The influence of hole shape on the nonlinear optical properties of metallic subwavelength hole arrays is investigated. It is found that the amount of second harmonics generated can be enhanced by changing the hole shape. In part this increase is a direct result of the effect of hole shape on the linear transmission properties. Remarkably, in addition to enhancements that follow directly from the linear properties of the array, we find a hot hole shape. For rectangular holes the effective nonlinear response is enhanced by more than 1 order of magnitude for one particular aspect ratio. This enhancement can be attributed to slow propagation of the fundamental wavelength through the holes which occurs close to the hole cutoff. Metal nanostructures are known to greatly enhance electromagnetic fields in certain geometries. Therefore they can also boost nonlinear optics. Surface-enhanced Raman scattering (SERS) [1], for instance, makes use of nobel metal nanoparticles to amplify the spectroscopic Raman signature of even a single molecule [2]. Sharp nanosize tips [3] provide a more controlled route to enhance the optical field. Another powerful example of nonlinear enhancement is a single circular hole, surrounded by an ordered plasmonic structure [4]. In addition to the use of single structures, ensembles of multiple nanostructures or extended metal objects can also be used to induce nonlinear enhancement randomly positioned metal nanoclusters show nonlinear effects [5]. A popular example of multiple nanostructures acting together is a subwavelength hole array which exhibits extraordinary optical transmission [6] in the linear regime. Calculations indicate that the local field enhancement associated with the transmission through the holes is exceptionally large [7]. Blair and co-workers showed that second-harmonics generation (SHG) on a hole array is possible [8].

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Carbon deposition on extreme ultraviolet (EUV) optics was observed due to photon-induced dissociation of hydrocarbons in a EUV lithography environment. The reflectance loss of the multilayer mirror is determined by the carbon layer thickness and density. To study the influence of various forms of carbon, EUV-induced carbon, hot filament and e-beam evaporated carbon were deposited on EUV multilayer mirrors. Spectroscopic ellipsometry was used to determine the carbon layer thickness and the optical constants ranging from ultraviolet to near infrared. The carbon density (and thus reflectance loss) was determined from the optical constants using both Bruggeman's effective medium approximation and the Clausius–Mosotti equation. Both approaches result in a similar EUV reflectance loss, with an accuracy of about 4%. The application of this process to ultrathin carbon films is further discussed.

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