Micro-LED displays have rapidly become one of the most compelling technologies in advanced display engineering. By using inorganic light-emitting diodes shrunk to microscopic dimensions, they deliver exceptional brightness, fast response times, strong contrast, and remarkably low power consumption. These properties make them especially attractive for near-eye applications such as AR and VR headsets, where pixels must be both extremely small and extremely efficient. Yet building a full-color micro-LED display remains one of the field’s most stubborn engineering challenges, because red, green, and blue emitters must all be fabricated and aligned with near-perfect registration at the micrometer scale and below.

One of the most practical strategies for achieving full color is quantum-dot color conversion. In this scheme, the display is built from efficient blue micro-LEDs, and selected pixels are overlaid with films of red- or green-emitting quantum dots—nanoscale semiconductor crystals whose emission wavelength is set by their size. When blue light from the underlying LED excites the dots, they re-emit it as red or green, while the untouched blue pixels provide the third primary color. The approach elegantly sidesteps the difficulty of making efficient red micro-LEDs directly, since red indium gallium nitride emitters suffer from sharply declining efficiency as their size shrinks.

The bottleneck, however, lies in patterning. Different quantum dots must be placed into precisely defined, uniform, and closely spaced pixel wells without contaminating neighboring pixels, spreading across the surface, or degrading the delicate optical properties of the nanocrystals themselves. Conventional patterning techniques—photolithography, inkjet printing, transfer stamping—each bring trade-offs involving multi-step processing, limited resolution, material bleed, or chemical and thermal damage to the quantum dots.

The Beijing Institute of Technology team, led by Professor Xueqiang Zhang with PhD student Muhammad Farhan as first author, addressed this problem with a spatially shaped femtosecond laser combined with a selective filling process. Their workflow begins with a glass substrate coated with a thin layer of SU-8, a common epoxy-based photoresist polymer. Using a femtosecond laser, the researchers drill ordered arrays of microholes directly into the polymer, creating an empty pixel mold before any quantum dots are ever introduced.

The key innovation lies in how the laser beam is shaped. A conventional Gaussian beam concentrates its intensity in a smooth, bell-shaped profile that tends to produce holes with ragged entrances, rough walls, and a characteristic taper that widens toward the surface. Moreover, high intensities deep in the focus can damage the underlying substrate. The team instead employed a Bessel beam, a non-diffracting beam profile with an elongated axial focus that distributes the laser energy differently. Compared with Gaussian drilling, the shaped Bessel beam produced microholes with cleaner entrances, smoother sidewalls, reduced taper, and markedly less damage to the glass below. By tuning laser parameters such as pulse energy and focusing conditions, the researchers could sweep the hole diameter across a broad range, ultimately demonstrating well-formed nanoholes as small as 100 nanometers—a scale at which the feature is smaller than most wavelengths of visible light.

Once the mold was in place, the delicate luminescent material entered the picture. Red or green cadmium selenide (CdSe) quantum dots were deposited into selected holes and cured in place, after which excess material was removed from the surface, leaving clean, isolated pixel wells. Because the femtosecond laser never touches the quantum dots directly—the light-emitting nanocrystals are introduced only after the drilling is complete—the method avoids the photochemical damage and thermal stress that laser-based patterning often inflicts on quantum materials. This separation of steps is central to the technique’s appeal: the laser provides lithographic precision while the quantum dots retain their pristine optical quality.

The team demonstrated the versatility of the approach by fabricating well-defined monochromatic red and green pixel arrays, with individual luminescent pixels reaching diameters of approximately 700 nanometers. By performing sequential rounds of drilling and filling—first patterning one set of wells and populating them with one color of dots, then drilling a second aligned array and filling it with another—the researchers also produced red-and-green dual-color arrays with crisp, well-resolved boundaries between neighboring pixels of different colors. Such spatial registration is precisely what a full-color display demands.

The optical performance of the resulting color-conversion layers was equally impressive. The patterned films exhibited narrow-band red and green fluorescence, indicating high color purity—a critical figure of merit for wide-gamut displays. Pixel-to-pixel emission was highly uniform, with the red and green arrays achieving luminous uniformities of approximately 90 percent and 97 percent, respectively. Taken together, the color performance of the fabricated layers corresponded to a color gamut covering 111 percent of the NTSC standard, exceeding even the benchmark set for high-end display color reproduction.

What makes the work particularly notable is the combination of attributes that rarely coexist in a single patterning method: it is maskless, requiring no expensive photomasks and permitting rapid design iteration; it is solvent-free in the patterning step, simplifying processing; and it is fundamentally quantum-dot-compatible, since the light-emitting material is shielded from all direct laser exposure. The resolution—sub-micrometer pixels and 100-nanometer holes—far exceeds what most printing-based approaches can achieve, while the smooth, low-damage hole walls produced by the Bessel beam ensure that filled dots emit efficiently and uniformly.

The researchers acknowledge that the path from laboratory demonstration to manufacturing will require further development. Scaling the approach to large display areas will demand parallel laser processing, since sequentially drilling billions of pixel wells with a single beam is impractical for mass production. Improvements in quantum-dot filling efficiency and in integration with actual micro-LED driver backplanes will also be essential before the technique can populate real devices. Yet the demonstration of reliable, uniform, sub-micrometer luminescent pixels with excellent color purity establishes a flexible and promising foundation for that scale-up effort.

If those engineering challenges can be met, the implications extend beyond AR and VR headsets. High-pixel-density quantum-dot color-conversion layers could benefit microdisplays for projectors, heads-up displays, ultra-dense sensor arrays, and any application where light of different colors must be generated in pixels smaller than the eye can resolve. For now, the Beijing team’s spatially shaped femtosecond laser offers a vivid illustration of how advances in light-shaping and precision manufacturing continue to push the physical limits of what a display pixel can be.

Among the emerging technologies, optical encryption has garnered significant attention due to its inherent advantages of high-speed processing, vast data capacity, and parallelism capabilities. Optical metasurfaces, ultrathin materials engineered to manipulate light at subwavelength scales, have revolutionized this domain. They enable multi-dimensional control over various properties of light, including phase, amplitude, and polarization. The adaptability of metasurfaces, particularly their integration with external stimuli such as thermal inputs, electrical fields, and phase-change materials, paves the way for dynamic, flexible, and scalable encryption platforms. However, practical deployment is hampered by two primary limitations: the necessity for cumbersome decryption setups and the reliance on expensive, high-precision fabrication methods like electron beam lithography.

Addressing these challenges, a promising approach leverages the inherent randomness and quasi-ordered structures achievable in metasurfaces. These configurations provide greater manufacturing tolerances and expand accessible design parameters without compromising encryption fidelity. Nevertheless, producing such metasurfaces at scale demands a new fabrication technique that balances complexity, efficiency, and precision. This is where femtosecond laser maskless direct writing (fs-LMDW) enters as a game-changing technology for metasurface engineering.

Femtosecond laser processing uses ultra-short laser pulses to induce localized, precise modifications on material surfaces without the need for photomasks or pre-defined masks. Its versatility spans various materials, including challenging refractory metals, due to its ability to induce both physical and chemical modifications synergistically. Fs-LMDW technology can create complex micro- and nano-scale structures across broad spatial scales, ranging from micrometers down to nanometers, with remarkable speed and minimal environmental constraints. This unique combination of attributes enables rapid prototyping of metasurfaces with finely tuned optical properties, suitable for encryption applications.

Despite its promise, femtosecond laser writing on non-transparent substrates traditionally handles only single-band information encoding—either visible light or infrared—falling short in integrating multi-band data into a single platform. Integrating visible and infrared information within one metasurface without interference, or crosstalk, represents a formidable technical challenge. Such integration is critical for creating versatile, high-density encryption matrices that benefit from dual-band operation, enabling secure multi-channel data embedding and selective retrieval with elevated security against unauthorized decryption.

The research team led by Associate Professor Dongshi Zhang and Professor Zhuguo Li from Shanghai Jiao Tong University has unveiled a groundbreaking solution to this problem. Their strategy hinges on utilizing pure zirconium, a refractory metal known for its excellent thermal stability and resistance to oxidation at high temperatures, as the substrate for femtosecond laser maskless direct writing. By finely tuning the laser parameters and processing environment, they have achieved integrated dual-band information embedding with zero crosstalk between visible and infrared domains.

The process begins with the inscription of infrared-encoded information, such as a QR code linked to Shanghai Jiao Tong University’s official webpage, on the zirconium substrate in an ambient air environment. This inscription employs gradient micro-nanostructures that manipulate infrared light absorption and reflection selectively. Interestingly, the surface appears as a uniform black due to abundant oxygen vacancies generated during laser processing, effectively camouflaging the infrared data to casual observers. Following this stage, the substrate is immersed in ethylene glycol, an environment conducive to precise visible light information inscription.

In ethylene glycol, the femtosecond laser facilitates the writing of visible light patterns—such as the acronyms “SJTU,” “Shanghai,” and “Jiaotong”—with nanometric precision. Notably, these visible patterns do not disrupt the larger-scale microstructures critical for infrared encoding, thereby maintaining signal integrity across both bands. This hierarchy of spatial and spectral control ensures that the visible and infrared information coexist harmoniously without crosstalk, a significant milestone in metasurface encryption technology.

A remarkable feature of this dual-band metasurface is its temperature-responsive behavior. The visible light information is engineered to be thermally erasable and rewritable, whereas the infrared counterpart demonstrates temperature-dependent visibility, operating as a temperature-controlled encryption key. For instance, upon heating the metasurface to around 300°C, the visible patterns such as “SJTU” and “Shanghai” disappear, effectively erasing any visible traces of encrypted data. The pattern “Jiaotong,” however, exhibits partial rewritability under the femtosecond laser, resisting complete erasure at this temperature, allowing verification of unauthorized access or modifications.

Simultaneously, the infrared-encoded QR code gains enhanced visibility with increasing temperature due to the thermal modulation of material properties, enabling graded information display. At 300°C, the QR code becomes fully decipherable by common infrared scanning devices like smartphone cameras, unlocking the embedded confidential data. This selective and dynamic control of information visibility based on external thermal stimuli introduces an additional layer of security, rendering the metasurface encryption platform not only robust against tampering but also adaptive to environmental changes.

Delving into the structural underpinnings responsible for this sophisticated functionality, the researchers investigated the micro- and nano-scale modifications induced by the femtosecond laser in various processing environments. The formation of laser-induced periodic surface structures (LIPSS) plays a pivotal role in defining the optical responses for visible light encoding. Furthermore, the redox chemistry within the ethylene glycol environment influences the oxidation states and defect densities within the zirconium oxide layer, significantly affecting color contrast and erasure capabilities upon heating.

The erasure mechanism for visible light information is attributed to increased oxidation at high temperatures, which alters surface chemistry and morphology, nullifying the encoded optical patterns. This reversible and controllable oxidation process is fundamental for achieving high-security erasable and rewritable encryption. In contrast, the infrared structures fabricated in ambient air remain thermally stable, preserving the integrity of the infrared information even under elevated temperatures, which is essential for reliable temperature-controlled decryption.

By integrating these features into a single metasurface platform, this research transcends traditional approaches relying on phase-change materials that often exhibit limited thermal stability and require intricate control systems. Zirconium’s refractory nature permits higher operational temperatures, enhancing encryption reliability under harsh conditions, a key consideration for applications spanning from secure financial transactions to tactical defense communications.

The implications of this research extend beyond academic curiosity. The demonstrated femtosecond laser maskless direct writing technique empowers rapid, cost-effective manufacturing of high-security metasurfaces scalable for industrial deployment. This method addresses long-standing bottlenecks in optical encryption, including the reliance on complex nano-lithography and bulky decoding apparatus. Moreover, the dual-band crosstalk-free information encoding, combined with thermal erasure and rewritability, positions this platform as an attractive solution for next-generation secure data storage, anti-counterfeiting labels, and multi-factor authentication systems.

As the digital landscape intensifies its demand for stronger and more versatile security approaches, this advance offers a novel paradigm that merges cutting-edge laser technology, materials science, and optical engineering. Further studies could optimize the spectral range, enhance multi-dimensional control, and explore integration with electronic components for seamless data encryption and retrieval. The versatility of femtosecond laser processing further opens possibilities to extend this methodology to other refractory metals and complex material systems, fostering a new generation of resilient and adaptive security devices.

In conclusion, the work by the Shanghai Jiao Tong University team presents a substantial advancement in optical metasurface encryption technology. The dual-band, crosstalk-free, thermally controllable encryption metasurface fabricated through femtosecond laser maskless direct writing heralds a promising future where secure information can be stored and selectively accessed with unprecedented precision, speed, and robustness. As cybersecurity demands continue to escalate globally, such innovations are imperative to safeguard the confidentiality and integrity of digital data in an increasingly interconnected world.

Subject of Research: Optical metasurface encryption using femtosecond laser processing on refractory metals.

Article Title: Femtosecond laser maskless direct writing of dual-band crosstalk-free information for all-in-one high-security encryption metasurface.

News Publication Date: Not specified in the content.

Web References:

• DOI link: http://dx.doi.org/10.29026/oea.2026.250303
• Shanghai Jiao Tong University homepage QR code as an example (implied).

References: Not explicitly provided.

Image Credits: OEA

Keywords

all-in-one metasurface, femtosecond laser maskless direct writing, high-security encryption, temperature-controlled decryption, erasability, rewritability, dual-band information encoding, zirconium metasurface, optical encryption, laser-induced periodic surface structures (LIPSS), refractory metals, crosstalk-free encryption