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Description
Strong local enhancement of electromagnetic field of light is a key requirement in many fields of science ranging from nearfield microscopy, plasmonic biosensors, chemical catalysis or nonlinear photonics. One of the most common nanostructures capable of strong local electromagnetic field enhancement is a metallic nanotip. Computational methods are capable to accurately predict the properties of local nearfields in simplified geometries, but due to their sub-wavelength dimensions and duration in the range of femtoseconds, experimental methods allowing to measure field enhancement started to emerge only recently [1]. Most notable method for direct visualization of the field distribution is photon-induced near-field electron microscopy (PINEM), which can quantify the effect of the Lorentz force component aligned with the electron propagation direction through changes in the electron energy spectrum [2].
In earlier works, it has been predicted that the nearfield enhancement factor of a nanotip does not depend only on the radius of curvature of the tip apex, but it also depends on the opening angle of the conical tip [3]. However, an experimental verification is still missing for fields polarized perpendicular to trajectory of electrons, most notably along the symmetry axis of the nanotip, since the change of the electron trajectories cannot be resolved by PINEM which is only sensitive to the longitudinal momentum component of the electrons.
Here we use ultrafast 4D STEM method to image near-fields induced by infrared light at wavelength of 1930 nm with transverse polarization (perpendicular to the electron beam propagation direction) in the vicinity of a tungsten nanotip [4,5]. Lorentz force of the optical nearfield deflects the electrons in the transverse plane and broadens the electron distribution on a pixelated detector placed downstream the specimen. The broadening is proportional to the magnitude of the Lorentz force. An extensive set of finite difference time domain (FTDT) simulations is compared with the experimental data to retrieve the near-field enhancement factor as a function of the radius of curvature of the tip apex and the opening angle of the tip cone.
References
[1] M. Schenk et al., Phys. Rev. Lett. 105, 257601 (2010).
[2] B. Barwick et al., Nature 462, 902 (2009).
[3] S. Thomas et al., New Journal of Physics 17, 6 (2015).
[4] P. Koutenský et al., ACS Photonics 12, 4452 (2025).
[5] P. Koutenský et al., arXiv:2604.20463, (2026).