Anti-Counterfeiting White Light Printed Image Multiplexing by Fast Nanosecond Laser Processing

Bi-path color tunable plasmonic micro-nano hybrid structures for encrypted printing

Colored information is crucial for humans to perceive the world. Plasmonic spectra modulation can serve as an effective means to create different colors. Although several solutions for plasmonic color-printing have been proposed, further information encryption has not received any attention. Herein, we exhibit a fine color modulation strategy to construct noble-metal-based micro-nano hybrid structures in the bi-path of photo-thermal deformation and liquid-phase-chemical reaction. Ag/Ta2O5 bi-layer films are ablated at the center of the machined lines of nanosecond pulsed laser, while silver nanoparticles are formed in other regions by thermal radiation of the infrared laser, which can be further dissolved and shape-modulated in KCl solution under different periods. The variation of size and spacing of nano-Ag particles results in a precise shift of plasmonic spectra in visible region. Colored information can be hidden by adjusting the scan number and the energy density during laser processing, and will emerge after the subsequent chemical dissolution reactions. The bi-path color adjustment strategy is easy to operate and can play a role in key information protection and color image switching.

Physically Unclonable Holographic Encryption and Anticounterfeiting Based on the Light Propagation of Complex Medium and Fluorescent Labels

Physically unclonable function (PUF) methods have high security, but their wide application is limited by complex encoding, large database, advanced external characterization equipment, and complicated comparative authentication. Therefore, we creatively propose the physically unclonable holographic encryption and anticounterfeiting based on the light propagation of complex medium and fluorescent labels. As far as we know, this is the first holographic encryption and anticounterfeiting method with a fluorescence physically unclonable property. The proposed method reduces the above requirements of traditional PUF methods and significantly reduces the cost. The angle-multiplexed PUF fluorescent label is the physical secret key. The information is encrypted as computer-generated holograms (CGH). Many physical parameters in the system are used as the parameter secret keys. The Diffie-Hellman key exchange algorithm is improved to transfer parameter secret keys. A variety of complex medium hologram generation methods are proposed and compared. The effectiveness, security, and robustness of the method are studied and analyzed. Finally, a graphical user interface (GUI) is designed for the convenience of users. The advantages of this method include lower PUF encoding complexity, effective reduction of the database size, lower requirements for characterization equipment, and direct use of decrypted information without complicated comparative authentication to reduce misjudgment. It is believed that the method proposed in this paper will pave the way for the popularization and application of PUF-based anticounterfeiting and encryption methods.

Er3+/Tm3+ co-activated core@shell nanoarchitectures: tunable upconversion luminescence and high-security anti-counterfeiting

Currently, developing advanced optoelectronic materials is of great importance to solving serious problem of fake and shoddy products. Lanthanide-doped nanomaterials are particularly suitable for addressing this issue, but limited by the realization of multiple upconverison (UC) emissions upon a single-wavelength laser excitation. Herein, it is proven that the co-dopant of blue/near-infrared (NIR)-emitting activators (Tm3+) and green/red-emitting centers (Er3+) in a particular designed core-shell nanoarchitecture allows the achievement of multiple luminescence over wide spectral region for optical security. In our study, cubic-phased NaYbF4:Tm/Er@CaF2 nanocrystals have been successfully synthesized through a layer-by-layer coprecipitation strategy, which presents visible multicolor UC luminescence and invisible NIR UC emission upon 980 nm laser excitation by just regulating the laser power and temperature. Significantly, the unique luminescent characteristics enables the designed UC nanoparticles a promising candidate for advanced anti-counterfeiting. This works offers a reference to develop advanced optoelectronic materials for practical application in optical security.

Hybridization between plasmonic and photonic modes in laser-induced self-organized quasi-random plasmonic metasurfaces

Plasmonic metasurfaces made of perfectly regular 2D lattices of metallic nanoparticles deposited on surfaces or close to waveguides can exhibit hybridized plasmonic and photonic modes. The latter arise from the excitation of surface or guided modes through the in-plane coherent scattering of periodic arrays. Recently, laser-induced self-organization of random plasmonic metasurfaces has been used to create nanoparticle gratings embedded in protective layers. Despite the broad size distribution and positional disorder of nanoparticles, the resulting nanostructures exhibit strong coupling between plasmonic and photonic modes in transverse electric polarization, leading to dichroism, which is well-reproduced from one laser printing to another. Here, we examine quantitatively the effect of inhomogeneities at the nanoscale on the hybridization between localized plasmonic modes and delocalized guided modes by considering realistic laser-induced self-organized nanoparticle arrays embedded in a two-layer system. By referring to regular samples, we describe the optical mechanisms involved in the hybridization process at characteristic wavelengths, based on far and near field simulations. Two kinds of real samples are considered, featuring different levels of coupling between the plasmonic and photonic modes. The results demonstrate that controlling the statistical properties of plasmonic metasurfaces, such as the nanoparticle size distribution and average position, over areas a few micrometers wide is enough to control in a reproducible manner the hybridization mechanisms and their resulting optical properties. Thus, this study shows that the inherent irregularities of laser-induced self-organized nanostructures are compatible with smart functionalities of nanophotonics, and confirms that laser processing has huge potential for real-world applications.

Rapid Electrohydrodynamic-Driven Pattern Replication over a Large Area via Ultrahigh Voltage Pulses

Despite the prospects of electrohydrodynamic instability patterning (EHIP), poor process parameter controllability is a significant challenge in uniform large-scale nanopatterning. Herein, we introduce a EHIP process using an ultrahigh electric field (>108 V/m) to effectively accelerate the pattern growth evolution. Owing to the strong dependence on a temporal parameter (1/τm) of the field strength, our method not only reduces the completion time of pattern growth but also overcomes critical parametric restrictions on the pattern replication, thereby enhancing the replicated pattern quality in three dimensions. The pattern can be uniformly replicated over the entire film surface even without a perfectly uniform air gap, which has been severely difficult in the conventional method. To further demonstrate how straightforward yet versatile our approach is, we applied our EHIP approach to successfully replicate the densely packed nanostructures of cicada wings.

High-Precision and Low-Damage Microchannel Construction via Magnetically Assisted Laser-Induced Plasma Ablation for Micro-Thermoelectric Devices

Thermoelectric devices are developing toward high density and miniaturization with a large filling factor for new applications in chip thermal management and microenergy harvesting. Pulsed laser etching has become one of the most effective tools for the patterning construction of highly integrated micro-thermoelectric devices. However, the laser spot size and Gaussian laser energy distribution restrict the processing size and accuracy of microchannels. Moreover, the rapid temperature rise caused by laser energy injection would also raise serious problems such as element volatilization, cracks, and recast layers. Herein, a liquid-assisted nanosecond laser ablation technology with magnetically controlled plasma is proposed to etch microchannels on thermoelectric thick films. By evaluating the size and shape of microchannels, we theoretically investigated the influence of cavitation bubbles on the laser optical path and surface roughness in laser-induced plasma ablation. In addition, the energy criterion for high-precision ablation is revealed, and the effect of magnetic field on ablation threshold is explained by magnetic constraint on energy and kinetic properties of the laser-induced charged plasma plume. Finally, the high-precision and low-damage microchannels are achieved on Bi₂Te₃ thermoelectric thick films with a minimum line width of 19.12 μm and a small sidewall inclination degree of tan θ = 0.085. This work provides a promising alternative for the fabrication of high-density three-dimensional (3D) patterning in semiconductor microdevices.

Laser Fabrication of Highly Ordered Nanocomposite Subwavelength Gratings

Optical nanogratings are widely used for different optical, photovoltaic, and sensing devices. However, fabrication methods of highly ordered gratings with the period around optical wavelength range are usually rather expensive and time consuming. In this article, we present high speed single-step approach for fabrication of highly ordered nanocomposite gratings with a period of less than 355 nm. For the purpose, we used commercially available nanosecond-pulsed fiber laser system operating at the wavelength of 355 nm. One-dimensional and two-dimensional nanostructures can be formed by direct laser treatment with different scan speed and intensity. These structures exhibit not only dispersing, but also anisotropic properties. The obtained results open perspectives for easier mass production of polarization splitters and filters, planar optics, and also for security labeling.

Self-Aligned Plasmonic Lithography for Maskless Fabrication of Large-Area Long-Range Ordered 2D Nanostructures

This paper proposes a one-step maskless 2D nanopatterning approach named self-aligned plasmonic lithography (SPL) by line-shaped ultrafast laser ablation under atmospheric conditions for the first time. Through a theoretical calculation of electric field and experimental verification, we proved that homogeneous interference of laser-excited surface plasmon polaritons (SPPs) can be achieved and used to generate long-range ordered 2D nanostructures in a self-aligned way over a wafer-sized area within several minutes. Moreover, the self-aligned nanostructures can be freely transferred between embossed nanopillars and engraved nanoholes by modulating the excitation intensity of SPPs interference through altering the incident laser energy. The SPL technique exhibits further controllability in the shape, orientation, and period of achievable nanopatterns on a wide range of semiconductors and metals by tuning processing parameters. Nanopatterned films can further act as masks to transfer structures into other bulk materials, as demonstrated in silica.

Predicting Laser-Induced Colors of Random Plasmonic Metasurfaces and Optimizing Image Multiplexing Using Deep Learning

Structural colors of plasmonic metasurfaces have been promised to a strong technological impact thanks to their high brightness, durability, and dichroic properties. However, fabricating metasurfaces whose spatial distribution must be customized at each implementation and over large areas is still a challenge. Since the demonstration of printed image multiplexing on quasi-random plasmonic metasurfaces, laser processing appears as a promising technology to reach the right level of accuracy and versatility. The main limit comes from the absence of physical models to predict the optical properties that can emerge from the laser processing of metasurfaces in which random metallic nanostructures are characterized by their statistical properties. Here, we demonstrate that deep neural networks trained from experimental data can predict the spectra and colors of laser-induced plasmonic metasurfaces in various observation modes. With thousands of experimental data, produced in a rapid and efficient way, the training accuracy is better than the perceptual just noticeable change. This accuracy enables the use of the predicted continuous color charts to find solutions for printing multiplexed images. Our deep learning approach is validated by an experimental demonstration of laser-induced two-image multiplexing. This approach greatly improves the performance of the laser-processing technology for both printing color images and finding optimized parameters for multiplexing. The article also provides a simple mining algorithm for implementing multiplexing with multiple observation modes and colors from any printing technology. This study can improve the optimization of laser processes for high-end applications in security, entertainment, or data storage.