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Uniform homeotropic alignment of nematic liquid crystals is achieved on a square array of metallic nanoholes through vapor-phase deposition of a fluorinated silane-coupling agent. We show through polarization optical microscopy and optical spectroscopy that the monolayer formed can induce homeotropic alignment on the nanohole array without disturbing its optical properties. The proposed technique is a step towards realizing electro-tunable metamaterials with controlled liquid crystal orientation.
© 2012 Optical Society of America
1. Introduction
Metamaterials based on metallic nanostructures are currently being investigated actively in prospect of realizing ultra-compact optical devices that can manipulate light [1]. One of the challenges in metamaterial research is to make them tunable so as to enable on-demand switching of their properties. Because most metamaterials exhibit an unconventional dispersion relationship, its optical properties can be changed drastically by changing the wavelength of light impinging on the device [2]. However, this configuration is not practical since compact wavelength-tunable light sources are not readily available and is also not cost effective. The challenge lies in changing the dispersion relationship itself so that the optical properties experienced by monochromatic light can be varied.
Nematic liquid crystals (NLCs) are viewed as candidate materials which enable tuning of metamaterial properties based on refractive index modulation. In a NLC, rod-like molecules spontaneously align along a single direction and exhibit anisotropy in both refractive index and dielectric constant. Thus, application of a DC or AC field can induce molecular reorientation, which subsequently changes the refractive index [3].
Several groups have previously investigated metamaterials infiltrated with NLCs [4–9]. One of the popular metamaterial structures that has been investigated is the fishnet structure based on metallic nanohole arrays (NHAs). They are suited for this kind of investigation since they possess both the freedom in design to engineer their dispersion relationship (being a planar device), and ”holes” that can be infiltrated with various media, including NLCs. However, most experimental studies performed to date have been undertaken in the microwave region where the device size is large (∼ mm). Although LC-based tuning is possible even in such low frequency regions, the large device size deteriorates the tuning characteristics in terms of both the driving voltage and response speed. While optical fishnet metamaterials (which have smaller footprints) can merit from faster response times and lower driving voltages comparable to that of LC displays, they have been mainly studied through simulation, and the number of experimental studies is limited [10].
An important issue regarding LC-tunable optical metamaterials is alignment control of NLCs on nanostructured surfaces. It is known that the orientation of LC molecules is easily affected by corrugations on the surface [11]. Although one-dimensional structures, i.e. gratings usually induce uniaxial alignment, alignment on two-dimensionally patterned surfaces are quite complex and can even induce multistable alignment in various directions [12–14]. Unfortunately, however, this issue has not been addressed in detail in most works performed to date on metallic nanostructure - LC hybrids [15, 16], although from a device perspective, alignment control should be of high importance, since non-uniform alignment can give rise to unwanted light scattering which seriously deteriorates optical performance. Recently, Liu et al. have proposed to coat plasmonic structures with hexadecyl trimethyl ammonium bromide (CTAB) to induce homeotropic alignment of NLCs [17, 18]. However, they did not succeed in obtaining uniform alignment inside nanoholes: defect-induced LC domains were observed in their sample, leading to strong light scattering and low transmittance [18]. It could be that the nanoholes were actually not covered with CTAB, since they used a dip-coating method to treat their samples: presence of air-bubbles inside nanoholes can easily prevent the bottom surface from being covered with CTAB.
In this study, we propose to apply a silane-coupling agent (SCA) in the vapor phase to obtain large-area, uniform alignment of NLCs. The SCA vapor can penetrate deeply into nanostructured surfaces, and allows the surface properties to be modified without affecting its optical properties. Specifically, we apply a fluorosilane monolayer on a metallic NHA with square holes and show that uniform homeotropic alignment can be obtained in a sandwich-type device. This is in constrast to devices prepared with non-treated substrates, in which non-uniform, polydomain alignment is obtained as a consequence of structure-induced alignment bistability. We also show that the transmission properties of our device can be tuned via refractive index modulation of NLCs.
2. Sample preparation
Figures 1(a) and 1(b) show the design of the NHA substrate used in this study. The square holes have side length of 450 nm and are arranged in a square array with a period of 900 nm. To fabricate the substrate, SiO2 and Al were first deposited by sputtering on a fused quartz substrate at the following thicknesses: 50 nm of SiO2, 40 nm of Al and 50 nm of SiO2. The substrate was then patterned by electron beam lithography (Elionix, ELS-7000), and was etched sequencially, layer by layer. SiO2 was etched by reactive ion etching (Samco, RIE-200NL) using CHF3 as the etchant gas, while Al was processed by inductively coupled plasma etching (Samco, RIE-101iPH) using Cl2 as the etchant gas. The scanning electron microscope (SEM) images of the substrate shown in Fig. 1(c) shows that the nanoholes are fabricated with high precision.
Fig. 1 (a) Schematic illustration of the NHA pattern, (b) Side-on view of the NHA pattern. (c) SEM image of the NHA substrate. scale bar: 1 μm.
The SCA used in this study was trichloro(1H,1H, 2H,2H-heptadecafluorodecyl)silane (Tokyo Chemical Industry, Co., Ltd.), whose chemical structure is shown in Fig. 2(a). This SCA forms a monolayer through chemisorption on SiO2, and makes the surface strongly hydrophobic, reaching water contact angles as high as 111° [19]. The strong hydrophobicity results from the fluorinated alkyl-chains being polar oriented, and is also believed to be responsible for the homeotropic anchoring it imposes on LCs. Homeotropic orientation of NLCs using this SCA agent was confirmed in a reference sandwich cell using two cleaned glass substrates.
Because of its extreme sensitivy to moisture, the coupling reaction was performed in a nitrogen-purged glove box. The NHA substrate was first cleaned by UV/ozone (Filgen, UV253), and was placed on a hotstage set at 150° C. The SCA was also placed on the hot-stage and left for 2–4 hours to complete monolayer formation. The substrate was rinsed in anhydrous toluene before being taken out of the glove box to assemble a sandwich cell. Formation of the fluorosilane monolayer was confirmed by making ellipsometric measurements on a reference quartz substrate treated by the same process: the obtained layer thickness was was 1.5 nm, comparable to the value reported in literature [19]. A BK7 substrate coated with homeotropic alignment material (JSR, JALS-2021-R2) was used as the counter-substrate to assemble cells with thickness between 0.4 – 3.5 μm. The samples were then infiltrated with a NLC (Merck, 4-pentyl-4′-cyanobiphenyl) at 45°C which is in the isotropic phase, and cooled at a rate of −1°C/min to 25°C. The sample was evaluated by polarization optical microscopy and microscopic spectroscopy (measurement spot of ∼ 25μm) in the visible and near infra-red regions, respectively.
3. Results and discussion
Figure 3 shows the transmittance spectra of the NHA substrate before and after SCA treatment, measured using the through-hole as the reference. The two spectra look almost the same, indicating that the fluorosilane monolayer has no effect on the optical properties of the NHA.
Shown in Fig. 4 are microscope images of the SCA-treated sample and a reference sample, prepared using an NHA substrate without SCA treatment. Although the tranmsission image looks similar in both samples [Figs. 4(a) and 4(b)], careful observation reveals the presence of domain boundaries in the non-treated sample, but not in the SCA-treated sample. Between crossed polarizers [Figs. 4(c) and 4(d)], the difference in the texture becomes more obvious: in the SCA-treated sample, a uniform, dark texture is obtained, while a bright texture with thread-like dark lines is observed in the non-treated sample. This thread-like texture is well-known as the Schlieren texture, and is an indication of planar NLC alignment. Considering that the counter-substrate imposes homeotropic anchoring, it can be surmised that the NLC molecules are in a hybrid alignment, gradually changing from homeotropic alignment at the counter-substrate to planar alignment on the NHA. Planar alignment on the NHA is also supported by the extinction positions of the sample, which exist along the diagonal of the square array (Fig. 4(d), right). The fact that defect lines (appearing as bright lines) are observed in the extinction position suggests that the alignment is bistable in two directions, along either diagonal of the array. Alignment bistability is a phenomenon characteristic to two-dimenional patterns with four-fold symmetry [14], and proves that planar anchoring is imposed by the nanostructured hole array. However, although this is an interesting phenomenon, unwanted scattering can be generated through the presence of domain boundaries, and thus needs to be avoided in an optical device.
Fig. 4 (a), (b) Microscope image of samples with and without SCA-treatment, and (c), (d) POM image of samples with and without SCA-treatment.
Homeotropic alignment in the SCA-treated sample is further evidenced by the conoscopic images shown in Fig. 5. The images were acquired using a x40 objective with numerical aperture of 0.6, by inserting a Bertrand lens in the optical path of the microscope. The white circle in the center of the figure shows the aperture of the condenser lens, and surrounding this circle, diffracted light spots are observed in two orthogonal directions, reflecting the four-fold symmetry of the NHA. Importantly, a dark cross (isogrye) is observed in Fig. 5(a), indicating that the optical axis is in the cell-normal direction. This is in contrast to Fig. 5(b), in which no dark lines are observed. This is a consequence of the NLC molecules lying parallel to the cell-plane and in two orthogonal directions, angled at 45° to the polarizers.
Finally, we demonstrate that the extraordinary optical transmission (EOT) characteristics of the NHA can be tuned in this device based on refractive index modulation of NLCs [20]. We use the fact that the minima in transmittance are observed where Wood’s anomaly is fulfilled: that is, , where a0 is the period of the holes (900 nm), mi and mj denote the order of diffraction and nd is the refractive index of the medium in contact with the metal [15]. Figure 6(a) shows the transmittance spectrum of the sample below and above the clearing point of the NLC (which occurred at ∼36.2 °C on our home-built hot-stage), measured using the through-hole as the reference. The spectrum of the sample before LC infiltration is also shown as a dotted line. It can be seen that all spectra share a common transmittance dip at ∼ 1300 nm: this corresponds to first-order resonance ( ) of the substrate, with refractive index nSiO2 ∼ 1.45. In the LC infiltrated sample, two extra dips are observed: one in the range 1400 – 1500 nm, and the other between 1000 – 1100 nm. Since these peaks are only observed in the LC-infiltrated sample, these peaks must correspond to first and second-order ( ) resonances of the NLC. As expected, the refractive index extracted from the dip positions [Fig. 6(b)] are close to those reported in literature [21], thus supporting this hypothesis. Figure 6(b) also shows a phase transition behavior at 36.2 °C (clearing point of the NLC): this proves that the change in EOT characteristics is caused by a change in the LC refractive index. The extracted refractive index values are close to the extraordinary refractive index of the NLC, and is believed to be a consequence of p-polarized surface plasmon polaritons being responsible for the EOT phenomenon. This characteristic provides an opportunity to tune the EOT phenomenon by an electric field using the vertically-aligned (VA) LC switching mode. The slight decrease in the transmittance of the dip occurring at shorter wavelengths is possibly due to the change in the coupling efficiency of surface plasmon-polaritons, determined by the wavelength-dependent dielectric constant of aluminum.
Fig. 6 (a) Temperature dependence of the transmittance spectrum of the SCA-treated sample. (b) Temperature dependence of the transmittance dip positions and corresponding refractive indices.
It should be noted here that the side-walls of the nanoholes are also covered with the fluorinated monolayer and thus could be inducing alignment of NLCs in the cell-plane direction. While experimental verification of molecular alignment inside such small holes is difficult, one can discuss its effect on the optical properties by considering the length scale over which the LC molecules are planarly aligned. If uniaxial alignment persists over a length scale longer than the wavelength of light, then light propagating through the medium will be retarded due the birefringence of LCs. However, if the alignment direction varies within a length scale shorter than the wavelength of light, the medium will effectively appear isotropic. Since the holes investigated here (450 nm) are small compared to the wavelength of light at which it is intended to be used (> 1μm), no phase retardation would be induced, even if the alignment inside the nanoholes were inhomogeneous: this is supported by Fig. 4(c) which shows a completely dark texture (meaning the sample is retardation-free), although visible light is incident on the sample, which has a wavelength much closer to the size of the nanoholes.
The effectively-isotropic LC inside the nanoholes, when they exist, can have a detrimental effect on the optical performance, since the tuning range of the device would be reduced due to reduced effective birefringence. In reality, the extent of this effect will vary from device to device, depending on how the electric field is distributed with respect to the LC alignment. The device investigated in this study yielded NLC refractive indices very close to that reported in literature for a pure NLC without the nanostructure [21]. This means that partial isotropization hardly affects the optical performance of this system, but could also mean that there is no partially isotropizated region in the first place. Therefore, to gain better understanding of the system, one would need to perform numerical simulations, analyzing both the LC director distribution and the electro-magnetic field distribution. Construction of such a platform is currently underway and should allow us to design optimized device structures for various specific applications.
4. Conclusion
Defect-free homeotropic NLC alignment was achieved on a square array of metallic nanoholes using a fluorosilane monolayer. The monolayer has no effect on the optical properties of the NHA, while strongly influencing the alignment of LC molecules. Although a single-layered substrate was used in this study, the proposed method has the flexibility to be applied to bulk metamaterials [2] and other complex nanostructures. We believe this simple yet effective technique is a step towards realizing practical, reliable tunable metamaterials with controlled LC alignment behavior.
Acknowledgments
This work was supported by NEDO. H. Yoshida acknowledges financial support from the JST PRESTO Program. The authors thank Mr. Takuya Higashi for experimental assistance.
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