Abstract
Near field scanning optical microscopy exploiting differential interference contrast enhancement is demonstrated. Beam splitting in the near field region is implemented using a dual color probe based on plasmonic color sorter idea. This provides the ability to illuminate two neighboring points on the sample simultaneously. It is shown that by modulating the two wavelengths employed in exciting such a probe, phase difference information can be retrieved through measuring the near field photoinduced force at the difference of the two modulation frequencies. This difference in frequency is engineered to correspond to the first resonant frequency of the cantilever, resulting in improved SNR, and sensitivity. The effect of both topographical and material changes in the proposed near field differential interference (NFDIC) technique are investigated for CNT and silica samples. This method is a promising technique for high contrast and high spatial resolution microscopy.
Highlights
High resolution imaging, beyond the conventional diffraction limit has lead to the development of near-field microscopy (SNOM), in which the evanescent waves containing high spatial harmonic frequencies are collected in the near field region
We demonstrate the implementation of a near-field differential interference contrast microscopy (NFDIC) technique by the means of combining a dual color SNOM system and a photoinduced force microscope (PiFM)
Diffraction-unlimited excitation of the two neighboring points can be done through utilizing the novel SNOM probes reported in our previous works[7,14], a new category of SNOM probes known as dual color probe
Summary
Near field scanning optical microscopy exploiting differential interference contrast enhancement is demonstrated. Several challenging issues have prohibited the implementation of DIC in SNOM These include high resolution excitation of the two neighboring points on the specimen, near-field interference of the two scattered beams from the specimen, and detection of very low phase changes resulting from nanometric changes of optical path length during the near field scanning of the specimen’s topography. Www.nature.com/scientificreports where ∆φβ (β = x, y, z) is equal to φβ1 − φβ[2] These equations clearly demonstrate that by converting two optical signals to photoinduced forces, the interference signal in the near field regions can be retrieved at the difference modulation extracted by firnetqeurfeenricnyg(∆forfmce)s. Considering Eq 10 in a simple case of having single wavelength illumination (ignoring the effect of the second hot spot), we can correspond the real optical phase induced by the first illuminating wavelength to that for photoinduced force
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