What can we learn from the Two-Photon Luminescence properties of Single Colloidal Gold NanoParticles ?

Although bulk gold is known to present very low luminescence quantum yield, a huge luminescence is observed from gold nanostructures, the two-photon brightness of a single gold nanorods (GNR) being a few millions higher than... [ view full abstract ]

Although bulk gold is known to present very low luminescence quantum yield, a huge luminescence is observed from gold nanostructures, the two-photon brightness of a single gold nanorods (GNR) being a few millions higher than the two-photon brightness of fluorescein molecules ! Although directly correlated to the excitation of the rods longitudinal surface plasmon modes, the detailed origin of the important luminescence in GNRs is still a matter of debates : are the particles plasmon modes determining their luminescence spectra or, inversely is the spectral shape of luminescence directly determined from the gold band structure and thus the crystalline properties of particles ?

The two-photon luminescence (TPL) of small 10 nm x 40 nm colloidal gold nanorods (GNR) was investigated at the single object level, combining polarization resolved TPL and simultaneously acquired topography. A very high dependence of the TPL signal with both the nanorods longitudinal axis and the incident wavelength is observed confirming the plasmonic origin of the signal. The spectral analysis of the TPL evidences two emission bands peaks: in the visible and in the infrared (IR) ; the signal emitted in either the visible or the IR exhibiting however different polarization properties.
We show that the important TPL observed in these GNRs results from resonance effects both at the excitation and emission level : local field enhancement at the longitudinal surface plasmon resonances (LSPR) first results in an increase of the electron-hole generation. Further relaxation of electron-hole pairs then mostly leads to the excitation of the GNR transverse plasmon mode and its subsequent radiative relaxation (see figure). This conclusion was confirmed by the results obtained in the case of nanobipyramids. A deeper insight in this model was further developed after investigating the properties of gold nanorods having close aspect ratio and plasmonic resonances but increasing volume. Experimental data were correlated with BEM (Boundary Elements Method) simulations. It is evidenced that the TPL signal directly depends on field enhancements effects at both the exciting and emission wavelengthes, the efficiency of the transverse plasmonic mode excitation by hot plasmons playing a key role.