Nonlinear Photoemission Electron Microscopy Research Articles

Plasmonic vortices host magnetoelectric interactions

The vector E×H and pseudoscalar E·H products of electric and magnetic fields are separately finite in vacuum transverse electric and magnetic (TEM) plane waves, and angular momentum structured light. Current theories of interactions beyond the standard model of particle physics invoke E·H≠0 as the source term in the axion law that describes interactions with the cosmological dark matter axion particles outside of the quartet of Maxwell's equations. E·H≠0 also drives relativistic spin-charge magnetoelectric excitations of axion quasiparticles at a distinctively higher condensed matter scale in magnetic and topological materials. Yet, how to drive coherent E·H response is unknown, and provides motivation to examine the field polarizations in structured light on a deep subdiffraction limited spatial scale and suboptical cycle temporal scale by ultrafast nonlinear photoemission electron microscopy. By analytical theory and ultrafast coherent photoemission electron microscopy, we image E·H fields in surface plasmon polariton vortex cores at subwavelength scales, where we find that the magnetoelectric relative to the dipole density is intensified on an ∼10-nm-diameter scale as a universal property of plasmonic vortex fields. The generation and nanoscale localization of E·H fields introduces the magnetoelectric symmetry class, having the parity P and time reversal T broken, but the joint PT symmetry preserved. The ability to image the optical fields of plasmonic vortex cores opens the research of ultrafast microscopy of magnetoelectric responses and interactions with axion quasiparticles in solid state materials. Published by the American Physical Society 2024

Monitoring the surface quality of silver plasmon waveguides with nonlinear photoemission electron microscopy and in-situ ion sputtering

We use photoemission electron microscopy (PEEM) with single- and two-photon excitation to study the properties of surface plasmon polaritons (SPPs) in tapered silver stripe waveguides. Arrays of waveguides with different widths, grating coupler geometries, and taper angles are fabricated using electron beam lithography (EBL) on Si and indium-tin-oxide (ITO)-covered glass substrates. The presence of propagating SPP modes is indicated by modulations of the non-linear photoemission intensity which run along the length of the waveguide - the result of interfering SPPs reflected from the edges. Surface roughness also results in the presence of random emission hot spots and strong edge diffraction, which hinders an analysis of the effect of waveguide geometry on SPP propagation length and nanofocusing. Accordingly, we develop a relatively simple, in-situ method of reducing the surface roughness by argon ion sputtering using low-energy (∼ 300eV) ions. Surface roughness is qualitatively assessed by the ratio of the photoemission from the central interference fringes to that from the edges, and from the hot spot intensity. The improvement of the surface quality is a critical step for reducing radiative losses and therefore increasing the propagation length of plasmonic waveguides.

Enhanced Propagating Surface Plasmon Signal Detection

Overcoming the dissipative nature of propagating surface plasmons (PSPs) is a prerequisite to realizing functional plasmonic circuitry, in which large-bandwidth signals can be manipulated over length scales far below the diffraction limit of light. To this end, we report on a novel PSP-enhanced signal detection technique achieved in an all-metallic substrate. We take advantage of two strategically spatiotemporally separated phase-locked femtosecond laser pulses, incident onto lithographically patterned PSP coupling structures. We follow PSP propagation with joint femtosecond temporal and nanometer spatial resolution in a time-resolved nonlinear photoemission electron microscopy scheme. Initially, a PSP signal wave packet is launched from a hole etched into the silver surface from where it propagates through an open trench structure and is decoded through the use of a timed probe pulse. FDTD calculations demonstrate that PSP signal waves may traverse open trenches in excess of 10 μm in diameter, thereby al...

Characterization of a circular optical nanoantenna by nonlinear photoemission electron microscopy

We report on the investigation of an advanced circular plasmonic nanoantenna under ultrafast excitation using nonlinear photoemission electron microscopy (PEEM) under near-normal incidence. The circular nanoantenna is enhanced in its performance by a supporting grating and milled out from a gold film. The considered antenna shows a sophisticated physical resonance behavior that is ideal to demonstrate the possibilities of PEEM for the experimental investigations of plasmonic effects on the nanoscale. Field profiles of the antenna resonance for both possible linear polarizations of the incident field are measured with high spatial resolution. In addition, outward propagating Hankel plasmons, which are also excited by the structure, are measured and analyzed. We compare our findings to measurements of an isolated plasmonic nanodisc resonator and scanning near-field optical microscopy (SNOM) measurements of both structures. All results are in very good agreement with numerical simulations as well as analytial models that are also discussed in our paper.

Visualizing surface plasmons with photons, photoelectrons, and electrons.

Both photons and electrons may be used to excite surface plasmon polaritons, the collective charge density fluctuations at the surface of metal nanostructures. By virtue of their nanoscopic and dissipative nature, a detailed characterization of surface plasmon (SP) eigenmodes in real space-time ultimately requires joint nanometer spatial and femtosecond temporal resolution. The latter realization has driven significant developments in the past few years, aimed at interrogating both localized and propagating SP modes. In this mini-review, we briefly highlight different techniques employed by our own groups to visualize the enhanced electric fields associated with SPs. Specifically, we discuss recent hyperspectral optical microscopy, tip-enhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, as well as correlated scanning transmission electron microscopy-electron energy loss spectroscopy measurements targeting prototypical plasmonic nanostructures and constructs. Through selected practical examples from our own laboratories, we examine the information content in multidimensional images recorded by taking advantage of each of the aforementioned techniques. In effect, we illustrate how SPs can be visualized at the ultimate limits of space and time.

Ultrafast imaging of surface plasmons propagating on a gold surface.

We record time-resolved nonlinear photoemission electron microscopy (tr-PEEM) images of propagating surface plasmons (PSPs) launched from a lithographically patterned rectangular trench on a flat gold surface. Our tr-PEEM scheme involves a pair of identical, spatially separated, and interferometrically locked femtosecond laser pulses. Power-dependent PEEM images provide experimental evidence for a sequential coherent nonlinear photoemission process, in which one laser source launches a PSP through a linear interaction, and the second subsequently probes the PSP via two-photon photoemission. The recorded time-resolved movies of a PSP allow us to directly measure various properties of the surface-bound wave packet, including its carrier wavelength (783 nm) and group velocity (0.95c). In addition, tr-PEEM images reveal that the launched PSP may be detected at least 250 μm away from the coupling trench structure.

Interferometric Plasmonic Lensing with Nanohole Arrays.

Nonlinear photoemission electron microscopy (PEEM) of nanohole arrays in gold films is used to map propagating surface plasmons (PSPs) launched from lithographically patterned structures. Strong near-field photoemission patterns are observed in the PEEM images, recorded following low angle of incidence irradiation of nanohole arrays with sub-15 fs laser pulses centered at 780 nm. The recorded photoemission patterns are attributed to constructive and destructive interference between PSPs launched from the individual nanoholes which comprise the array. By exploiting the wave nature of PSPs, we demonstrate how varying the array geometry (hole diameter, pitch, and number of rows/columns) ultimately yields intense localized photoemission. Through a combination of PEEM experiments and finite-difference time-domain simulations, we identify the optimal array geometry for efficient light coupling and interferometric plasmonic lensing. We show a preliminary application of inteferometric plasmonic lensing by enhancing the photoemission from the vertex of a gold triangle using a nanohole array.

Nonlinear Photoemission Electron Micrographs of Plasmonic Nanoholes in Gold Thin Films

Nonlinear photoemission electron microscopy of isolated nanoholes in gold thin films maps propagating surface plasmon polaritons (SPPs) launched from the lithographically patterned plasmonic structures. A damped elongated ringlike photoemission beat pattern is observed from the nanoholes, following low angle of incidence irradiation of these structures with sub-15 fs 780 nm laser pulses. A notable agreement between finite difference time domain simulations and experiment corroborates our assignment of the observed photoemission patterns to SPPs launched from isolated nanoholes and probed through nonlinear photoemission. We also demonstrate how the efficiency of coupling light waves into isolated plasmonic holes can be tuned by varying hole diameter. In this regard, a simple intuitive geometrical model, which accounts for the observed and simulated diameter dependent plasmonic response, is proposed. Overall, this study paves the way for designing nanohole assemblies where optical coupling and subsequent plasmon propagation can be rationally controlled through 2D SPP interferometry.

Direct imaging of optical diffraction in photoemission electron microscopy

We report the visualization of optical diffraction at the boundaries of semiconductor and metal nanostructures in non-linear photoemission electron microscopy. We observe light diffracting into photonic and plasmonic modes of planar samples, and into photonic vacuum modes above sample surfaces. In either case, the electron photoemission rate from the sample material is spatially modulated resulting in photoemission images with information on the electric field distribution at the sample/vacuum interface. The resolution in these images is typically ∼30 nm, i.e., significantly below the wavelengths of the exciting light. Optical phase shifts and absorption losses for the diffracted modes can be determined.

The interplay of topography and energy dissipation in pentacene thin films

Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(001) and (√3×√3)R30°-Ag/Si(111). After an optical excitation of electrons by a λ=400nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(001), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3×√3)R30°-Ag/Si(111), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay.

Local 2PPE-yield enhancement in a defined periodic silver nanodisk array

Well-prepared periodic arrays of silver nanoparticles are investigated by means of linear and non-linear photoemission electron microscopy. The structures show homogeneous photoemission for UV excitation in the linear photoemission regime whereas striking inhomogeneities are mapped in the case of the nonlinear (2 photon) excitation using ultrashort 400 nm laser pulses. A detailed analysis enables to assign these inhomogeneities to defect induced electron momentum transfer processes only effective for the 2 photon excitation process. We propose this mechanism to be of relevance for the appearance of so-called hot spots in nonlinear photoemission as identified in other 2PPE studies in the past. Furthermore, the complementarity between all-optical studies and nonlinear photoemission studies of localized surface plasmons in nanoparticles is discussed.