Light-Driven Pitch Tuning of Self-Assembled Hierarchical Gratings

: Gratings are of vital importance in modern optics. Self-assembled cholesteric liquid crystal (CLC) gratings have attracted intensive attention due to their easy fabrication and broad applications. However, simultaneously achieving arbitrary patterning and delicate tuning of CLC gratings remains elusive. Here, light-driven pitch tuning is accomplished in hierarchical gratings formed in a molecular switch doped CLC. We fabricate a checkerboard hierarchical CLC grating for a demonstration, whose pitch is optically tuned from 4.6 µ m to 10.7 µ m. Correspondingly, the ﬁrst-order diffraction angle continuously changes from 9.4 ◦ to 4.8 ◦ and a signiﬁcant polarization selectivity is also observed. In addition, hierarchical CLC gratings with triangular wave pattern, Archimedean spiral, and radial stripes are also demonstrated. This work creates new opportunities for soft-matter-based intelligent functional materials and advanced photonic devices.


Introduction
Optical gratings are key components of spectrometers, sensors, tomography, media storage, and beam steering devices widely adopted in nanofabrication, biological medicine, and military areas. Usually, they can be made by physical techniques, such as mechanical ruling, electron beam ablation, lithography, and holography, or by bottomup chemical nanofabrication [1][2][3][4]. However, these methods are often complicated and limited to producing gratings with a fixed grating constant. Liquid crystal (LC), a typical soft building block, has the ability to self-assemble into various hierarchical superstructures [5][6][7]. Thanks to the high responsiveness to diverse external stimuli, LC gratings have played important roles in tunable optical components for light manipulation [8][9][10]. Recently, fingerprint textures of cholesteric LC (CLC) have attracted intensive attention due to their controllable pitch of self-organized periodic configurations [11][12][13]. Vast potentials have been investigated, including nonmechanical beam steering [12], spectrum scanning [14], particle assembly [15], optical vortices generator [16], tunable laser [17], anti-counterfeiting [18], and photolithography [19]. For all these applications, it is a fundamental and key requirement to obtain desired and reliable helical superstructures.
So far, much effort has been devoted to the patterning of helical superstructures and continuous tuning of the fingerprint gratings. Usually, the homogeneously aligned CLC exhibits a unidirectional grating vector [20,21]. In 2007, two-dimensional (2D) diffractive gratings were proposed in photo-responsive dye-doped CLCs [22]. Chen et al. presented interlaced fingerprint textures via sequential UV-induced polymer stabilization [23].
Direction-controllable CLC gratings were further obtained by introducing patterned electrodes or alignments under appropriate electric fields [24,25]. In addition, the vector or pitch of grating can be tuned via changing the helical pitch p with light stimulation, electric field actuation, or concentration regulation [13,14,[26][27][28]. Nevertheless, the above methods are usually complicated; and most reported results are limited to either structure patterning or dynamic tuning of the grating pitch L. If one could realize desired patterning of fingerprint landscapes with tunable L, new functionalities based on smart CLC gratings would be reasonably expected.
In this work, we report a light-driven pitch tuning of hierarchical CLC gratings generated by photopatterning a photo-responsive CLC doped with chiral molecular switches. Upon UV irradiation, the pitch of a checkerboard hierarchical CLC grating presents a continuous increase. The corresponding diffraction regulations are also investigated. Moreover, a triangular wave pattern, an Archimedean spiral, and radial stripes are achieved as well, verifying the flexibility of the photopatterning technique. This work demonstrates the combination of photopatterning and photo-tuning of self-assembled hierarchical gratings, which bring us more possibilities in dynamic functional optics.

Materials
The CLC mixture is composed of 99.54 wt.% nematic liquid crystal E7 (Jiangsu Hecheng Display Technology, Nanjing, China) with a positive dielectric anisotropy and 0.46 wt.% left-handed chiral molecular switch ChAD-3C-S (BEAM Company, Orlando, FL, USA). The incorporation of ChAD-3C-S in E7 yields an initial helical twisting power (HTP) of −42.6 µm −1 in the dark [29]. Under the irradiation of UV light, HTP decreases due to the transformation of ChAD-3C-S. It shows a reverse change when providing a high ambient temperature or upon green light irradiation [30]. The HTP in green-light photo-stationary state is about −30.7 µm −1 [31]. The p of CLC is determined by where c and ee are the concentration and the enantiomeric excess of the chiral molecular switch, respectively. Thus, for c = 0.46 wt.%, the initial p is calculated as 5.1 µm (ee = 1) and measured as 3.9 µm. The ps in the UV photo-stationary state and the photo-stationary state achieved by green light exposure are 8.1 µm and 4.8 µm, respectively. To implement the photopatterning of CLCs, sulfonic azo dye SD1 (Dai-Nippon Ink and Chemicals, Tokyo, Japan) is used as the photoalignment agent, which is dissolved in dimethylformamide (DMF, Sigma-Aldrich, Saint Louis, MO, USA) at the concentration of 0.3 wt.%. SD1 molecules tend to reorient their long axes perpendicular to the incident linear polarization of UV light in order to minimize the photon absorption, thus guiding the alignment of LC molecules [32,33].

Methods
The CLC cell is assembled with two transparent glass substrates (1.5 × 2 cm 2 ). Their inner surfaces are coated with indium tin oxide and SD1. The cell thickness d is determined by the spacers (5 µm) between the two substrates and measured as 4.9 µm by the optical interference method. The empty cell is placed at the image plane of the digital micromirror device (DMD, home-made) based micro-lithography system to record the desired in-plane molecular orientations by the photoalignment technique [34]. The CLC mixture is stirred in the isotropic phase (73 • C) and capillary filled into the cell. After the temperature is gradually decreasing to room temperature, a planar cholesteric texture appears and CLC gratings are formed when AC voltage is applied. The textures are observed under a polarized optical microscope (POM) (Nikon 50i, Tokyo, Japan) with an illuminating light of low intensity and recorded by a CCD (Nikon DS-Ri1, Tokyo, Japan). A He-Ne laser (Thorlabs HNLS008L-EC, Newton, MA, USA) at 632.8 nm is used as the probe laser in the diffraction measurements. The diffraction patterns are captured by a digital camera (Canon EOS 600D, Tokyo, Japan).

Results and Discussion
After the CLC mixture is filled in a homogeneously photoaligned cell, the LC molecules self-assemble into a helical structure with helix axes perpendicular to the substrates (Figure 1a). A planar texture in uniform orange is observed under the POM (Figure 1b). When applying an AC voltage, where k 11 , k 22 and k 33 are the splay, twist and bend elastic constants, respectively, ε 0 is the vacuum permittivity and ε is the dielectric anisotropy of the liquid crystal [35], unidirectional stripes begin to grow parallel to the alignment direction. In our experiment, U = 2.5 V (f = 1 kHz). Due to the antagonistic energies between the anchoring energy and helical twisted power, the helix axes are field-induced parallel to the substrates (Figure 1c). Due to the periodic modulation of the refractive index along the helical axis, a one-dimensional phase grating in the Raman-Nath regime is generated with L = 4.9 µm (Figure 1d). For a given cell, L strongly depends on p. When U exceeds 3 V, grating stripes begin to disappear and finally the CLC grating is switched off. A homeotropic nematic-like configuration is also obtained after U reaches 10 V. By means of photoalignment, a 2D checkerboard hierarchical grating is created and a delicate light control is further performed. As shown in Figure 2a, the size of the checkboard unit is 150 µm × 150 µm and the alignment directions in checkerboard units are perpendicular to each other indicated by yellow arrows. When applying a voltage of 2.3 V, orthogonal gratings gradually fill the checkerboard. To investigate the optical tuning property, the checkerboard hierarchical grating is irradiated under UV light (λ = 365 nm, intensity, I = 3 µW cm −2 ). With the exposure time T increasing from 0 min to 25 min, L is gradually elongated from 4.6 µm to 10.7 µm, which is 2.3 times the initial L (Figure 2b). L stays almost stable for 40 min after the light is turned off. The corresponding dependency of L on T is presented in Figure 2c. The increase of L originates from the photo-induced transformation of the chiral molecular switch, which reduces the HTP and results in the unwinding of helical structure. After 25-min irradiation, the decreased HTP is not sufficient to maintain stripes. Thus, the CLC grating begins to disappear, leading to a uniform texture with metastable unwound nematic configuration. A reverse optical tuning process is given in Supplementary Materials. To further inspect the performance of CLC hierarchical gratings, a He-Ne laser at 632.8 nm with circular polarization is normally incident onto the cell. The diameter of the probe laser beam is~1 mm. The resultant diffraction pattern is projected onto a screen and captured by a digital camera. The schematic diagram of the light path is shown in Figure 3a. The screen is set to receive diffraction patterns. As shown in Figure 3b, up to fifth-order diffraction spots along the x and y-axis are clearly observed (distance between the sample and the screen, S = 67.75 cm), indicating a large-area and well-ordered checkerboard configuration. The field-induced fingerprint grating diffracts light into four diagonal directions, visible up to the fourth order (S = 5.15 cm). Once the grating is irradiated under UV light, the first-order diffraction angle θ decreases from 9.4 • to 4.8 • due to the expanding of L (Figure 3c). The excellent regularity of L during light irradiation implies a predictable and reliable method for beam steering. In addition, the polarization sensitivity of the CLC grating is also investigated. As shown in Figure 3d, it is obvious that the spots diffracted from stripes parallel to the incident polarization possess higher intensity than those diffracted from orthogonal stripes, stemming from the optical anisotropy of the LCs and the periodic lying-helix structure inside the CLC grating [16,36,37]. Besides checkerboard hierarchical gratings, more complex and photo-responsive CLC hierarchical gratings are fabricated by rationally designing the alignments. As shown in Figure 4a, a micro-pattern of alternating ±45 • alignment is constructed with a two-step photo-exposure process [34]. Consequently, a triangular wave CLC grating is obtained accordingly (Figure 4b). To obtain a continuous space-variant orientation of SD1, the designed LC director distribution is divided into 90 equal subregions, endowed with uniform director values from π/90 to π in intervals of π/90. A sum-region of five adjacent subregions is exposed simultaneously with an exposure dose of 1 J cm −2 , which is one fifth of the sufficient exposure dose for the reorientation of SD1. The subsequent exposure of the sum-region shifts one subregion with the polarization changing 2 • synchronously. After the 90-step five-time partly overlapping exposure, a nearly arbitrary continuously varied alignment can be obtained. Thus, the Archimedean spiral and radial gratings are easily achieved by presetting angular and radial alignments (Figure 4c-f). Their optical tuning performances are similar to the abovementioned checkerboard gratings. It is worth mentioning that the light-induced variation of L is accompanied with the relaxation and unwinding of the Archimedean spiral, due to the limited number of stripes (usually only one stripe).
So far, we have achieved the complex photopatterning of CLC gratings and the lightdriven tuning of pitches. The hierarchical architecture of CLCs in the checkerboard grating originates from three distinct levels. One is the self-assembled helical structure and the other two are the field-induced fingerprint gratings and the periodic checkerboard pattern introduced by the photoalignment technique. Thanks to the intrinsic self-assembly of CLC, the photopatterned CLC gratings show the merits of simple-fabrication, reconfigurable, cost-efficiency, and easy-miniaturization. The photo-responsive chiral molecular switch induces the winding/unwinding of CLC helices and subsequently tunes the pitches of generated gratings. A drastic tuning range of 4.6 µm to 10.7 µm is achieved. The pitch tuning and beam steering exhibit a delicate, remote, and reversible control, which is promising in various active diffraction elements. It only needs a stimulating UV light with weak intensity of~3 µW cm −2 , which can be supplied by an incoherent light-emitting diode and eliminates the light damage to the sample [38]. In addition to the light controllability, the three-dimensional architecture of directors can be multi-manipulated by combining different stimuli, including heat, magnetic, and acoustic fields. Although only growingmodulation type fingerprints are demonstrated here, more fruitful complex gratings can be reasonably expected by introducing developable-modulation type fingerprints, whose formation is similar to the development of a photograph [39].

Conclusions
In conclusion, we propose electrically induced, photopatterned, and optically tuned hierarchical CLC gratings in a photosensitive helical medium. A checkerboard grating is photopatterned and optically tuned, providing a continuous L tuning from 4.6 µm to 10.7 µm. The corresponding beam steering capability is also investigated. Furthermore, a triangular wave pattern, an Archimedean spiral, and radial fingerprint gratings are also demonstrated thanks to the flexibility of the photopatterning technique. This work will open a new approach for fantastic, dynamic, and intelligent optical devices that may be widely utilized in beam steering, virtual reality, measurements, optical data storage, communications, etc.