Politecnico di Torino Master Thesis in Nanotechnologies for ICTs Spiral plasmonic lens for new generation photoemitters

For decades, metal photocathodes were used in the UV-regime by up-converting the fundamental harmonic of the excitation laser to reach the required energy for single-photon photoemission process. The main problems in this experimental approach are represented by low quantum efficiency at these frequencies, the high power required together with the efficiency for the third harmonic generation. Nowadays, the photocathode technology is paying great attention on a new generation of photoemitters, able to directly exploit the fundamental harmonic, showing the ability to simultaneously enhance and focus the electric field in a sub-wavelength focal spot, leading to non-linear optical processes as the multiphoton photoemission of the 3rd and 4th order. Behind this new technology, the field of plasmonics plays the role of protagonist. Nanometric emission areas are of fundamental importance in the physics of electron accelerators. Indeed, modern science is asking for even brighter electron beams able to detect matter with incredibly high spatial resolution, reaching at the same time high imaging quality. The requirement of a small emission area is due to the fact that the smaller the electron extraction source is, the higher the brightness will be. The final aim of this project consists in the development of a new photoemitter with all the characteristics mentioned until now, following the way paved by different reaserches conducted on this field at the Molecular Foundry, part of the Lawerence Berkeley National Laboratory. In particular, the project will focus on the simulation, fabrication and characterization of a new photoemitter to be implemented for the ultrafast electron diffraction technique. Ultrafast electron diffraction and microscopy are new promising techniques, able to guarantee high spatialand temporal-resolved analysis of organic, so as inorganic materials, from amorphous to crystalline smaples, where a laser triggers the whole process. Together with the requirement of high-brightness, the electron bunches used as probes in these types of characterizations must show high temporal resolution. This can be obtained with the acceleration of electron pulses up to MeV, where the electron source is positioned inside an RF-gun. One could think to extract electrons from tip-like sources to reach the sub-wavelength dimensions required for the emission area. The main problem of tip-emitters is that it becomes too difficult to control the desired emission for performing ultrafast analysis, since the high electric field will cause field emission at every cycle in addition to the emission caused by the laser triggering. The main goal of this project, entirely developed at Molecular Foundry, will be to exploit the physics behind the collective oscillations of conduction band electrons in a metal photocathode. If well designed, the structure will be able to enhance and focus the electromagnetic energy arriving from a pulsed laser toward a sub-wavelength region where electron extraction can occur. At the basis of this process, surface plasmon polaritons will represent the main carrier by means of which light can be focused beyond the diffraction limit, making this nanostructure a real plasmonic lens. An archimedean spiral plasmonic lens is designed and optimized through the use of FDTD simulations; the optimized structure is fabricated with the nowadays consolidate template stripping process, which satisfies all the requirements for photoemission applications. All the fabrication steps are performed in a cleanroom class 100, present at the NanoFabrication Division at the Molecular Foundry. Finally, the fabricated plasmonic lens is characterized by means of the cathodoluminescence technique, offering a complex but performant way to detect its plasmonic characteristics. Spiral plasmonic lens for new generation photoemitters Kostas Kanellopulos