An antireflective silicon grating working in the resonance domain for the near infrared spectral region

We have designed and fabricated a silicon grating which shows antireflection properties, in normal incidence, in the [4μm; 6μm] spectral region. This configuration is of utmost importance for stealth applications when laser telemeters are used to localize moving targets. It is shown that although the grating parameters are outside the validity domain of diffraction homogenization theories, the opto-geometrical parameters given by the effective medium theory can be used as starting solutions for designing the structure. This allows the use of a standard low-resolution etching technique for fabricating the grating. The reflectance is calculated with a modal method and compared successfully with the experimental results. It is shown that the grating reduces the silicon substrate reflectance in the whole [4μm; 6μm] spectral domain by a factor greater than 10.