P. Perrier, D. Lepoittevin, L. Delrey, S. Pairis, T. Fournier et al., We thank A. Anane, J. Grollier and R. Mattana for experimental help and useful discussions. We thank the European Synchrotron Radiation Facility (ESRF), and in particular the staff of beamline ID08, where several preliminary experiments were carried out. Nanofabrication was performed at the 'Plateforme de Technologies Avancées' and at the Institut Néel/CNRS 'Nanofab' facility, both in GrenobleC. acknowledge financial support through projects HF2007- 0071, S2009/MAT-1726, and CSD 2007-00010. V.U. was financially supported by grants No. MSM0021630508, No. KAN400100701 and No. 2E13800101-MSMT, and by the project INGO No. LA287 of the Czech Ministry of Education. This work was partially supported by the ANR-07-NANO-034 'Dynawall

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