Near-field scanning optical microscopy in reflection: A study of far-field collection geometry effects

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Abstract
We have designed and demonstrated two simple and versatile reflection mode near‐field scanning optical microscopes (NSOMs). In one scanner far‐field collection is coaxial with the NSOM tip, and in the other scanner, the far‐field collection is at a 45° angle to the NSOM tip. We quantitatively compare images obtained with the two scanners. While off‐axis collection offers a significantly higher signal‐to‐noise ratio, it also introduces tip shadowing in samples with topographic features larger than approximately 40 nm. The additional contrast from the shadowing further complicates image interpretation and must be considered when performing NSOM in reflection with off‐axis collection. In addition, we discuss some general issues that should be considered when designing a reflection NSOM.