Finalist EPFL doctorate Award 2015 – Flavio Mor

The standard optical microscopy cannot satisfy the today's requirements of extremely high spatial resolution, which is necessary to obtain a deeper insight into intracellular phenomena. Therefore, to drastically circumvent the resolution limit imposed by the wave nature of light, several imaging techniques based on super-resolution fluorescence microscopy have been developed. In parallel, optical trapping methods, such as optical tweezers and photonic force microscopy (PFM), have shown their capability to confine and manipulate individual nanoparticles in liquid environments.

In this Ph.D. thesis, at first, we studied the 3D Brownian motion of a single sphere confined in a Newtonian fluid by the harmonic potential of the PFM optical trap. The optimization of our custom-built PFM allowed detecting, for the first time, resonances in the Brownian motion arising from hydrodynamic memory, which indicates that Brownian motion is not entirely random.

Subsequently, we reported on the multicolor upconversion luminescence (UCL) for arbitrarily shaped nano/micro sized particles of NaYF4:Yb,Er (UCP), individually held in the optical trap. Moreover, the energy transfer between an optically trapped UCP and molecules of an organic dye adsorbed on its surface was demonstrated.
The next milestone towards making this technology useful for biological research would be to validate, within a crowded environment of a living cell, the selective optical trapping of nanoparticles.

Figure:

Left: Log–linear representation of the normalized power spectral density of a trapped resin sphere in acetone for increasing laser power (green < red < blue). The yellow arrow indicates the resonances. Inset: 3D position trajectory of a trapped resin sphere.

Middle: Representation of a typical multicolor UCL spectrum from a trapped UCP.

Right: Excitation of fluorescent molecules through energy transfer for two different laser powers (blue < magenta).