Three-photon absorption in seeded CdSe/CdS nanorod heterostructures

Size-dependent three-photon absorption (3PA) and three-photon-excited photoluminescence (PL) of CdSe/CdS nanorod heterostructures have been investigated using Z-scan and upconversion PL techniques with 150 fs laser pulses at 1300 nm. The PL from these rods shows clear cubic power dependence, and the 3PA cross section (σ₃) was found to be as high as 1.5 × 10^-75 cm⁶ s² photon^-2 for 39 nm long rods, which is 2-4 orders of magnitude larger than those previously reported for spherical semiconductor nanocrystals under ∼100 fs laser pulse excitation. Importantly, by exploiting the unique property of the seeded nanorods to exhibit strong quantum confinement even at relatively large rod sizes, a superlinear dependence of the 3PA cross section on nanoparticle volume was found and validated over a wide volume range. A simple derived relation that quantitatively describes σ₃ and takes into account the laser pulse duration is proposed, thereby providing a clear basis of comparison between the σ₃ values of various II-VI semiconductor nanomaterials and facilitating a more judicious choice of their use in 3PA applications.