Near-field vector imaging of nanoparticles using s-SNOM

The optical local field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices.  Understanding the correlation between particle size and shape, its spectral plasmon response, and the related details of the local field distribution is the goal of this project.  Using interferometric homodyne scattering Scanning Near-field Optical Microscopy (s-SNOM), we spatially map the strong field variations around plasmonic  metal nanostructures and optical antenna geometries from the visible to the mid-infrared.

Fig. 1: Topography (a) and corresponding s-SNOM image (b) for a small rounded single-crystalline triangular Ag nanoprism. This illustrates a single dipolar surface plasmon polariton excitation. The in-plane electric field vector has been probed under in-plane optical excitation conditions as indicated by the double-arrow.

Imaging at visible wavelengths

Triangular nanoprisms with sizes ranging from 50 to 400nm were synthesized from aqueous solution of silver nitrate and poly(vinyl-pyrrolidone). The localized plasmon resonance of these metal particles is characterized by a dipole resonance which broadens and red-shifts with increasing particle size. In addition, excitation of in-plane and out-of-plane quadrupolar modes lead to additional spectral features at higher energies for larger prisms.

Fig. 2: Scattering spectra of an individual Ag nanoprism (a) with an edge length of approximately 200 nm. Calculated optical extinction spectra for Ag nanoprism particles of varying size with truncated tips under s-polarized incident excitation.

By measuring of the near-field distribution for a single excitation wavelength (633nm) across prisms of differing size, we were able to map the spatial characteristics of both the dipole and quadrupolar modes. The field around smaller triangles takes on a characteristic dipolar field pattern exhibiting two lobes of high field intensity as shown in Fig. 1 Larger particles exhibiting quadrupolar resonances display a much richer field distribution as seen Fig. 3 with reduced field enhancement at the particle tips - in contrast to intuitive expectations.

Fig. 3: Topography (a) and corresponding tip-scattered near-field images at 633nm for s- (b) and p- (c) polarizations. Strong variations occur on short length scales as short as 20 nm as indicated by linecuts through the optical signal for p-polarization (d, e).

Imaging at infrared wavelengths

Optical antennas promise highly efficient coupling between light and nanoscale structures and nanoparticles below the classical diffraction limit. By engineering the structural details of an optical antenna, one may control the characteristics of its near-field. The ability to manipulate light at the nanoscale enables new devices in photonics, plasmonics, optical computing, and chemical spectroscopy.

Fig. 4: The topography (a) of a Au monomer IR optical antenna. The normalized out-of-plane near-field signal (b) with interferometric homodyne amplification, resulting in regions of constructive and destructive interference representing the optical phase corresponding to the direction of the Ez near-field vector component.

To better understand the effects of antenna scaling, Au linear antennas with length ranging from 1.65 μm to 7.0 μm were fabricated by electron beam lithography. Additionally, linear dimers were created to investigate the effects of antenna coupling on the near-field. Using interferometric homodyne detection with s-SNOM, we separately probe the in-plane (Ex) and out-of-plane (Ez) vector fields near the surface of the wires with mid-IR excitation. In analogy to the radio frequency regime, strong dipolar behavior is seen for short rods, while longer rods sustain multipolar resonances.

This work is in collaboration with the groups of Prof. Younan Xia (Washington University), Prof. Fei Zhou (Chinese Academy of Science), and Prof. Glenn Boreman (University of Central Florida, CREOL).

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