“…Ferrofluids can easily be manipulated by an external magnetic field, and thus they can be used for precise drug delivery and in hyperthermia applications (see, e.g., [5,6] and references therein). They also have large potential for applications in optical devices [7], such as tunable beam splitters [8], tunable gratings [9], interferometers [10], and optical logic devices [11]. They are also used in fiber optics including optical fiber modulators, optical filters, and fiber sensors, in which the ferrofluid is used as a fiber cladding or coating [12,13].…”