The search for new low loss nematic liquid crystal mixtures with enhanced birefringence and low temperature of nematic-to-isotropic phase transition plays a pivotal role in a development of new applications in the emerging field of thermally tunable metamaterials. Here we maximize thermally induced tunability of a terahertz metamaterial by using a specially designed nematic liquid crystal mixture. It is shown that the resonant response of a metamaterial device can be effectively tuned both in terms of its magnitude and wavelength with the spectral tunability approaching the theoretical limit of 8 GHz. Electromagnetic simulations confirm our tests and match the experimental observations well. The suggested approach opens new routes for next-generation soft-matter-based filtering and sensing components and devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metamaterials (MMs) have seen increasing growth in research activity over the past decade. New and exotic electromagnetic properties can be realized by structuring these materials on a subwavelength scale and this has led to new fields being explored in electromagnetic studies, ranging across the entire frequency spectrum from zero to the far optical. The major advantage of metamaterials is that it is possible to control their electromagnetic properties by simply adjusting the material assembly process .
Engineering metamaterials with properties that can be actively tuned play fundamental roles in the development of various metamaterial-based devices. Current research into metamaterials is thus focused on attaining dynamic material functionalities, including tunability and the switching and modulation of electromagnetic waves. The control and tuning of the properties of metamaterials can be achieved using superconductors, nonlinear media, carrier injection schemes, graphene and coherent control methods [2–8]. However, one of the most efficient methods available for creation of tunable metamaterials is infiltration of the materials with nematic liquid crystals (NLCs), thus enabling a variety of tuning strategies to be used, including those based on temperature, application of external voltages or magnetic fields, and all-optical methods that use the materials’ strongly nonlinear responses [1, 9–20]. Several phenomena are known to allow the modulation of scattering parameters through active control of the LC molecules using external electric or magnetic fields [11, 12, 17–20]. However, especially thermally tunable properties of metamaterials employing liquid crystals have been studied insufficiently despite numerous experimental investigations [17, 21]. The tuning efficiencies, high temperatures, and high losses remain major limitations of thermally tunable metadevices.
Here, an experimental demonstration of a metamaterial device that uses a heat-induced reversible spatial reconfiguration from the nematic phase to the isotropic phase is reported. This enables switching of the magnitude of the transmittance by up to 7% with respect to its value at room temperature. The thermal tunability of this metamaterial device is thus attributable to the temperature sensitivity of the permittivity of the LC contained in the metamaterial cavity. Importantly, the approach presented here allows continuous tunability of the device’s plasmonic properties, in contrast to most reported methods to date, which usually allow only switching between the on and off states. In addition, to understand the interaction of terahertz radiation with metamolecules, the resonant behaviour of a single split-ring resonator (SRR) was simulated using full-wave electromagnetic simulation tools and the electric and magnetic field distributions were determined. It is essential to understand these temperature-controllable properties of plasmonic structures and their dependence on the molecular properties of the system because of the roles played by anisotropic materials in various reconfigurable metadevices.
A thorough comparison between the approach in this work and prior publications on thermally controlled metamaterials loaded with LCs [17, 21] clearly shows the differences in the metamaterial structure proposed in this work and the advantages of this structure when compared with those reported in previous works. The proposed scheme offers the following three strategic advantages: (i) the nematic liquid crystal mixture has high birefringence and also requires low temperature to tune metadevice; (ii) the change in the resonance frequency is up to 6GHz, which is the best reported result for thermal tuning of terahertz metamaterials infiltrated with LCs to date; and (iii) the designed metadevice shows lower losses than previously reported systems and this adds another level of novel functionality.
2. Formulation of nematic mixture
In order to optimize thermally induced tunability of a metamaterial, low loss nematic liquid crystal mixture 3073 with enhanced birefringence (> 0.3) and low temperature of nematic-to-isotropic phase transition (< 50°C) has been formulated. Recently, our group has shown examples of mixtures with the birefringence ∆n in the range of 0.2–0.5 [22–28]. One of the most effective methods for their formulation is mixing polar higher birefringence compounds with nonpolar low birefringence hydrocarbons having very small rotational viscosity. The relatively low dielectric losses observed in these liquid crystal mixtures mean they are ideal materials for tunable microwave and terahertz components [22–25]. Following our recent studies, here we demonstrate multicomponent mixture dedicated to thermally tunable metamaterials. The molecules with a large anisotropy of the electron polarizability αe, which is involved by the presence of the long conjugated π electron system of bonds were employed as they exhibit high birefringence. Moreover, the most profitable molecules were the ones with a large length to breadth ratio and rigid cores composed of aromatic rings. For this reason laterally fluorosubsituted-4- isothiocyanato-4’-4-alkyl tolanes were used. These materials have temperature of isotropisation near 40°C and birefringence around 0.3. NCS group connected with benzene ring system give strongly enhacement π electron system and makes birefringence higher [24,25]. The longer terminal alkyl chain the lower both melting and clearing points. Laterally fluorosubstituted- 4-isothiocyanato-4’-(4-alkylphenyl) tolanes have birefringence of about 0.5 but melting as well as clearing points are much higher, which makes them suitable only as dopants (8%) for the designed system with increased birefringence and makes them keep temperature of nematic-to-isotropic phase transition near 50°C. Chemical composition of the designed nematic mixture and structure of components, are shown in Table 1.
3. Metadevice employing nematic mixture
The metamaterial was fabricated as a planar square array of subwavelength 200-nm-thick gold electric SRRs on a 1-mm-thick quartz substrate, as shown in Fig. 1, using conventional photolithography, electron-beam metal film deposition and lift-off processes. The geometrical dimensions of the SRR unit cell are shown in Fig. 1(a). A 15-µm-thick layer of the NLC mixture was sandwiched between two parallel quartz substrates that were coated with a thin film of uniformly rubbed polyimide. The alignment layers required for liquid crystal orientation were deposited with a thickness of 50 nm by spin-coating of the Sunever SE-130 (Nissan Chemical Industries, Ltd.) polyimide precursor solution, followed by imidization at 80°C for 1 h. The SRR pattern was oriented parallel to the rubbing direction.
The LC material used is the nematic mixture 3073, which is characterized by high birefringence (∆n ≈0.32 at 0.5 THz) and viscosity (η = 16.66 mPa·s). The 3073 liquid crystal is in the nematic phase at room temperature, but becomes isotropic at TNI≈48.5°C. Significant temperature variation allows a phase transition to be produced, at which the nematic mesophase microstructure and, consequently, its symmetry have changed considerably [29–31]. This means that an increase in the applied temperature causes the internal energy of the LC molecules to increase, and the LC then undergoes a transition to the isotropic phase with random molecular orientation, as shown in Fig. 1(c). Thermodynamically, this type of change implies a change in the system entropy, which is described by , where F is the free energy at a given temperature T and volume V. The nematic-to-isotropic phase transition is a first-order transition, and is characterized by the order parameter S vanishing at the transition temperature, TNI.
4. Electromagnetic simulations
The numerical simulations shown in Figs. 2(a)–2(e) are full three-dimensional Maxwell calculations based on the finite-difference time-domain numerical method and performed using Quick Wave 3D commercial software. In our simulations, we use a nonuniform spatial grid with sizes varying from 2 µm in the x and y directions to 10 µm in the z direction. To simulate a periodic structure in the (x, y) plane, we use periodic boundary conditions on the planes that are orthogonal to x and y, and perfectly matched layer boundary conditions on the sides that are orthogonal to z. In the following analysis, εII = 3.7, ε⊥ = 2.56 and the conductivities σII = 0.3·10−6 S/m and σ⊥ = 0.25·10−6 S/m were used to describe the dielectric properties of the highly-birefringent nematic LC mixture. In the isotropic state, . The modelled SRR structure was set to be a perfect electric conductor (PEC). An incident beam with its electric field polarized along the x direction and its magnetic field polarized along the y direction was assumed to illuminate the structure along the z direction, as shown in Figs. 1(b) and 1(c).
The SRR [see Fig. 1(a)] is perhaps the most common resonant element to be fabricated using metamaterials [1–21]. The SRR is equivalent to an inductor-capacitor (L-C) circuit, in which the metal loops function as inductors and the gap between the metal strips acts as a capacitor of capacitance C . In sub-wavelength MMs, the electric field penetrates the entire volume of the metallic structure [see Figs. 2(a) and 2(b)], resulting in an equivalent dipolar response under the collective effect of the induced currents [see Fig. 2(e)]. This means that the incident electric field (in the x-direction) perpendicular to the gap can excite loop currents on the SRR and thereby induce an artificial magnetic moment m, which results in negative permeability. Also, because the currents are not uniformly distributed on the front and back strips, the oscillating currents are thus equivalent to an oscillating electric dipole along the incident electric field direction. The electric energy is concentrated around the gap [see Figs. 2(a) and 2(b)] and the magnetic energy is concentrated on the rear metallic strip [see Figs. 2(c) and 2(d)], which demonstrates that the SRR supports inductor-capacitor (L-C) resonance.
5. Thermal tunability of metamaterial transducer
To characterize the designed NLC thermal tunability, we first measured a traditional liquid crystal cell (i.e. without the metamaterial substrate) and determined the transmittance as a function of temperature at THz frequencies, as shown in Fig. 3. The measured transmittance signals were well separated, so the NLC we formulated fulfilled its role, and may be employed in thermally tunable terahertz components and devices.
Under normal incidence conditions, the metamaterial device behaviour was characterized as a function of temperature using a terahertz time-domain spectroscope (THz-TDS) that incorporated a continuous flow liquid helium cryostat . A metamaterial transducer is placed between the THz transmitter and the detector. To avoid any complications due to the presence of water vapour, the spectrometer is initially purged using dry air. Both the THz phase and the amplitude are acquired as functions of frequency through a Fourier transform of the time-domain data. To understand the behaviour of the metamaterial when loaded with a nematic liquid crystal mixture during the heating process, THz-TDS (TeraView TPS 3000) measurements of the metamaterial transducer were performed at a heating rate of 5°C min−1.
The proposed tuning system is based on the use of a thermotropic NLC mixture. To determine tunable properties of a metadevice we first measured the transmittance of an empty metamaterial cell [see Fig. 4(a)]. Then the cell was filled up with a liquid crystal under test and again the transmittance of filled metamaterial cell was determined at applied temperatures of 25°C and 50° [see Fig. 4(b)]. In Fig. 4(a), for the sample without LC, a transmission dip was observed at around 0.525 THz. The insertion of homogenous ordered LC layer red-shifted the resonance by 60 GHz, which was also confirmed by the results of numerical simulation [see Fig. 4(b)]. In Fig. 4(b), at room temperature (25°C), when the NLC molecules are all ordered parallel to a specific direction defined by the unit vector n [see Fig. 1(b)], the transmittance has its lowest value, and reaches 0.22 at 0.464 THz. When the temperature gradually increases from 25°C to 50°C, which results in the order parameter falling and the molecules becoming completely unordered, the transmittance decreases to 0.20 at 0.458 THz. As presented in Fig. 5, the most rapid change in the transmittance occurs at 0.416 THz, reaching up to 7%. When the applied temperature increased to 50°C, the shift became saturated because the NLC molecules were almost completely unordered at that temperature. The shift in resonance wavelength is caused by a phase transition in the LC, which occurs at the applied temperatures above 48 °C. The resonant shift occurs in both simulation and experimental results. Please note, that in the case of simulation there is a cross point between two curves, opposed to the experimental curves. This is due to the fact that the resonance width in the case of simulation is constant, and its shift is associated with the change of the refractive index of the liquid crystal only. In experimental research, as the temperature rises, the quality factor of materials from which metamaterial cell is made decreases. This causes an increase in losses as well as broadening of the resonance band. Therefore, the experimental curves do not intersect. Furthermore, there is a sticking effect – the LC molecules at the aligning layer are stuck to the aligning layer and the heating above the clearing point does not affect their orientation. By varying the applied temperature in the range from 25 to 50°C, we could control both the intensity (up to 7%) and spectrum (up to 6GHz) of the resonant response of metamaterial transducer, as evident from Fig. 5. The demonstrated tuning range is close to the absolute theoretical limit of 9% for intensity tunability, and 8GHz for spectral tunability [see Fig. 4(b)]. These values were determined assuming that the liquid crystal could be replaced with isotropic layer with niso = 2.94. Please note, that we simulate metamaterial’s unit cell and use periodic boundary conditions on the planes that are orthogonal to x and y. Consequently, the metamaterial structure is supposed to be perfect in the numerical simulation, which is not quite the case for the experimental sample which consists of exactly 12816 unit cells and there are defects. As a result, there are differences in the resonance strengths between the simulation and experimental results [see Figs. 4(a) and 4(b)]. In the theoretical model we can observe a perfect deep resonance only.
It would be advantageous to achieve a reversibility behaviour of the process; therefore, we tested the terahertz response of the sample with a cooling/heating cycles rate of 5°C min−1, as presented in Fig. 6. Changes of temperature also result in a change of the terahertz response but with a slightly decreased magnitude of the plasmonic band shift . Switching behaviour of the designed metadevice is fully reversible both in the view of structure and properties, if slow cooling/heating is used.
Figure 7 shows the average temperature of nematic-to-isotropic phase transition with respect to the birefringence of nematic mixtures. Graph clearly shows the advantages of nematic mixture 3073 when compared with those reported in the previous works. In contrast to the previously reported nematic mixtures dedicated for applications in tunable photonic devices, the newly designed mixture 3073 has a high value of birefringence and also allows to lower the phase transition temperature, assuring long-term stability of the material.
In summary, a thermally tunable terahertz metamaterial is proposed using a combination of a thermotropic NLC mixture with a sandwiched SRR structure. The terahertz radiation can be self-regulated by simply tuning the device temperature, which is of significant importance for extension of the applications of metamaterials. Spectral tunability of 6 GHz has been observed without any significant variation in the resonance strength. The resonance tuning occurs because of the temperature-dependent order parameter of the NLC mixture, which results in a nematic-to-isotropic phase transition. This thermal tuning of metamaterial resonance using NLC materials will enable the realization of multi-functional THz metamaterial [34–36] sensors [37–39], filters [40,41], modulators [42, 43], and switches [44,45].
This research was supported by the Polish Ministry of Sciences and Higher Education under grant Iuventus Plus no. 0365/IP2/2016/74, entitled “Tunable properties of metamaterial transducers with short response times in the THz range.”
The authors would like to thank Prof. J. Wrobel of the Institute of Physics PAS for preparing metasurface and Dr. Katarzyna Garbat from Institute of Chemistry in Military University of Technology for delivering the liquid crystal material.
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