27 Nov 2018

Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission

tELIgraph, ELI-ALPS’ internal newsletter, interviewed Judit Budai and Zsuzsanna Pápa about the most recent paper of the Ultrafast Nanoscience Group  – Plasmon-plasmon coupling probed by ultrafast, strong-field photoemission with < 7 Å sensitivity, Nanoscale, 10, 16261, (2018): http://doi.org/10.1039/c8nr04242j.

Could you summarize this latest publication for us?

This paper investigates a fundamental electromagnetic coupling phenomenon which is very important for real-life applications. For this, we built upon our new method published in 2017 which introduced and validated a new experimental technique for measuring the electric field strength near nanostructures (Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission, Nano Lett., 17, 1181, (2017). In this paper, we developed a non-destructive ultrafast near-field enhancement measurement technique based on the photoemission of electrons from the surface and their subsequent acceleration by the local field. With this method, the field strength could be measured with a sensitivity of 1 nm above the probed material. Our current paper takes this technique and uses it to investigate the coupling between localized plasmons and propagating plasmon waves.

Zsuzsanna Pápa and Judit Budai

Why silver?

Silver shows plasmonic behaviour in the wavelength range provided by the probing laser enabling the excitations of propagating plasmonic waves. These waves can couple with localised plasmon oscillations enhancing the local surface electric field by as much as a factor of 30. This means a local intensity enhancement of almost 1000. This coupling and subsequent enhancement depends on the size of the surface structures. In this work, we looked at these resonances experimentally and found that corresponding behaviour could be described using a localised dipole model. Our models accurately match the experimental measurements, taken under with various conditions, and confirms the validity of our models and theoretical understanding of the phenomena.

Measurement scheme for the investigation of plasmon–plasmon coupling probed by photoelectron spectroscopy. Plasmons are generated on surfaces with precisely controlled roughness by femtosecond laser pulses to achieve sufficient intensity for the photoacceleration of the electrons in nano-optical near-fields.

What’s next?

The next step is applying these techniques to the design and construction of nanostructured materials. Here at ELI-ALPS, there is an electron beam lithography setup which will enable us to create custom-formed nanoarchitectures with unique localised electric field enhancements. Tailored nanostructures have important applications in single-molecule spectroscopy, ultrasensitive sensorics and ultrafast switching, therefore, we expect interesting results in the future, too.

Dipole-like behavior of the surface nanostructures. Scheme of(a) a modeled dipole, (b) modeled rough silver surface (rms roughness of 4.5 nm) with a ∼65 nm-diameter surface nanostructure in the centre and (c) silver surface with a semi-ellipsoid (diameter: 65 nm and height: 6.5 nm). (d), (g) and ( j) Normalized field distribution maps for the x, y and z-components of the electric field of a dipole being parallel to the x-axis. (e), (h) and (k) Field distribution maps in a plane 2 nm above a 65 nm-diameter surface nanostructure on the Ag layer with 4.5 rms roughness. (f ), (i) and (l) Field distribution maps in a plane 2 nm above a 65 nm-diameter surface nanostructure on a smooth Ag layer. The color bars are offset in (e), (f ), (h) and (i) to indicate only the dipole component, and the residual background in the Ex and Ey field amplitude components can be attributed to the presence of propagating plasmons on the surface. They propagate along the y-axis and have 0 electric field components in the z-direction, as expected.