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Effective THz fields on nano-structured surfaces

November 13, 2019

Polarization-sensitive field reconstruction in a THz/XUV pump-probe experiment at FLASH

Intense THz fields can be used to excite specific low-frequency modes in condensed matter or drive confined currents in nano-structured arrays, paving the way for a new class of optoelectronic devices. The long wavelength enables spatially confined field enhancement due to interaction with metallic structures and interference effects at dielectric interfaces. We use THz photoelectron streaking for the polarization-sensitive reconstruction of all electric field components at dielectric and metallic interfaces. This is realized in a THz/x-ray pump–probe experiment where x-ray emitted photoelectrons probe the effective THz field in close surface proximity. We observe distinct differences in the THz response of a nano-structured Pt thin film and a Pt(111) bulk crystal, in particular for the parallel field component. Simulations of the nanoscale spatial field modulation at metallic islands on the thin film provide design parameters for different sensitivity regimes with respect to local fields.


Fig. 1. Terahertz streaking scheme and polarization-sensitive pulse reconstruction. (a) Superposition of incoming and reflected parts of the THz pulse generate a complex field grating above the sample surface. The magnified inset shows possible momentum modifications of emitted electrons due to interaction with the E-field. (b) Stack of raw detector data of the Pt valence level at different THz/x-ray delays, showing the effect of tilting and shifting of the electron momentum distribution. (c) Schematics of the individual effects of the perpendicular (left) and parallel (right) field components on the momentum distribution. (d) Angle-integrated delay trace of the thin-film sample (top) and reconstructed field components parallel (center) and perpendicular (bottom) to the sample surface. Dashed lines indicate the delays of the detector images in (b).
 
Reference: Waltar, K., Haase, J., Pan, R., Golz, T., Kliuiev, P., Weinl, M., Schreck, M., Bajt, S., Stojanovic, N., van Bokhoven, J.A., Hengsberger, M., Osterwalder, J., and Castiglioni, L. (2019). Polarization-sensitive reconstruction of transient local THz fields at dielectric interfaces. Optica 6, 1431-1436. (www.osapublishing.org/optica/abstract.cfm?URI=optica-6-11-1431)

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