Broadband all-optical modulation using iron-oxide-nanoparticle-functionalized microfiber resonators

We propose and experimentally demonstrate broadband, in-line all-optical modulation of a microfiber knot resonator (MKR) coated with iron oxide nanoparticles (IONPs). The resonance wavelength of the MKR coated with IONPs can be tuned using non-resonant pump lasers (at 690, 980 and 1562 nm). Owing to the enhanced light-matter interaction enabled by the broadband optical absorption and strong photothermal effect of IONPs, the IONP-functionalized MKR enables broadband, in-line all-optical modulation covering the visible to near-infrared band. The modulation efficiencies of 25 pm/mW, 21 pm/mW, and 17 pm/mW are achieved when the pump wavelengths are 690 nm, 980 nm, and 1562 nm, respectively. A maximum tuning range of 0.53 nm has been reached, exceeding one free spectral range, and the measured rise/fall time is ∼780/480 μs following the 10%-90% rule. This device features small footprint, high modulation efficiency, low insertional loss, broadband operation, and rapid response time, demonstrating promising application potential in all-optical modulation, optical filtering, and fiber sensing.