There have been extensive research activities on functionalizing metamaterials by combining switchable functionalities with metamaterial leading to the concept of metadevices [1]. One simple example is to control metaresonances by external means such as light, electric field, magnetic field and so on. Our previous work has reported an optical switching of near-IR light transmission resonance in metamaterial-liquid crystal cell, where a positive meta-structure possessing two orthogonal polarization-dependent resonances was employed [2]. Recently, an electric switching of light transmission has been demonstrated in a similar metamaterial-liquid crystal cell [3, 4]. Noting that a liquid crystal display can be configured by electric switching of either transmission or reflection of lights, we look into how metamaterial can be incorporated in a twisted nematic liquid crystal cell to control an optical reflection. When incorporated in a nematic liquid crystal cell, a positive meta-structure is useful to control an optical transmission, and a negative meta-structure can be utilized more readily in a reflection control. Furthermore, an advantage of the negative meta-structure in constructing an electro-optic controllable liquid-crystal cell lies in that the large area of metal surface serves as an electrode necessary for application of an external voltage. In this work, we demonstrate an electric switching of near-IR reflection resonances in a reflective metamaterial twisted nematics cell by incorporating a negative meta-structure possessing two orthogonal polarization-dependent resonances [5].

Figure 1 shows how an electric switching of optical reflection operates in a reflective meta-material twisted nematic liquid crystal cell (RMNLC). In a twisted nematic liquid crystal cell incorporating a negative meta-structure as the bottom alignment layer, the polarization direction of an incident beam follows the director of nematics, and twisted nematic liquid crystal acts as a polarization rotator [6]. In the absence of an external voltage as shown in Fig. 1(a), the vertical polarization of the incident beam follows the twisting of nematics director, being converted to the horizontal polarization through a 90° rotation, and the incident beam experiences the horizontal reflective resonance upon reflecting from the metamaterial bottom alignment layer and reflects back to the top transparent alignment layer with the polarization recovered back to the original vertical polarization. We note the reflected beam is the phase conjugate of the incident beam [7]. When an external voltage above the threshold voltage for nematics tilting is applied across the cell as shown in Fig. 1(b), the twisting of nematics director is destroyed. The vertical polarization of the incident beam is maintained throughout reflection, and the incident beam experiences the vertical reflective resonance upon reflecting from the metamaterial bottom alignment layer and the reflected beam is vertically polarized. That is, for a vertically polarized incident beam, the reflection spectrum of the reflected beam depends on whether an external voltage is on or off. In other words, an electric control of the reflection spectrum can be achieved in an RMNLC.

 

Fig. 1 Schematics of a reflective metamaterial twisted mematics cell are shown, when (a) no external voltage is applied and (b) an AC voltage is applied. Vertical polarization of an incident beam experiences different reflective resonances upon reflecting from the metamaterial bottom alignment layer.

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We design a reflective metamaterial possessing two orthogonal reflective resonances. In order to facilitate a switching between two meta-resonances at λ_S and λ_L in near-IR spectral range, a nano-sized double-split ring resonator (DSRR) possessing two-fold rotation symmetry is adopted as constituents of the reflective metamaterial. While a positive DSRR exhibits two orthogonal polarization-dependent absorption resonances, two orthogonal polarization-dependent reflection resonances exist in a negative DSRR, the polarization dependence being complementary each other as described by Babinet principle [2, 5, 8]. Figure 2(a) shows schematics of the reflective metamaterial with nano-sized DSRR apertures, with the outer and inner radii of aperture R = 100nm and r = 75nm and the lattice constant p = 300nm and the gap size of split ring g = 40nm. Reflection spectrum simulated by FDTD is shown in Fig. 2(b). Two reflection resonances are located at λ_S= 750nm and λ_L=1000nm.

 

Fig. 2 (a) Schematics of metamaterial with nano-sized double-split ring resonator apertures is shown. (b) FDTD simulated reflection spectrum is shown. (c) SEM image of metamate-rials fabricated by an FIB milling is shown. (d) Measured reflection spectrum of metamate-rial is shown. Red and black curves correspond to the spectrum for horizontal and vertical polarizations of a linearly polarized incident beam, respectively.

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Fabrication of the reflective metamaterial is carried out by an FIB milling. Once 30nm-thick Au film is e-beam evaporated on a 1mm-thick fused quartz substrate deposited with 3nm-thick Ti adhesion layer, an FIB milling is performed. With the accelerating voltage fixed at 30kV, nano-sized DSRR array was successfully milled at the ion beam current 1.5pA with milling depth setting of 40nm as can be seen in the SEM image of Fig. 2(c). By use of a micro-spectrophotometer, the reflection spectra are measured for horizontal (θ = 0°, red curve) and vertical (θ = 90°, black curve) polarizations of a linearly polarized incident beam, as shown in Fig. 2(d). When compared with the FDTD simulated reflection spectra of Fig. 2(b), two reflection resonances are quite broadened with center wavelengths λ_S= 750nm and λ_L= 1000nm. In an FIB milling, Au layers are remnant inside the arc-patterned nano-apertures, namely, the dielectric fused quartz substrate is not fully shaped as an arc pattern, which results in broadened reflective resonances of nano-sized DSRR apertures.

Before proceeding to investigate an electric switching of reflection resonances in a reflective metamaterial twisted nematics cell, we examine the behavior of voltage dependence of optical reflection in a generic twisted nematic cell. When an external voltage is applied to a twisted nematic liquid crystal cell, changes in transmission and reflection exhibit different behaviors, aside from a common threshold behavior. In a reflective twisted nematic cell, the Jones matrix analysis of complex amplitude reflectance shows that the reflective intensity does not change monotonically but goes through an initial dip in magnitude and then reaches the maximum and decreases again, upon increasing phase retardation Γ related to the difference between the ordinary index and the effective extra-ordinary index which is dependent on the tilt angle of nematics. Furthermore, the dependence of phase retardation Γ on an external voltage is nonlinear [7, 9, 10]. Therefore, a calibration is important when identifying an electric switching of optical reflection in an RMNLC.

As a control sample, we prepared a liquid crystal cell by sandwiching a rubbed polyimide(SE-5291, Nissan Chemical Industries, Ltd.)-coated ITO glass and a rubbed poly-vinyl-alcohol(PVA, M_w ≈ 205K, Aldrich)-coated fused quartz substrate deposited with 30nm-thick Au without any patterning. By capillary-filling the cell of 12μm cell gap with nemtaics, we obtained a reflective twisted nematic cell. Nematic liquid crystal is ZLI-2293 (Merck) with n_e=1.6313 and n_o=1.4990 at 589.3nm, ε_|| = 14.0 and ε_⊥ = 4.1 at 1.0kHz, possessing the clearing point 85°C. In Fig. 3(a) is shown the theoretically calculated reflection as a function of phase retardation Γ, and Fig. 3(b) shows the experimental measurement of voltage dependence of the optical reflection as well as the theoretical curve for the control sample.

 

Fig. 3 (a) Theoretical reflection intensity is plotted as a function of the phase retardation Γ. (b) Experimental measurement of the voltage dependence of reflection intensity in a generic twisted nematic liquid crystal cell, i.e., a control sample, is shown. Red curve corresponds to the theoretical reflection intensity as a function of the external voltage for the control sample.

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Now we are ready to fabricate an RMNLC as shown in Fig. 1(a). Top alignment layer is the same as that of the control sample, and bottom alignment layer is a rubbed PVA-coated reflective metamaterial, and the cell is subsequently capillary-filled with nematics. Red curve in Fig. 4(a) shows reflection spectrum for a vertically polarized incident beam in the absence of an external voltage. When compared with reflection spectrum of the reflective metamaterial, red curve in Fig. 2(d), the main spectral feature is kept unchanged except for a red-shift. Upon applying an external voltage of 10V across the cell to obtain a structure as shown in Fig. 1(b), reflection spectrum for a vertically polarized incident beam switches over to black curve in Fig. 4(a), which corresponds black curve in Fig. 2(d).

 

Fig. 4 (a) Reflection spectrum of the reflective metamaterial twisted nematic liquid crystal cell is shown for a vertically polarized incident beam in the absence (red curve) and presence (black curve) of an external voltage. (b) Changes in the reflected intensity are plotted as a function of the external voltage at wavelengths 700nm (gray square) and 1010nm (blue triangle). See text for the description of ①, ②, ③, and ④.

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Change in reflection spectrum is monitored as a function of the external voltage, and is plotted in Fig. 4(b) after a calibration to sort out a net change associated with reflection resonances of the reflective metamaterial. We pay attention to two wavelengths 700nm marked as ① and 1010nm marked as ③, where reflections are of the same level ≈60% in the absence of an external voltage. Upon increasing the external voltage, ① and ③ develop to ② and ④, respectively, each ending up at the levels of reflection of ② and ④ in Fig. 4(a). Threshold voltage of the electric switching is ≈3.5V. Three dimensional plots of reflection spectra are shown as a function of the external voltage for a vertically polarized incident beam in Fig. 5.

 

Fig. 5 Three dimensional plots of reflection spectra are shown as a function of the external voltage for a vertically polarized incident beam.

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Now we examine the speed of electric switching in an RMNLC. Switch-on and switch-off time of a twisted nematic liquid crystal cell depends on the thickness of liquid crystal cell, tilt angle of nematics, anchoring energy in the rubbed alignment layer, temperature, and so on [6]. As far as liquid crystal alignment layer is concerned, polyimide is widely employed in liquid crystal displays adopting a twisted nematic liquid crystal cell structure owing to its excellent material properties such as high optical transparency, low thermal expansion coefficient, low dielectric constant, and high glass transition temperature. A high temperature excursion necessary for imidization, typically above 250°, however, renders polyimide inadequate as alignment layer when a metamaterial with negative meta-structure is adopted, mainly due to the difference in thermal expansion coefficient between polyimide and Au thin film. PVA, on the other hand, is an aqueous polymer, and no high temperature processing is involved in preparation as an alignment layer. That is why we employed PVA in the bottom alignment layer when fabricating a reflective metamaterial twisted nematics cell. We fabricated a twisted nematics liquid crystal cell with polyimide as alignment layer in both top and bottom ITO substrate as a reference to compare the switching speed.

AC square voltage was applied across the cell in a cross-polarized configuration, and transmission intensity is monitored by an oscilloscope. Photographs of oscilloscope traces are shown in Fig. 6. The upper channel, CH1 (upper trace), shows the external voltage in a 5V/div scale, while the lower channel, CH2 (lower trace), shows the voltage across the load in photodiode detecting transmitted light intensity in a 5mV/div scale. Figures 6(a) and 6(b) correspond to the reference sample and the RMNLC, where switch-on and switch-off times are found to be 20ms and 2ms in (a) and 100ms and 10ms in (b), respectively.

 

Fig. 6 Oscilloscope traces are shown in the measurement of switch-on and switch-off time in (a) the reference sample and (b) the reflective metamaterial twisted nematics cell.

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In summary, we experimentally demonstrated an electric switching of reflection resonances in a reflective metamaterial twisted nematic liquid crystal cell by incorporating a negative meta-structure possessing two orthogonal polarization-dependent resonances. Switching operation in reflection provides a new device architecture when metamaterials with near-IR resonance are adopted in fiber-optic applications.