Nanometer level resolving near field optical microscope under optical feedback in the observation of a single-string deoxyribo nucleic acid

We present the observation results of a single string deoxyribo nucleic acid (DNA) obtained by a collection mode near field optical microscope (NOM) operated under constant distance mode with the optical near field intensity as the feedback signal. The observed width of the narrowest string is around 4 nm. To our knowledge, this is not only the successful optical observation of a single string DNA by purely optical means but also under such high resolving capability. This high resolution is mainly attributed to the special care taken during the preparation of the sample and of the probe to avoid unwanted scattered light. The probe efficiently picks up the high spatial frequency component of the scattered near field and simultaneously rejects out effectively the lower spatial frequency components.