Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-6-8900
Timestamp: 2019-04-21 09:00:41+00:00

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Transmission optical diffraction gratings composed of periodic slices of a ferromagnetic liquid crystal and a conventional photoresist polymer are demonstrated. Dependence of diffraction efficiencies of various diffraction orders on an in-plane external magnetic field is investigated. It is shown that diffraction properties can be effectively tuned by magnetic fields as low as a few mT. The tuning mechanism is explained in the framework of a simple empirical model and also by numerical simulations based on the rigorous coupled wave analysis (RCWA). The obtained results provide a proof of principle of operation of magnetically tunable liquid crystalline diffractive optical elements applicable in contactless schemes for control of optical signals.
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Fig. 1 (a) Schematic drawing of the polymeric grating structure fabricated by a TPP-based direct laser writing technique. (b) Polarization optical microscopy (POM) image of the grating structure with Λ = 20 μm that is filled with a ferromagnetic LC material. Arrow-ended lines in the top left corner indicate the orientations of the polarizer (P) and the analyzer (A). (c) Position of the sample between the two poles of electromagnet core.
Fig. 2 Polarization optical microscopy (POM) image of a ferromagnetic LC-filled grating structure with a grating period of Λ = 5 μm (a) at zero magnetic field and (b) at a magnetic field of B = 57 mT. The field was oriented at 45° deg with respect to the grating planes. Arrow-ended white lines denote the orientations of the polarizer (P) and the analyzer (A). Yellow arrows indicate the coordinate axes used in the theoretical description. The insets in the lower left corners indicate orientation of the LC molecules. Red squares denote the region of interest (ROI) that was selected for analysis of the average grayscale level of the image. (c) Average grayscale level in the selected ROI as a function of the magnetic field B. Full circles: values obtained for increasing field, open circles: values obtained for decreasing field. Practically no hysteresis is observed.
Fig. 3 Schematic drawing of the diffraction experiment. A linearly polarized laser beam with either s or p polarization direction enters the sample at normal incidence with respect to the grating plane. The intensities of the 0th, + 1st and + 2nd diffraction orders are measured. In the top right corner, the far field diffraction patterns at B = 0 are shown for s and for p polarized light, respectively.
Fig. 4 Diffraction efficiencies of different diffraction orders as a function of an applied magnetic field (a) for an s-polarized beam and (b) for a p-polarized beam.
Fig. 5 Time dependence of diffraction efficiencies of different diffraction orders after switching on and switching off a magnetic field of 35 mT (a) for an s-polarized beam and (b) for a p-polarized beam.
Fig. 6 Calculated diffraction efficiencies of the 0th and the 1st diffraction orders as a function of the rotation angle β obtained from Eq. (4). (a) Results obtained for an s-polarized beam and (b) for a p-polarized beam. The definition of β is shown Fig. 3. The value β = 0 corresponds to n pointing along the grating planes, i.e. along the y axis.
Fig. 7 Schematic drawing of the unit cell used for the RCWA simulations with the S4 solver.
Fig. 8 Diffraction efficiencies of the 0th, the 1st and the 2nd diffraction orders as a function of rotation angle β obtained by numerical calculation of the electromagnetic field propagation in a one-dimensional grating structure composed of the unit cells shown in Fig. 7. (a) Results obtained for an s-polarized beam and (b) for a p-polarized beam. The definition of β is shown Fig. 3. The value β = 0 corresponds to n pointing along the grating planes, i.e. along the y axis.

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