1. Introduction

In the fields of optics and photonics, we have experienced significant progress with notable advancements in past decades, for examples, optical communication, beam shaping, and light-field displays. These developments have revolutionized the way that we transmit, manipulate, and visualize light. One of the key components to drive these progresses is the diffractive optical elements (DOEs), which play a pivotal role in enabling and enhancing various applications within the optical domain. DOEs are a powerful optical element that can generate desired wavefront for specific functions. Typically, specific surface relief structures or photomask patterns on a substrate are used to modulate the amplitude or phase of the incident light. With advantageous features of being thin, compact, and fabrication-easy [1,2], DOEs have been widely applied for various applications including spectral optimization [3], fiber couplers [4], beam shaping [5–7], laser coherent combination [8–11]. Compared to traditional optical components, DOEs can generate the desired wavefront by effectively changing the grating parameters, including the period, width, thickness, and geometrical structures, hence greatly increasing the freedom of design. However, to date, most DOEs are only passive since they are made of nondeformable materials and fabricated on rigid substrates, leaving a grand challenge to post-fabrication modification of their properties. Therefore, it is highly demanded to develop actively tunable DOEs to suit the need of applications. Fabricating DOEs on a flexible substrate and changing the period of the DOEs via mechanical deformation could be an effective way to realize tunable DOEs. A laser speckle suppression scheme has been proposed based on flexible DOE rings that can be tracked by a specific mechanical device [12,13].

As known, liquid crystal elastomers (LCEs) [14–16] are a class of smart materials that can undergo large deformations in response to external stimuli, such as electricity [17], magnetism [18] and light [19]. Due to the elasticity of the polymer network and the anisotropy generated by the liquid crystal orientation, reversible deformation and versatile shape morphing can be achieved based on LCEs, making them highly promising for artificial muscles and soft robots [20–22]. From another perspective, it is also possible to develop reconfigurable optical devices based on the LCEs’ unique properties [23–26]. Thus far, different LCE photonic structures have been developed for optical control of optical switch [19], diffraction [24], lasing [25], spectral tuning [27] and optical resonators [28,29]. However, it is still challenging to fabricate photonic micro/nanostructures on the LCE substrate. Very recently, Liu et al. demonstrated a tunable photoelastic metasurface [27] using the nanosphere lithography (NSL) technique [30,31]. However, this technique lacks the freedom of design since it is only limited to the fabrication of hexagonal structures.

Here we report a light-driven LCE-based DOE. The DOE consists of a square gold microarray that is photolithographically fabricated on the LCE, where the LCE serves as a deformable substrate. The LCE substrate undergoes reversible deformation upon excitation of an external laser beam, allowing the gold microarrays to be transformed from a square lattice to a central rectangular lattice, and subsequently resulting in the reconfigurable light diffraction pattern. The use of the well-developed photolithography technique provides a universal approach to fabricate arbitrary photonic structures on the LCE substrate, hence greatly increasing the design freedom.

2. Method

2.1 Sample preparation

The LCE substrate was prepared with a commercially available prepolymer mixture (see Fig. 1(a)). The prepolymer mixture was consisted of 78.55 mol% LC monomer, 4-methoxybenzoic acid 4-(6-acryloyloxyhexyloxy)phenyl ester (RM006, Synthon Chemicals), 20 mol% LC crosslinker, 1,4-bis-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2methylbenzene (HCM009, Synthon Chemicals), 0.53 mol% Disperse Red 1 acrylate (DR1A, Merck), and 0.92 mol% photoinitiator, 2,2-dimethoxy-2-phenylacetophenone (initiator 651, Sigma Aldrich). All the materials were used as received and ultrasonically mixed to form a homogenous prepolymer mixture. The prepolymer mixture was then filled into a cell with a controlled gap of 50 µm and then subjected to exposure of a UV lamp (center wavelength: 365 nm, intensity: 5 mW/cm²) at 25 °C for polymerization. After complete polymerization, the cell was opened and a free-standing LCE film was obtained by deliberately peeling off from the cell substrate, which serves as the substrate for further fabrication of the gold microarrays.

 

Fig. 1. (a) Chemical structures of the materials used to synthesize the LCE. (b) Schematic illustration of the fabrication process. (1) Deposition of a thin titanium layer on the LCE substrate; (2) Spin-coating the positive photoresist on the sample; (3) UV exposure by a direct writing laser; (4) Removal of the exposed areas using the developer; (5) Deposition of a 50 nm-thick gold layer; (6) The achieved gold disc microarray on the LCE substrate after the lift-off process in acetone.

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For the gold microarrays fabrication, a 3 nm titanium thin layer was first deposited on the surface of the LCE substrate using an electron beam evaporator (TF500, HHV, UK). There are two purposes for the thin layer of titanium deposition. One purpose is to serve as an adhesion layer for gold deposition. The other one is to act as a protective layer to minimize the impact of acetone on the LCE surface to the great extent in the subsequent lift-off process. After that, the LCE substrate was spin-coated with the positive photoresist (RZJ-304-10, Suzhou Ruihong Electronic Chemicals, China) and then patterned by a direct-write lithography machine (MicroWriter Ml3, Durham Magneto Optics, UK) at the UV exposure intensity of 120 mJ/cm². Subsequently, the patterned photoresist was developed in the developer (RZX-3038, Suzhou Ruihong Electronic Chemicals, China) for 20 s. Following a 50 nm gold layer deposition using an electron beam evaporator, residual photoresist was completely removed by soaking and rinsing with acetone. Finally, the tetragonally distributed gold microarray was fabricated on the LCE substrate. The whole fabrication process of the gold microarray is schematically shown in Fig. 1(b).

2.2 Characterization

The fabricated LCE substrates with planar molecular alignment can exhibit reversible deformation upon thermal excitation [16]. In our experiment, the LCE film was placed at a hot stage and observed under an optical microscope, and the LCE deformation at different temperatures was measured accordingly. The LCE film was cut into a size of 500 × 500 µm². The transmission spectra were measured using a UV–vis–NIR microspectrophotometer (CRAIC Technologies Inc., USA) with the reference signal taken from the quartz substrate. To investigate the actively tunable diffraction pattern, two optical setups were constructed with a hot plate (Fig. 2(a)) and a pump laser (Fig. 2(b)), respectively. In the optical setup in Fig. 2(b), a 633 nm laser (Pacific Lasertec, USA) was used as the probe light. During the testing process, the 633 nm laser beam was deflected by a mirror and diffracted upon incidence onto the sample. The diffracted light then passed through a beam splitter and was converged into the CCD with a lens. A 532 nm laser (Changchun New Industries Optoelectronics Technology Co., China) was placed above the sample as the pump light to control the LCE deformation. An optical filter was placed in front of the CCD to block the 532 nm laser beam, while leaving only the 633 nm laser beam entering the CCD.

 

Fig. 2. Optical setups to record the diffraction patterns with the LCE deformation (a) thermally controlled by a heating stage, and (b) photothermally controlled by the pump laser light, respectively.

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3. Results and discussion

In our experiment, the LCEs were prepared in uniaxial alignment, hence illustrating strong optical anisotropy and polarization dependence. Figures 3(a) and (b) show typical images observed under a polarizing optical microscope (POM). When the LCE alignment direction is parallel to the polarization direction of either polarizer, the LCE demonstrates a completely dark state (Fig. 3(a)). While the alignment direction has a 45° angle with respect to the polarization direction, a clear bright state can be observed, as shown in Fig. 3(b). These observations confirm the uniaxial alignment of the LCE film. Figure 3(c) shows the measured absorption spectra of the LCE film. From Fig. 3(c), we can see that the absorption of the LCE film is ∼70% at the wavelength of 532 nm, while the absorption of the LCE film is <10% at the wavelength of 633 nm. This is mainly attributed to the absorption of the dye DR1A doped in the LCE. In addition, by changing the dyes doped in LCE, we will be able to achieve higher transmittance and absorbance at specific wavelengths in the future.

 

Fig. 3. (a, b) POM images to confirm the uniaxial alignment of the LCE. (c) The absorbance curve of the LCE. The green dashed line indicates the wavelength (532 nm) of the excitation laser. (d, e) Deformation curve of the LCE film (negative value for contraction and positive for expansion) as a function of the temperature (d) and the pump light power (e), respectively. The shaded area is the error area obtained from multiple measurements.

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When the LCE file was placed at a hot plate, the uniaxially aligned LCE underwent a phase transition from an anisotropic to an isotropic state, disrupting the molecular orientation originally maintained by the cross-linked network. This order-to-disorder phase transition results in macroscopic deformation of the entire LCE film [32]. Macroscopically, contraction occurs along the alignment direction, while expansion takes place at the same time perpendicular to the alignment direction. The temperature-dependent LCE deformation was measured under the optical microscope, as shown in Fig. 3(d). It can be clearly seen that the fabricated LCE film reaches its maximum deformation at ∼250°C with a shape variation of ∼20%. Figure 3(e) illustrates the shape deformation curve of the LCE film as a function of the pump power. We can clearly see that the deformation reaches its maximum at the light power of ∼150 mW. The light-induced deformation is essentially also attributed to photothermal effect. The LCE film doped with DR1A has strong absorption for the 532 nm laser beam, hence resulting in an elevated temperature due to the photothermal effect. Therefore, in both thermal heating and photothermal heating cases, the LCE film demonstrated quite similar deformation behaviors. However, comparatively, the light-driven approach is much more advantageous in terms of remote control, noncontactness, and flexibility.

With knowing the alignment and deformation characteristics of LCEs, we further fabricate the gold microarray on them. In our experiment, we prepared a square lattice gold micropillar array. Figure 4(a) shows the typical morphologies of the gold microarray observed under the scanning electron microscope (SEM). Figures 4(b) and (c) illustrate the magnified top and 45°-tilt views of the highlighted area in Fig. 4(a). The array has a period of ∼1.8 µm and each disk has a diameter of ∼1.2 µm and the height of ∼50 nm. The total size of the fabricated array is 500 × 500 µm². Figures 4(d) and (e) show the POM images over a large area when the alignment direction of the LCEs has 0° and 45° angles respectively with respect to the optical axis of the analyzer. We can observe a similar effect under POM with Figs. 3(a) and (b) caused by the uniaxial alignment of the LCE substrate, indicating the complicated photolithography processes for the gold microarray fabrication on the LCE film have negligible impact on the LCE film.

 

Fig. 4. (a − c) SEM images of the gold microarray on the LCE substrate: (a) typical large-area top view, (b) magnified top view of a single unit, and (c) cross-section view of a single Au disk of the Au microarray. Scale bar in (a): 5 µm. (d, e) POM images of the gold microarray on the LCE substrate. Scale bar in (d) and (e): 5 µm.

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From the above discussion, we can achieve ∼20% shrinkage and expansion in two orthogonal directions. It is straightforward that upon a gold microarray situating on the LCE substrate, the deformation of the LCE substrate causes the lattice change of the gold microarray, hence resulting in a distinctive change of the diffraction pattern. In a previous report, Yu et al. analytically and experimentally demonstrated the phase transition of plasmonic lattices between two arbitrary 2D Bravais lattices under certain strain configurations [33]. They fabricated the gold microarray via electron-beam lithography and then transferred to a PDMS substrate. The lattice transformation of the gold microarray can be achieved by stretching the PDMS. However, the fabrication is much more complicated, and the mechanical stretching makes the whole system bulky, which is not conducive to miniaturization and light weight of the device. In our design, we propose light-driven phase transition of the gold microarray, which can be reconfigured between the square lattice and the rectangular one. Figure 5 shows the schematic diagram of the lattice transformation of the gold microarray.

 

Fig. 5. Schematic diagram of the phase transformation of the gold microarray from a square lattice to a central rectangular lattice.

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Due to the strong light absorption caused by the doped dye DR1A, the LCE substrate can be photothermally activated, demonstrating similar deformation behavior as the heat causes. Figures 6(a) and (b) show the optical images before and after the laser pump. The pump light power used in our experiment was 150 mW and the pump time was 1 s. It can be clearly seen that there is a significant change in the lattice structure after the pump. A fast Fourier transform was carried out for the corresponding square lattice and central rectangular lattice, respectively, as shown in Figs. 6(c) and (d). We have experimentally taken the pictures of their corresponding diffraction patterns, as shown in Figs. 6(e) and (f). Both the experimental and simulated results are in good agreement. In the diffraction pattern of the square lattice, the distances between three adjacent spots are defined as w and l (see Fig. 6 (g)). We therefore measured the quantitative changes of the diffraction pattern under thermal and photothermal deformation, respectively, as shown in Figs. 6 (h) and (i). Compared Fig. 6(f) with Fig. 6(e), we can observe that the diffraction pattern changes from square to rectangle. Moreover, a closer look reveals that the uppermost and lowermost diffracted light spots disappear, while the leftmost and rightmost spots approach closer to the center. The observed changes in the diffraction pattern can be mainly attributed to the deformation-induced lattice transformation by the LCE substrate. As shown in Fig. 5, when the lattice structure transforms from square to central rectangle, the distance among the four gold disks closest to the central one in a square lattice structure has changed from equal to unequal, hence resulting in the change of the diffraction pattern. In our experiments, the measured response times of the phase change upon the excitation and deexcitation are ∼1 s, respectively. We can conclude that upon thermal and photothermal excitation/deexcitation, the gold microarray on the LCE substrate will undergo a reversible phase transition, demonstrating an important reconfigurable feature.

 

Fig. 6. Gold microarray with (a) square and (d) central rectangular lattice structures at initial and excited states. (b, e) Calculated FFT patterns and (c, f) Experimentally measured diffraction patterns corresponding to square and central rectangular lattices, respectively. (g) The representation of the distances w, l in the diffraction pattern. Length variation of distances w and l when LCE is deformed by (h) thermal and (i) photothermal excitation, respectively. The shaded area in (h) and (i) is the error area obtained from multiple measurements.

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4. Conclusion

In summary, we have demonstrated the light-driven phase transition of DOEs based on the gold microarray situated on the LCE substrate. We experimentally showed the reconfiguration between the square and center rectangular lattices under either thermally or photothermally induced LCE deformation, hence realizing the control of the diffraction pattern. The photothermal driving features all-optical, noncontact (remote), low-power control of the LCE deformation. Such a proof-of-concept demonstration verified that with proper and deliberate design, a more comprehensive crystallography library can be realized with the LCE deformation-based phase transition. The deformation-induced structural change also provides a dynamic platform for light field control, playing an essential role in adaptive or reconfigurable photonics and plasmonics.