We report systematic measurements of the dephasing of the excitonic ground-state transition in a series of $\mathrm{InGaAs}∕\mathrm{GaAs}$ quantum dots having different quantum confinement potentials. Using a highly sensitive four-wave mixing technique, we measure the polarization decay in the temperature range from 5 to 120 K on nine samples having the energy distance from the dot ground-state transition to the wetting layer continuum (confinement energy) tuned from 332 to 69 meV by thermal annealing. The width and the weight of the zero-phonon line in the homogeneous line shape are inferred from the measured polarization decay and are discussed within the framework of recent theoretical models of the exciton-acoustic phonon interaction in quantum dots. The weight of the zero-phonon line is found to decrease with increasing lattice temperature and confinement energy, consistently with theoretical predictions by the independent Boson model. The temperature-dependent width of the zero-phonon line is well reproduced by a thermally activated behavior having two constant activation energies of 6 and 28 meV, independent of confinement energy. Only the coefficient to the 6-meV activation energy shows a systematic increase with increasing confinement energy. These findings rule out that the process of one-phonon absorption from the excitonic ground state into higher energy states is the underlying dephasing mechanism.

Quantitative differential interference contrast microscopy (qDIC) is demonstrated here as a label free method able to image and measure the thickness of lipid bilayers with 0.1nm precision. We investigate the influence of the substrate on the thickness of fluid phase DOPC supported lipid bilayers, and find a thinning of up to 10% depending on substrate hydrophilicity, local bilayer coverage and ionic strength of the medium. With fluorescently labelled lipid bilayers, we also observe changes in the bilayer thickness dependent on the choice of fluorophore. Furthermore, liquid ordered domains in bilayers formed from DOPC, cholesterol and sphingomyelin, are measured, and the corresponding thickness change between the liquid ordered and liquid disordered phase is accurately determined. Again the thickness difference is found to be dependent on the presence of the fluorophore label, highlighting the need for quantitative label free techniques.

We demonstrate a phase-sensitive four-wave mixing microscopy in heterodyne detection to resolve the ultrafast changes of the real and imaginary parts of the dielectric function of single small (<40 nm) spherical gold nanoparticles at the surface plasmon resonance. The results are quantitatively described via the transient electron temperature and density in gold considering both intraband and interband transitions. We find that the effect of interband transitions in the excitation is important to explain not only the magnitude of the measured four-wave mixing, but also its initial dynamics, which is dominated by the formation of hot electrons via Auger electron-hole recombination with 70-fs time constant, much faster than the well-characterized 500-fs electron thermalization dynamics for intraband excitation. This microscopy technique enables background-free detection of the complex susceptibility change even in highly scattering environments and can be readily applied to any metal nanostructure.

Mammalian oocytes contain lipid droplets that are a store of fatty acids, whose metabolism plays a significant role in pre-implantation development. Fluorescent staining has previously been used to image lipid droplets in mammalian oocytes and embryos, but this method is not quantitative and often incompatible with live cell imaging and subsequent development. Here we have applied chemically-specific, label-free coherent anti-Stokes Raman scattering (CARS) microscopy to mouse oocytes and pre-implantation embryos. We show that CARS imaging can quantify the size, number and spatial distribution of lipid droplets in living mouse oocytes and embryos up to the blastocyst stage. Notably, it can be used in a way that does not compromise oocyte maturation or embryo development. We have also correlated CARS with two-photon fluorescence microscopy simultaneously acquired using fluorescent lipid probes on fixed samples, and found only a partial degree of correlation, depending on the lipid probe, clearly exemplifying the limitation of lipid labelling. In addition, we show that differences in the chemical composition of lipid droplets in living oocytes matured in media supplemented with different saturated and unsaturated fatty acids can be detected using CARS hyperspectral imaging. These results demonstrate that CARS microscopy provides a novel non-invasive method of quantifying lipid content, type and spatial distribution with sub-micron resolution in living mammalian oocytes and embryos.

Nanoparticles have attracted enormous attention for biomedical applications as optical labels, drug-delivery vehicles and contrast agents in vivo. In the quest for superior photostability and biocompatibility, nanodiamonds are considered one of the best choices due to their unique structural, chemical, mechanical and optical properties. So far, mainly fluorescent nanodiamonds have been utilized for cell imaging. However, their use is limited by the efficiency and costs in reliably producing fluorescent defect centres with stable optical properties. Here, we show that single non-fluorescing nanodiamonds exhibit strong coherent anti-Stokes Raman scattering (CARS) at the sp³ vibrational resonance of diamond. Using correlative light and electron microscopy, the relationship between CARS signal strength and nanodiamond size is quantified. The calibrated CARS signal in turn enables the analysis of the number and size of nanodiamonds internalized in living cells in situ, which opens the exciting prospect of following complex cellular trafficking pathways quantitatively.

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