02 April 2009 PhD Thesis Defense
PETRU GHENUCHE \'Probing the near-field optical response of plasmon nanostructures with two-photon luminescence microscopy\'

Petru Ghenuche

Thursday, April 2, 11.00, ICFO\'s Auditorium. The thesis describes the design, fabrication and optical characterization of plasmon-resonant systems able to confine and enhance light fields down to the sub-wavelength scale. Extensive 3D numerical modeling was first used to design different geometries of coupled plasmonic nanostructures through the calculation of their far field and near field optical response. On the basis of simulations, the nanostructures were fabricated by e-beam lithography and thin film deposition. Special efforts were devoted to increasing the resolution and optimizing the reproducibility of critical parameters such as particle shape and interparticle gaps. Finally, far-field spectroscopy combined with two-photon induced luminescence (TPL) spectroscopy was used to probe the local optical response of the optimized architectures.

The candidate focused his attention on different families of structures: metal dimers, bar antennas, finite chains of nanoparticles and star-like particle arrangements. Particle dimers feature strong field enhancements in their sub-wavelength gap due to near-field coupling of their dipolar localized plasmon resonances. Based on the same physics, gap antennas, formed by two adjacent gold bars supporting multipolar resonances can efficiently couple to propagating light and concentrate it into tiny volumes. While finite particle chains were previously shown by other authors to be good candidates to guide light through subwavelength cross-sections, the candidate shows that they can also be used as efficient nanolenses able to concentrate light at their extremity. Finally, the near-field distribution in star-like arrangements of gold nanoparticles exhibits a strong dependence with the incident field polarization which can be exploited for dynamical optical addressing of nano-objects.

The candidate has compared the far field spectroscopy of large ensembles of dimers and finite chains with TPL spectroscopy. His main result is to show that TPL is preferentially sensitive to local fields and that it enables the assessment of spectroscopic features which cannot be resolved otherwise. In order to overcome the limitations of measurements on large ensembles, a considerable effort was dedicated to mounting and optimizing an optical set-up enabling TPL measurement of single structures.

By using the developed TPL micro-spectroscopy, spatially resolved spectral mode mapping on single resonant gap-antennas was achieved. As predicted by calculations, the candidate was able to directly visualize at resonance the strongly enhanced TPL signal within the gap. Results show how TPL scans can be directly compared with the convoluted distribution of the fourth power of the calculated local mode field. By monitoring the evolution with the incident wavelength of the TPL signal within the gap and at the antenna extremities, the candidate got further insight into the physical mechanism behind the buildup of the antenna’s resonance.

Finally, TPL microscopy was used to probe the local fields under different orientations of the incident linear polarization near star-like arrangement of gold disks. It is shown that, unlike the scattering spectrum, the TPL distribution over the structure is found to depend drastically on the incident polarization state.

This study brings a significant contribution to the field of plasmon optics by proposing novel geometries able to efficiently confine optical fields down to the nanometric scale, but also by providing deep insight into the use of TPL microspectroscopy to probe their local optical response. The findings are foreseen to be important in applications such as enhanced spectroscopy, bio-sensing and enhanced light-matter interaction, where one needs to assess the actual field experienced by small amounts of matter.

Thursday, April 2, 11.00, ICFO\'s Auditorium

Thesis advisors: Prof. Romain Quidant and Prof. Gonçal Badenes