Resonant Diffraction Into Symmetry-Prohibited Orders of Metal Gratings

We study, by numerical simulations and experimentally, the resonant diffraction by metal (Ag) gratings in the mid-IR (~10 μm) spectral range. The excitation of surface plasmon-polaritons facilitates narrow-band resonantly enhanced diffraction into symmetry-prohibited diffraction orders. We show that, even though the propagation losses for plasmonic modes are greatly reduced at longer wavelengths, the magnitude of the resonant diffraction peak remains limited due to diffraction into other, normally allowed, diffraction orders. The grating depth dependence of the resonant diffraction spectra indicates that with shallower gratings, the Q-factor of the resonance may become as large as 10³-10⁴, while the magnitude of the peak is reduced.