Nonlinear Photoemission Electron Micrographs of Plasmonic Nanoholes in Gold Thin Films

Abstract

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. ■ INTRODUCTIONTrapping light waves at a metal−dielectric interface 1,2 in the form of surface plasmon polaritons (SPPs) is the concept behind several emerging nanotechnologies. These include the fabrication of optical devices 3 and optoelectronic circuits, 4 as well as the development of ultrasensitive plasmonic detectors for chemical and biological sensing applications, 5,6 all operating well beyond the diffraction limit. Over the past decade, several techniques have been devoted to imaging SPPs, including dark field scattering microscopy, 7 fluorescence microspectroscopy, 8−10 coherent anti-Stokes Raman scattering microspectroscopy, 11 near-field scanning optical microscopy (NSOM), 12 and nonlinear photoemission electron microscopy. 13,14 Among the various tools of trade, PEEM has the distinct advantage of directly mapping SPPs at nanometer resolution through photoemission, without the need for molecular reporters or a scanning probe tip. Herein, we employ multiphoton PEEM to probe propagating SPPs launched from isolated plasmonic holes lithographically etched in a gold thin film.That light incident on an otherwise nominally flat metal surface can be coupled to a SPP mode through subwavelength structures is not a new concept. The most commonly used plasmonic couplers are ridges 15 and slits, 16,17 engineered to reflect and refract SPPs at metal/dielectric interfaces. 18−20 Nanohole arrays have also been previously exploited to achieve extraordinary optical transmission, 21,22 high brightness photoemission, 23 and enhanced energy conversion when coupled to photovoltaic devices. 24,25 Nonetheless, few works 12,26 have been devoted to understanding the operative physics that governs light coupling into and SPP generation from individual nanoholes, fundamental geometric coupling structures. Here, we report on the generation and manipulation of SPP fields through isolated nanoholes etched in thin films of gold. We take the first step towar...

Ultrafast Imaging of Surface Plasmons Propagating on a Gold Surface

“…28 In this report, the two beams are spatially separated. Therefore, power dependent imaging using the distinct beams can be particularly informative about what process(es) contribute to the nonlinear time-resolved PEEM images.…”

“…For a single laser beam experiment (or two-beam excitation with indistinguishable beams), a coherent three photon photoemission process has been postulated to be the dominant mechanism. 28 For a threephoton process, the photoemission intensity, I P , is proportional to the sixth power of the total polarization field integrated over time (following the discussion in ref 19).…”

The information content in single-molecule Raman nanoscopy

“…Using the same experimental scheme illustrated in Figure 3(A), scanning the metallic AFM probe allows mapping TERS across the diffraction-limited laser-illuminated area of the substrate [38]. In Figure 4, TERS images of DMS molecules adsorbed on a thin silver film reveal that different vibrational resonances (ν 1 − ν 4 ) of the probed molecules are optimally enhanced at different sites of the metal surface.…”

High-Brightness Plasmon-Enhanced Nanostructured Gold Photoemitter

“…(d) FDTD simulation of the nanohole array with s-polarized laser pulse. The laser pulse propagates from left to right in both simulations.Previous reports and analyses[12,13] of isolated nanohole and nanohole arrays indicate efficient light coupling into the metal film with the formation of propagating surface plasmons. It appears that the photoemission from the nanostructures is further enhanced in the regions of propagating surface plasmons, and the overall collective 'hotspot' pattern traces the surface plasmon propagation pattern.…”

“…With advances in nanofabrication technology, nanostructures such as holes [8], ridges [9] and slits [10] can be easily etched into metal surfaces. These structures can efficiently couple the incident radiation field [6,[11][12][13] into the metal and generate propagating surface plasmon Eigenmodes [14]. In this regard, surface plasmons can be focused, interfered, and wave-guided [15] using designed nanostructures.…”