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PGI Colloquium: Prof. Dr. Albert Polman, NWO-Institute AMOLF, the Netherlands


Online Talk

Please note: You will receive the link to the online talk in the e-mail invitation, usually sent out a few days before the lecture takes place. It is also available on request from the contact person below.

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22 Oct 2021 11:00

Nanoscale coherent optical excitations in the electron microscope    


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We use cathodoluminescence imaging spectroscopy (CL) as a powerful tool to characterize optical metasurfaces at deep-subwavelength spatial resolution.[1] In CL, a 5-30 keV electron beam is raster-scanned over the surface while the emitted radiation in the optical and near-infrared spectral range is detected. In the coherent excitation mode the electron beam passing through the metasurface creates a femtosecond electric field oscillation that couples strongly to polarizable electrons in the material, providing a spectrally broadband nanoscale probe of the local optical density of states. In the incoherent excitation mode, optical transitions in semiconductors are efficiently excited and can be used as a sensitive  probe of temperature and thermal conductivity at the nanoscale.[2]

We use CL to image localized modes of resonant plasmonic and dielectric nanostructures and reconstruct their scattering wavefronts using CL holography. We determine the phase and amplitude of wavefronts scattered by single-crystalline Au nanocubes and nanoholes, and derive from that the dominant scattering dipoles.[3] We discuss how the electron wavepacket may collapse as it generates a coherent superposition of surface plasmon polaritons and plasmonic transition dipole radiation that interfere in the far field. 

We correlate CL data with photon-induced near-field electron microscopy (PINEM) of plasmonic nanotips in Au nanostars. As shown before,[4] strong plasmonic near fields can dress the high-energy electron energy spectrum into a ladder of coherent harmonics, shaping the electron quantum wavepacket. We will show that this enables probing of the plasmon charge distribution and the corresponding near-field intensity of plasmonic nanotips at the true nanoscale.[5] Finally, we will introduce how PINEM can be effectively realized in a scanning electron microscope, enabling entirely new ways to shape electron wavepackets in space and time to probe coherent optical excitations and relaxations at the nanoscale.

[1] Polman, A., Kociak, M. & García de Abajo, F.J. Electron-beam spectroscopy for nanophotonics, Nature Mater. 18, 1158 (2019)

[2] K.W. Mauser, M. Solà-Garcia, M. Liebtrau, B. Damilano, P.-M. Coulon, S. Vézian, P. Shields, S. Meuret, and A. Polman, Employing cathodoluminescence for nanothermometry and thermal transport measurements in semiconductor nanowires, ACS Nano 15, 11385 (2021)

[3] Schilder, N., Agrawal, H., Garnett, E.C. & Polman, A. Phase-resolved surface plasmon scattering probed by cathodoluminescence holography, ACS Photon. 7, 1476 (2020)

[4] Feist, A., Echternkamp, K.E., Schauss, J., Yalunin, S.V., Schäfer, S. & Ropers, C. Quantum coherent optical phase modulation in an ultrafast transmission electron microscope, Nature 521, 200 (2015).

[5]  M. Liebtrau, M. Sivis, A. Feist, H. Lourenco-Martins, N. Pazos-Perez, R.A. Alvarez-Puebla, F.J. Garcia de Abajo, A. Polman, and C. Ropers, Spontaneous and stimulated electron–photon interactions in nanoscale plasmonic near fields, Light Sci. Appl. 10, 82 (2021) 

Contact

Dr. Martina Luysberg
Phone: +49 2461 61-2417
Fax: +49 2461 61-6444
email: m.luysberg@fz-juelich.de