Letter pubs.acs.org/NanoLett
Normal-Incidence PEEM Imaging of Propagating Modes in a Plasmonic Nanocircuit Gary Razinskas,† Deirdre Kilbane,‡ Pascal Melchior,‡ Peter Geisler,† Enno Krauss,† Stefan Mathias,‡,¶ Bert Hecht,*,†,§ and Martin Aeschlimann*,‡ †
Nano-Optics and Biophotonics Group, Department of Experimental Physics 5, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany ‡ Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin-Schrödinger-Str. 46, D-67663 Kaiserslautern, Germany ¶ I. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany § Röntgen Center for Complex Material Systems (RCCM), Am Hubland, D-97074 Würzburg, Germany S Supporting Information *
ABSTRACT: The design of noble-metal plasmonic devices and nanocircuitry requires a fundamental understanding and control of the interference of plasmonic modes. Here we report the first visualization of the propagation and interference of guided modes in a showcase plasmonic nanocircuit using normal-incidence nonlinear two-photon photoemission electron microscopy (PEEM). We demonstrate that in contrast to the commonly used grazing-incidence illumination scheme, normal-incidence PEEM provides a direct image of the structure’s near-field intensity distribution due to the absence of beating patterns and despite the transverse character of the plasmonic modes. Based on a simple heuristic numerical model for the photoemission yield, we are able to model all experimental findings if global plane wave illumination and coupling to multiple input/output ports, and the resulting interference effects are accounted for. KEYWORDS: Plasmon propagation, plasmonic functional device, nanocircuitry, control, near-field imaging, photoemission electron microscope
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bridge forming a short circuit). Therefore, nanoimaging tools are needed that provide detailed information about the propagation of plasmonic modes in order to verify functionalities. In this framework, multiphoton photoemission electron microscopy21 (PEEM) has been established as a powerful tool to image and characterize propagating SPPs along planar metal films,22−26 chemically grown27 and microfabricated28 noblemetal nanowires, organic nanofibers,29 and photonic waveguides.30−32 However, the observation of the propagation and interference of a finite spectrum of eigenmodes on a complex, functional plasmonic waveguide device relevant for future nanooptical circuits has never been attempted. Conventional PEEM setups have successfully exploited grazing-incidence illumination in many studies, e.g., coherent superposition of multiple localized surface plasmon (LSP) modes in small rice-shaped silver nanoparticle structures using few cycle laser pulses33 and polarization of the LSP resonances in metallic flat nanoprisms.34 However, when investigating
otivated by the potential to use noble-metal nanostructures supporting propagating plasmonic modes in future ultrafast integrated nano-optical circuitry and devices, a variety of different proof-of-concept functionalities1−3 such as signal splitting and filtering4 have been demonstrated. Also, coherent control of nonpropagating near-fields in nanosystems was predicted theoretically5 and realized experimentally by using closed-loop learning algorithms6,7 and open-loop control schemes.8 Manipulation of propagating surface plasmon polaritons (SPPs) was demonstrated in networks of chemically grown silver nanowires9−12 or slotless gold nanostructures.13 Recent progress in the quantitative selective excitation of multiple eigenmodes14 in plasmonic nanocircuits composed of optical antennas15,16 and two-wire transmission lines17,18 (TWTLs) has prepared the ground for an experimental demonstration of deterministic coherent control of plasmon propagation and routing19 based on multimode SPP interference in a nanoplasmonic device.20 So far the investigated structures remained rather simple and their design was based on computer simulations. More complex devices, however, are less tolerant toward fabrication uncertainties such as substrate inhomogeneities or small structural defects (e.g., a conductive © XXXX American Chemical Society
Received: June 22, 2016 Revised: October 5, 2016
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DOI: 10.1021/acs.nanolett.6b02569 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters more complicated structures, due to phase retardation effects such asymmetric illumination leads to complex photoemission patterns thereby obstructing the direct visualization of optical near-fields. Only recently normal-incidence illumination PEEM was introduced35 removing these difficulties. Normal-incidence illumination PEEM will have a major impact as it is expected to allow direct imaging of SPPs near-field intensity distributions with, e.g., the observed fringe spacing in a standing wave directly resembling the effective wavelengths of the involved plasmons. So far, just proof of principle measurements of plasmon polaritons have been published using this technique.35 Here, for the first time we use normal-incidence PEEM to visualize the propagation of distinct superpositions of multiple plasmonic modes responsible for directional coupling in a plasmonic y-splitter (directional coupler). The photoemission process induced by plasmons propagating a one-dimensional transmission line is expected to differ markedly from that of surface plasmon propagation on a two-dimensional (2D) metal film. Indeed it is unknown which field components at which position of the sample contribute most to the emission of photoelectrons. It is therefore beneficial if plasmon modes and their superpositions have been characterized before by independent methods.20 In that study, employing diffractionlimited far-field optical characterization only emission from the structure’s terminations was observed since propagating modes do not result in any far-field radiation. Of course, in the presented PEEM study the imaged near-field distribution differs from the one present in Rewitz et al.20 due to the different excitation scheme. Here, due to the widefield plane wave illumination of the structure launching of plasmon modes occurs simultaneously at all the device’s input/output ports. In addition, for certain polarization, the plane wave illumination directly excites an off-resonant near-field intensity in the gap of the TWTL that coherently superimposes to the guided mode field. Here we demonstrate (i) that excellent agreement between PEEM images and simulation data is found by using the fourth power of the mostly transverse field components propagating in the transmission line gaps as a measure for the photoemission yield. We show (ii) that PEEM signals for different pure plasmonic modes vary strongly in amplitude depending on the mode. Furthermore, we demonstrate (iii) that interference of plasmonic mode fields forming standing waves within the device with the illumination field has to be taken into account to correctly explain the observed patterns in PEEM. (iv) The observation of a distinct switching behavior with PEEM can be obscured by the above-mentioned launching of plasmon modes at all device’s input/output ports. Figure 1 shows a sketch of the directional coupler design and the experimental conditions. The investigated structure is a multiwire junction extending into TWTLs36 that act as plasmon waveguides. A linear optical antenna16,37 (right) as well as terminations called mode detectors14 (left) are used for efficient conversion of far-field laser light into propagating plasmon modes and vice versa. The sample is illuminated with a mode-locked Ti:Sapphire laser of 25 fs pulse duration and an 80 MHz repetition rate at a center wavelength of λ = 795 nm. The laser beam comes in at normal incidence from the air half space and is focused to a roughly 100 μm diameter spot on the sample resulting in an evenly illuminated structure. The studied photoemission process is a two-photon photoemission process (2PPE), i.e., two photons must be absorbed in order to free one electron. The spatial distribution of photoemitted electrons is
Figure 1. Directional coupler design and principle of the experiment. The entire structure is illuminated by a short plane wave laser pulse at normal incidence resulting in the excitation of counter-propagating plasmon modes forming a standing wave pattern. The spatially varying photoemission of electrons (sketched as blue dots) is imaged by PEEM. Inset: SEM image of investigated structure. The structure is fabricated by FIB milling of a single-crystalline gold flake that is deposited on a glass substrate. The total length of the structure is approximately 5 μm.
imaged using a photoemission electron microscope (Focus IS PEEM) with a spatial resolution of
