Electrically active nanoantenna array enabled by
varying the molecular orientation of an interfaced
liquid crystal
Yu-Cheng Hsiao,aChen-Wei Su,bZong-Han Yang,bYevheniia I. Cheypesh,c Jhen-Hong Yang,bVictor Yu. Reshetnyak,cKuo-Ping Chen*aand Wei Lee*a
A system comprising a gold nanoantenna array covered with a high-birefringence liquid crystal (LC) material is introduced. By applying voltage across the LC bulk, we demonstrate that the refractive-index and polarization changes significantly modify the hybrid plasmonic– photonic resonances in the system. The hybrid structure enables the active control of the spectrum as well as a large shift in resonance wavelength of the metallic nanoantennas by means of an externally applied electricfield. Our modeling supports the observed results, by assuming that the nanoantenna array leads to two orthogonal easy axes with a finite anchoring energy. In combination of the nano-structured surface with birefringent LC, tunability up to 90 nm is achieved in the visible wavelengths, opening the door towards nanoscale displays or nano-optical switches.
1.
Introduction
The oscillation of free electrons in metallic nanostructures causing large electricelds near the metallic particle's surface is known as localized surface plasmon resonance (LSPR). The LSPR transforms free-space radiation into localized energy. As such, the term—optical antenna or nanoantenna—has appeared to describe metallic nanostructures with LSPRs coupled to receivers or light sources.1Nanoantennas possess many intriguing
prop-erties such as the directivity gain,2–5 polarization control,6,7
intensity enhancements,8–10 and spectral shaping.11 They are formed by pairs of metal nanostructures. The resonance wave-length and intensity of the localizedelds in nanoantennas are strongly dependent of the structural geometry and the refractive index of the surrounding medium. It is practically signicant to
enable active control of these optically coupling properties by means of an external tuning technology. Such tunability can be accomplished by incorporating other materials submitted to, say, an applied voltage, heat, or illumination prole.12–14 Recently,
graphene has been integrated into nanogaps of coupled plas-monic antennas for electrical tuning of antenna resonance in the mid-infrared region.15However, the electrical tuning of antenna resonances in the visible is highly desired. Liquid crystals (LCs) are interesting materials in that their electrically induced re-orientation of molecules can modify the resonance conditions of optical resonators. Indeed, LCs can be employed to control resonances in metallic nanostructures, including LSPR-based nanoantennas and surface plasmon polaritons in metallic lms.16–19To the best of our knowledge, a hybrid LC/nanoantenna
structure has not been proposed until now, not to mention the investigation of the corresponding tuning properties. Besides, the tuning ranges of typical or conventional LC-based metallic nanostructures are quite limited, merely several nanometers.20,21
In this study we demonstrate an active LC tuning of electro-magnetic (EM) resonances in periodic arrays of metallic nano-antennas. The structure consists of pairs of square gold nanobulges covered with a high-birefringence LC material. Such nanoantennas own two resonance modes that are useful for realizing extraordi-narily broad shi in wavelength. The active tuning is achieved by applying voltage on the LC-plasmonic coupled system conned by the nanostructured substrate and a conducting glass substrate coated with a rubbedlm of polyimide (PI) for planar alignment of the LC. Evidenced by the voltage-dependent transmission spec-trum, the shi in optical resonance of 90 nm is accomplished. Based on the assumption that the nanoantenna array produces two orthogonal easy axes withnite anchoring energies, our simulation is in good agreement with the observed results.
2.
Fabrication of liquid
crystal-nanoantenna coupled devices
Fig. 1 shows two scanning electron microscopic images of two-dimensional gold nanostructures fabricated on an
indium-tin-aInstitute of Imaging and Biomedical Photonics, College of Photonics, National Chiao
Tung University, Guiren Dist., Tainan 71150, Taiwan. E-mail: [email protected]; [email protected]
bInstitute of Photonic System, College of Photonics, National Chiao Tung University,
Guiren Dist., Tainan 71150, Taiwan
cPhysics Department, Taras Shevchenko National University of Kyiv, Kyiv, 01601,
Ukraine
Cite this: RSC Adv., 2016, 6, 84500
Received 3rd May 2016 Accepted 30th August 2016 DOI: 10.1039/c6ra11428h www.rsc.org/advances
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Published on 06 September 2016. Downloaded by University of California - San Diego on 12/09/2016 03:11:43.
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oxide (ITO)-coated glass substrate by well-controlled e-beam lithography. Each square gold nanoparticle in the array in die 1 (Fig. 1(a)) has x–y dimensions of 100 nm by 100 nm. The thickness (or height) of the nanoantenna array is 50 nm, and the gap between two paired square nanoparticles is also 50 nm. The periodicity of the nanoantenna dimers is 400 nm in both the x-and y-directions. Comparatively, the paired strip nanoantennas in die 2 (Fig. 1(b)) have the same dimension in the x-direction and the y dimension is 100 mm, which allows die 2 to be regarded as a grating. The substrate with the nanostructured surface serves as the bottom substrate for the LC cell as shown in Fig. 2. The other substrate is a typical ITO-coated glass slide spin-coated with a PI alignment layer and treated with mechanical buffing along the x-direction. The assembled cell has a thickness (i.e., cell gap) of ca. 10mm, as determined by silica spacers. A high-birefringence LC material22with a wide nematic range from30.0 to 95.0C was introduced into the empty cell. The optical properties of the LC at 589 nm and 20C are: birefringenceDn ¼ 0.333; refractive indices ne¼ 1.851 and
no¼ 1.518. The top and bottom electrodes permit AC voltage to
be applied across the cell thickness. Linearly polarized white light transmits through the top glass substrate and the LC layer successively, and then couples to the nanoantennas with plas-monic resonance. The LSPR can be adjusted by the dielectric constant of the surrounding material; i.e., the LC, which is, in turn, determined by the reorientation of the nematic director induced by the externally applied electric eld at 1 kHz. The
square dimers of nanoantennas combined with LC susceptible to stimuli such as an electric eld provide the tunability in resonance wavelength (and color), whereas the paired strips of nanoantennas are utilized to demonstrate the switchability in intensity as a binary grey-level controller.
3.
Results and discussion
Fig. 3 compares the far-eld transmission spectra acquired with nite element method (FEM) simulations for die-1 nano-antenna array at wavelengths from 600 to 900 nm. In the simulation the loss factor of the gold Drude model is 3.23The refractive index of the substrate is 1.52, and the ITO layer is not included in the simulation model. As the surrounding medium is anisotropic LC with high birefringence, normal-incidence spectra illuminated by linearly polarized EM waves with both x- and y-polarizations were simulated and the results are shown in Fig. 3. One can see that, in the x-polarization condition, the resonance wavelength is 780 nm and there is strong localized electric-eld enhancement in the small gap correspondingly. When the polarization changes to the y-direction, the LSPR wavelength shis to 680 nm.
Fig. 4 and 5 illustrate the observed transmittance of the LC cell at various applied AC voltages in the x- and y-polarization conditions. The experimentally measured transmittance for a specic nanoantenna pattern, either die 1 (Fig. 4) or die 2 (Fig. 5), is dependent of both the wavelength and the polariza-tion direcpolariza-tion of the incident light, and the spectrum varies with the applied voltage which determines the orientation of the optic axis of the uniaxially anisotropic LC. A much greater voltage is required to reorient the LC molecules near the nanostructure due to the signicant strength of the anchoring force by the gold nanostructure. Fig. 4 shows that the polari-zation remains unchanged and the primary resonance occurs at the wavelength of 780 nm when the x-polarized incident light goes through the planar-aligned LC layer at 0 Vrms(le panel in
Fig. 1 Scanning electron micrographs of gold nanoantenna arrays in (a) die 1 and (b) die 2.
Fig. 2 Schematic of the LC cell with a nanoantenna-patterned surface on the bottom substrate. An applied electric field across the cell reorients the nematic molecules and, in turn, changes the wavelength of resonance.
Fig. 3 Simulated far-field spectra of the die-1 nanoantenna array for linearly polarized EM waves with the x- and y-polarizations. The resonance wavelength shifts from 780 to 680 nm when the polari-zation changes from the x- to the y-polaripolari-zation. The insets depict the localized EMfield distributions.
Fig. 4). This unperturbed condition favours the alignment of the LC in the neighbourhood of the bottom substrate to be in parallel to the x-direction. The nanostructured surface does not render a strong unidirectional aligning effect on the anchoring of LC molecules near the bottom substrate. As a result, some ill-aligned LCs along the y-direction can bring about the noise resonances at 0 Vrms. At applied voltages 1–4 Vrms the LC
molecules in the mid-plane tilt, causing the slight blueshi of the 780 nm resonance as a response to the reduced effective index refraction (neff< ne). When the applied voltage increases
to 5 Vrms, the LC starts to take the twisted nematic conguration
to impose the incident x-polarized light to change its polariza-tion when the lightnally emerges from the bottom substrate. The resonance wavelength blueshis to 690 nm, virtually following the 90-twist waveguide effect of LC. Noticeably, the primary resonance at a voltage beyond 9 Vrms reverts to the
initial wavelength of 780 nm (as is at null voltage) for the inci-dent x-polarized light in that the LC becomes unwound at such a high voltage. This can be understood because the optic axis along the vertical direction in the resulting homeotropic conguration does not alter the polarization state. Moreover, the spectral features for the y-polarized light impinging onto the cell at increasing voltages can be explained by the same reason;
namely, the change in LC conguration from the planar state at 0 Vrms, to the 90-twisted state to permit the adiabatic following
of the incident light at mid-voltages, andnally to the homeo-tropic state at, say, 10 Vrms(right panel in Fig. 4). The observed
wavelength shi of 90 nm is reasonably close to the predicted 100 nm shi for the bare nanostructured substrate (see Fig. 3). The 10% deviation is presumably due to fabrication tolerances. Here the change in plasmonic resonance wavelength enables optical switching in nanophotonic devices.
The numerical modelling of the transition from the planar state to the twisted nematic conguration under an external electriceld is essential to examine the observed phenomenon. Because our nanostructures is two dimension, the two easy axis model is used legitimately. Let us consider an initially planar-aligned nematic LC cell with strong anchoring and the easy axis direction given by et¼ (1, 0, 0) at the top substrate (z ¼ L).
Assume that the LC on the bottom substrate (at z¼ 0) has two mutually orthogonal easy axes described by eb¼ (cos 40, sin 40,
0), where one is at 40¼ 0 with the azimuthal anchoring energy
coefficient W2, and the other easy axis corresponds to 40¼ p/2
with the anchoring energy coefficient W1. Following the work of
Fukuda et al.24the energy density can be written as the form
with functional dependence of the director anchoring in the azimuthal plane: Fs¼ 1 2W1sin 4 4ð0Þ 1 2W2cos 44ð0Þ (1)
At the bottom substrate the anchoring is assumed to be strong for the polar director angle q(0) ¼ 0. We shall describe the LC director distortion in terms of the angles q and 4 given by n ¼ (cos q cos 4, cos q sin 4, sin q) (2) The strong director anchoring in the polar plane results in the following boundary conditions for the angle q
q(0) ¼ q(L) ¼ 0 (3)
The free energy per unit surface area of the LC layer is given by F ¼ 1 2 ðL 0 K11cos2q þ K33sin2q ðq0Þ2þK 22cos2q þ K33sin2q cos2qð40Þ2 3 03aE2sin2q dz 1 2W1sin 4 4ð0Þ 1 2W2cos 44ð0Þ (4) The corresponding Euler–Lagrange equations and boundary conditions are
q00(K33sin2q + K11cos2q) + (q0)2(K33 K11)sin q cos q
+ sin q cos q(K33(sin2q cos2q)
+ 2K22cos2q)(40)2+ 303aE2sin q cos q ¼ 0 (5a)
Fig. 4 Transmission spectra of the LC cell with the die-1 nanoantenna array on the bottom substrate at various applied voltages under the x-and y-polarizations.
Fig. 5 Transmission spectra of the LC cell with the die-2 nanoantenna array on the bottom substrate at various applied voltages under the x-and y-polarization conditions.
d dz cos2qK 33sin2q þ K22cos2q 40 ¼ 0 (5b) and
K2240(0) + 2(W1sin24(0) W2cos24(0))sin 4(0)cos 4(0) ¼ 0
(6a)
q(0) ¼ q(L) ¼ 0; 4(L) ¼ 0 (6b)
respectively. One has to accompany these equations with Maxwell's equations for the electriceld:
div D ¼ 0; curl E ¼ 0 (7)
where
D ¼ 30^3E (8)
and the elements of the dielectric tensor^3 can be expressed in terms of the orthogonal components as
3ij¼ 3tdij+ (3k 3t)ninj (9)
in the initial state (when there is no electric eld applied). Depending on the ratio between the values of W1and W2, the LC
director orientation may be planar (W1 W2); namely,
q(z) ¼ 0; 4(z) ¼ 0 (10)
or twisted (W1[ W2)
qðzÞ ¼ 0; 4ðzÞ ¼ 40
L z
L (11)
We have numerically solved eqn (5)–(9) for the high-birefringence LC used, which has the elastic constants K11 ¼
10.6 1012N, K22¼ 6.36 1012N, K33¼ 13.5 1012N (at 20
C) and the dielectric anisotropy 3
a ¼ 10.4. The effective
anchoring energy coefficients for the die-1 nanoantenna array are not known. For some insights on what may happen under the applied voltage, we adopted reasonable values for these coeffi-cients assuming that W1> W2. Through numerical modelling we
found the following interesting results. At voltages 1–4.5 V and W1 ¼ 3 105J m2, W2 ¼ 1 105J m2, only the planar
conguration appears in spite of starting to seek a numerical solution from twist as therst guess. However at 5 V, in addition to a planar director conguration a twist conguration also exists when starting to seek a numerical solution from the initial guess of twist. The total free energy for the twist conguration is lower than that for the planar one:5.1214 105J m2(twist) vs. 4.1223 105J m2(planar). Therefore one may expect that,
up to 5 V in the dye-1 sample, the LC director is in a planar conguration, becoming twisted at voltages above 5 V, and with a further reorientation to a homeotropic state at a sufficiently high voltage. The detailed modelling of the planar–twist–home-otropic transition will be presented elsewhere.
While the spectra displayed in Fig. 4 are associated with die 1, Fig. 5 demonstrates the weak voltage dependence of plasmonic
resonance from a nanoantenna grating. Note that the resonance only occurs in the x-direction in the case. The LC molecules structure a 90-twisted nematic conguration initially. The x-polarized incident light then takes a 90 rotation of the polarization state—a phenomenon known as the adiabatic following. When the applied voltage goes beyond 3 Vrms, the LC
molecules are reoriented vertically and the optical waveguide effect diminishes accordingly such that the polarization of the incident light remains unmodied. It is clear from Fig. 5 that a shutter-like behaviour can be found at 640 nm with a threshold voltage around 3 Vrms.
Fig. 6 shows the images of micrographic textures of the hybrid plasmonic–photonic cell placed between two linear polarizers under the parallel- and crossed-polarizer schemes at various applied voltages. With knowledge of the behaviours of LC on nanoantenna arrays, the color and brightness level of the textures can be easily understood. One can see from Fig. 6 that die 1 exhibits color changes by applied voltages under either polarizer scheme. In contrast, the die-2 plasmonic device exhibits simply a grey-level change between the two voltage regimes separated by the transition voltage of3 Vrms. It should
be reminded that, unlike the case where a regular metallic grating structure is considered,25 the plasmonic absorption observed in this study relies on multiple periods of the nano-structures. Based on E-beam lithography and suitable LC cell designs, a nanosize-pixeled display or nanoscale optical switch can be further developed.
4.
Concluding remarks
In summary, a nanoantenna device comprising a two-dimensional gold nanostructure array (die 1) covered with a high-birefringence LC layer has been studied. By applying voltage on the anisotropic LC layer, we demonstrate that the coupling effect between surface plasmons and photons through the LC can modify the hybrid plasmonic–photonic resonances Fig. 6 Polarizing micrographs of the LC cell covering regions of die 1 and die 2.
in the system. This enables electrical control of the trans-mission properties rectied by the corresponding plasmonic absorption, yielding a dramatically large wavelength shi of 90 nm in LSPR of the metallic nanoantennas. Our modeling supports the observed results, by assuming that the nano-antenna array leads to two orthogonal easy axes with anite anchoring energy. The square grooves discussed in Fukuda et al.26may be considered as a model for our die 1 nanoantenna
conguration. Unfortunately we do not know the constant ksfor
the LC used in our study, but one could legitimately speculate that the easy axes in the die-1 conguration are along the sides of the squares. We are unaware of any other study else which deals with the electriceld-induced director reorientation from the planar to twist and then to the homeotropic conguration with increasing voltage across a LC cell with two easy axes. What one would normally expect is a reorientation from a planar to a homeotropic director conguration; here the “planar–home-otropic” sequence cannot fully explain our experimental observations. Understanding why the “planar–twist–homeo-tropic” sequence of the LC director prole may occur in our study is important to interpret the observed change in trans-mittance under increasing applied electriceld. As a reference, the spectrum of the LC cell containing an array of strip nano-antennas (die 2) has also been investigated. The transmittance, as can be clearly seen at 640 nm, exhibits a two-level switching effect. An investigation of the LC anchoring for the die-1 and die-2 congurations is beyond the scope of the current paper and will be presented elsewhere. Our current study is about the hybrid plasmonic–photonic resonances in the system and its active control of the spectrum of the metallic nanoantennas. The hybrid structure can be used as a display or a two-level attenuator, making the nanoantenna device promising for photonic applications. A study along the line of optimizing the geometrical distribution of nanoantenna arrays is underway to achieve high contrast for the proposed colorlter.
Acknowledgements
This research isnancially supported by the Ministry of Science and Technology, Taiwan, under Grant No. MOST 104-2112-M-009-008-MY3 and MOST 104-2221-E-009-130-MY3. VYR and YIC acknowledge the EOARD/STCU grant P521a for nancial support.
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