From Dark to Light in a Flash

Design concept for the switchable 3D scatterer. (Source: KAIST)

Researchers from the Korea Advanced Institute of Science and Techno­logy (KAIST) have developed a new easy-to-use smart optical film tech­nology that allows smart window devices to auto­nomously switch between transparent and opaque states in response to the surrounding light conditions. The proposed 3D hybrid nano­composite film with a highly periodic network structure has empiri­cally demonstrated its high speed and perfor­mance, enabling the smart window to quantify and self-regulate its high-contrast optical trans­mittance.

As a proof of concept, a mobile-app-enabled smart window device for Internet of Things (IoT) appli­cations has been realized using the proposed smart optical film with successful expansion to the 3-by-3-inch scale. This energy-efficient and cost-effective tech­nology holds great promise for future use in various appli­cations that require active optical trans­mission modu­lation. Flexible optical transmission modulation techno­logies for smart appli­cations including privacy-protection windows, zero-energy buildings, and beam pro­jection screens have been in the spotlight in recent years. Conven­tional technologies that used external stimuli such as electricity, heat, or light to modulate optical trans­mission had only limited applications due to their slow response speeds, unneces­sary color switching, and low durability, stability, and safety.

The optical trans­mission modulation contrast achieved by control­ling the light scattering interfaces on non-periodic 2D surface structures that often have low optical density such as cracks, wrinkles, and pillars is also generally low. In addition, since the light scattering interfaces are exposed and not subject to any passi­vation, they can be vulnerable to external damage and may lose optical trans­mission modu­lation functions. Furthermore, in-plane scattering interfaces that randomly exist on the surface make large-area modulation with uniformity difficult.

Inspired by these limi­tations, a research team led by Seokwoo Jeon and Jung-Wuk Hong used proximity-field nano­patterning (PnP) technology that effectively produces highly periodic 3D hybrid nano­structures, and an atomic layer depo­sition (ALD) technique that allows the precise control of oxide deposition and the high-quality fabri­cation of semi­conductor devices. The team then success­fully produced a large-scale smart optical film with a size of 3 by 3 inches in which ultrathin alumina nanoshells are inserted between the elastomers in a periodic 3D nano­network.

This mechano-responsive 3D hybrid nano­composite film with a highly periodic network structure is the largest smart optical trans­mission modulation film that exists. The film has been shown to have state-of-the-art optical trans­mission modu­lation of up to 74% at visible wavelengths from 90% initial trans­mission to 16% in the scattering state under strain. Its dura­bility and stability were proved by more than 10,000 tests of harsh mechanical deformation including stretching, releasing, bending, and being placed under high tempera­tures of up to 70°C. When this film was used, the trans­mittance of the smart window device was adjusted promptly and auto­matically within one second in response to the surrounding light conditions. Through these experi­ments, the underlying physics of optical scattering phenomena occurring in the hetero­geneous inter­faces were identified.

Donghwi Cho, a PhD candidate in materials science and engi­neering, said, “Our smart optical film techno­logy can better control high-contrast optical trans­mittance by relatively simple operating prin­ciples and with low energy consumption and costs. When this technology is applied by simply attaching the film to a conventional smart window glass surface without replacing the existing window system, fast switching and uniform tinting are possible while also securing durability, stability, and safety. In addition, its wide range of appli­cations for stretchable or rollable devices such as wall-type displays for a beam projection screen will also fulfill aesthetic needs.” (Source: KAIST)

Reference: D. Cho et al.: High‐Contrast Optical Modulation from Strain‐Induced Nanogaps at 3D Heterogeneous Interfaces, Adv. Sci., 201903708 (2020); DOI: 10.1002/advs.201903708

Link: Flexible Device and Metamaterials Lab (FDML), Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea

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