Measurement of Two‐Photon Absorption of Silicon Nanocrystals in Colloidal Suspension for Bio‐Imaging Applications. Issue 4 (8th January 2018)

• Record Type: Journal Article
• Authors: Furey, Brandon J.; Silbaugh, Dorothy A.; Yu, Yixuan; Guillaussier, Adrien C.; Estrada, Arnold D.; Stevens, Christopher; Maynard, Jennifer A.; Korgel, Brian A.; Downer, Michael C.
• Is Part Of: Physica status solidi. Volume 255:Issue 4(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2018-01-08
• Subjects: bio‐imaging -- colloidal suspensions -- nanocrystals -- photoluminescence -- silicon -- two‐photon absorption; Solid state physics -- Periodicals; Solids -- Periodicals; Atomic structure -- Periodicals; 530.41
• DOI: 10.1002/pssb.201700501
• Languages: English
• ISSNs: 0370-1972

Abstract

Abstract : 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. Abstract : Water‐soluble silicon nanocrystals attached to cells luminesce efficiently when excited by laser light, enabling bio‐imaging. Excitation of the nanocrystals by two‐photon absorption of deeply penetrating near‐infrared light (2PA) enables imaging of deeper‐lying cells than one‐photon excitation by shallow‐penetrating ultraviolet light (1PA). The two‐photon excitation efficiency is measured by observing the luminescence versus excitation intensity (top figure), and mouse cell imaging is demonstrated by confocal microscopy (bottom figures).