Research

SMART POLYMERS

Many photoresists have been developed so far for single- and two-photon lithography. With no exception though, these materials only respond to the light intensity profile. Polymers containing azobenzene units as side-chains (less often in the main polymer chain) have a very peculiar behavior, responding in fact to both the light polarization and intensity. For instance, if a linearly polarized Gaussian laser beam is focused on the surface of a film made of one of such polymers, a hollow will form with two protrusions on the side along the illuminating light polarization direction (with preserved volume). The phenomenon, known as mass migration, is driven by the photo-isomerization cycles of the azobenzene units. This enables in fact vectorial lithography where the combination of intensity and polarization patterns in the illuminating beam results in complex structures. Even more interesting, in 2012 I discovered that such polymers may also respond to the incident light wavefront. For this experiment, I used a linearly polarized vortex beam focused by a high NA microscope objective. The intensity profile of the vortex beam is a ring of light (donut beam) but the topographical features that I observed on the sample surface were spirals (see figure) with handedness depending upon the handedness of the helical wavefront of the beam. This phenomenon, that we described in terms of an anisotropic molecular diffusion, further enrich the complexity achievable in the surface structuring of these smart polymers.
azo-polymers

Selected Publications:

*corresponding author; +equal contribution

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OPTICAL NANO-IMAGING
Overcoming the diffraction limit of classical optical microscopy is the main motivation behind Scanning Probe Microscopy (SPM) techniques. In particular, Scanning Near-field Optical Microscopy (SNOM) allows optical imaging of surfaces with a resolution better than 50nm. Over the years I developed several SNOM setups for nano-imaging and nano-spectroscopy, in the visible and the mid-IR. I used these techniques to study polymers (including the Smart Polymers reported above), plasmonic metasurfaces, 2D materials, etc.

More recently, I have been interested in the concept of performing optical nano-imaging without detecting any light. This is a principle common to a few recently-developed SPM techniques. In such setups, the mechanics of an Atomic Force Microscope (AFM) cantilever is used to detect the weak forces originated between the sample and the cantilever tip when both are illuminated by an external light beam. I showed that such mechanism can be used e.g. to produce maps of the local refractive index of dielectric metasurfaces as well as to detect the phonon polariton dispersion of hexagonal Boron Nitride (a 2D material).

near-field imaging

 

Selected Publications:

*corresponding author; +equal contribution

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METASURFACES
Metasurfaces are nanostructured devices designed to control propagating light and surface waves in amplitude, phase and polarization state. For transmitted light, dielectric metasurfaces are usually preferred to reduce the losses associated with metals. Metasurfaces already allowed unprecedented control and structuring of light from visible to microwaves. I recently designed a hologram that project different light patterns according to the incident light polarization. What I find more interesting of that design is that we used the old holographic approach of detour phase, first introduced for binary holograms 50 years ago. However, the possibility to fill small areas with functional nanostructures allows in this case to completely cancel the wavelength dependence of detour phase with the dispersion of the nanostructures. This is in fact a device that works at any wavelength in the sense that it projects the same image whatever the wavelength of the incident light. More recently, we realized a device that is the dielectric metasurface equivalent of a q-plate, a device that turns a Gaussian beam into a vortex beam whose helical wavefront has opposite handedness with respect to the incident light polarization. We then generalized the concept shoving a new device, that we called J-plate, that can turn any two orthogonal polarization states of the incident light into two arbitrary orbital angular momentum states (vortex beams), even with the same wavefront handedness.
metasurface q-plate

Selected Publications:

*corresponding author; +equal contribution

more..........