Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes

Abstract

We identify weak and strong coupling regimes between a near-field probing tip and a plasmonic sample by imaging plasmon-resonant gold nanodisks with scattering-type scanning near-field optical microscopy ͑s-SNOM͒. By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response. DOI: 10.1103/PhysRevB.79.125439 PACS number͑s͒: 73.20.Mf, 78.67.Ϫn Surface plasmons in metallic nanoparticles have become a powerful driving force in the scientific and technological development of nanooptics. [1][2][3] The ability of plasmons to act as the interface between far-field radiation and nanoscale confined near fields has generated promising prospects in plasmon-enhanced spectroscopy, 4 optical nanoimaging, 5 electromagnetic signal guiding, 6,7 and biosensing applications, 8,9 among others. The potential of surface plasmons to decay radiatively, as well as nonradiatively, depending on the particular conformation and environment around them is the basis for most of these applications. In spite of the general and straightforward techniques available to obtain information on the far-field radiation by a surface plasmon such as in dark-field optical spectroscopy, 10 the amplitude and phase of its near field is still more than a challenge to access. Electron-energy-loss spectroscopy for example has provided a tool for spatial mapping of the plasmon modes with unprecedent resolution, 11 but it only provides information on the amplitude of the modes. However, in nanophotonics it is often the local near-field phase that is of extreme importance, as for example in coherent control applications, 12 in nanoantenna-assisted molecular emission 13 and spectroscopy, 14 and in plasmon dynamics of complex metallic systems. 15 A promising method to access both local amplitude and phase relies on near-field optical methods that have progressively succeeded in imaging nanoscale field patterns in metallic particles ͑optical nanoantennas͒. [16][17][18] In particular, scattering-type scanning near-field optical microscopy ͑s-SNOM͒ ͑Ref. 19͒ has managed to map both the local amplitude and phase of the plasmon modes by interferometric detection of the antenna fields scattered by a scanning atomic force microscope tip. [20][21][22][23][24] However, the antenna optical response is extremely sensitive to environmental changes, 5,8,9 thus the process of measurement of its near field may result in the modification of the antenna modes, similar to probe-induced modifications in other nanophotonic systems. [25][26][27][28] In this work we address this issue, presenting a basic understanding of the near-field coupling between s-SNOM probes and plasmonic nanoantennas ͑here gold nanodisks͒. We find that weak dielectric probes allow for plasmon mode mapping, whereas metallic probes introduce substantial m...

The Morphology of Narrow Gaps Modifies the Plasmonic Response

“…95 Notably, the exact field distribution in a plasmonic cavity determines the performance of many field-enhanced spectroscopies, such as in tip-enhanced Raman spectroscopy (TERS) 96,97 and of scattering-type scanning near-field optical microscopy (s-SNOM). 98 The variation of the morphology of the tip-onsubstrate cavities in these spectroscopies can indeed explain the large tip to tip dependencies in signal quality and spectral behavior often found in experiments. 99 Similarly, modifications of the gap can also determine the yields and properties of many optoelectronic processes such as in photoemission 100,101 or in nonlinear plasmonics.…”

Mapping the electromagnetic field confinement in the gap of germanium nanoantennas with plasma wavelength of 4.5 micrometers

“…The use of the bare silicon tip is essential to avoid the interference of the field enhancement due to the gold-coating of the tip with the field pattern of the nanoantenna. 35 The maps with the E//x polarization were also acquired for reference, to eliminate the unavoidable mechanical and thermal inhomogeneity of the PMMA coating. At k vib ¼ 5.8 lm, IR absorption by the PMMA molecules generates an AFM-IR signal approximately proportional to the square of the local electric field value.…”

Direct Characterization of Plasmonic Slot Waveguides and Nanocouplers

“…Background contributions were suppressed by demodulating the detector signal at a high-order harmonic frequency nΩ (in our case n = 2), providing background-free nearfield amplitude and phase images. It should be pointed out that, in most s-SNOM experiments, the illumination is done in the reflection mode (side-illumination scheme), where the incident light is focused on the tip with the same parabolic mirror that collects scattered light, a configuration that creates many problems for obtaining clear near-field images due to strong tip-sample coupling 24 and phase-retardation effects 25 . However, in our transmission-mode configuration, the sample was illuminated from below with an in-plane direction of polarization, allowing us to achieve uniform illumination and efficient excitation of the plasmonic antenna while avoiding the direct tip excitation 22,25,26 .…”