A metamaterial-liquid crystal cell structure is fabricated with the metamaterial as one of the liquid crystal alignment layers. Nano-sized double-split ring resonator in the metamaterial accommodates two distinct resonances in the near infrared regime. By adopting an azo-nematic liquid crystal in a twisted nematic liquid crystal cell structure, a photo-isomerization process is utilized to achieve an optical switching of light transmissions between two resonances. A single device of the metamaterial-liquid crystal cell structure has a potential application in the photonic switching in optical fiber telecommunications.
© 2010 OSA
Control and tuning of the metamaterial electromagnetic response are gaining research interests as a natural research development to attain a functionality in metamaterials . In the micron- and millimeter-size patterned metamaterials operating in THz and GHz regime, a variety of means have been adopted to control the characteristic metamaterial resonances, such as temperature, electric field, and light. By use of these external variables, the dielectric functions or the geometric structures of the metamaterials are tuned in a controllable way. Examples include superconducting metamaterial , a bimaterial anisotropic metamaterial , a memory metamaterial , a frequency-agile metamaterial , and a negative permeability metamaterial tuned electrically by nematic liquid crystals .
In a photonic device application, liquid crystal is the choice material when a control and tuning is pursued, as evidenced in wide applications to the optical display and photonic switching devices. A thermotropic liquid crystal phase transition is one example of employing liquid crystal to control the optical response, as shown in the resonance shift in the magnetic metamaterial [7, 8], where the dielectric function of liquid crystal surrounding the metatmaterial is varied by temperature to induce a change in the metamaterial resonance.
In the optical wavelength regime, there are comparatively few examples of electro-optic or all-optical control of liquid crystal structure to tune the metamaterial resonance, when we note that various photonic device schemes are available based on the optical anisotropy of mesogens of liquid crystal. In other words, the anisotropic optical response of nematic liquid crystal, characterized by the extra-ordinary and ordinary refractive indices, ne and no, can be fully utilized to attain a functional metamaterial. Furthermore, in applications of liquid crystal to the integrated optic devices, an optical control of metamaterial response is preferred.
In this work, we propose a novel scheme to achieve an optical switching of metamaterial response by adopting metamaterial-liquid crystal cell structure. First, we design a nano-sized planar metamaterial accommodating resonances in the near infrared wavelength regime, which exhibits a polarization-dependent transmission characteristics. Second, we introduce an azotwisted nematic liquid crystal cell, where the light transmission can be optically controlled via photo-isomerization process. Among the liquid crystal structures, the twisted nematics(TN) is one of the most widely adopted structures in the optical display devices for a light transmission control . By combining the two ideas, an optical switching of the near infrared light transmission is achieved.
2. Metamaterial-liquid crystal cell structure
We construct a metamaterial-liquid crystal cell structure by adopting a planar nano-structured metamaterial as one of the alignment layers in the twisted azo-nematic liquid crystal cell. See Fig. 1(a). Trans-isomer of azo-nematics is in the energy ground state, and when irradiated with an ultra-violet(UV) light, it goes through a conformational change to cis-isomer via photo-isomerization process. Upon heating, cis-isomer recovers back to trans-isomer. When the azonematics is incorporated, the resulting metamaterial-liquid crystal cell structure works as a single photonic device where the wavelength-selective light transmission can be controlled in the near infrared regime by a photo-isomerization.
Figure 1 shows the schematics of metamaterial-liquid crystal cell structure and optical switching behavior. The metamaterial was coated with a thin polyimide (SE-5291, Nissan Chemical Industries, Ltd.) alignment layer and rubbed along x-axis. After preparing the top substrate fused silica coated with the polyimide alignment layer rubbed along y-axis, a TN cell was fabricated with the 12 µm thick spacer. Azo-NLC 1005 (Beam Co.) filled the cell by capillary action to yield a metamaterial-liquid crystal cell.
For a linearly-polarized incident light with the polarization direction fixed along the alignment direction of the top fused silica substrate (y-polarized), the transmission spectrum is measured before and after UV irradiation. Before UV irradiation, the electric-field of the incident light inside the liquid crystal cell follows the director of nematics through a TN wave-guiding, and undergoes a 90° rotation by passing through the TN. Upon exiting the liquid crystal cell, the x-polarized light is incident upon the planar metamaterial, and the transmission spectrum is shown in Fig. 1(b). Upon UV irradiation, the photo-isomerization occurring in the azo-nematic LC destroys the TN structure, resulting in an isotropic structure as shown in Fig. 1(c). Hence, there results no change in the polarization direction by passing through the isotropic liquid crystal cell. That is, the exiting light has the same polarization direction as that of the incident light. Upon exiting the liquid crystal cell, the y-polarized light is incident upon the planar metamaterial, and the transmission spectrum is shown in Fig. 1(d). By use of UV irradiation only, the transmission is switched-off at 1650nm, while the transmission is switched-on at 1170nm.
3. Planar metamaterial
Let us consider the metamaterial structure in detail. Figure 2(a) shows schematics of the planar metamaterial, a square array of nano-sized double-spilt ring resonator (DSRR). The outer dimension of DSRR is D=220nm, the line width is 30 nm, and the lattice constant is A=300nm, possessing two gaps with opening size of 30nm. Standard electron-beam-lithography on a fused-silica substrate, sputtering of 5nm adhesion layer of chromium and 35nm thick gold, and lift-off process yielded a metamaterial composed of nano-sized DSRRs. Figure 2(b) shows the scanning electron microscope picture of the fabricated metamaterial, which has the total area of 500µm×500 µm consisting of about 2.8 × 10 6 individual unit meta-particles. Once the fabrication is completed, a thermal annealing at 400°C for 2 min. was carried out to stabilize the gold film structure. For a linearly polarized light normally incident on the metamaterial, Fig. 2(c) shows the transmission spectrum of the metamaterial as a function of the polarization angle θ with respect to the gap orientation. The metamaterial exhibits two pronounced absorptions at 1176nm and 1656nm for the electric fields at the angles θ = 90° and 0°, corresponding to the excitation of H- and L-oscillators indicated in Fig. 2(a), respectively.
The building block of the metamaterial is a two-dimensional DSRR meta-particle with two symmetric gap openings, which possesses the point group D 2h symmetry. A single DSRR can accommodate two orthogonal plasmonic oscillators H and L, perpendicular and parallel to the gap orientation with the high and low resonance frequencies ωH and ωL, respectively.
Since the resonances in H- and L-oscillators belong to different irreducible representations of the point group D 2h, they are excited independently by an electromagnetic wave . Hence, the induced dipole moment of a DSRR is composed of dipole moments induced in H and L, which can be described in terms of the plasmonic polarizabilities αH and αL.
Figure 2(d) and 2(e) show the finite-difference time-domain simulation of the electric field distributions of H- & L-oscillators. The electric fields are highly concentrated near the split gap areas. Figure 2(f) and 2(g) show the surface current density schematics of H-& L-oscillators, which correspond to two independent excitations. 
For a normally incident linearly-polarized light with the electric field making an angle θ with the x-axis, the induced dipole moments along x- and y-direction of the DSRR can expressed in terms of αH, αL, and the x-& y-components of electric field.
The induced dipole moment of the metamaterial is a coherent linear sum of the induced dipole moments of individual meta-particles. Therefore, the induced dipole moments oscillating at the resonance frequencies ωH and ωL are a linear sum of the induced dipole moments of N DSRRs.
We find that the two resonances of the metamaterial are excited selectively depending on the direction of electric field .
When the transmission spectra Fig. 2(c) and Fig. 1(b) and 1(d) are compared, we find that the transmission spectra of the metamaterial-liquid crystal cell structure before and after UV irradiation reproduces those of the metamaterial for θ = 0° and 90°, corresponding to the excitation of L- and H-oscillators, respectively. In other words, the light transmission is optically controlled with the wavelength selected.
The advantage of split-ring shaped oscillator is two-fold. First, the separation of two spectral positions of plasmonic resonance ωH and ωL is ≈0.3eV, which permits an optical switching between ωH and ωL with a single near infrared white illuminator light source. Second, the ring shaped oscillator allows a continuous variation of absorption as the polarization direction is rotated.
4. Photo-isomerization in twisted-azo-nematic LC cell
Next, we look at the photo-isomerization process in the azo-nematic LC cell. To examine the nematic-isotropic phase transition induced by a trans-cis photo-isomerization [12–14], a standard TN liquid crystal cell is fabricated by filling the azo nematic liquid crystal (Azo-NLC 1005, Beam Co.) through a capillary action into a 12-µm-thick liquid crystal cell. The inner surfaces of two fused quartz substrates are coated with the polyimide alignment layer and rubbed.
In the TN cell, the rubbing directions of two alignment layers are perpendicular to each other, and the resulting azo-nematic TN liquid crystal cell is such that it is bright (dark) state in the crossed (parallel)-polarizers configuration. In the crossed-polarizers configuration, a temporal change in the light transmission of the azo-nematic TN cell is monitored with a 355nm UV light irradiated. See Fig. 3. During the initial 40 sec., the TN structure is maintained as bright state. After 40 sec., the TN alignment starts to get destroyed rapidly, resulting in dark state. The trans-isomer azo-nematic liquid crystal maintains the TN structure, while the cis-isomer azo-nematic liquid crystal destroys the TN structure and the cell becomes optically isotropic. Upon heating the isotropic cis-isomer azo-nematic liquid crystal cell at 70° for 10 min., the TN structure was restored via cis-trans isomerization.
We studied the transmission change in the azo-nematic TN cell in two different polarizer/analyzer configurations as shown in the inset of Fig. 3. A mask with an aperture of ’TN’ letter was attached on the azo-nematic TN cell, and to monitor the light transmission, a near infrared white illuminator is incident on both the aperture and the bare fused silica substrate. The transmission through the half-circle-shaped area on the substrate is an indicator of the polarizer/analyzer configuration, parallel- or crossed-polarizers. The two top pictures are images taken before UV irradiation, the left one for parallel- and the right one for crossed-polarizers. Before UV irradiation, in parallel (crossed)-polarizers, the half-circle is bright (dark), while the ’TN’ letters are dark (bright), confirming the wave-guiding behavior of the twisted-nematics in the ’TN’ letters region. The two bottom pictures are images taken after UV irradiation, the left one for parallel- and the right one for crossed-polarizers. After UV irradiation, in the parallel (crossed)-polarizers, both the half-circle and ’TN’ letters are bright (dark), showing that the wave-guiding property of twisted nematics is destroyed through the trans-cis photo-isomerization process and the liquid crystal cell is optically isotropic.
5. Optical switching as a function of the UV irradiation time
In the metamaterial-liquid crystal cell structure, we further investigated changes in the transmission of two pronounced resonances as a function of the UV irradiation time. Figure 4 shows the optical switching behavior shown in Fig. 1(b) and 1(d) in detail. Changes in the near infrared transmission occurring in H- and L-oscillators are denoted as red-squares and black-circles, respectively. A complete optical on-off switching at 1650 nm and 1170 nm is achieved in 60 sec. Once optically switched, the transmission spectrum did not undergo any change for 14 hr. when kept dark at room temperature, which is due to the long life-time of cis-isomer in azo-nemaitcs. Upon heating at 70°C for 10 min., the thermal cis-trans isomerization process recovers the metamaterial-liquid crystal structure back to the twisted-nematics structure. Difference in the amount of changes in H- and L-oscillators (ΔH and ΔL in Fig. 4) incurred by an optical control is simply from the different transmission characteristics of metamaterial shown in Fig. 2(c).
In summary, by employing a planar metamaterial as one of the alignment layers in the twisted azo-nematic liquid crystal cell, a metamaterial-liquid crystal cell structure is fabricated, and an optical control of wavelength-selective light transmission in the near-IR regime is demonstrated. The novel single device of the metamaterial-liquid crystal cell structure has a potential application in the photonic switching in optical fiber telecommunications.
This work was supported by National Center of Nanomaterials Technology through Yeungnam University as well by the Quantum Metamaterials Research Center program (Ministry of Education, Science, and Technology, Republic of Korea).
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