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Spectroscopy of the nonlinear refractive index of PbSe nanocrystals

par Moreels, Iwan;Kockaert, Pascal ;Loicq, Jérôme;Thourhout, D.;Hens, Zeger
Référence European Materials Research Society, page (6-2)
Publication Publié, 2006

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Résumé :

A spectroscopic study of the optical nonlinearities of PbSe colloidal solutions was performed with the Z-scan technique, at wavelengths between 1200 and 1750nm. No nonlinear absorption is observed, while the third order nonlinear refractive index n2 shows clear resonances, somewhat blueshifted relative to the exciton transitions in the absorbance spectrum. The occurence of thermal effects are ruled out by time-resolved measurements. At resonance, measured n2 values exceed typical bulk semiconductor values by two orders of magnitude. At high optical intensity, the refractive index change saturates, indicating that state-filling lies at the origin of the observed effect. PbSe nanocrystals exhibit a bandgap absorption tunable around 1310 and 1550nm, offering a great potential for the use of these particles on a Si platform. We present a spectroscopic study on the nonlinear refractive index of colloidal PbSe nanocrystal suspensions (Q-PbSe sols) using the Z-scan method. Monodisperse Q-PbSe sols with a bandgap absorption tunable between 1250 and 1900nm were prepared by a wet chemical synthesis. Particles of 3 different sizes have been studied (3.9,5.2 and 5.9nm, with respective absorbance peaks of 1245,1550 and 1700nm). The nonlinear refractive index n2 and absorption beta have been measured as a function of light intensity in the range of 1200-1350nm and 1550-1750nm, using an 82MHz femtosecond pulsed laser. Our results demonstrate that n2 exhibits a clear resonant behaviour, correlated with the single photon transitions of the nanocrystals. Maximal n2 values of ~10^(-11) cm2/W were obtained for 1textmuM solutions. We attribute these high values to the strong confinement in Q-PbSe. Time-resolved experiments on the largest Q-PbSe sols enabled us to exclude thermal effects. As the long exciton lifetime (~1textmus) causes a saturation of the first exciton in the Q-PbSe at the light intensities used in these experiments, we suggest that bi-excitons contribute to the large nonlinearities observed. Changing the concentration of the Q-PbSe sols allowed us to calculate the n2 of the Q-PbSe, an important value when comparing with different systems, eg. thin films.