We have developed a photothermal method for far-field optical detection of nanometer-sized metal particles, combining high-frequency modulation and polarization interference contrast. We can image gold colloids down to 5 nm in diameter, with a signal-to-noise ratio higher than 10. This is a considerable improvement over commonly used optical methods based on resonance plasmon scattering which, for background reasons, are limited to particles of more than about 40 nm in diameter. We also show that in addition to its intrinsic sensitivity, our photothermal method is totally insensitive to non-absorbing scatterers as 10 nm nanoparticles can be imaged in cells.
By severely reducing the number of solute molecules in the illuminated sample, the optical resonances of
individual molecules can be resolved in a fluorescence excitation spectrum. Single molecule lines can be
studied as a function of time and temperature: sudden jumps of their resonance frequencies are due to
spectral diffusion processes. The signal from a single molecule displays specific correlations which makes
time-resolved studies possible. Here, emphasis is put on photon-bunching arising from intersystem
crossing (ISC). ISC rates are deduced from the observed decay rates of the correlation and are found to
differ from molecule to molecule. A single molecule is a truly local probe of its environment by means of
which fundamental studies of the matrix dynamics as well as nanophysics experiments may be
undertaken.
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