Fabrication of OPAs. As shown in Fig. 1a, an OPA is composed of a photo-responsive elastomeric composite sheet with an OFT embedded in the middle as the waveguide. The elastomeric composite sheet is made of a polydimethylsiloxane (PDMS) film, which is doped with Au nanorods (AuNRs) as the photothermal agent, and a graphene oxide (GO) film. When a control light is launched into the elastomeric composite via the OFT, photothermal heating induced by the AuNR will cause a significant expansion of the PDMS/AuNR layer due to the high coefficient of thermal expansion (CTE) of PDMS37,38. On the other hand, GO layer undergoes negligible thermal expansion due to its low CTE. Thus, the mismatch between the CTE and deformations of two layers leads to a dramatic bending of the OPA. When the light is switched off, the OPA recovers back to its initiate state. Figure 1b shows a detailed preparation process of the OPA. Firstly, a thin layer of PDMS doped with AuNRs was spin-coated onto a glass substrate as the first layer of the elastomeric composite sheet. Then, an array of OFTs were embedded in the PDMS layer with an interval of ~ 5 mm, which were subsequently covered with a thin layer of GO sheet. Finally, as fabricated photo-responsive elastomeric composite sheet was cut into small pieces to obtain OPAs. AuNRs used in OPAs, with average length and diameter of ~ 51 and 23 nm respectively (Fig. 1c, left inset), show an ensemble longitudinal surface plasmon resonance peak around 638 nm (Supplementary Fig. 1), which matches well with the peak wavelength of the control light. Due to the high absorption cross section (Fig. 1c) and strong local-field enhancement (Fig. 1c, right inset), the AuNRs can efficiently generate heat in the PDMS layer via the nonradiative decay of LSPR excited by the control light. OFTs were fabricated by flame-heated taper drawing34,35 of standard optical fibres. Figure 1d shows an optical microscope of a typical OFT with a taper length of ~ 1.75 cm and a tip diameter of ~ 700 nm. The relatively long taper can effectively suppress the intermodal energy transfer in the taper region and satisfy the adiabatic condition34,39, guaranteeing the high optical coupling efficiency, while the small tip diameter is advantageous for the seamless shrink of beam size down to micrometer scale. The cross-sectional view of an as-fabricated elastomeric is shown in Fig. 1e, in which a layer of PDMS/AuNR with a thickness of ~ 70 µm and a layer of GO with a thickness of ~ 1.5 µm can be clearly seen. Despite of the thin thickness, the GO layer shows a fine lamellar structure and typical wrinkled surface, benefiting from the outstanding self-assembly ability of GO sheets with large lateral dimension (Supplementary Fig. 2). When guiding a 635-nm-wavelength laser into an OPA, a clear light propagation path in the sheet can be observed due to the scattering of light by the AuNRs (Fig. 1f). The width of the OPA in Fig. 1f was designedly broadened to completely display the light propagation path, while the typical size of OPA is usually ~ 500 µm in width and ~ 1 cm in free length (Fig. 1g and Supplementary Fig. 3).
Light-driven actuation. As previously mentioned, the actuation mechanism of OPA is based on the CTE mismatch and asymmetric deformation of the PDMS/AuNR-GO bilayer under photothermal heating. We first investigated the deformation behavior of an OPA when a temperature rise is directly applied onto the bilayer structure. As shown in Fig. 2a, the PDMS/GO bilayer (70 µm/1.5 µm) undergoes a reversible bending when the environmental temperature is switched between room temperature (RT, 20 ℃) and 60 ℃, which is simulated based on finite element analysis (FEA, see details in Methods). The simulated bending angle of the OPA increases linearly with the increase of environmental temperature (Fig. 2b and Supplementary Fig. 4), which agrees very well with the experimental results by putting it on a hot plate. Thus, by taking advantage of the photothermal effects of embedded AuNRs, OPA can undergo similar bending. As shown in Fig. 2c, when a 635-nm-wavelength laser (75 mW) was launched into the OPA, the temperature rise induced by the photothermal effect leads to a significant bending of OPA, with the highest temperature (Tmax) of ~ 110 ℃ located near the OFT tip of the OPA. It is worth to note that there is a diminishing bending curvature along the longitudinal direction due to the gradient distribution of temperature. Figure 2d compares laser power-dependent Tmax of OPAs fabricated with different components. As expected, the AuNR plays an important role in the light-to-heat conversion of the OPA, and the GO also shows notable conversion ability due to its light adsorption property in the UV-Vis spectral range (Supplementary Fig. 5). The Tmax of an OPA composed of PDMS/AuNR-GO under 150 mW laser is as high as 170 ℃, which is benefited from the significantly enhanced energy density near the OFT tip and the high photothermal conversion efficiency of AuNRs.
The light-driven deformation amplitude of OPAs can be readily regulated by controlling the laser power. With the increase of control laser power, the bending angle increases gradually (Fig. 3a and Supplementary Fig. 6). The highest bending angle is larger than 270° at 150 mW, which is much larger than the bending angle (< 60°) of other types of waveguide actuators31–33. In addition, the bending angle of OPA is also much larger than that of the actuator driven by free-space light under the same laser power of 150 mW (Fig. 3b, I). When illuminated with a free-space laser beam, the PDMS/AuNR-GO film can only absorb a small portion of the light, while the rest is wasted since the illuminating area is larger than the film. As a result, the energy efficiency is pretty low and the bending angle is only 62°. Using OFT to guide light into the PDMS/AuNR-GO film can effectively overcome this problem since the light can be guided along the film and absorbed continuously, allowing for a higher energy efficiency and larger bending angle. The energy consumption to drive the OPA is as low as < 0.55 mW/°, indicating the high actuation efficiency with the use of OFT, which is favorable for the cost-cutting and miniaturization of the integrated device in practical applications.
On the other hand, the small tip diameter of the OFT (Fig. 1d) allows for reducing the thickness of OPA to less than 100 µm, which is favorable for large bending angles. In contrast, the actuator fabricated with a standard optical fibre has a total thickness of ~ 220 µm due to the large diameter of the standard optical fibre (Supplementary Fig. 7a, b), and the bending angle under 150-mW laser is only 56° (Fig. 3b, II). The detailed influences of thicknesses of each layer on the bending angles of OPA are shown in Supplementary Fig. 8. In addition, OPA with similar thickness (~ 225 µm, Supplementary Fig. 7c) also shows larger bending angle (85°) than that of standard optical fibre-based actuator (Fig. 3b, III), which indicates that the enhanced energy density, produced by the small tip diameter of the OFT, also favors the efficient actuation of the OPA. In summary, the thickness as thin as 70 µm, along with the improved energy density and optical coupling efficiency, endow the OPA with excellent light-driven actuating performances.
Dynamic deformation of the OPA under 120 mW laser indicates a rapid response of the OPA (Fig. 3c and Supplementary Movie 1). It takes only 1.8 s to reach a bending angle greater than 180° (~ 70% of the total deformation), whereas the recovery of the 70% bending takes 2.3 s after the switch off of the laser. This is understandable because the cooling process of PDMS is slower than the heating process. The fast response of OPAs is probably ascribed to the high energy density and thin thickness of active layer benefiting from the small tip of OPT. Moreover, the OPA shows negligible decay in deformation after more than 3000 actuations with a standard deviation of 3.62° and coefficient of variation as low as 0.14 (Fig. 3d and Supplementary Fig. 9). The excellent stability and maneuverability endow OPAs with significant advantages in accurate control when executing a task that involves location changes, while the free-space light-driven actuators suffer from the difficulties in accurately controlling the illuminating spot.
Biomimetic applications. Many natural activities of creatures can be regarded as a series of actuations performed through various actuators with different functions. For example, when a chameleon on a branch preys an insect, its tail winds the branch to keep balance (function i), and its tongue reaches out and captures the insect (function ii), as shown in Fig. 4a. Inspired by this, an OPA is used to mimic the predation of chameleons. As shown in Fig. 4b, the OPA winds rapidly onto a plastic pipe under 635-nm laser within 2.2 s (panels 1–3), and unwinds gradually after switching the control laser off (panels 4–6), which clearly imitates the function of the tail (see more details in Supplementary Movie 2). Similarly, the OPA can grasps an ant (~ 20 mg) stayed on a tip immediately with the switch on of control laser (Fig. 4c and Supplementary Movie 3), which imitates the function of the tongue. Benefiting from the large bending deformation, the OPA can hold the ant firmly and take it off from the tip, showing great potential in biomimetic target capture.
In fact, target capturing/moving is an important and challenging application of photoactuators. For the use of free-space light-driven actuators to capture/move objects, the illuminating spot needs to be accurately controlled to follow the moving of actuators or a large illuminating beam covering the whole operating area should be supplied, which greatly restricts the maneuverability and operating area. However, for OPAs demonstrated here, they are seamlessly connected to optical fibres, allowing the control light to intrinsically follow the moving of actuators. Thus, OPAs hold significant advantages in accurately handling and moving objects in a wide operating area. A soft gripper composed of two OPA strips was fabricated to capture objects with different shapes (Fig. 5 and Supplementary Movie 4–6). Initially, the two OPA strips bend outwards with the GO sides aligned face to face to offer an opening angle large enough for accommodating different objects with complex shapes. With the switch on of 635-nm laser, the two arms of the OPA gripper bend inwards to capture and lift targets such as a cuboid, a ball and three balls, as shown in Fig. 5. When the laser is switched off, the gripper arms bend reversely to release the object. Alternatively, the OPA strips can also initially form an inward bending configuration for the gripper to capture small objects (Supplementary Fig. 10 and Supplementary Movie 7, 8).