A hybrid tunable THz metadevice using a high birefringence liquid crystal

Abstract

We investigate a hybrid re-configurable three dimensional metamaterial based on liquid crystal as tuning element in order to build novel devices operating in the terahertz range. The proposed metadevice is an array of meta-atoms consisting of split ring resonators having suspended conducting cantilevers in the gap region. Adding a "third dimension" to a standard planar device plays a dual role: (i) enhance the tunability of the overall structure, exploiting the birefringence of the liquid crystal at its best, and (ii) improve the field confinement and therefore the ability of the metadevice to efficiently steer the THz signal. We describe the design, electromagnetic simulation, fabrication and experimental characterization of this new class of tunable metamaterials under an externally applied small voltage. By infiltrating tiny quantities of a nematic liquid crystal in the structure, we induce a frequency shift in the resonant response of the order of 7-8% in terms of bandwidth and about two orders of magnitude change in the signal absorption. We discuss how such a hybrid structure can be exploited for the development of a THz spatial light modulator.

Figure 1. Two-dimensional metamaterial.

(a) 3 × 3 array based on “four gap” SRR unit cells. Geometrical parameters are: unit cell size 50 μm, ring side 40 μm, ring width 5 μm, gap 7 μm. Metal thickness is 200 nm. (b) Simulated transmission response for the device with polarized (black curve) and unpolarized (red curve) LC. (c) Electric field distribution on the array surface (planar view) and in the gap region (transverse view) respectively. Intensity values are given in a pseudocolor scale.

Figure 2. Three-dimensional metamaterial.

(a) 3 × 3 array based on “four gap” SRR unit cells with stand up capacitors. Geometrical parameters in the plane are the same as in Fig. 1. At the centre of each gap, there is a pedestal with a rectangular 2.5 μm × 7 μm shape, and two cantilevers forming the suspended structure with 12 μm overall length and 600 nm thickness. The vertical gap distance is 400 nm. (b) Simulated transmission response for the device with polarized (black curve) and unpolarized (red curve) LC. (c) Electric field distribution on the array surface (planar view) and in the gap region (transverse view) respectively. Intensity values are given in a pseudocolor scale.

Figure 3

(a) Transmission spectra of the hybrid metamaterial measured at zero bias (state OFF, full black square points) and at 10 V (state ON, open red square points). Dashed curves refer to the results of simulations (OFF, black line; ON, red line), assuming a LC not perfectly aligned with the THz field in the unpolarized state and with a complex anisotropic dielectric permittivity (see text for details). Dashed vertical lines highlight the frequency blue shift switching the metadevice from OFF to ON. (b) The measured percentage modulation factor as a function of frequency in the interval 1.0–1.2 THz.

Figure 4

Schematic of the main fabrication steps: (a–c) base electrode; (d–f) pedestal; (g–i) cantilevers; (j) final vertical armature.

Figure 5. Details of the fabricated meta-device.

SEM magnification of (a) the mushroom-shaped structure formed by two adjacent cantilevers on each side of the gap and (b) a single cantilever. (c) Optical image (×100) of the final hybrid meta-device after LC infiltration (and without the cover PET/ITO electrode).

Figure 6. Sketch of the experimental set-up used to test the tunability of the metamaterial response.