Thermally Triggered Multilevel Diffractive Optical Elements Tailored by Shape Memory Polymers for Temperature History Sensors

Shape memory polymers (SMPs) have been widely used as sensory material due to capability to induce morphological transition in response to external stimuli. Diffractive optical elements (DOEs) have shown their attractive potential as sensing platforms owing to the high sensitivity provided by the real-time response of far-field diffraction patterns. Many studies have demonstrated a series of reconfigurable DOEs with SMP based on conventional template molding process, which is fast, inexpensive and has no limitation in materials. However, in most cases, simple surface relief structures such as micro-groove grating were chosen as a proof of principle model due to their straightforward relationship between ridge height and diffraction intensity. Thus, they still need to be validated in a much more complex diffraction pattern which facilitates achieving higher information content to provide greater flexibility in device design and operation. Here, by combining the temperature-sensitive characteristic of SMP and the display characteristic of multilevel DOEs, an optical temperature sensing film showing the temperature information as a change of far-field diffraction images is proposed. To achieve this, micropatterns of the DOEs are imprinted on epoxy based SMP film, which can be programmed to hold a temporary optical image and revert to its original image when exposed to a specific temperature. Convolutional neural networks (CNNs) are utilized to detect and measure the optical image transformation depending on temperature. Furthermore, the threshold temperature where the image transformation occurs can be customized by varying the chain flexibility of the SMP. Based on a range of transition points, imprinting the desired combination of SMP-DOEs on a film, a sensor that can record and inform the temperature history is demonstrated. This work can be used for the compact and reliable optical temperature indicator, which can be applied in temperature-sensitive industries such as food and pharmaceutical.