02 April 2009 ICFO Congratulates a New PhD Graduate

Dr. Petru Ghenuche

Thesis Committee

Dr. Petru Ghenuche obtained his Doctoral Degree under the supervision of Prof. Quidant and Prof. Badenes. Petru Ghenuche holds an MSc degree in Physics (2004) from the University of Bucharest, Romania. Since 2004 he has been working in the field of plasmon nano-optics.

Dr. Ghenuche’s research project studies probing the near-field optical response of plasmon nanostructures with two-photon luminescence microscopy.

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.

Dr. Ghenuche 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, Dr. Ghenuche 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.

Dr. Ghenuche 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, Dr. Ghenuche 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, Dr. Ghenuche 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.