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 cm4 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.

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap E g (QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Spectra of the degenerate two-photon absorption coefficient β(ω), anisotropy parameter σ(ω), and dichroism parameter δ(ω)=[σ(ω)+2η(ω)]/2 of crystalline 110-cut GaAs, GaP, and Si, at 300 K were measured using femtosecond pump–probe modulation spectroscopy over an excitation range in the vicinity of each material’s half-bandgap Eg/2 (overall 0.62&lt;ℏω&lt;1.91 eV or 2000&gt;λ&gt;650 nm). Together, these three parameters completely characterize the three independent components of the imaginary part of the degenerate third-order nonlinear optical susceptibility tensor Im{χabcd(3)(ω)}. In direct-gap GaAs, these components peak at ℏω≈0.78Eg, which is close to the peak at ℏω=0.71Eg predicted by the Jones–Reiss phenomenological model. The dispersion is comparable with ab initio calculations. In indirect-gap GaP and Si, these components tend to increase with ℏω over our tuning range. In Si, the dispersion differs significantly from predictions of semi-empirical models, and ab initio calculations do not account for transitions below the two-photon direct bandgap, motivating further investigation. Kleinman symmetry was observed to be broken in all three materials. We also note anomalies observed and their possible origins, emphasizing the advantages of a 2-beam experiment in identifying the contribution of various nonlinear effects.