Optical switching spectroscopy of PbS quantum dots with dual-wavelength pump-probe

Optical transmission modulation in PbS quantum dots is investigated using a dual-wavelength pump–probe technique. High-energy excitation at 800 nm e2ciently alters the transmission of a probe beam 1100–1600 nm spectral range, with a clear enhancement observed at longer wavelengths towards the exciton peak (2100 nm). A decay time of less than 2:5 ps is measured for pump intensities of up to 3:8 GW=cm2. The maximum modulation is delayed by approximately 1 ps relative to the perfect overlap of the pump and probe pulses, the time required for the electrons to reach lower energy states. Long-term exposure shows photodegradation but the process is reversible and recovers on its own after several hours.