Characterization of laser-induced optothermal effect in magnetic fluids based on a coreless-fiber-coupled microcavity Mach-Zehnder interferometric sensor

Abstract

We present an integrated coreless-fiber-coupled microcavity Mach-Zehnder interferometric sensor to investigate the optothermal effect of magnetic fluids in the near-infrared wavelength region. The Brownian motion of magnetic nanoparticles and solvent molecules intensifies along with the change of medium refractive index under 473 nm laser illumination, and consequently the interference wavelengths are sensitive to the applied laser power. The maximum wavelength tuning sensitivity is up to 4.7475 nm/(mW/mm2) at a microcavity length of 262.26 µm. Based on interferometric spectral analysis, the refractive index sensitivities of the magnetic fluids are found to be about −3.0909 × 10-4 RIU/(mW/mm2) and −3.3319 × 10-4 RIU/(mW/mm2) for the laser power increase and decrease processes, respectively. The slight difference between the refractive index sensitivities could be attributed to the accumulation of laser energy and Sorét effect. The work presented in this paper provides important references for understanding the optothermal mechanism of magnetic fluids and the design of magnetic-fluids-functionalized laser-controlled photonic devices.