Unique apertured and apertureless NSOM protocols with low background


MultiProbe Apertureless NSOM ANSOM 

Link to pdf version of app note

 In standard approaches to apertureless NSOM (ANSOM) the probe is simply a modulated scatterer with gross far-field optical illumination using elements such as lenses or mirrors, which cause large spot sizes of radiation around the scattering probe. 

It has been very chellenging to develop ways to reduce the artifacts that arise from the background that is created by these far-field optical elements; this background can interfere with the desired signal from the nanometric tip of an atomic force sensor capturing the near-field surface component in a large far-field radiation background.

The MultiView 4000 with its unique multiprobe capability enables the reduction of both the optical and mechanical background and thus increase the overall S/N.

One probe can be used as a limited illumination spot onto a second scattering probe.  This both reduces the optical background and generates the correct k vectors to excite the scattering probe.  In addition, since the feedback employs tuning forks, the scattering probe can be kept with an oscillation amplitude of 1 nm.  This has been shown to be critical due to the fact that the probe tip has to be modulated close to the surface without a jump to contact and without variation in z which adds additional background to the signal from mechanical sources.  Furthermore, the rigid tuning fork frequency significantly enhances heterodyne and homodyne lock-in detection schemes to further reduce background.

This scheme diagrammatically describes the two probe configuration where the illuminating probe excites the scattering probe and this probe can even be a single gold nanoparticle with high scattering contrast at the exposed tip of the AFM probe for increased signal from the very tip of the probe.  These probes have been developed by Nanonics for Tip Enhanced Raman Spectroscopy
   

Near-Field Plasmon Excitation & Apertureless Scattering and Collection  Apertureless NSOM ANSOM

A Multiview 4000 with a two Probe SPM setup has been used for effective localized illumination of a plasmonic structure with an apertured NSOM probe which produces all k-vectors, and so it is most efficient for such plasmonic propagation. The propagating plasmons are scattered and then collected with a second probe, which has a very low dielectric constant and minimal perturbation of the plasmonic propagation

Two Probe Setup Scheme: An apertured probe to produce an evanescent field with a spectrum of k vectors to effectively excite SPP (Left Probe). Right probe is a very low dielectric contrast, highly exposed, non-interfering scanning and work in Photon Tunneling mode to scatter SPP and directly collect the photons produced by such scattering MV4000 picture of two probes in close nanometric proximity. MV4000 provides flexible probe and sample piezo scanning stages for fine and coarse probe positioning and scanning. The image above shows two probe attached to Tuning Forks for the ultimate in AFM force sensitivity. A microscope picture (100x objective) top view shows two tips of apertured (Left) NSOM fiber probe providing 532nm near-field illumination and an Apertureless NSOM probe (Right). Surface plasmons are generated on top of an Au strip and scattered by the scanning ANSOM tip.

 

Left: AFM Height image of the Au strip performed with the ANSOM scanning tip. The circle at the bottom shows the effect of the illumination apertured tip when scanning in its close proximity. Middle: ANSOM image performed with the scanning tip.  Rich contrast is seen by the apertureless probe doing the AFM and ANSOM imaging. Right: 3D ANSOM image shows sustained plasmon propagation and then rapid decay

 Apertureless Probes  

Standard probes that need to be used in order to effectively scatter the plasmonic signal have significant perturbation on the plasmonic propagation because of the need to use probes with high dielectric constant to obtain effective signal to noise in such scattering experiments. 
Nanonics exclusively provides Apertureless probes of glass with plasmonic or Non-Plasmonic Scattering Particles.  Nanonics ANSOM probes are low dielectric constant, provide non-interfering scanning, highly exposed and work in Photon Tunneling Mode to scatter SPP. For such plasmonic probes, glass provides for high dielectric contrast for exceptional antenna effects at the tip of the probe.  Such probes can be provided with a nanoparticle as small as 10 nm or simple 5nm diameter glass probes.

Apertureless IR NSOM, nanoIR  

In the IR ANSOM regime standard far-field optical elements give large spot sizes of microns to tens of microns which seriously compromise the nature of the signal detected by the nanometric tip of an atomic force sensor.  Applying a Dual Probe system allows for ultra low background with minimal spot illumination size through an infrared fiber probe, which is nanometrically held in close proximity using the dual probe geometry to a scattering low dielectric glass probe or a single  gold nanoparticle probe or a silicon exposed tip probe.  Such a tip is generally modulated in close proximity to a surface in order to delineate the near-field interactions.

NanoIR Probe: Unique methods for IR illumination with a 100 nm point heat source for broad band IR irradiation of a scattering probe tip using multiple probe capabilities and with subsequent interfero-metric IR spectral resolution.  Overcome tens of micron spot sizes with lens based IR optical illumination.

Nanonics MultiProbe Apertureless NSOM: 

  • Multiprobe systems are singularly capable of exceptional apertureless and scattering NSOM imaging.

  • Ideal Apertureless solution with minimum stray light & maximum plasmonic excitation

  • MultiProbe ANSOM appears to have significant potential to reduce background and maximizing signal at the highest resolution

 

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