Direct Near-Field Optical Imaging of Higher Order Plasmonic Resonances

Abstract

We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.When nanoscopic metallic structures are illuminated at adequate frequencies, the incoming radiation can couple to charge density oscillations and excite so-called localized surface plasmon polaritons (LSPPs). Recently, the nearfield enhancing qualities of LSPPs have been realized to hold promise for a bounty of novel applications in optics and photonics. 1 These applications often rely on the fine details of LSPPs and their interactions with nanostructures. For example, in the fields of ultra sensitive bio(chemo)detectors 2,3 or plasmonic metamaterials, 4-6 direct near-field optical microscopy of LSPPs would be of great benefit. While spectroscopic far-field properties of LSPP resonances are routinely accessible, real-space near-field information is difficult to obtain.To assess LSPPs of real nanostructures, microscopy techniques are required that are capable of spatially resolving the relevant structure sizes. One approach is the subsequent investigation by means of atomic force microscopy (AFM) of chemical or mechanical changes induced in suitable substrates by the excitation of optical eigenmodes. 7,8 A second approach uses electron energy loss or induced cathode-luminescence scanning electron microscopy to map LSPPs with nanometer resolution. 9,10 This technique requires vacuum compatible samples.For many applications in plasmonics, an all-optical detection of LSPPs with ultimate spatial resolution is called for. Characterizing local optical fields under ambient conditions has been achieved with AFM by carrying nanoscopic optical probes to the immediate vicinity of the nanostructures. [11][12][13][14][15][16] However, such an optical probe has to perform contradicting tasks: as an "optical nano-antenna", its reception/emission efficiency improves with larger size, 17 and as a near-field detector, the achievable spatial resolution improves with smaller size, which can also reduce parasitic interference and coupling effects between probe and sample. 14 In this communication, we demonstrate that these competing demands can be concerted by polarization control of the exciting and scattered radiation, even with off-the-shelf AFM tips as optical probes. We are able to clearly map dipolar and quadrupolar LSPPs, both in phase and in amplitude.In apertureless scanning near field optical microscopy (aSNOM), parasitic background signal from bulk scattering in the sample and tip is often suppressed by anharmonic lockin detection techniques, 18,19 and homodyne, heterodyne, or pseudoheterodyne inte...

Strong near-field optical localization on an array of gold nanodisks

“…We also observed that the light intensity rapidly decays in the vertical direction above the structure, showing a strong nearfield localization. In contrast to experiments performed with fiber-type probes [21][22][23][24] or scattering tips, 19,20 our measurements directly represent the intensity of the total field located on the surface (or more precisely, the square of the total field intensity). They indicate, in three dimensions, the zones of the structure that would be active if we use it as a chemical sensor or for performing near-field nanolithography.…”

“…In this work, our aim is to directly observe the electromagnetic field distribution using a scanning near-field optical microscope (SNOM). Recently, many SNOM experiments have been performed on isolated or coupled nanostructures, [17][18][19][20] but only very few studies have been achieved on arrays. Salerno et al 21 and Salomon et al 22 observed the localization of light on an array of gold nanoparticles with a SNOM, but the structures were separated from each other by one micrometer, which makes them almost isolated from each other.…”

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

“…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.…”

Real-space imaging of nanoplasmonic resonances

“…[43][44][45][46] To push the spatial resolution down to less than 10 nm, apertureless SNOM (aSNOM or aNSOM) uses point-or needlelike probes, whose apex radius largely determines the achievable lateral resolution, 47 essentially independent of wavelength. aSNOM is increasingly being applied to individual nano-plasmonic structures like disks, 13,23 holes, 48 triangles, 49 linear wireantennas 14,50-52 ( Fig. 2).…”

“…Ad hoc, this is verified by the favourable comparison with simulations of the bare sample in the absence of any probe. 13,14,51 3. Photon-in/electron-out methods Photoemission (PE) of electrons (or ''photo-electric effect'') is not only one of the pillars of quantum-physics.…”