Measurement of Two‐Photon Absorption of Silicon Nanocrystals in Colloidal Suspension for Bio‐Imaging Applications

Two‐photon absorption (2PA) of ligand‐passivated silicon nanocrystals (ncSi) in liquid suspensions is difficult to measure directly because of their low 2PA cross section and competing nonlinear optical processes in the suspension medium at high light intensity. Here we overcome these difficulties for small (diameter d < 5 nm) ncSi by measuring background‐free, 2PA‐induced photoluminescence (PL) at intensities below the threshold for high‐order processes, and at wavelengths shorter than the excitation wavelength λ (2) = 810 nm. We calibrate the instrument response by measuring PL induced by one‐photon absorption, and confirm previously reported 2PA cross sections σ (2) for a control rhodamine B dye solution. We find σ (2) = (0.28 ± 0.05, 1.25 ± 0.17, and 18 ± 3) × 10 −50 cm 4 s/photon, respectively, for d = 2.0, 2.6, and 5.1 nm diameter ncSi suspended in toluene. For d = 5.1 nm ncSi, the PL spectrum overlaps λ (2) , and PL scales sub‐quadratically with intensity at lower intensities in our range, indicating one‐photon excitation of the upper PL level. Thus the indirect method reported here appears best suited for ncSi with d < 5 nm. 2PA confocal microscopy of mouse cells incubated with d = 2.7 nm ncSi is demonstrated.