Simulation of a metallic SNOM tip illuminated by a parabolic mirror

Scanning near-field optical microscopy measurements and simulations of regularly arranged silver nanoparticles

Silver nanoparticles on a glass substrate are experimentally investigated by aperture scanning near-field optical microscopy (a-SNOM). To understand the experimental results, finite-element-method simulations are performed building a theoretical model of the a-SNOM geometry. We systematically vary parameters like aperture size, aluminum-coating thickness, tip cone angle, and tip-surface distance and discuss their influence on the near-field enhancement. All these investigations are performed comparatively for constant-height and constant-gap scanning modes. In the end, we establish a reliable and stable optical model for simulating a-SNOM measurements, which is capable of reproducing trends observed in experimental data.

Revealing the local crystallinity of single silicon core-shell nanowires using tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy is combined with polarization angle-resolved spectroscopy to investigate the nanometer-scale structural properties of core-shell silicon nanowires (crystalline Si core and amorphous Si shell), which were synthesized by platinum-catalyzed vapor-liquid-solid growth and silicon overcoating by thermal chemical vapor deposition. Local changes in the fraction of crystallinity in these silicon nanowires are characterized at an optical resolution of about 300 nm. Furthermore, we are able to resolve the variations in the intensity ratios of the Raman peaks of crystalline Si and amorphous Si by applying tip-enhanced Raman spectroscopy, at sample positions being 8 nm apart. The local crystallinity revealed using confocal Raman spectroscopy and tip-enhanced Raman spectroscopy agrees well with the high-resolution transmission electron microscopy images. Additionally, the polarizations of Raman scattering and the photoluminescence signal from the tip-sample nanogap are explored by combining polarization angle-resolved emission spectroscopy with tip-enhanced optical spectroscopy. Our work demonstrates the significant potential of resolving local structural properties of Si nanomaterials at the sub-10 nanometer scale using tip-enhanced Raman techniques.

Subwavelength Nanostructuring of Gold Films by Apertureless Scanning Probe Lithography Assisted by a Femtosecond Fiber Laser Oscillator

Optical methods in nanolithography have been traditionally limited by Abbe's diffraction limit. One method able to overcome this barrier is apertureless scanning probe lithography assisted by laser. This technique has demonstrated surface nanostructuring below the diffraction limit. In this study, we demonstrate how a femtosecond Yb-doped fiber laser oscillator running at high repetition rate of 46 MHz and a pulse duration of 150 fs can serve as the laser source for near-field nanolithography. Subwavelength features were generated on the surface of gold films down to a linewidth of 10 nm. The near-field enhancement in this apertureless scanning probe lithography setup could be determined experimentally for the first time. Simulations were in good agreement with the experiments. This result supports near-field tip-enhancement as the major physical mechanisms responsible for the nanostructuring.

Grating enhanced apertureless near-field optical microscopy

We examine the possibility of optimizing the emission and the near-field signal of apertureless silver and gold tips by using an optimized non-periodic grating. In this context, we consider the emission of a single quantum emitter in close proximity to optimized tips. Additionally, we study the far-field coupling efficiency of a tightly focused beam to the near-field of the tip. The gain in performance is compared with unstructured tips and the comparison with a pure plasmonic excitation of an unstructured tip is discussed. The optimized, structured tips show a significant enhancement of the total decay rate, as a result of standing plasmonic waves between the grating and the tip apex, leading to a resonant behavior. The resonances can be explained well with a Fabry-Pérot model. Furthermore, the total decay rate of an emitter near a structured tip can also be decreased as compared to an unstructured tip, when the grating is shifted from the optimal resonant position. The proposed scheme represents an interesting novel nano-antenna, for which the resonance as well as the directivity can be controlled by the grating.

Hybrids of carbon nanotube forests and gold nanoparticles for improved surface plasmon manipulation

We report the fabrication and characterization of hybrids of vertically-aligned carbon nanotube forests and gold nanoparticles for improved manipulation of their plasmonic properties. Raman spectroscopy of nanotube forests performed at the separation area of nanotube-nanoparticles shows a scattering enhancement factor of the order of 1 × 10(6). The enhancement is related to the plasmonic coupling of the nanoparticles and is potentially applicable in high-resolution scanning near-field optical microscopy, plasmonics, and photovoltaics.