Nanoplasmonics offer a pathway for creating a new class of ultrathin display pixels that exhibit highly tunable optical properties using tailored arrays of subwavelength metallic nanostructures [1–3]. Metallic nanohole arrays can be used to excite surface plasmon polaritons (SPPs), leading to wavelength-dependent enhanced optical transmission [4]. The optical properties of the nanohole layer are dependent on the geometry of the nanohole and the material of the metallic layer. Through design, SPP excitation can be utilized to create a myriad of plasmonic-based pixels operating across the visible spectrum [3,5–9]. These include multicolor polarization-dependent filtering, in which output color is controlled via linear polarizer rotation [10,11], and complex wavefront shaping, based on arrays of spatially variant metallic nanostructures [12,13]. Towards the goal of thinner, higher-resolution, and multifunctional electro-optic display pixels, these plasmonic pixels offer an innovative solution [2]. However, challenges to incorporate wideband tunable optical properties remain [1,2], and generally, to induce a shift in plasmonic resonance, the manipulation of surrounding materials’ refractive index is utilized [3,14,15]. Liquid crystals (LCs) are optically anisotropic, and, possessing permanent dipole moments, will reorientate under an applied electric field, providing an electro-optically tunable medium. Widely used as a fundamental building block for modern displays, LCs are promising for integration with metallic nanostructures for highly tunable plasmonic pixels [3,16,17].

In this Letter, we experimentally demonstrate an active color tunable LC-nanohole transmissive pixel device operating in the visible spectrum with unpolarized input light. Through integration of nematic LC with Al nanoholes on glass, an applied voltage across the pixel results in variable color output. The Al nanohole arrays are designed to exhibit plasmonic resonance at a specific wavelength, and in effect act to linearly polarize and filter the light. The LCs are used to modulate the polarization state of light as it propagates through the device, followed by a linear polarizer acting to selectively filter the transmission. Using this principal, we show voltage-controlled plasmonic color from single pixels. The devices shown are attractive for display applications and may help to reduce the dependency on RGB pigment-based filters, shrink pixel technology, and offer additional optical functionality from a single pixel. The devices show advantages over similarly demonstrated electro-optic devices [3,15], such as ease of fabrication, visible spectrum operation, and transmissive functionality.

A schematic of the LC-nanohole (LC-NH) pixel is shown in Fig. 1(a). An individual Al nanohole will exhibit localized surface plasmonic resonance (LSPR) [18], spectrally dependent on hole geometry, whereas periodic nanohole arrays can provide the additional momentum required to excite SPPs [1,4]. For the former, the transmission maxima wavelength is a function of the aspect ratio of the nanohole; for the latter, the transmission minima wavelength is proportional to array periodicity. Unlike nanohole arrays in optically thick metallic layers [19,20], here SPPs result in transmission minima [21]. As a result, depending on design, the periodic hole arrays will possess both localized and nonlocalized characteristics. The square nanoholes in this work, under unpolarized light, exhibit two orthogonal modes, shown with finite-difference time-domain (FDTD) simulations [22] in Fig. 1(b): aligned $x$-polarized (0°) and $y$-polarized (90°) to the nanohole lattice. The two resonant modes, in an isotropic refractive index ($n_o=n_e$), at any given wavelength, are in phase and of equal intensity. Hence the transmitted light from the nanohole surface has a linearly polarized output (at 45°). With LC integration, the surrounding anisotropic index ($n_o\ne n_e$) leads to two distinct modes, as now the index is dependent on the orientation of the LC.

 

Fig. 1. (a) Schematic of the LC-nanohole device operation with and without applied voltage. (b) FDTD simulations of the Al nanohole layer (${\mathrm{\Lambda }}_{x/y}=140\mathrm{nm}$ and ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$) illuminated with unpolarized light. The two orthogonal SP modes are exhibited shown through the normalized $E$-field enhancement maps at $\lambda =580\mathrm{nm}$. These modes determine the spectra and polarization of light transmitted through the nanohole layer and into the device. When $n_o=n_e$, both modes are equal, and the light is linearly polarized at 45° to the nanohole lattice. (c) Subset of the fabricated nanohole pixels under SEM.

Download Full Size | PDF

The devices in this work are constructed from Al (30 nm) on glass, patterned using electron-beam lithography (EBL) to form rectangular nanoholes, shown in Fig. 1(c). Al is an excellent plasmonic material [23], exhibits good chemical stability, and has a low cost [8,24]. Furthermore, it is preferable over other noble metals, such as Ag, due to the preferential planar alignment of LCs to its surface. 80-kV EBL (Nanobeam Ltd. nB1) is used, in conjunction with Ma-N 2405 negative tone photoresist, for the high-resolution patterning of the nanoholes on glass. To overcome the issue of charge accumulation associated with EBL patterning on insulating substrates [25], two charge dissipation layers (the first, 2-nm Al prior to spin-coating the resist; the second, a 35-nm Al layer on top of the resist) are used. Deposition of Al ($\sim 30\mathrm{nm}$) is performed using a thermal evaporator. The Al nanohole array—shown under SEM in Fig. 1(c)—is used as the bottom electrode of the LC cell (pixel). The top layer of the cell consists of commercial indium–tin–oxide (ITO) glass with a polyimide layer, mechanically rubbed, giving preferential alignment to the LCs. The cell is constructed using UV-cured glue containing 2-μm glass spacer beads resulting in a cell thickness of 2.5–3.3 μm. The cells were filled with nematic LC BL006 (Merck) in the isotropic phase by heating to 130°C, phase formation occurring by cooling to room temperature. Optical characterization is with an Olympus BX-51 polarizing optical microscope, with a halogen bulb light source, attached to a spectrometer and DSLR. A 1-kHz sinusoidal AC signal was applied to the cell with $V_{p-p}$ varying from 0 to 10 V ($\mathrm{\Delta }V=0.25$). Full-wave 3D FDTD modelling is performed using commercial software (Lumerical FDTD Solutions [22]). A single nanohole is modelled with periodic boundary conditions at the unit cell spacing. A complex dispersive material model is used for Al (Palik). An anisotropic refractive index is used to describe a LC layer with $n_e=1.82$ and $n_o=1.53$. The voltage-dependent index is modelled as a 3D matrix rotation of the LC index, and planar surface anchoring incorporated up to 200 nm above the nanohole array and below top alignment surface.

The Al nanohole transmission spectra are dependent on the pitch, size, and shape of the nanoholes, along with the thickness, density, and material of the nanohole layer and surrounding index. The nanohole dimensions utilized were: width, ${\mathrm{\Lambda }}_{x/y}$, ranging from 100 to 200 nm; and pitch, ${\mathrm{\Gamma }}_{x/y}$, ranging from 250 to 450 nm [Fig. 3(a)]. The pixel is formed encapsulating nematic LCs between the nanohole layer (bottom electrode) and ITO-covered glass (top electrode). A linearly rubbed polyimide alignment layer is used on the ITO-glass side only, aligned parallel to the square holes. Nematic LC preferentially align planar to the Al surface, with bulk LC rotation induced through the top-electrode alignment layer [26]. Bulk LCs re-orientate relative to the nanohole surface with applied voltage.

The LC birefringence ($\mathrm{\Delta }n=n_{\Vert }-n_{\perp }$) causes the propagating transmitted light from the two nanohole modes to travel through the device at different velocities, altering the phase (${\varphi }_{x/y}$) between the two components. This phase difference ($\gamma ={\varphi }_x-{\varphi }_y$), at a given wavelength ($\lambda$), is dependent on the $\mathrm{\Delta }n$ and thickness ($t$) of the LC layer such that $\gamma =2\pi \mathrm{\Delta }nt/\lambda$. $\mathrm{\Delta }n$ between the two modes is dependent on the LC orientation, which is dependent on applied voltage. With no applied field, the LC is planar aligned, whereby $n_{\Vert }=n_e$ and $n_{\perp }=n_o$, thus $\mathrm{\Delta }n=n_e-n_o$. With an applied voltage (AC signal) sufficiently large enough for homeotropic alignment, $n_{\Vert }=n_{\perp }=n_o$ and $\mathrm{\Delta }n=0$. In the intermediary situation, $n_{\Vert }$ is a function of applied voltage between the two LC indices. This operation is demonstrated through FDTD simulations [22], shown in Fig. 2. The square nanohole array exhibits an optical response dependent on geometry and LC state. As this light propagates through the LC layer, depending on propagation distance, the polarization state oscillates between linearly ($\gamma =0,\pm \pi$) and circularly ($\gamma =\pm \pi /2$) polarized. Thus by varying the amplitude of the applied voltage between the nanohole and ITO layers, the phase difference between propagating orthogonal field components and the spectra from the LC-NH device can be controlled.

 

Fig. 2. FDTD simulations of the LC-NH device: ${\mathrm{\Lambda }}_{x/y}=140\mathrm{nm}$ and ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$. (a) LC orientation ($x\text{-}z$ plane) in off, on, and fully aligned states. (b) Transmission spectra, polarized at 45° (shown with red arrow) to the nanohole lattice, as light propagates through the LC-NH device. (c) Transmission spectra with output anaylzer at 45° for the chosen cell thickness of $\sim 3.3\mathrm{\mu m}$. The dashed lines are the unfiltered nanohole response of the two orthogonal SP modes and solid line post-LC interaction.

Download Full Size | PDF

 

Fig. 3. Experimental optical transmission. (a) Schematic of nanohole surface, indicating width (${\mathrm{\Lambda }}_{x/y}$) and pitch (${\mathrm{\Gamma }}_{x/y}$). (b) Photos of LC-NH device in off (left) and on state, containing a range of pixels with ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$ and varying ${\mathrm{\Lambda }}_{x/y}$. Under an optical microscope, the same devices are shown in (c), with analyzer at 45° (the iterated dimensions are indicated on the subplot). A single pixel is selected (indicated by colored box) from (c) and the applied voltage is now iterated in smaller increments; results shown in (d). The dimensions are ${\mathrm{\Lambda }}_x=155\mathrm{nm}$, ${\mathrm{\Lambda }}_y=140\mathrm{nm}$, and ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$. The results are shown with no output analyzer (top row); output analyzer aligned orthogonal to nanohole lattice; 135°; and 45° (bottom row).

Download Full Size | PDF

Moreover, introducing an output analyzer results in an electro-optic device that has an output from a combination of the nanohole array spectra 2c, the optical retardance from the LCs, and output selected to be polarized at 45° to the nanohole lattice, which can operate in on/off mode or variable applied voltage color output mode.

Figure 3 shows the experimental optical transmission results for the LC-NH pixels for varying nanohole geometries, illustrated in (a). The device is illuminated with an unpolarized plane wave at normal incidence to the nanohole surface, captured with both (b) a DSLR and (c–d) polarizing optical microscope. An output analyzer is aligned 45° to the lattice. The device is shown in on/off (0/4 V) states, and color switching from red to green is visually observed. The total pixel widths are chosen to be large enough (400 μm) in order to be easily visible to the naked eye. Figure 3(c) shows optical microscopy imagery of the LC-NH pixel arrays in the same on/off states. The pixels contain nanohole arrays with ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$ and ${\mathrm{\Lambda }}_{x/y}$ ranging from 110 to 170 nm. The color variation in the pixels is due to the size variation of the nanohole geometry.

A single pixel is now selected for investigation into finer control of color, shown in Fig. 3(d). The pixel has fixed nanohole grid geometry: ${\mathrm{\Lambda }}_x=155\mathrm{nm}$, ${\mathrm{\Lambda }}_y=140\mathrm{nm}$, and ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$. The applied voltage is varied in incremental steps of 0.5 V. The four rows show images of the selected pixel with unpolarized illumination for four output analyzer states: (1) no output analyzer; (2) analyzer orientated at 0° to the lattice; (3) at 135°; and (4) at 45°. For the unpolarized state, there is no significant change in output color, due to limited realigment of LCs around the nanohole surface, resulting in a small shift in plasmonic resonance of the nanohole modes. For an analyzer at 0°, again there is little shift in output color. This analyzer state selects a single orthogonal SP mode [with reference to Fig. 1(b)], and as this mode passes through the LC layer (polarized parallel to this analyzer state), regardless of optical retardance, the polarization of the single SP mode is unaltered. Hence the spectra, filtered by the output analyzer, are unaltered.

For the 45°/135° state, a combination of the modes can be selected by the output analyzer. The resulting spectra from the two orthogonal analyzer states is such that $T_{\lambda ,noA}=T_{\lambda ,A:45°}+T_{\lambda ,A:135°}$. The LC layer, through voltage-controlled retardance, acts to mix the relative contributions of the two modes, and more specifically, acts as a tunable band-pass filter. This effect is indicated by the field-dependent color shift, shown in Fig. 3(d), and supported through the simulation results in Fig. 2(c).

Figure 4 shows pixel operation with the output analyzer at 135° in further detail. Two pixel geometries are investigated: ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$ and ${\mathrm{\Gamma }}_{x/y}=250/350\mathrm{nm}$, keeping ${\mathrm{\Lambda }}_{x/y}=140\mathrm{nm}$. The experimental transmission spectra for the two LC-NH pixels are shown in Fig. 4(a), whereby two SP modes are observed for orthogonal output states. The pixel in which ${\mathrm{\Gamma }}_{x\ne y}$ results in much greater spectral distinction between orthogonal modes due the variation in orthogonal grating vectors, and hence GCSPs. Figures 4(b) and 4(c) show the experimental and simulated transmission spectra, respectively, for the two LC-NH pixel geometries under varying applied voltage. Figure 4(d) shows optical microscope images of the respective pixels. Initially the spectra of light are toward the red part of the spectrum. As the electric field increases, the retardance of the LC layer is modulated, and the output transmission peak blue-shifts. As the phase difference between the two propagating SP modes cycles between $\gamma =0$ and $\gamma =\pi$, the spectra shifts back and forth from 650 (red) to 550 nm (green). In the case where ${\mathrm{\Gamma }}_{x/y}=300\mathrm{nm}$, both orthogonal SP modes have their transmission maxima towards the red part of the spectrum, hence the red-dominated output transmission spectra. At sufficient voltage-induced retardance, for which the red part of the spectrum is filtered, the remaining color spectra (in the green part of the spectrum) are broad and weak. Two similar SP modes are present, with variation in spectral position due in part to the orientation of the LC (anisotropic index) surrounding the nanoholes. Hence, the combined output is a broadened red peak. In contrast, for the pixel in which ${\mathrm{\Gamma }}_{x\ne y}$, the more spectrally distinct SP modes result in both narrower transmission spectra and a wider color palette over the full tunable range. The two respective pixel outputs are shown in Fig. 4(d) under the same exposure conditions, and in combination with Figs. 4(b) and 4(c) demonstrate greater color vividness based on the narrower output peaks. The brightness of the LC-NH pixels may be improved simply by enlarging the size of the nanohole (${\mathrm{\Gamma }}_{x/y}$). The location of the transmission maxima wavelength (${\lambda }_{pk}$) is dependent on the aspect ratio of the nanohole, and hence the transmission may be increased while keeping ${\lambda }_{pk}$ unchanged. This, however, comes at a cost, broadening the transmission peak. Any discrepancies between experimental and simulation results are speculated to be due in part to fabrication tolerances (including rounded nanohole geometry, LC surface alignment issues due to resist residue, and Al surface roughness), and simulation methodology inaccuracies (including approximations for LC anchoring and electric-field-induced LC reorientation).

 

Fig. 4. Experimental and simulated transmission spectra for two LC-NH pixel geometries: (i) ${\mathrm{\Gamma }}_{x=y}=300\mathrm{nm}$ and (ii) ${\mathrm{\Gamma }}_{x\ne y}=250/350\mathrm{nm}$, with constant ${\mathrm{\Lambda }}_{x/y}=140\mathrm{nm}$. Pixels are illuminated with unpolarized light and output analyzer at 135° to the lattice. (a) Experimental transmission spectra from orthogonal nanohole SP modes. (b) Experimental and (c) simulated transmission spectra as a function of voltage. (d) Shows the pixels’ color under optical microscope for varying applied voltages.

Download Full Size | PDF

In summary, we have experimentally demonstrated an active color tunable LC-NH pixel device operating in the visible spectrum with unpolarized input light. The ultrathin ($<40\mathrm{nm}$) Al nanohole surface acts to linearly polarize and spectrally filter the input light. The nanohole arrays typically have widths of 140 nm and periodicity of 300 nm. After propagation through a nematic LC layer, the combined color output is determined through an applied voltage between the nanohole surface and ITO-glass electrodes. The LC retardance actively filters the light passing through pixel in combination with an output analyzer at 45° to the nanohole lattice. The pixel design combines electrode, polarizer, and color filter, reducing the dependency on separate subpixel components, such as pigment-based filters, for commercial display devices [21].