Coherent Control of Colloidal Semiconductor Nanocrystals

Weak-field coherent control of a two-photon transition in colloidal semiconductor nanocrystals (NCs) by tailored femtosecond laser pulses is demonstrated at room temperature. Ensembles of cadmium sulfide (CdS) and cadmium selenide (CdSe) NCs forming a colloidal suspension were irradiated by ultrashort infrared laser pulses being phase-modulated in frequency domain. The luminescence generated by electron hole recombination after two-photon excitation (TPE) of the shaped pulses serves as a measure for NC excitation. In the experiment, we applied polynomial spectral phase functions of second- (GDD) and third-order (TOD), as well as phase jumps (theta-step), and studied the effect of the various shaped laser pulses on the excitation of both types of NCs. In view of potential applications in multilabel two-photon microscopy, both types of NCs are uniformely mixed in a single sample. We find that distinct pulse shapes enable selectivity among the excitation of both NC types in this mixture. Numerical simulations based on calculating the spectral overlap of the second order nonlinear optical spectrum and the TPE spectrum of the NCs are in good agreement with the measurement results. Analytic expressions derived for the second order power spectral density (PSD) because of combined GDD-TOD modulation and the theta-step rationalize this finding and show that GDD-TOD modulation provides a spectroscopic tool to investigate the TPE spectrum.