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42545-AC10
Quantitative Measurement of Field Enhancement Near Metal Nanostructures
In this project we developed a scattering-type scanning near-field optical microscope (sSNOM) for the quantitative measurement of local field enhancement near metal nanostructures. In this method, light incident on a metallic probe experiences an amplitude and phase change on scattering, which is dependent on sample properties. We implemented phase-shifting interferometry to extract amplitude and phase information from an interferometric near-field scattering system, and acquired images of standing surface-plasmon polaritons near a gold edge. This method is simpler than standard heterodyne methods and less sensitive to errors than the pseudo-heterodyne method, which is a limiting case of phase-shifting interferometry.
Scattering-type scanning near-field optical microscopy (sSNOM) combines the resolution of atomic force microscopy with the chemical sensitivity of optical spectroscopy. Most commonly, a sharp metal tip is brought into close proximity to a sample surface, and the tip-sample region is illuminated with an external laser beam, creating a region of enhanced electric near field. Light experiences a change in amplitude and phase relative to the incident light upon scattering from this region, and the amplitude and phase change depends of the local dielectric properties and the topology of the sample. The scattered light is usually collected interferometrically, and amplitude and phase shifts are extracted with lock-in techniques. There is much interest in measuring the amplitude and phase changes separately, since these signals contain different but complimentary information about the sample. In sSNOM, the tip is raster-scanned over the sample surface in close proximity and for each image pixel the optical phase and amplitude is recorded.
We developed an anternative method of amplitude and phase recovery for A-SNOM, drawing from the field of phase-shifting interferometry (PSI), which has been used for several decades in optical testing. The reference mirror is moved though at least three phase steps, which are usually equally spaced. Interferometric optical measurements at each of these steps are used to solve for amplitude and phase. This method exhibits the relative simplicity of the pseudo-heterodyne method while drastically reducing uncertainty and maintaining reasonable scan speeds. Furthermore, extensive work has already been published on error suppression in PSI, which may be ostensibly applied to its SNOM applications.
PSI-SNOM is significantly simpler than the heterodyne method of sSNOM, and more robust to sources of error than the pseudo-heterodyne method. We have demonstrated the application of this method by obtaining amplitude and phase images of surface plasmon polaritons near a gold edge. Application of analysis and techniques from the developed field of phase-shifting interferometry should lead to reliable quantitative characterization of local sample characteristics with good spatial resolution.
