5.5 Case Studies
5.5.1 NLO Emission of Ag 29 (DHLA) 12 —Playing
with Photons and Colors
As a first example, we investigated in details the nonlinear optical response of
Ag 29 (DHLA) 12 . The two-photon excited fluorescence spectra were recorded, and
their first hyperpolarizability is obtained. A general observation is these NCs have
huge two-photon absorption cross sections typically 5 Â 10
4 GM for Ag 29 while
their corresponding two-photon excited fluorescence cross section is only 0.5 GM
[11, 12]. For NLO applications, they are still less effective than dyes, but usually
these clusters present a good biocompatibility and low toxicity. We have also
observed for the Ag 29 an interesting behavior for potential applications, which
concerns its emission properties following the absorption of 1 or 2 photons, as
shown in Fig. 5.8. With an excitation with one photon at 400 nm or by two photons
at 800 nm, the resulting emission spectrum is either blue or red (Fig. 5.8).
Interestingly, this unique Ag 29 nanocluster displays two colors, playing with the
number of photons involved in the excitation.
Fig. 5.7 Schematic view and photograph of the classical 90° angle hyper-Rayleigh scattering
geometry. The setup was recently upgraded in a “tout-en-un” experimental setup able to record
TPA, TPEF cross sections along with the first hyperpolarizability b values for liganded silver and
gold quantum clusters
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