Abstract
Evanescent-wave illumination is applied to synthetic-aperture microscopy on a transparent solid substrate to extend the resolution limit to lambda/2(n+1) (where n is the substrate refractive index) independent of the lens NA. Using a 633 nm source and a 0.4 NA lens, a resolution to 150 nm (lambda/4.2) is demonstrated on a glass (n = 1.5) substrate. Further extension to approximately 74-nm resolution (lambda/8.6) is projected with a higher index substrate (n = 3.3).
Highlights
Improvement of optical microscopy resolution is an age-old quest
The interferometric reintroduction of a zeroorder beam allows unambiguous capture and retrieval of the information corresponding to the propagating diffracted waves scattered by the object from the evanescent-wave illumination. This expands the frequency space coverage of interferometric microscopy (IIM) to a circle of radius NAeff ≤, where nsub is the refractive index of the substrate
Evanescent wave illumination has been used to extend the resolution of IIM to λ/2(n+1)
Summary
Improvement of optical microscopy resolution is an age-old quest. The classical resolution limits, first described by Abbé [1], are a pitch of λ/NA and a corresponding resolution of λ/2NA for normal incidence coherent illumination where NA is the objective lens numerical aperture. We have demonstrated [4,9] resolution of complex patterns almost to these linear systems limits, ~ λ/3.5 ~ 180 nm at a 633 nm wavelength, by imaging interferometric microscopy (IIM), consisting of dark-field illumination along with interferometric reintroduction of a zero-order reference beam at the square-law detection plane We combine this synthetic aperture approach with evanescent-wave illumination to extend the frequency space coverage to λ/(n+1) [resolution to λ/2(n+1)] and demonstrate a large field (~ 10×10 μm2) resolution to ~ 150 nm (λ/4.2) with a glass substrate (refractive index, nsub = 1.51) at a 633-nm wavelength. Much of the scientific excitement associated with metamaterials has been driven by the possibility of “perfect lenses” that operate without any transverse spatial-frequency bandpass limitation [15] This improved resolution is necessarily restricted to near-field domains for flat lenses and remains limited by materials, fabrication and impedance-matching constraints [16]. In the case of a 2D, planar, hyperlens structure [19] the image is necessarily restricted to a 1D line image
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