1. Introduction

Augmented reality (AR) is an emerging technology that projects useful information directly into the user’s field of vision, integrated into environmental scenery [1–3]. A bright, multicolored background with fine details can significantly degrade the performance of such see-through displays. To circumvent the crosstalk between the virtual graphics and the background, a projector screen with controllable local translucency is necessary. Existing smart window technologies that enable switching between clear (transparent) and diffuse (translucent) states include suspended particle device (SPD), polymer-dispersed liquid crystal (PDLC, including polymer-network liquid crystal), bistable cholesteric texture (BCT), and dynamic scattering mode (DSM) nematic liquid crystal. However, for safety reasons, normally translucent modes (e.g., SPD [4] and normal-mode PDLC [5,6]) and bistable modes (e.g., BCT [7–11]) are unfavorable for practical use. For instance, in automotive applications, such as AR head-up display [2,3], the driver’s visibility can be reduced dramatically if the circuit suddenly fails and turns the whole screen scattering. Normally transparent shutters are therefore preferred. In this context, nematic DSM and reverse-mode PDLC are good candidates. DSM [12] exploits the electro-hydrodynamic instabilities in a liquid crystal to create optical scattering, yet, the induced ion movement could establish a permanent inner electric field that leads to performance degradation over time. In a reverse-mode PDLC [13–16], the clear–diffuse switching is driven under the action of a dielectric torque (no ionic mechanism is involved). However, the driving voltage and other properties of the PDLC depend strongly on the morphology of the polymer network, thus less feasible for large-scale fabrication. High angular dependence of the clear-state transparency (which originates from the liquid crystal/polymer index-matching mechanism [16]) is also of serious concern, especially for automotive applications.

Here we report a normally-transparent dynamic diffuser based on a newly observed field induced dielectric instability in cholesteric liquid crystal (CLC). Besides exhibiting infrared filtering ability due to the CLC’s photonic bandgap and angle-independent clear-state transparency since no heterogeneous optical composite is used [17–20], this field-induced transparent–translucent change can potentially be utilized to construct smart windows with desirable characteristics such as fast and reversible switching (10’s–500 ms), fast self-restoration from the diffuse state to the clear state (subsecond–seconds), and polymerization-free fabrication.

2. Sample preparation

CLCs are liquid crystals of which the molecules are self-assembled into one-dimensional helices. It was formulated by mixing a nematic liquid crystal with negative dielectric anisotropy DNM-C (ordinary index n_e ≈1.59, extraordinary index n_o ≈1.48, and dielectric anisotropy Δε = ε_// – ε_⊥ ≈–3.6; same as the nematic host used in [21]) with a trace of a chiral agent R5011 (helical twisting power ≈130 μm^–1, from HCCH). The samples were fabricated by filling glass sandwich cells with a dielectrically negative cholesteric liquid crystal via capillary action. The inner surfaces of the glass cells were pre-coated with indium tin oxide to serve as transparent electrodes and poly(vinyl alcohol) films to induce planar alignment. Cells with two different cell gaps, 32 and 45 μm, were used in the following experiments. The reflection band was tuned to the desired near-infrared spectral region (in this case around 800 nm) by controlling the concentration of R5011 in the CLC. The experiments were performed with single-pixel samples; for practical use, the CLC cells will be pixelated so as to achieve local translucency control.

3. Results and discussion

In the absence of an external field, the helices are aligned along the surface normal by the strong anchoring force of the alignment layer―known as the planar state. The CLC hence acts as a Bragg reflector in the near infrared regime and is highly transparent in the visible [Fig. 1, left panel]. When an alternating-current electric field is applied to the CLC cell, the degree of translucency, characterized by the specular transmission spectra shown in Fig. 2(a), can be either improved or degraded depending on the field strength (E) and frequency (f). The spectra were measured by using a spectrometer (USB4000, Ocean Optics) that has a finite aperture of 5 mm in diameter and was set at 10 cm away from the sample.

 

Fig. 1 Schematic and photographs of the proposed CLC smart window (d ≈45 μm).

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Fig. 2 (a) Specular transmission spectra at different field strengths (d ≈32 μm and f = 1 kHz). (b) Dependence of instability threshold on driving frequency.

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In the low applied field regime (e.g., E < 2.8 V/μm at f = 1 kHz), an increase of the electric field strength leads to an overall rise of the transmittance in the visible range and sharpening of the photonic bandgap, cf. Fig. 2. This is due to the negative dielectric anisotropy of the CLC; the field-induced dipole moment is perpendicular to the principal molecular axis, and so the electric torque brings the molecules away from the field axis. As revealed by the microscope images in Fig. 3(a), the grain boundaries of the planar domains diminish with increasing field, smoothening the texture.

 

Fig. 3 (a) Microscope images, (b) far-field diffraction patterns, and (c) undulation periods of CLC at different field strengths above the threshold field E_c ≈2.8 V (d ≈32 μm and f = 1 kHz).

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In response to fields above a certain threshold [i.e., the solid curve in Fig. 2(b)], the planar state becomes structurally unstable and therefore scatters light. The data points in Fig. 2(b) were collected by probing the specular transmission of the CLC cell with a HeNe laser at λ = 633 nm and recording the field strength at which the transmittance begins to drop. The measured threshold field scales with the square root of driving frequency, suggesting that the observed blurring of the sample was caused by the field-induced instability in the dielectric regime [22]. A planar CLC can be regarded as layers of different director orientations stacked along the helical axis. In a CLC that is experiencing field-induced instability, the associated layers tend to tilt towards the field axis.

The instability result in platelets of two-dimensional (2D) periodic undulations, as a consequence of balancing the induced torque with the confining substrates and the surface anchoring by poly(vinyl alcohol) [Fig. 3(a)]. At wavelengths far from the photonic bandgap (e.g., in the visible regime), each platelet acts as an optical grating with micrometer-scaled periods and thus gives rise to optical diffractions. Figure 3(b) depicts that the diffractions are quasi-omnidirectional (cf. the round diffraction patterns appearing at E ≈3.8 and 5.1 V/μm) in that the 2D grating orients differently from one platelet to another. The grating period (Λ) ranges from 13 to 22 μm and increases with the field [Fig. 3(c)]. Given that the cell gap (d) is ~30 μm, the grating operates in the Raman-Nath regime (Λ² ≫ λ⋅d), leading to multiple orders of diffraction. The quasi-omnidirectional multi-order diffractions effectively generates spatial crosstalk in the optical image that passes through the CLC cell. Meanwhile, the reflection band becomes broadened. According to the Bragg’s law, the bandgap (λ_B) of the CLC blue-shifts as the helix tilts away from the viewing direction―λ_B = n̅⋅p⋅cosθ, where n̅ ≈(n_e + n_o)/2 is the average index of the CLC and θ is the angle between the helical axis and the surface normal [21]. In an undulated CLC, the tilt angle of the helix varies in space and so does the magnitude of the blue-shift, cf. Figures 1 and 2(a) [22]. The bandgap is expanded toward the visible regime with increasing applied field, indicating the possibility of grayscale control. The platelet (grain) boundaries and inhomogeneous period of undulation also contribute to light diffusion. The translucency can be further improved by increasing the interaction length (increasing the cell gap or stacking cells).

To gain more insight into the switching dynamics, we monitored the specular transmission change of the CLC cell in electric fields above the instability threshold. The HeNe laser was employed as a probe that passed through the cell and arrived at a photodiode (818-SL, Newport; the active diameter is 11.3 mm) which was connected via a power meter (1830-C, Newport) to an oscilloscope (TDS360, Tektronix). The distance between the cell and photodiode was ~0.7 m. Figure 4(a) shows the clear–diffuse switching with a 1 kHz AC field (E_H) of ~3.8 V/μm. The molecules were first aligned by the field, revealed by a sharp rise in specular transmittance (T) from 86 to 89% within 5 ms. At ~65 ms upon the application of the field, the undulation instability became visible and caused a transmittance drop from 89 to 11%. The corresponding fall time was ~40 ms. This was followed by some fluctuations and soon became steady at T ≈26%. The field was then removed at ~2.49 s. The CLC hence relaxed to the clear state, taking ~525 ms to restore 90% of the initial transmittance (henceforth defined as the rise time). Such a field-off response is in contrast to the CLC smart windows that are driven by the field-induced instability in the conduction regime (usually at low driving frequencies), of which the diffuse state is stable upon the removal of the applied field [10,11].

 

Fig. 4 (a) Time-resolved specular transmittance revealing the clear–diffuse switching and self-restoration processes (E_H ≈3.8 V/μm, f = 1 kHz, and d ≈32 μm). (b) Response time for the clear–diffuse (fall time, circles) and diffuse–clear (rise time, squares) at different field strengths (E_H) and thicknesses (filled symbols: 32 μm, open symbols: 45 μm).

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As depicted in Fig. 4(b), the response times vary with the driving frequency and cell gap. The fall time is in general one order of magnitude shorter than the (self-restoration) rise time. In the 32 μm-thick cell, the fall time is on the order of 10’s ms and drops with increasing field strength. The rise time follows an opposite trend, from 5 ms at 3.2 V/μm to ~1 s at ~5 V/μm. Although increasing the cell gap to 45 μm can improve the translucency, both the deformation and relaxation processes become slower. Adopting the layer stack configuration [12,17,20] or increasing the anchoring force of the surface alignment could prove effective in reducing the response time of the self-restoration process while retaining high translucency in the diffuse state. Note that the spontaneous diffuse–clear transition is the key to circumventing a reduction of visibility caused by unexpected failure of the driver circuit that otherwise turns the whole window into a permanent diffuse state. This can be a serious concern when using a normally translucent or bistable window, but not the case for a normally transparent window. Under normal operating conditions, this relatively slow process (compared to the fall time) can be sped up to several 10’s ms by applying a small electric field.

Recalling the field strength dependence of specular transmission in Fig. 2, applying an electric field below the instability threshold to a deformed CLC can assist the diffuse–clear restoration by exerting a torque to reorient the LC molecules until they are aligned perpendicular to the field axis. Figure 5 demonstrates the spontaneous and field-induced restoration processes with a 45 μm-thick CLC. When a high field (E_H ≈4.5 V/μm) is applied, the CLC first experiences a transient higher-transmittance clear state (T ≈90%) that lasts for only a few 10’s ms and then transitions into a diffuse state (T ≈20%). Switching off the high field brings the CLC back to the stable clear state through the self-relaxation process (induced by the surface anchoring solely). A successive application of E_H drives the sample into the diffuse state again. Subsequently, reducing the field strength to 2.7 V/μm (E_L), the clear state is restored with rise time of only ~70 ms.

 

Fig. 5 Time-resolved specular transmittance diagram showing the field-induced clear–diffuse transition, spontaneous restoration, and field-induced restoration process (E_H ≈4.5 V/μm, E_L ≈2.7 V/μm, d ≈45 μm, and f ≈1 kHz).

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4. Summary

A normally transparent smart window has been demonstrated, enabled by the electrically induced undulation instability in a CLC with negative dielectric anisotropy. The window is switchable between the clear and diffuse states with controllable translucency by varying the field strength. The clear state, in which the cholesteric helices are uniformly oriented along the window normal, is the only stable state in the absence of an electric field, an important characteristic for safety concerns. By applying an electric field higher than the threshold for the undulation instability to occur, the CLC window is switched to the diffuse state taking 10’s–100’s ms. When the applied field is removed, the clear state is restored spontaneously with response time of subsecond–seconds depending on the cell gap and field strength. A much faster restoration of < 100 ms can be gained by directly reducing the field strength below the instability threshold instead of switching off the voltage. Featuring single stable state, subsecond-scaled response, and polymerization-free fabrication, the CLC smart window shows great potential for AR applications.