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Probing Coherent Surface Plasmon Polariton Propagation Using Ultrabroadband Spectral Interferometry Jue-Min Yi, Dongchao Hou, Heiko Kollmann, Vladimir Smirnov, Zsuzsanna Pápa, Peter Dombi, Martin Silies, and Christoph Lienau ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00821 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017
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ACS Photonics
Probing Coherent Surface Plasmon Polariton Propagation Using Ultrabroadband Spectral Interferometry 1
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Jue-Min Yi,* Dongchao Hou, Heiko Kollmann, Vladimir Smirnov, Zsuzsanna Pápa, 1 1 Silies, and Christoph Lienau
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Péter Dombi,
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Martin
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Institute of Physics and Center of Interface Science, Carl von Ossietzky Universität Oldenburg, D-26129 Oldenburg, Germany. 2 3
ELI-ALPS, ELI-Hu Nonprofit Kft., 6720 Szeged, Hungary.
Department of Optics and Quantum Electronics, University of Szeged, 6720 Szeged, Hungary.
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Wigner Research Centre for Physics, 1121 Budapest, Hungary.
KEYWORDS: Surface plasmon polaritons (SPPs); spectral interferometry; SPP propagation; group velocity; group velocity dispersion
Abstract: Surface plasmon polaritons (SPPs) are short-lived evanescent waves that can confine light at the surface of metallic nanostructures and transport energy over mesoscopic distances. They may be used to generate and process information encoded as optical signals to realize nanometer-scale and ultrafast all-optical circuitry. The propagation properties of these SPPs are defined by the geometry and composition of the nanostructure. Due to their short, femtosecond lifetimes it has so far proven difficult to track this propagation in the time domain and to directly study the effect of the propagation on the shape of a coherent SPP wavepacket. Here, we introduce an ultrabroadband far-field spectral interferometry method, allowing for the reconstruction of the plasmonic field in the time domain, to characterize coherent SPP propagation in metallic nanostructures. Group velocity and dispersion of SPPs are determined with high precision in a broad frequency range in the visible and near-infrared region, and the propagating SPP field is tracked with high time resolution over distances of tens of microns. Our results shed new light on the interplay between nanostructure geometry and coherent SPP propagation and hence are important for probing plasmon-matter interactions as well as for implementations of ultrafast plasmonic circuitry.
Surface plasmon polaritons (SPPs), surface-bound electromagnetic waves at metal-dielectric interfaces, have potential applications in a broad range of fields such as spectroscopy1, photochemistry,2 photovoltaics3 and biomedicine.4 Most promising applications are expected in the field of all-optical integrated circuitry and optical communications,5, 6 since they can transport energy in the form of ultrafast SPP wavepackets which, unlike light waves, may be confined to small length scales of only a few tens of nanometers. As such, they combine the favorable geometric features of conventional electronic devices with the ultrafast information processing that is only possible at optical frequencies.5, 7 Consequently, intensive research has been carried out in the last few years on proposing and designing novel nanostructures with tailored plasmonic properties. Different functionalities that are considered to be building blocks for all-optical devices, including lasing,8, 9 waveguiding,10 splitting,11 modulation12 and detection13 have been demonstrated. Furthermore, the coupling to active gain media may allow for loss compensation and amplification of SPPs which inherently suffer from radiative losses14 and the intrinsic
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Ohmic losses in the metal.15, 16 As such, the implementation of plasmonic components with logic functions via active and coherent control can be performed.15, 17, 18 These studies promise the realization of plasmonic-based nanometer-scale circuitry. Understanding the propagation properties of SPPs is essential for accurately evaluating and optimizing the performance of plasmonic circuitry. They propagate as coherent wavepackets carrying bits of information, and the propagation properties of these wavepackets are defined by the geometric structure and the composition of the material,19, 20 e.g., grain boundaries and surface quality. Tracing these dynamics with high spatial and temporal resolution is rather challenging, since collective plasmonic oscillations not only decay on a few femtosecond timescale but are also spatially confined in a deep subwavelength range. A number of approaches such as photoelectron emission microscopy (PEEM),21-27 attosecond streaking,28, 29 ultrafast near-field microscopy30, 31 and far-field spectral interferometry (SI),20, 32-37 have been developed to study the fundamental physics involved in plasmon propagation. Linear SI38, 39 probes the interference between an incident pulse with known spectrum and a replica of this pulse propagating along or across a certain piece of material. It provides a direct measurement of the linear response of the material, i.e., it probes both the attenuation and the phase shifts of the pulse acquired upon propagation. Since it is a linear optical technique, it is easier to implement than more advanced nonlinear nanooptical measurement schemes. Also, the result of the measurement does not depend on the spectral phase of the incident pulse and hence identical results are obtained when using coherent, pulsed lasers or incoherent, white-light sources for illumination. Such a technique is regularly used for phase-resolved spectroscopy,38 and more recently in particular to study strongly coupled nanosystems.32, 36, 40, 41 By measuring the linear response in a sufficiently broad spectral range, SI has the potential to completely characterize the effect of SPP propagation on the time-dependent electric field of a known ultrashort incident pulse. As we will show for a prototypical slitgroove nanostructure, plasmon propagation introduces not only a time delay and attenuation of the incident pulse but also a significant reshaping of the plasmon wavepacket stemming from SPP higher-order dispersive effects. Therefore, experimental time-domain studies of plasmon propagation are highly desired as they can give precise information on the evolution of the time structure of plasmonic waves and in particular on coherent excitation and control of ultrafast few-cycle plasmon wavepackets in nanostructures with different types of functionalities.42 In this paper, we present an ultrabroadband far-field SI method to characterize coherent SPP propagation which is capable of providing a complete reconstruction of the plasmonic fields in the time domain. We used a prototypical metallic nanostructure consisting of a straight slit-groove pair in a planar metallic film. SPPs are generated through the slit by nonresonant excitation with broadband spectral pulses. The generated SPPs propagate between the slit and groove and their propagation properties are determined by measuring their partial scattering at either the groove or the slit. We measured group velocity and dispersion of SPPs with high precision in a wide frequency range in the visible and near-infrared regions, and the temporal structure of the SPP field at large propagation length is obtained with high time resolution.
Results and discussion For the ultrabroadband SI setup, schematically depicted in Figure 1a and described earlier,41 optical pulses from a coherent, ultrabroadband light source (Fianium SC-450-4) are split into sample pulses and time-delayed and attenuated reference pulses = ∙ + in a dispersion-balanced Mach-Zehnder interferometer. Details of the experimental setup are described in the Methods section. The sample pulse is focused to a 2-µm spot size onto the sample by a dispersion-free, all-reflective objective (ARO, 15x/0.28). All-reflective objectives are used through the setup to minimize undesired temporal chirp. Essentially, the sample pulse locally excites the sample at the slit. This launches propagating SPP fields which are scattered into the far field at either slit or groove. The reemitted field through the sample is collected by another ARO (36x/0.52). It is connected to the incident field by a convolution with the linear response function = ̃e dω, =
− ′d′.Here, denotes the response of the sample to a fictitious delta-pulse excitation with ̃ as the complex transmission coefficient. The transmitted pulse and the reference pulse are superimposed using a second beam splitter and recorded with a spectrometer to obtain the spectral interferogram, given as = | ∙ e + ̃| !" ! . Here !" ! is the spectral amplitude of the laser field. One can select transmitted signals from the different spatial positions on the sample plane independently by a tip-tilt mirror and a spatial filter using a pinhole. Hence, subsequently measuring SI from different positons on the sample allows us to
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measure local response functions , and hence to characterize SPP excitation and propagation in nanostructures. Figure 1c shows an experimental SI mapping recorded for the bare setup without inserting a sample. The signal is acquired with 0.1 s acquisition time per time delay and the time delay is varied from 340 to 380 fs in 0.5 fs delay steps. As can be inferred from cross sections along both the time-delay and frequency axes in the mapping, the obtained SI signals show strong fringe contrast and high stability.
Figure 1. (a) Schematic of the far-field ultrabroadband SI setup. BLS = Broadband light source, BS = beam splitter, ARO = All-reflective Objective (Cassegrain), PH = pinhole. (b) Scanning electron microscope (SEM) image of a slit-groove nanostructure in a 200 nm thick epitaxially grown gold film. The scale bar is 1 µm, slit width: 150 nm; groove width: 150 nm and groove depth 60nm. (C) SI mapping of the bare setup with variant time delay . Inset: intensity at photon energy 1.70 eV as a function of time delay (the white curve), right: spectral interferogram at the time delay = 370 fs (the black curve).
To study optical excitation and propagation of SPPs on planar metal films, we use a prototypical nanostructure with straight slit-groove pairs. These nanostructures were fabricated in 200 nm thick epitaxially grown Au (111) films deposited on mica substrates (from PHASIS, Switzerland). With focused-ion (Ga+)-beam lithography (FEI Helios 600i), the slit is milled through the Au film while the groove has a depth of 60 nm. Both the slit and the groove have a width of 150 nm and length of 15 &m, and the slit-groove distance ' is varied from 5 to 120 &m. Here the maximum distance ' is larger than the SPP propagation length. A scanning electron microscope (SEM) image of a fabricated nanostructure is shown in Figure 1b. For measuring the plasmon propagation as a function of slit-groove distance, we first blocked the reference arm in the setup and measured spectra transmitted (scattered) from the slits (grooves) for excitation with linear polarization perpendicular to the slit. The spectra in Figure 2a and 2b show distinct periodic modulations, whose oscillation visibility becomes weaker with increasing slit-groove distance '. The spectra recorded from the slits are in accordance with earlier studies.33, 43 In addition to the periodic modulation that is also seen in the spectra taken from the grooves, the spectra measured at the groove positions (Fig. 2b) also show a lower carrierfrequency modulation which may result from the interference of light components from the slit and the groove. The high-frequency oscillations in the spectra can be assigned to multiple reflection of SPPs propagating between grooves and slits. SPPs that are excited at the slit propagate along the slit normal direction and are multiply reflected between the slit and the groove until they are reemitted into the far field. The resulting interference can not only be seen at the slits when recording the directly transmitted light and the SPPs scattering, but also at the groove through multiple scattering. The round trip time for the SPP propagation between slits and grooves can directly be determined by calculating the Fourier transform of the measured spectra. Figure 2c shows the obtained amplitude of the Fourier-transform of the spectra from the slit in Figure 2a. The time delay at two symmetric sidebands gives the round-trip time for SPP propagation between slit and groove (as schematically shown in inset of Figure 2a). With the extracted time delays (( = 60, 106, 140, 227 fs for groove distances ' = 8.2, 13.2, 18.2 30.0 μm we can estimate a SPP group velocity 567 = 2'⁄ (( = 0.87 9 0.03: at the central photon energy of 1.65 eV (750 nm), where : is the speed of light in vacuum. The value is in good agreement with the theoretical value 567 = 0.91: , given by 567 =
