Engineers at the University of California San Diego have developed an optical device that reveals hidden images and changes colors in response to different levels of humidity. The technology, published in Light: Science & Applications, could lead to the development of new anti-counterfeiting labels, secure data storage, interactive displays, and environmental sensors.
The device works by displaying different images depending on moisture levels in the air. Under normal conditions or low humidity levels, one image (UC San Diego Triton logo) is visible. When humidity increases, a second image (UC San Diego library logo) emerges and conceals the first. This transition can be triggered even when a person breathes on the device. It happens in a fraction of a second and can be repeated many times.
“You can imagine using this as a built-in security feature with the environment acting like a key that unlocks different pieces of information,” said First Author Asad Nauman, an electrical and computer engineering postdoctoral researcher at UC San Diego. “One example would be something like a credit card security tag, where you can blow on it and reveal a hidden code. Another application would be an environmental sensor that changes color as the humidity changes.”
The new optical device is a postage stamp-sized chip that consists of two layers. The bottom layer is what is known as a phase-change material called antimony trisulfide, which functions as a reusable storage medium. Images can be written, erased and rewritten on this layer using a laser. The top layer is a hydrogel made of azido-grafted carboxymethyl cellulose, a soft material that swells in humid conditions and shrinks in drier ones. An image can be permanently patterned into this layer using UV light.
Here is an exclusive Tech Briefs interview, edited for length and clarity, with Nano Devices and Applied Optics (NDAO) Lab Lead and Senior Author Abdoulaye Ndao, Professor, Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
Tech Briefs: What was the biggest technical challenge you faced while developing this optical device?
Ndao: One of the biggest hurdles in developing the new optical device was integrating two fundamentally different classes of materials into a single working platform. Phase-change materials, such as antimony trisulfide (Sb₂S₃), are widely used for rewritable optical storage because they can be repeatedly switched between different optical states, while hydrogels are soft, water-responsive materials that expand and contract in response to humidity.
We initially designed the device with the 20-nanometer-thick Sb₂S₃ layer positioned on top of the hydrogel and a thin silver semi-reflector beneath it. However, the design quickly ran into a major obstacle. Because Sb₂S₃ is hydrophobic, it effectively blocked moisture from reaching the hydrogel, preventing the material from responding to changes in environmental humidity. At one point, the project seemed destined to fail.
Then we decided to completely rethink the device architecture. Instead of placing the phase-change layer above the hydrogel, we reversed the order of the layers, positioning Sb₂S₃ beneath the hydrogel and depositing silver on top. The designed silver layer was intended not to form a continuous film but rather a porous layer of nanoscale islands. The gaps between the silver nanoislands allowed water molecules to penetrate and interact with the hydrogel, enabling the fast and reversible humidity response needed for dynamic image switching.
Even after solving this challenge, we still faced the difficult task of ensuring that both the phase-change layer and the hydrogel could be programmed independently while maintaining a strong optical signal. Successfully balancing these competing requirements ultimately enabled the team to create a device capable of storing, concealing, and revealing different images under ambient light.
Tech Briefs: Can you please explain in simple terms how it works?
Ndao: You can imagine the device as an ultrathin storage drawer that displays multiple stored information on demand stored in different stacked layers. The demonstrated device contains two stored images, one image is written and erased using a laser on phase change material, while another image can be programmed into a humidity-responsive hydrogel using photolithography. Under normal conditions, only one image is visible which is written on phase change material. As the humidity changes, the hydrogel layer expands slightly, altering how light passes through the device and revealing a different image. In this way, the same device can display different information depending on the surrounding environment, all under ordinary white light.
Tech Briefs: Do you have any updates you can share?
Ndao: We are actively exploring electrically controlled versions of the platform. One promising direction is to replace or augment the humidity-responsive mechanism with electrically responsive soft materials and electro-optic structures. The long-term goal is to create devices that can switch information states on demand using a small electrical signal rather than relying solely on environmental stimuli. While this work is still in its early stages, we believe it could enable faster switching speeds, greater integration with electronic systems, and entirely new applications in adaptive optics and programmable photonic devices.
Tech Briefs: Do you have any set plans for further research/work/etc.? If not, what are your next steps?
Ndao: Our next steps are to expand the platform beyond data storage and encryption toward large-area adaptive optical surfaces and dynamic displays. One direction we are particularly excited about is the development of "dynamic wall paintings" and architectural photonic surfaces that can autonomously change their appearance in response to environmental conditions such as humidity, temperature, or seasonal changes. For example, a wall painting or decorative panel that autonomously changes its appearance with seasonal humidity variations or user-controlled indoor humidity, without requiring electronics, power consumption, or digital screens. We are also exploring electrically controlled versions of the technology to enable on-demand switching and greater user control. More broadly, we envision a new generation of intelligent optical materials that seamlessly integrate sensing, information display, and environmental responsiveness into everyday spaces.
Tech Briefs: Do you have any advice for researchers aiming to bring their ideas to fruition?
Ndao: Many impactful ideas emerge at the intersection of different fields. This project was made possible by combining expertise in phase-change photonics, soft materials, optics, and device engineering. My advice is to remain open to collaboration, communicate frequently with researchers outside your immediate specialty, and be willing to explore unconventional combinations of ideas. Some of the most exciting discoveries occur when concepts that seem unrelated at first are brought together in a new way.
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