Polarimetric Study of an Embeddable, Flexible, Textile-Compatible Ferromagnetic Microfiber Platform

In the information age, much effort and attention remain focused on the processing and transmission of information. Of equal importance is data acquisition and execution, which is made even greater with today’s big data and AI. In this work, we present an embeddable, flexible, wearable, and scalable sensing and actuation platform for monitoring, communicating physical and chemical conditionals, as well as extracting/delivering molecular substances. The platform is based on a novel ferromagnetic microfiber and can be configured into various embedded structures such as 2D arrays and 3D stacked sheets,<br/>It offers many degrees of freedom to exploit, such as electrical, magnetic, and optical activities. The microfiber itself consists of a 30 μm diameter conductive ferromagnetic alloy (CoFeSiB) core, encased in a 10 μm thick borosilicate glass (Schott Duran) shell. This choice of materials and feature sizes allows us to exploit magnetic and electrical degrees of freedom, and strong polarization dependent electromagnetic activity that can be embedded in coating layers, fabrics and textiles and then read-out and/or activated from remote, over a broad spectral band from RF to visible to deep infrared while maintaining flexibility, chemical stability, cost effectiveness, and manufacturing lengths up to kilometers. In addition to individual fibers or their random ensembles, we can also structure them into 2-/3-dimensional arrays of these fibers (e.g. photonic band-gap engineering) to exploit their collective optical activities or functionalities, which are therefore variable via their spacings, fillings, and externally applied electric and/or magnetic fields.<br/>Applications of these tunable degrees of freedom include but are not limited to sensing, actuation, remote-tracking, and thermal property management. Some configurations would also allow for delivering payloads (e.g., drug) via inductive heating of the conductive ferromagnetic core combined with thermal expansion and the Laplace force.<br/>As an example, we report on a polarimetric study of a 1-D array of parallel microfibers, embedded between two PET double-sided acrylic adhesive layers, that are invisible to human eyes and indiscernible by touch. A polarimetric reflectance spectra, however, can read out this array and determine its structure features (e.g. optical thickness, orientation, and effective spacing). The experiment was conducted over the 600~1750 nm spectral range with a polarized broadband (Halogen) light source. To isolate the optical activity of the fibers from their ‘concealing cover’ (PET), the normalized reflectance spectra was subtracted from a control spectra of the PET films. The resulting spectra revealed a polarization-dependent cavity effect. When the polarization was both parallel and rotated 45° counterclockwise to the longitudinal axis of the fibers, we observed a free spectral range of 7.14 THz (optical thickness ~21 μm). When the polarization was both orthogonal and rotated 45° clockwise from the longitudinal axis of the fibers, we observed a free spectral range of 7.89 THz (optical thickness ~19 μm). The measured effective optical thickness of the array is consistent to the ~200 μm periodic spacing of the array. But the polarization preference off of the principal axes is curious. One contributor is the anisotropy of the PET films, resulting from a residual strain from the rolling and pressing process during manufacturing. Since the fiber spectrum was isolated from the control, the measurements also suggest an additional and likely mesoscopic physical origin (e.g. the ferromagnetic alloy grain/domains). However, the current 1D ferromagnetic array at a layer thickness of 50 μm already shows promise for a wide range of sensing and photonic management applications.