Multichannel remote polarization control enabled by nanostructured liquid crystalline networks

Abstract

In this article we demonstrate that a grating fabricated through nanoscale volumetric crosslinking of a liquid crystalline polymer enables remote polarization control over the diffracted channels. This functionality is a consequence of the responsivity of liquid crystal networks upon light stimuli. Tuning the photonic response of the device is obtained thanks to both a refractive index and a shape change of the grating elements induced by a molecular rearrangement under irradiation. In particular, the material anisotropy allows for nontrivial polarization state management over multiple beams. Absence of any liquid component and a time response down to 0.2 milliseconds make our device appealing in the fields of polarimetry and optical communications. * email: simone.zanotto@nano.cnr.it MANUSCRIPT ACCEPTED BY APPLIED PHYSICS LETTERS [ 114, 201103 (2019) ]; https://doi.org/10.1063/1.5096648dictated by the material. Photonic nanostructures solve this issue, as they allow to obtain optical properties on demand by appropriate subwavelength patterning of dielectric or metallic materials [9]. By these means it is possible to implement very general polarization response from isotropic materials [10][11][12][13], and to develop division-of-amplitude static polarimeters which show unprecedented speed and compactness [14,15]. Moreover, these operations can be reconfigured statically or dynamically by exploiting thermo-optic, electro-optic, electrochromic, or optomechanic effects [16][17][18][19][20][21][22][23][24][25]. While several proposed devices rely on plasmonic elements to exploit field enhancement effects, dielectric photonic structures are also of great interest because they are almost lossless. In this view, a new class of material, named Liquid Crystalline Networks (LCNs), has acquired a lot of attention within the photonic community due to its unique properties. In fact, these materials exhibit low optical loss, intrinsic optical anisotropy, machinability to subwavelength precision, and tunability. LCNs are polymeric networks where liquid crystalline chains are interconnected, giving rise to an elastomeric material [26,27] that sustains different phase transitions. The materials employed in the present work -i.e., polyacrylates -show a nematic-paranematic transition, across which birefringence, stress, and strain change, with a direct impact on the photonic properties of a LCN nanostructure [28,29]. While certain photonic devices based on nanostructured LCN have been already demonstrated [30][31][32][33][34][35][36], our aim is here to illustrate a different application: a remotely controllable multichannel polarization conversion element. It is indeed the important feature of remote, non-invasive tuning that makes our device a significant advancement with respect on previous findings on multichannel photonic components [37].

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