PLASMENTA · Nanoscale detection of entangled surface plasmon polaritons

Nanoscale detection of entangled surface plasmon polaritons

Surface plasmon polaritons SPPs are combined oscillations of conduction electrons together with electromagnetic waves at the interface between a metal and a dielectric along the interface. Their electromagnetic energy is strongly localised in the vicinity to the surface, allowing the nanoscale confinement of optical waves. The possible applications of their properties have led to rapid growth of the field of plasmonic research, helped by the parallel development of nanoscale fabrication techniques for metal surfaces. At the same time, the field of quantum optics has seen in boom in research and development of first applications. The prospect of combining quantum optics together with the miniaturisation of plasmonic circuits has attracted large interest and led to first experiments investigating the quantum character of SPPs. The results suggest that, despite being a collective oscillation of an ensemble of conduction electrons, SPPs posses a quantised character. Still, the quantised character of SPPs and their entanglement have not been properly investigated. The main goal of the project was to explore the quantum nature of SPPs by measuring entanglement directly on the nanoscale. To achieve this goal the project was split in two steps: 1) A detector for plasmons Previous experiments to investigate the entanglement of plasmons relied on the conversion back into photons and measurements in the far-field. The detector proposed for this project works in the near-field and measures directly the entanglement on the nanoscale. It relies on the upconversion of light by a single star-shaped gold nanoparticle placed close to a gold surface, as these nanoparticles have shown high second-harmonic generation due strong field enhancements at their tips. The stars are excited by SPPs leading to emission of an up-converted photon. The high up-conversion efficiency allows detecting the signal of only two plasmons, which is up-converted to a single photon. 2) An source of entangled plasmons The first step to measure entangled SPPs is to create them. By using entangled photons as excitation source and coupling them to plasmons, we will have access to entangled SPPs. As the source, we have chosen a Sagnac-design, which allows a high flux of entangled photons and will also be able to work in pulsed mode. The coupling of entangled photons to SPPs is finally being performed using grating couplers. The final goal of the project was to combine these elements to measure the interaction of those entangled plasmons using the two-plasmon detector and reveal the grade of entanglement directly on the SPP level. All the steps relied on the Dr Kuttge's existing expertise in photonics, plasmonics and nanofabrication, but also gave the fellow the opportunity to explore the new fields of nanoparticle photonics and quantum optics. In the first weeks of the project, the fellow visited the collabourating Colloidal Chemistry group in Vigo and was introduced to the basics of nanoparticle synthesis and attachment. This allowed him to produce samples directly at ICFO and to adjust the required parameters like star density and spacing. The introduction was followed by measurements of propagating SPPs in random arrays of stars both with CW and pulsed excitation. The possibility of producing and measuring random arrays with nanostar density compensated the problems in producing ordered arrays of nanostars using electron beam lithography. Coupling to SPPs was improved using grating coupler structures, which were optimised by the fellow using FDTD and structured using the FIB milling. The main results of these measurements were a characterisation of the SPP-scattering coefficients of nanostars and the observation of a strong dependence of the nonlinear signal on the distance between the star and the surface. The original plan had proposed a two-colour experiment of two SPPs with different energies coming in from two sides. After extensive investigation this setup did produce the desired results, so that Dr Kuttge chose a new design which relied rather on polarisation than on energy entanglement. The resulting source was being set up at the end of the project time and will be characterised. Afterwards, the output will be coupled to the newly designed plasmon detector structures, which have been fabricated already. As scientific results, the fellow obtained important data on the coupling of SPPs and light to nanostars. Furthermore, the nonlinear properties of nanostars were characterised and the strong dependence of the SHG signal on the gap size was observed for the first time. The entangled photon source, which was built up during the project, will be further used by the host group. This will also allow continuing experiments of coupling entangled photons to SPPs will deliver interesting results. During the fellowship, Dr Kuttge gained important knowledge in nanophotonics during and could diversify his expertise and skills by entering a new research field. Furthermore, he supervised several students and was responsible for the collabouration with other groups, preparing him for a possible position with more responsibility.

Data: CORDIS, © European Union