The physical mechanism underlying multiphoton luminescence in gold is still the subject of debate. To obtain a better understanding of the mechanism, experiments that study the luminescence spectra of single particles are necessary. In this study, the multiphoton luminescence spectrum was measured for surrounding media of different refractive indices. The resulting spectra of single gold nanospheres with diameters in the range of a few tens of nanometers were found to
be strongly dominated by the absorption peak of the plasmon resonance. This is in agreement with the theory proposed by Boyd et al. (1986)1. According to Lorenz-Mie Theory, an increase in the refractive index of the surrounding medium results in a redshift of the plasmon resonance spectrum; a corresponding shift in the multiphoton luminescence spectrum has now been found experimentally.
Noble metal nanoparticles are characterized by a strong peak in the scattering and absorption spectrum, termed the
plasmon resonance. Researchers have taken advantage of this to create a new label for biological molecules. A
disadvantage of techniques based on scattering and absorption is that the detected signal is at the same wavelength
as the incident light, making it more challenging to discriminate between signal and background. Gold
nanoparticles also luminesce, suggesting an alternate method for their detection. A tightly focused ultra-short
pulse laser beam can be used to achieve multiphoton excitation of the particles; the resulting luminescence
exhibits a peak in the same region of the spectrum as the plasmon resonance. Because excitation is nonlinear,
significant luminescence is only observed when the particle is in the focus, permitting localization with both high
lateral and axial resolution. The physical mechanism underlying multiphoton luminescence in gold is still the
subject of debate. Here, we present a systematic study in single gold nanospheres with diameters between 15 nm
and 100 nm using peak laser intensities between 10 and 350 GW/cm2. A scattering confocal microscope
incorporated in the setup was used to distinguish single particles from clusters. We observed that not all gold
nanospheres have a detectable multiphoton luminescence signal; however, laser intensities above an exposure-time
dependent threshold can alter such particles so that they do. In addition, we found that gold nanoparticles
exposed to laser intensities above about 150 GW/cm2 can exhibit behavior reminiscent of the bleaching and
blinking of conventional fluorophores.
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