Near infrared light absorption in magnetic nanoemulsion under external magnetic field

We study the magnetic field dependent near infrared photon absorption behavior in a magnetically polarizable oil-in-water emulsion of droplet radius ~110 nm. The absorption of near infrared photons in magnetic nanoemulsion is found to be dependent on the volume fraction and applied magnetic field, which is attributed to the variation in the Mie absorption efficiency during the structural transitions of nanoemulsion droplets in dispersion. Also, the absorption linearly increases with incident near infrared photon energy up to certain external magnetic field. The imaginary part of the refractive index (k1) of magnetic nanoemulsion obtained from the near infrared absorption profile in the Rayleigh regime is found to vary with external magnetic field and the sample volume fraction (ϕ). The measured k1 follows a power law increment with sample volume fraction (k1~ϕp, where p is the exponent). The exponent (p) decreases with external magnetic field implying that the structural transition of nanoemulsion droplets increases k1. After a critical magnetic field (beyond Rayleigh regime), field induced absorption of near infrared photons decreases because of the increase in the aspect ratio of the chain like aggregates and interchain spacing which in turn reduces the Mie absorption efficiency.