Reaching the theoretical resonance quality factor limit in coaxial plasmonic nanoresonators fabricated by helium ion lithography

Optical antenna structures have revolutionized the field of nano-optics by confining light to deep subwavelength dimensions for spectroscopy and sensing. In this work, we fabricated coaxial optical antennae with sub-10-nanometer critical dimensions using helium ion lithography (HIL). Wavelength dependent transmission measurements were used to determine the wavelength-dependent optical response. The quality factor of 11 achieved with our HIL fabricated structures matched the theoretically predicted quality factor for the idealized flawless gold resonators calculated by finite-difference time-domain (FDTD). For comparison, coaxial antennae with 30 nm critical dimensions were fabricated using both HIL and the more common Ga focus ion beam lithography (Ga-FIB). The quality factor of the Ga-FIB resonators was 60% of the ideal HIL results for the same design geometry due to limitations in the Ga-FIB fabrication process.

Nonperturbative visualization of nanoscale plasmonic field distributions via photon localization microscopy

We demonstrate the nonperturbative use of diffraction-limited optics and photon localization microscopy to visualize the controlled nanoscale shifts of zeptoliter mode volumes within plasmonic nanostructures. Unlike tip- or coating-based methods for mapping near fields, these measurements do not affect the electromagnetic properties of the structure being investigated. We quantify the local field manipulation capabilities of asymmetric bowtie antennas, in agreement with theoretical calculations. The photon-limited localization accuracy of nanoscale mode positions is determined for many of the measured devices to be within a 95% confidence interval of +/-2.5 nm. This accuracy also enables us to characterize the effects of nm-scale fabrication irregularities on local plasmonic mode distributions.

Functional plasmonic antenna scanning probes fabricated by induced-deposition mask lithography

We have fabricated plasmonic bowtie antennae on the apex of silicon atomic-force microscope cantilever tips that enhance the local silicon Raman scattering intensity by approximately 4 x 10(4) when excited near the antenna resonance. The antennae were fabricated using a novel method, induced-deposition mask lithography (IDML), capable of creating high-purity metallic nanostructures on non-planar, non-conducting substrates with high repeatability. IDML involves electron-beam-induced deposition of a W or SiO(x) hard mask on the material to be pattered, here a 20 nm Au film, followed by Ar ion etching to remove the mask and the unmasked gold, leaving a chemically pure Au bowtie antenna. Antenna function and reproducibility was confirmed by comparing Raman spectra for excitation polarized parallel and perpendicular to the antenna axis, as well as by dark-field spectroscopic characterization of resonant modes. The field enhancement of these plasmonic AFM antennae tips was comparable with antennae produced by electron-beam lithography on flat substrates.

Manipulating nanoscale light fields with the asymmetric bowtie nano-colorsorter

We present a class of devices called Asymmetric Bowtie nano-Colorsorters. These devices are specifically engineered to not only capture and confine optical fields, but also to spectrally filter and steer them while maintaining nanoscale field distributions. We show that spectral properties and localized spatial mode distributions can be readily tuned by controlled asymmetry. Nano-Colorsorters can control light's spatial and spectral distributions at the nanoscale and thus significantly impact applications ranging from broadband light harvesting to ultrafast wavelength-selective photodetection.

Label-free in situ imaging of lignification in the cell wall of low lignin transgenic Populus trichocarpa

Chemical imaging by confocal Raman microscopy has been used for the visualization of the cellulose and lignin distribution in wood cell walls. Lignin reduction in wood can be achieved by, for example, transgenic suppression of a monolignol biosynthesis gene encoding 4-coumarate-CoA ligase (4CL). Here, we use confocal Raman microscopy to compare lignification in wild type and lignin-reduced 4CL transgenic Populus trichocarpa stem wood with spatial resolution that is sub-microm. Analyzing the lignin Raman bands in the spectral region between 1,600 and 1,700 cm(-1), differences in lignin signal intensity and localization are mapped in situ. Transgenic reduction of lignin is particularly pronounced in the S2 wall layer of fibers, suggesting that such transgenic approach may help overcome cell wall recalcitrance to wood saccharification. Spatial heterogeneity in the lignin composition, in particular with regard to ethylenic residues, is observed in both samples.

Dimerization boosts one-dimensional mobility of conformationally adapted porphyrins on a hexagonal surface atomic lattice

We employed temperature-controlled fast-scanning tunneling microscopy to monitor the diffusion of tetrapyridylporphyrin molecules on the Cu(111) surface. The data reveal unidirectional thermal migration of conformationally adapted monomers in the 300-360 K temperature range. Surprisingly equally oriented molecules spontaneously form dimers that feature a drastically increased one-dimensional diffusivity. The analysis of the bonding and mobility characteristics indicates that this boost is driven by a collective transport mechanism of a metallosupramolecular complex.

Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111)

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60 degrees.