As the field orientation in the surface plane rotates clockwise within each optical cycle, SPPs are excited at different locations along the groove. The curves are vertically shifted with respect to each other. (d) Azimuthally-averaged photoemission yield extracted from panels (b) and (c) in linear scaling showing the strong enhancement of the emission in the center ( J = 0, blue) and in the ring comprising the doughnut ( J = 2, red). (c) Logarithmically scaled PEEM image for excitation with S = −1 circularly polarized light yielding a focus spot in the center of the spiral with J = 0. (b) Logarithmically scaled PEEM image of the center of the spiral for excitation with S = 1 circularly polarized light yielding a “doughnut” mode with J = 2. The inset shows the gap of one SPP wavelength ( L = 1). (a) Au island with an Archimedean spiral milled into it. Accordingly, for a spiral with L = +1 that is comprised of only one revolution, a discontinuity of λ S is formed like the one shown (enlarged) in the inset of Figure 1(a).
#Electric field intensity full#
The radius r of the spiral increases as function of the polar angle φ as r φ = r 0 + L ⋅ φ 2 π λ s, i.e., after a full revolution φ = 2 π the radius has increased by L SPP wavelengths λ S. Figure 1(a) shows an example of a self-organized Au island with a groove in the shape of an Archimedean spiral.
#Electric field intensity how to#
In the following we will first describe the concept how to form a SPP focus and then create strong SPP fields and quantify their field strength. After fabrication of the grooves, the sample was placed in the load-lock system of the SPE-LEEM, plasma-cleaned, and directly transferred into the ultrahigh-vacuum preparation chamber where several cycles of standard Ar-ion sputtering and annealing were employed. A focused ion beam (Raith ionLINE Plus) was used to mill grooves into the surface that provide momentum matching and enable conversion of the laser pulses into SPPs. Mutually delayed laser pulses are created by a Pancharatnam’s phase stabilized Mach–Zehnder interferometer that can be bypassed for single-pulse experiments.įor the present study we use self-assembled Au platelets that were synthesized ex-situ by a single step thermolysis of (AuCl 4) −-tetraoctylammonium bromide. This microscope has been combined with a < 15 f s-pulsed Ti:Sapphire laser to enable nonlinear photoemission microscopy (PEEM) in a normal-incidence geometry and is equipped with a single-electron sensitive imaging CMOS detector. The experiment is based on the experimental setup around the spectroscopic photoemission and low energy electron microscope (ELMITEC SPE-LEEM III) at the University of Duisburg-Essen.
#Electric field intensity free#
The ponderomotive energetic shift is used to quantitatively derive the local SPP field strength in the focus without free parameters. In the present work we measure the ponderomotive energetic shift of electrons emitted from a flat Au(111) surface in an intense SPP focus that is spatiotemporally separated from the exciting laser pulse. While the orientation of SPP field vectors can be measured using near-field microscopy techniques or – with sub-femtosecond time-resolution – by employing the recently developed technique of vector microscopy, a quantitative measurement of the absolute field strength in an SPP focus is a challenging endeavor that has not been resolved yet. Understanding the nonlinear electron emission from such a focus requires detailed knowledge of the local SPP field, i.e., the field orientation and field strength. It was demonstrated that converting 800 nm wavelength laser pulses into short-range SPPs of 180 nm wavelength at a gold–silicon interface resulted in a nonlinear photoelectron emission spot of only 60 nm diameter. The diffraction limit, however, can be bypassed by converting laser pulses into shorter wavelength surface plasmon polaritons (SPPs), and by focusing the SPPs rather than the laser pulses. When creating strong fields by focusing of intense lasers, one encounters the fundamental problem of the diffraction limit: the attainable focus dimension of a Gaussian laser beam depends on the beam-diameter at the position of the focusing lens, its focal length, and on the wavelength of the light. For instance, field-enhancements in nano-optical systems can create strong-field situations and enable high-harmonic generation, photoinduced near-field electron micropscopy, field-enhanced Raman spectroscopy, and other nonlinear light–matter interactions. Nano-optics aims at controlling optical fields at the nanoscale to achieve novel functionality.
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