Soft-matter structures: From switchable diffraction gratings to active plasmonics

We present a review of a series of nano/micro, soft-matter-based, periodic, composite structures, along with a theoretical model, which explains their main physical and optical features as determined by a suitable choice of the values of given physical and geometrical parameters. The historical development is illustrated by starting from the fabrication technique, which enables obtaining a structure made of films of well-aligned Liquid Crystal alternated to polymer slices. Realised samples, which exhibit a spatial periodicity in the range 0.2-15 μm, are given the generic name POLICRYPS (as an acronym of POlymer LIquid CRYstal Polymer Slices) and are obtained by curing a homogeneous mixture of mesogenic material, monomer and curing agent under suitable conditions and by means of a UV radiation. A number of interesting applications are determined by the possibility of tuning, or even switching on/off, the spatial modulation (from polymer to LC) of the refractive index of these structures; this effect of tuneability can be obtained by applying an electric field of few V/μm or, in some cases, by irradiating the sample with a light beam of suitable wavelength. We show that, depending on the used geometry, these particular structures can be exploited as switchable diffraction gratings, tuneable beam splitters, switchable optical phase modulators, tuneable Bragg filters, soft matter templates for aligning different kinds of LCs, arrays of tuneable microlaser, or can enable realising "active plasmonics" devices.