefore, the same carriers are involved, which decay with the typical Auger recombination rate in these nanocrystals. Based on this, we assign this PA signal to intraband absorption, i.e., the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. This broadband response in the commercially interesting near to mid-infrared range is very relevant for ultra-high speed all optical signal processing.