I have applied FEM based numerical techniques to the design of several bi-periodic structures realizing various filtering functions in the infrared. I studied their spectral properties as a function of opto-geometrical parameters in order to achieve tunability.
Finally, two types of filters have been proposed and fabricated, and I took part in the thin film deposition and photolithography processes. I experimentally characterized the samples by Fourier Transform InfraRed (FTIR) spectroscopy. Experimental results were in good agreement with the different numerical approaches.
Bandcut filters in reflexion
We performed a modal analysis of metal–insulator–metal (MIM)-based metamaterials in the far infrared region. These structures can be used as resonant reflection bandcut spectral filters that are independent of the polarization and direction of incidence. We showed that this resonant reflection dip is due to the excitation of quasimodes (modes associated with a complex frequency) leading to quasi-total absorption. We have fabricated large area samples made of chromium nanorod gratings on top of Si/Cr layers deposited on silicon substrate. Measurements by Fourier transform spectrophotometry show good agreement with finite element simulations. A quasimodal expansion method has been applied to obtain a minimal resonant model that fits well full wave simulations and that highlights excitation conditions of the modes.
B. Vial, G. Demésy, F. Zolla, A. Nicolet, M. Commandré, C. Hecquet, T. Begou, S. Tisserand, S. Gautier, and V. Sauget. Resonant metamaterial absorbers for infrared spectral filtering: quasimodal analysis, design, fabrication and characterization. J. Opt. Soc. Am. B 31.6, pp. 1339–1346, June 2014. DOI | arXiv | download
Bandpass filters in transmission
The diffractive behavior of arrays of square coaxial apertures in a gold layer is studied. These structures exhibit a resonant transmission enhancement that is used to design tunable bandpass filters for multispectral imaging in the 7–13 μm wavelength range. A modal analysis is used for this design and the study of their spectral features. Thus we show that the resonance peak is due to the excitation of leaky modes of the open photonic structure. Fourier transform infrared (FTIR) spectrophotometry transmission measurements of samples deposited on Si substrate show good agreement with numerical results and demonstrate angular tolerance of up to 30 degrees of the fabricated filters.
A complete sensitivity analysis allows us to evaluate acceptable opto-geometric tolerances of fabrication and thus to improve reproducibility on large areas. The impact of fabrication defaults (rounded corners, aperture anisotropy, aperture edge roughness, sloping aperture edges) on the filtering performances is analyzed. The simulations of realistic structures allow to explain and reduce the differences between measured and simulated spectra.
B. Vial, F. Bedu, H. Dallaporta, M. Commandré, G. Demésy, A. Nicolet, F. Zolla, S. Tisserand, and L. Roux. Transmission enhancement through square coaxial apertures arrays in metallic film: when leaky modes filter infrared light. Opt. Lett. 39.16, pp. 4723–4726, Aug. 2014. DOI | arXiv | download
M. Commandré, B. Vial, S.Tisserand, L. Roux, H. Dallaporta, F. Bedu, G. Demésy, A. Nicolet and F. Zolla. Design, fabrication and characterization of resonant metamaterial filters for infrared multispectral imaging. Thin Solid Films, Vol. 592, Part B, pp. 296-304, Oct 2015. DOI | download