Quality of PMMA Images in the photoresist with and without the thin silver film
- Nov. 24, 2015
In conventional optics, image quality depends on the diffraction limit. Recently, transformative optics using a superlens is proposed  to restore image quality below the diffraction limit – the restoration not possible with conventional optics. The superlens comprises meta-materials having negative permittivity in contact with a dielectric with a permittivity of equal and positive sign. The noble metals such are silver natural candidates for transformative optics because negative permittivity is thought to be a consequence of the collective excitation of conduction electrons. Indeed, superlens experiments  using a 35 nm silver film in contact with a 40 nm PMMA spacer under UV illumination at λ = 365 nm showed sub–diffraction-
limited imaging of etched chromium objects down to 60 nm as depicted in the thumbnail. PMMA stands for polymethyl methacrylate and UV for ultraviolet. When the superlens is removed, the diffraction-
limited wavelength P* = λ / n is controlled by the refractive index n of PMMA, i.e., for n = 1.5, P* = 243 nm. With the silver film, the superlens was found to resolve the average cross section of the etched chromium objects to a line width of 89 nm.Problem
Transformative optics depends on the thickness of the superlens and the condition the permittivity of the silver film and that of the adjacent PMMA are equal and of opposite sign. Although a delicate resonance is essential to ensure the evanescent enhancement across the silver film, supporting calculations  assume the permittivity of silver from the literature instead of making direct measurements on the permittivity of the silver film used in the superlens experiment. In fact, all that can be confirmed with certainty is that the superlens is a 35 nm silver film. What this means is the sub-diffraction-
limited imaging observed with the superlens may have nothing to do with evanescent enhancement, but rather on another mechanism depending at least on the thickness of the silver film.Proposal
limited imaging in ibzng the super lens is proposed to be a natural consequence of QED induced EM radiation in the silver film. QED stands for quantum electrodynamics and EM for electromagnetic. For PMMA illuminated with UV at 365 nm, the diffraction-
limited wavelength P* for resolving periodic structures  is, P* = 243 nm, below which periodic spaced structures cannot be resolved. Prior to reaching the photoresist, the EM radiation constituting the diffraction-
limited image P*is absorbed in the silver film and induced by QED to create EM radiation at wavelength λ = 2 nd, where n is the refractive index of silver at wavelength P* and d is the silver film thickness. For d = 35 nm and silver at 243 nm  having n = 1.28, QED allows sub-diffraction-
limited resolution at λ = 89 nm consistent with that shown in the thumbnail.Theory
QM as described by the Planck law precludes the atoms in nanostructures from having the heat capacity to conserve absorbed EM radiation from any source by an increase in temperature. QM stands for quantum mechanics. Indeed, the heat capacity of the atom naturally vanishes under high EM confinement because of the high surface-to-volume ratios inherent in nanostructures, i.e., absorbed EM radiation from the diffraction-
limited PMMA image is almost totally confined to the surfaces of the silver film. Interior atoms between the surfaces separated by distance d are then placed under high EM confinement at nanoscale wavelengths that by the Planck law preclude atomic heat capacity. Conservation of absorbed EM radiation therefore proceeds by the creation of non-thermal QED induced EM radiation standing across the silver film thickness at wavelength λ = 2 nd, where n is the refractive index of silver at the wavelength of the incident EM radiation. Hence, QED in the silver film induces the conversion of EM radiation emitted from the diffraction-
limited image in the PMMA to EM radiation of sub-diffraction-
limited quality, where n is the refractive index of silver at 243 nm. QED applications are numerous and diverse, e.g., See http://www.nanoqed.org
, 2010 - 2015.Discussion
In transformative optics, near-field imaging with the superlens to sub-diffraction-
limited resolution requires nanoscale thin films < 100 nm necessary for significant enhancements of evanescent fields. However, experimental confirmation of evanescent fields as the mechanism of enhanced image quality is not yet fully demonstrated with permittivity measurements of the silver film used in the superlens. What this means is other mechanisms may explain the observed enhancement of image quality.
In this regard, QED induced enhancement of diffraction-
limited images in superlens thicknesses < 100 nm is proposed. QED does not rely on evanescent fields, but rather is a consequence of the size effect of QM that precludes conservation of EM radiation from the diffraction-
limited image in the PMMA at wavelength P* by an increase in silver film temperature. Hence, QED conserves the EM radiation from the diffraction-
limited P*image to shorter EM radiation at wavelengths < P* that enhance the quality of the diffraction-
limited image. However, the lens material is required to be absorptive at the wavelength P* of the PMMA image. Noble metals are therefore preferable, but not necessary.References
 J. Pendry, “Controlling Light on the Nanoscale, “ Progress In Electromagnetics Research, Vol. 147, 117-126, 2014.
 N. Fang, et al., “Sub–Diffraction-
Limited Optical Imaging with a Silver Superlens,” Science, Vol. 308, 534-537, 2005.
 D. Melville and R. Blaikie, “Super-resolution imaging through a planar silver layer,” Optics Express, Vol. 13, 2127. 2005.
 P. Winsemius, et al., "Temperature dependence of the optical properties of Au, Ag and Cu," J. Phys. F: Met. Phys., Vol. 6, 1583, 1976.