We have developed a novel multiphoton microscopy technique not relying on (and hence not limited by) fluorescence
emission, which exploits the third-order nonlinearity called four-wave mixing of gold nanoparticles
in resonance with their surface Plasmon. The coherent, transient and resonant nature of this signal allows its
detection free from backgrounds that limit other contrast methods for gold nanoparticles. We show detection
of single 10nm gold nanoparticles with low excitation intensities, corresponding to negligible average thermal
heating. Owing to the the third-order nonlinearity we measure a transversal and axial resolution of 140nm
and 470nm respectively, better than the one-photon diffraction limit. We also show high-contrast imaging of
gold-labels down to 5nm size in Golgi structures of HepG2 cells at useful imaging speeds (10 kHz pixel rate).
Thermal dissociation of gold nanoparticles from their bonding sites when varying the excitation intensity is also
We demonstrate frequency differential CARS (D-CARS) using femtosecond laser pulses linearly chirped by glass
elements of high group-velocity dispersion. By replicating the Pump-Stokes pair into a pulse train at twice the
laser repetition rate, and controlling the instantaneous frequency difference by glass dispersion, we adjust the
Raman frequency probed by each pair in an intrinsically stable way. The resulting CARS intensities are detected
simultaneously by a single photomultiplier as sum and difference using lock-in detection. We demonstrate
imaging of living cells with strongly suppressed non-resonant background. We also show D-CARS using a single
femtosecond laser source.
We demonstrate a novel multi-photon imaging modality based on the detection of four-wave mixing (FWM) from
colloidal nanoparticles. Four-wave mixing is a third-order signal which can be excited and detected in resonance
with the ground-state excitonic transition of CdSe/ZnS quantum dots. The coherent FWM signal is detected
interferometrically to reject incoherent backgrounds for improved image contrast compared to fluorescence methods.
We measure transversal and axial resolutions of 140nm and 590nm respectively, significantly beating the
one-photon diffraction limit. We also demonstrate optical imaging of quantum-dot-labeled Golgi structures of
We investigate the origin of radiative recombination in (InGa)(AsN)/GaAs single quantum wells by means of continuous wave and time-resolved photoluminescence (PL) measurements. Samples with different indium and nitrogen concentration were investigated. An analysis of the whole set of data for different excitation densities and lattice temperatures, T, is reported. This analysis provides insights into radiative and non-radiative processes ruling the recombination dynamics and shows the predominant contribution of localized state emission at low T. The nature of these states is further studied by measuring the time necessary (rise time) for their population. We find that the PL rise time in (InGa)(AsN) is independent of temperature and detection energy, thus being not conclusive about the origin of the states involved in the emission processes. On the contrary, magneto-PL measurements show that the shift of the PL peak energy induced by a magnetic field, B, decreases sizably and changes its dependence on B from linear to quadratic when going from low to high temperature. This counterintuitive result shows that radiative recombination at low temperature (T<100 K) is not excitonic, contrary to previous assignments, and is due to loosely bound electron-hole pairs in which one carrier is localized by N-induced potential fluctuations and the other carrier is delocalized.