Engineering transmitted light speckle statistics with field-tunable magnetic nanofluid scatterers

The active manipulation of electromagnetic wave scattering cross-sections within disordered media is crucial for understanding mesoscopic physics. An adaptable strategy for exploring the parameter space in order to control the scattering cross-section of such systems involves employing external field-tunable disorder. In this study, we utilize magnetic nanofluid as an in-situ tunable scattering medium, whose scattering parameters like scatterer number, density, size, and shape can be adjusted by an external magnetic field which is administered either constantly or with varying ramp rates. Our study on the transmitted light and the associated speckle statistics reveals intriguing behavior in response to changes in the strength and duration of the applied magnetic field. We observe that this tunable system remains within the weak scattering regime across various scattering scenarios: Rayleigh (for scatterer sizes smaller than the light wavelength, λ), Mie (when scatterer size approaches λ), and Geometric (for scatterer sizes exceeding λ) under both constant and ramped magnetic field application. Further, the transmitted speckle patterns deviate from Rayleigh behavior, with speckle contrast values increasing over time until it reaches a saturation point. However, the speckle patterns display intriguing dynamics with faster ramp rates resulting in larger speckle contrast values. This phenomenon arises from the inability of equilibrium field-induced structures to form rapidly under faster ramp rates, leading to a broader size distribution of scatterers and consequently a wider range of phase differences among scattered light components. Our study illuminates the dynamic behavior of wave transport through a tunable disordered media and underscores the potential of magnetic nanofluid as a versatile tool for controlling mesoscopic transport.