Soft tunable diffractive optics with multifunctional transparent electrodes enabling integrated actuation

We present a stretchable tunable transmission grating in which the optical surface serves simultaneously as an electrode for electrostatic actuation. Tunable optics based on elastomers allow for a large tuning range, but integrating an actuator generally significantly increases the device footprint. By combining the optical and electrical functions into one multifunctional transparent material, we use here the grating as an integral part of a dielectric elastomer actuator and hence avoid placing actuators around the grating. The grating/electrode consists of a 750 nm thick soft ionogel, which is bonded on both sides of a 13 μm thick silicone membrane. The top ionogel electrode is corrugated (2 μm pitch) and serves as the diffraction grating. The bottom electrode is planar. Applying a voltage between the electrodes generates a Maxwell pressure, leading to the in-plane expansion of the elastomer and electrodes. The linear actuation strain of 12.8% is obtained at 1300 V. The ionogel grating maintains accurately its period after 500 cycles and after one-month storage. The ionogel electrodes present self-clearing properties, allowing the operation of the actuator close to the breakdown voltage. This device presents an unprecedented level of integration by making accurate grating structures directly on a transparent soft ionogel conductor, which opens broad possibilities for making tunable optics.