Colloidal semiconductor nanocrystals
(NCs) are on the vanguard
of nonlinear optical materials due to their superb optical properties
including tunable multiphoton absorption (MPA) with large absorption
cross sections. So far, investigating MPA of NCs has been regarded
as a difficult task, mostly constrained by the lack of measurement
techniques with high precision and consistency. Here, an alternative
method to measure the MPA cross section of luminescent NCs is presented.
The new method, referred to as multiphoton absorption photoluminescence
saturation (MPAPS), does not depend on any specific property of an
NC sample, allowing for a precise comparative study of NC samples
either holding different structures (e.g., core-only or core–shell
heterostructure) or exhibiting slow multicarrier dynamics (e.g., engineered
NCs for reduced Auger rates).
Colloidal semiconductor nanomaterials present broadband,
with large
cross-section, two-photon absorption (2PA) spectra, which turn them
into an important platform for applications that benefit from a high
nonlinear optical response. Despite that, to date, the only means
to control the magnitude of the 2PA cross-section is by changing the
nanoparticle volume, as it follows a universal volume scale, independent
of the material composition. As the emission spectrum is connected
utterly to the nanomaterial dimensions, for a given material, the
magnitude of the nonlinear optical response is also coupled to the
emission spectra. Here, we demonstrate a means to decouple both effects
by exploring the 2PA response of different types of heterostructures,
tailoring the volume dependence of the 2PA cross-section due to the
different dependence of the density of final states on the nanoparticle
volume. By heterostructure engineering, one can obtain 1 order of
magnitude enhancement of the 2PA cross-section with minimum emission
spectra shift.
We report on the two-photon absorption spectra of a series of 2,6-disubstituted BODIPY dyes. Depending on the substituents, we observe increasing two-photon absorption cross sections with values up to 350 GM compared to 70 GM in the unsubstituted dye. Quantum chemical calculations are performed to assign the absorption bands and to understand the factors controlling the size of the two-photon absorption cross section. Both the maximum of the two-photon absorption band as well as the red-shift of the whole spectrum correlate with the ability of the substituents to extend the π-electron system of the dye. The above-mentioned intense two-photon absorption band corresponds to the absorption of photons with 1.3 eV, which is at the first near-infrared transparency window for biological tissues. The dyes could thus be suitable for bio-imaging applications.
Multiphoton absorption of nanocrystals is shown to be dependent on the band alignment of heterostructures, allowing for control over the multiphoton interactions in these materials with minimal change to their photoluminescence spectra.
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