Reconfigurable Bragg gratings for tunable all-optical filter

This work proposes what we believe to be a novel reconfigurable Bragg gratings composed of a gold nanoblock array and an indium tin oxide (ITO) waveguide, which overcomes the inherent obstacles of restricted tunability and low integration density in conventional all-optical filters. An external pump beam selectively excites localized surface plasmons (LSPs) in the gold nanoblock array, which induces a periodic electron concentration distribution in the ITO waveguide and forms two reconfigurable Bragg gratings via the concomitant refractive index modulation. The modulation depth of refractive index is controlled by varying the excitation efficiency of LSPs, while the spatial period of the refractive index distribution is tailored by adjusting the nanoblock array layout. This dual tunability facilitates efficient all-optical filtering of propagating surface plasmon polaritons in the ITO waveguide when the grating is rendered reconfigurable. Numerical simulations show that the proposed filter, with a compact footprint of 8 µm × 6 µm × 4 µm, achieves excellent performance: a minimum 3-dB bandwidth of 80 nm, a wavelength tuning range of 140 nm, a modulation depth of 78.3 dB, and an electric field stabilization time of 84.40 fs. This work not only provides what we believe to be a new route for dynamically tunable all-optical filtering but also offers critical support for advancing the integration and intelligence of next-generation ultrafast all-optical networks.