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Electrically tunable selective reflection of light from ultraviolet to visible and infrared by heliconical cholesterics

Cholesteric liquid crystals with helicoidal molecular architecture are known for their ability to selectively reflect light with the wavelength that is determined by the periodicity of molecular orientations. Here we demonstrate that by using a cholesteric with oblique helicoidal(heliconical) structure, as opposed to the classic right-angle helicoid, one can vary the wavelength of selectively reflected light in a broad spectral range, from ultraviolet to visible and infrared (360-1520 nm for the same chemical composition) by simply adjusting the electric field applied parallel to the helicoidal axis. The effect exists in a wide temperature range (including the room temperatures) and thus can enable many applications that require dynamically controlled transmission and reflection of electromagnetic waves, from energy-saving smart windows to tunable organic lasers, reflective color display, and transparent see-through displays.

2 can vary the wavelength of selectively reflected light in a broad spectral range, from ultraviolet to visible and infrared, by simply adjusting the electric field applied parallel to the helicoidal axis.
The effect exists in a wide temperature range (including the room temperatures) and thus can enable many applications that require dynamically controlled transmission and reflection of electromagnetic waves, from energy-saving smart windows to tunable organic lasers, and transparent "see-through" displays. Since the material is non-absorbing and transparent everywhere except the electrically preselected reflection band, the effect can be used in creating multilayered structures with a dynamic additive mixture of colours.
Development of materials capable to dynamically control transmission and reflection of visible light and near infrared (IR) radiation is one of the most important directions of research with potential applications such as energy-saving smart windows, [3] transparent displays, communications, lasers, [4] etc. The most challenging problem is to formulate materials in which the transmission of light, or, more generally, electromagnetic radiation, can be performed dynamically and independently for different spectral bands, preferably by controlling reflection (as opposed to absorption). Among the oldest known materials capable of selective reflection of light are the so-called cholesteric liquid crystals (LCs) formed by chiral elongated organic molecules in a certain temperature range between a solid crystal and an isotropic melt. [2,[4][5][6][7][8] The molecules are packed into a periodic structure in which the local orientation of molecules, called the director n , rotates around a helicoidal axis, remaining everywhere perpendicular to it,  The pitch 0 P and thus the wavelength p  of reflected light are sensitive to chemical composition and temperature, thus enabling applications such as temperature indicators and sensors of minute quantities of gases. [1,9,10] However, the most desirable mode to control light reflection, by an electric field applied parallel to the helicoidal axis, Figure 1a, has so far been elusive. The reason is that the field applied parallel to the axis, instead of changing the pitch while keeping the cholesteric axis intact to reflect light, rotates this axis perpendicularly to itself, as dictated by the dielectric anisotropy of the LC, [11] causing a light scattering structure called a "fingerprint texture", Figure 1b.

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In this work, we demonstrate electrically controlled selective reflection of electromagnetic radiation in a broad spectral range, from ultraviolet (UV) to visible and IR, by using an oblique helicoid structure of cholesteric, Figure 1c, as opposed to the right-angle helicoid shown in Figure 1a. In the oblique helicoid, the director is tilted, making some angle /2   with the helicoid axis. The electric field acting along the axis, realigns the molecules along itself and thus changes the pitch P without reorienting the helicoid axis. The oblique helicoidal state was made stable in a broad temperature range including the room temperatures by designing the chemical composition of cholesteric mixtures.
The oblique helicoid state has been predicted by Meyer and de Gennesto occur in the applied electric field in a cholesteric in which the elastic constant 3 K of bend is much smaller than the elastic constant of twist. [12,13] For a long time, the effect could not be explored because of lack of materials with the needed elastic properties. Recently, it was realized that the novel dimeric LCs, representing two rigid rod-like units connected by a flexible chain with an odd number of links, might have a small value of 3 K , Ref. [14] , which was confirmed experimentally. [15][16][17] The oblique helicoidal state of the corresponding cholesterics was observed in Raman-Nath diffraction experiments, [18] in which light propagates perpendicularly to the axis, and in Bragg reflection geometry, [19] but only at temperatures above 100 o C and short tuning range of the reflection band, not quite suitable for applications. In this work, we formulated a broad temperature range cholesterics with the small 3 K by mixing two dimeric LCs (1',7'-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) and 1-(4-cyanobiphenyl-4'-yl)-6-(4cyanobiphenyl-4'-yloxy)hexane (CB6OCB)), and a standard LC pentylcyanobiphenyle (5CB) higher temperatures, the effect disappears as 3 K in dimeric materials increases with temperature. [15][16][17] All data reported below were obtained at 25 o C .
In the experiments, the cholesteric is sandwiched between two glass plates with transparent indium tin oxide (ITO) electrodes. When a sufficiently strong electric field is applied, the material is switched into a uniform nematic with the director parallel to the field,   various field strengths that further demonstrate a very broad range of controlled reflectance, from UV to IR, covering the entire range of visible light. As the field decreases, the peak wavelength shifts to IR, Figure 3a,b. In absolute units, the reflection coefficient R was measured to be 41% for the reflection peak at 632 nm (field 1.1 V/μm ). By controlling the chemical composition of the mixture, one can optimize the reflection efficiency for any given pitch, and adjust the bandwidth that narrows down at higher electric field, Figure 3b. one can broaden the reflected bands by polymerization of the material, [8,11,22,23] or increase the reflectivity to 100% by using optical compensators. [2] Experimental results on the field dependence of p  , Figure 3b, support the theoretical model [12] in which . As demonstrated in Ref. [18] ,  decreases as the field increases, so that eff n  becomes smaller. This is why, for a given material, the reflectivity is typically weaker at high fields; it is also weaker at low fields, as the number of cholesteric layers in the cell of a fixed thickness decreases as the pitch increases, Figure 3a. 8 When the light beam is not strictly perpendicular to the cell, the oblique helicoid produces reflection not only at p nP   , but also at 2 p nP  , since the molecules separated by 2 P are tilted in opposite directions. [24] In the presented experiments with normally incident beams of small divergence; the peak at 2 p nP  , although observable, was about 10 times weaker than the main peak at p nP   .
To change the colour, the pitch needs to adjust to the field, either through slippage at the bounding plates or through nucleation and propagation of dislocations; some of these defect lines are seen in Figure 2b-e. An important advantage of the heliconical cholesteric over similar structures in smectics is absence of positional order in molecular packing, which allows the system to adapt easily and reversibly to the changes of period; all the colour changes observed in our experiments are fully reversible and relatively fast. For example, the homeotropic state switches into the heliconical state with reflective red colour within 0.8 second, Figure S1.
To conclude, we describe an electrically tunable selective reflection of light in UV, visible and IR by the heliconical cholesteric state that exist in a broad temperature range including the room temperatures. The tunable structural colour is achieved in a simple sandwich geometry, in which the cholesteric is confined between two plates with transparent electrodes, thus implying a low cost and easy fabrication process. The colour change occurs over the entire electrode area that can be designed as a desired pattern suitable, for example, for "see-through" colour displays. The electrically tunable colours can be additionally controlled by the cholesteric composition and by employing reflective stacks. The effect can be tuned to practically any spectral regions in UV, visible and IR by chiral additives of different twisting power or concentrations. The regular right-angle cholesterics are known to provide structural colours of certain birds, beetles [25] and plants; [26] it would be of interest to explore whether the oblique helicoidal states occur in the natural world.

Experimental
The main component of the explored mixtures that yields the necessary smallness of 3 K is CB7CB. [14,18] It shows a uniaxial N phase in the range (103-116) o C between the isotropic and the twist-bend nematic phase tb N . At the field frequency 10 kHz, the dielectric permittivities parallel and perpendicular to the director were measured to be || 7.3   and 5.9 respectively; the dielectric anisotropy of the material is thus positive [14] , so that the director prefers to align parallel to the electric field. At o 106 C , the elastic constants are 1 5.7 pN K  , 2 2.6 pN K  and 3 0.3 pN K  , [18] while the refractive indices are measured in our laboratory by the wedge cell technique [27] Figure 2. Second, we measured the reflection spectra of the cholesteric heliconical structure using Ocean Optics spectrometers USB2000 (visible) and NIRQuest256 (near-IR).
The absolute value of reflectivity coefficient R at 632 nm . The obtained value R =0.41 (or 41%) was used to normalize the values of R measured by the spectrometers. The characteristic times of the electro-optic response was measured by recoding the transmittance change of circularly polarized light (λ=632nm), Figure S1.

Supplementary Information
We determined the characteristic times of electrooptic response by recoding the transmittance change of the circularly polarized light through the cell and determining the levels of 10% and 90% of the maximum transmittance. The switching time is 0.8 second for switching from the homeotropic state (applied electric field