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Photon emission and absorption by plasmons at the nanoscale

The optical properties of metallic nanoparticles are dominated by surface plasmons (SP), which are resonant charge density waves confined to the surface of the nanoparticles. The energy of these excitations depends on the size and geometry of the nanoparticles. Theoretically, it is well established that the optical properties of metallic nanoparticles have nanometer-scale spatial variations, and that their absorption and diffusion properties should be different. However, this is experimentally very difficult to prove because conventional optical techniques, which measure the optical scattering and extinction sections (absorption + scattering), are limited by the diffraction. We combined within an electron microscope two spectroscopy techniques that are not affected by this limit : electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). By studying gold nanoprisms, we thus demonstrated experimentally and theoretically that EELS (resp. CL) measure optical extinction (resp. scattering) properties at the nanometer scale.

Since the SP is resonant, the color of nanoparticles depends solely on its energy. For example, a small particle will absorb in the blue range. A solution of such nanoparticles therefore appears red (complementary color) in transmission ; on the contrary, the color that will be strongly scattered is the one corresponding to the plasmon energy−blue. Such dichroism has been used since ancient times to create glasses with colors that change depending on whether they are viewed in reflection or transmission.

Due to the diffraction limit, the direct observation of these differential effects in absorption and scattering, and a fortiori the local study of the underlying energy transfers, are particularly difficult in single nanoparticles. We used a combination of techniques based on electrons, the electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). The EELS measures the energy lost by the electrons interacting locally with a nanoparticle, while the CL measures the emitted light resulting from this interaction. Because of its technical difficulty, the integration of these two technologies in a single scanning transmission electron microscope has been possible only recently, within our team.

A : Simulation of the charge distribution for a dipolar plasmon excitation (left) and a quadrupolar one (right). B. EELS (left) and CL (right) hyperspectral maps for a gold nanoprism 60 nm in size. The shape of the triangle is shown schematically in the EELS map. On the EELS, the dipole (D) and quadrupole (Q) modes are observed at two different energies with intensities localized at the tips (D) or edges (Q), while only the dipole mode is visible in CL.

We were able for the first time, in the case of very small particles (60 nm), to check an old theoretical prediction, which argued that the EELS is sensitive to plasmon modes regardless of symmetry (dipole, quadrupole see Figure A), whereas the CL is only sensible to modes of dipole type. We then showed, in the case of the dipole mode, that both techniques are mapping the surface plasmons in the same way (Figure B), and showed that the maps reflected in a first approximation the spatial variation of the potential created by the oscillation of the electron density due to the plasmon. However, the energies measured by the two techniques are slightly different. If they are almost complementary in the case of small particles, this is not the case for larger particles.

We showed theoretically that this effect is identical to that which would be measured, obviously without spatial resolution, in an extinction and scattering measurement. We have thus shown that the EELS and CL are spatially resolved counterparts of extinction and scattering cross-section experiments.

Reference :

Unveiling Nanometer Scale Extinction and Scattering Phenomena through Combined Electron Energy Loss Spectroscopy and Cathodoluminescence Measurements
Arthur Losquin, Luiz F. Zagonel, Viktor Myroshnychenko, Benito Rodríguez-González, Marcel Tencé, Leonardo Scarabelli, Jens Förstner, Luis M. Liz-Marzán, F. Javier García de Abajo, Odile Stéphan, and Mathieu Kociak
Nano Letters 15, 1229-1237 (2015).

Contact :

Mathieu Kociak