Fabrication of diffraction based security elements using direct laser interference patterning

Optical diffraction properties of three superimposed self-organized nanostructures induced by a laser process

Controlling the diffraction properties of materials over a large area holds great promise for a wide range of optical applications. Laser-based techniques have emerged as a viable solution to address this need. Here, we present the diffraction properties of laser-induced self-organized structures, which consist of three interlaced grating-like structures: self-organized nanoparticles, self-organized cracks, and laser marking lines. Under normal incidence external illumination, the sample exhibits an asymmetric diffraction pattern. However, when the incidence angle is tilted, circular diffraction patterns are observed in the plane perpendicular to both the sample and the incidence plane. These phenomena are attributed to the combination effect of the diffraction gratings. To elucidate the underlying physics of multiple diffraction, we use rigorous coupled-wave analysis (RCWA) and grating equations written in direction cosine space, extended to account for the presence of three superimposed gratings. Exploiting the laser-induced diffraction properties of these samples may have great potential for various industrial implementations, including security, display, and design.

Mechanically Tunable Flexible Photonic Device for Strain Sensing Applications

Flexible photonic devices based on soft polymers enable real-time sensing of environmental conditions in various industrial applications. A myriad of fabrication techniques have been established for producing optical devices, including photo and electron-beam lithography, nano/femtosecond laser writing, and surface imprinting or embossing. However, among these techniques, surface imprinting/embossing is simple, scalable, convenient to implement, can produce nanoscale resolutions, and is cost-effective. Herein, we utilize the surface imprinting method to replicate rigid micro/nanostructures onto a commonly available PDMS substrate, enabling the transfer of rigid nanostructures into flexible forms for sensing at a nanometric scale. The sensing nanopatterned sheets were mechanically extended, and the extension was remotely monitored via optical methods. Monochromatic light (450, 532, and 650 nm) was transmitted through the imprinted sensor under various force/stress levels. The optical response was recorded on an image screen and correlated with the strain created by the applied stress levels. The optical response was obtained in diffraction pattern form from the flexible grating-based sensor and in an optical-diffusion field form from the diffuser-based sensor. The calculated Young's modulus in response to the applied stress, measured through the novel optical method, was found in a reasonable range compared to the reported range of PDMS (360-870 kPa) in the literature.

Full Geometric Control of Hidden Color Information in Diffraction Gratings under Angled White Light Illumination

Under white light illumination, gratings produce an angular distribution of wavelengths dependent on the diffraction order and geometric parameters. However, previous studies of gratings are limited to at least one geometric parameter (height, periodicity, orientation, angle of incidence) kept constant. Here, we vary all geometric parameters in the gratings using a versatile nanofabrication technique, two-photon polymerization lithography, to encode hidden color information through two design approaches. The first approach hides color information by decoupling the effects of grating height and periodicity under normal and oblique incidence. The second approach hides multiple sets of color information by arranging gratings in sectors around semicircular pixels. Different images are revealed with negligible crosstalk under oblique incidence and varying sample rotation angles. Our analysis shows that an angular separation of ≥10° between adjacent sectors is required to suppress crosstalk. This work has potential applications in information storage and security watermarks.

Blazed grating enables highly decoupled optically variable devices fabricated by vibration-assisted diamond texturing

Optically variable devices (OVDs) are well received for anti-counterfeiting and decorative applications. In this study, new strategies to develop highly decoupled OVDs were proposed and demonstrated based on the fast patterning of blazed gratings by vibration-assisted diamond texturing. A unique surface generation mechanism was revealed as a combined cutting and forming process. One facet of blazed grating is generated by the cutting motion defined by the tool tip trajectory. The other facet is formed by the tool flank face, which establishes the blaze angle. This process is able to generate high-resolution, structurally colored graphics by modulating cutting velocity to control the grating distribution. Due to the unique surface generation mechanism, the orientation of the created blazed gratings is intrinsically perpendicular to the cutting direction. Thus, it enables the flexible control of concentration directions of diffracted light by tuning the orientation of blazed gratings. We designed and demonstrated two types of highly decoupled OVDs based on vibration-induced blazed gratings. The orthogonal-type OVD utilizes the azimuth angle dependence of blazed gratings to encode two images in orthogonal cutting directions. The in-plane-type OVD utilizes the optimized diffraction efficiency of blazed gratings in a given diffraction order to encode two images in opposite cutting directions. The fabricated OVDs are presented and compared with optical simulation results based on an extended scalar diffraction theory.

Label-free detection of C-Reactive protein using bioresponsive hydrogel-based surface relief diffraction gratings

Responsive hydrogel-based surface relief gratings have demonstrated great performances as transducers in optical sensing. However, novel and smart designs of hydrogels are needed for the appropriate detection of analytes and biomolecules since the existing materials are very limited to specific molecules. In this work, a biosensing system based on surface relief gratings made of bioresponsive hydrogels has been developed. In particular, the hydrogel contains phosphocholine moieties to specifically recognize C-Reactive protein (CRP). The CRP-Sensing hydrogel capacity to selectively detect CRP was fully demonstrated. Using Direct Laser Interference Patterning, micro-gratings were created on CRP-Sensing hydrogel substrates and applied for the label-free sensing of CRP using a simple laser-based homemade optical setup. Limits of detection (LOD) and quantification (LOQ) in human serum dilutions of 1.07 and 8.92 mg L^-1, respectively, were reached. These results demonstrate that the biosensing system allows the selective label-free detection of CRP within concentration ranges around those related to risks of cardiovascular diseases and sepsis. Besides, amplification strategies have been carried out improving the sensitivity, widening the linear range, and reaching better LOD and LOQ (0.30 mg L^-1 and 4.36 mg L^-1). Finally, all the approaches were tested for the quantification of CRP in certified human serum with recoveries of around 100%.

Bitmap and vectorial hologram recording by using femtosecond laser pulses

In this paper, we present two approaches for recording a quasi-hologram on the steel surface by femtosecond laser pulses. The recording process is done by rotating the polarization of the laser beam by a half-wave plate or a spatial light modulator (SLM), so we can control the spatial orientation of the formed laser-induced periodic surface structures (LIPSS). Two different approaches are shown, which use vector and bitmap images to record the hologram. For the first time to our knowledge, we managed to record a hologram of a bitmap image by continuously adjusting the laser beam polarization by SLM during scanning. The developed method can substantially improve hologram recording technology by eliminating complex processing procedures, which can lead to increasing the fabrication speed and reducing the cost.

Influence of Sulphur Content on Structuring Dynamics during Nanosecond Pulsed Direct Laser Interference Patterning

The formation of melt and its spread in materials is the focus of many high temperature processes, for example, in laser welding and cutting. Surface active elements alter the surface tension gradient and therefore influence melt penetration depth and pool width. This study describes the application of direct laser interference patterning (DLIP) for structuring steel surfaces with diverse contents of the surface active element sulphur, which affects the melt convection pattern and the pool shape during the process. The laser fluence used is varied to analyse the different topographic features that can be produced depending on the absorbed laser intensity and the sulphur concentration. The results show that single peak geometries can be produced on substrates with sulphur contents lower than 300 ppm, while structures with split peaks form on higher sulphur content steels. The peak formation is explained using related conceptions of thermocapillary convection in weld pools. Numerical simulations based on a smoothed particle hydrodynamics (SPH) model are employed to further investigate the influence of the sulphur content in steel on the melt pool convection during nanosecond single-pulsed DLIP.

Large-Beam Picosecond Interference Patterning of Metallic Substrates

In this paper, we introduce a method to efficiently use a high-energy pulsed 1.7 ps HiLASE Perla laser system for two beam interference patterning. The newly developed method of large-beam interference patterning permits the production of micro and sub-micron sized features on a treated surface with increased processing throughputs by enlarging the interference area. The limits for beam enlarging are explained and calculated for the used laser source. The formation of a variety of surface micro and nanostructures and their combinations are reported on stainless steel, invar, and tungsten with the maximum fabrication speed of 206 cm²/min. The wettability of selected hierarchical structures combining interference patterns with 2.6 µm periodicity and the nanoscale surface structures on top were analyzed showing superhydrophobic behavior with contact angles of 164°, 156°, and 150° in the case of stainless steel, invar, and tungsten, respectively.

Development of a Monitoring Strategy for Laser-Textured Metallic Surfaces Using a Diffractive Approach

The current status of research around the world concurs on the powerful influence of micro- and nano-textured surfaces in terms of surface functionalization. In order to characterize the manufactured topographical morphology with regard to the surface quality or homogeneity, major efforts are still required. In this work, an optical approach for the indirect evaluation of the quality and morphology of surface structures manufactured with Direct Laser Interference Patterning (DLIP) is presented. For testing the designed optical configuration, line-like surface patterns are fabricated at a 1064 nm wavelength on stainless steel with a repetitive distance of 4.9 µm, utilizing a two-beam DLIP configuration. Depending on the pulse to pulse overlap and hatch distance, different single and complex pattern geometries are produced, presenting non-homogenous and homogenous surface patterns. The developed optical system permitted the successfully classification of different pattern geometries, in particular, those showing single-scale morphology (high homogeneity). Additionally, the fabricated structures were measured using confocal microscopy method, and the obtained topographies were correlated with the recorded optical images.

Fabrication of superhydrophobic and ice-repellent surfaces on pure aluminium using single and multiscaled periodic textures

Fabricating aluminium surfaces with superhydrophobic and ice-repellent properties present nowadays a challenging task. In this work, multifunctional structures are manufactured by direct laser writing and direct laser interference patterning methods using pulsed infrared laser radiation (1064 nm). Different periodic patterns with feature sizes ranging from 7.0 to 50.0 µm are produced. In addition, hierarchical textures are produced combining both mentioned laser based methods. Water contact angle tests at room temperature showed that all produced patterns reached the superhydrophobic state after 13 to 16 days. In addition, these experiments were repeated at substrate temperatures from −30 °C to 80 °C allowing to determine three wettability behaviours as a function of the temperature. The patterned surfaces also showed ice-repellent properties characterized by a near three-fold increase in the droplets freezing times compared to the untreated samples. Using finite element simulations, it was found that the main reason behind the ice-prevention is the change in the droplet geometrical shape due to the hydrophobic nature of the treated surfaces. Finally, dynamic tests of droplets imping the treated aluminium surfaces cooled down to −20 °C revealed that only on the hierarchically patterned surface, the droplets were able to bounce off the substrate.

Design and Manufacturing Method of Fundamental Beam Mode Shaper for Adapted Laser Beam Profile in Laser Material Processing

Many laser material processing applications require an optimized beam profile, e.g., ring shape or Top-Hat profiles with homogeneous intensity distribution. In this study, we show a beam shaping concept leading to a phase shifting element with binary height profile as well as a very low periodicity with near diffraction limited spot size. Further advantages of so-called Fundamental Beam Mode Shaping (FBS) elements are the simplified handling, and a high efficiency and homogeneity. The calculated height profile of FBS elements are transferred in fused silica substrates using a combination of microlithography technologies, reactive ion etching (RIE) and ion beam etching (IBE). The experiments demonstrated a linear relation between the etching depth after RIE and IBE. The optical evaluation of the manufactured FBS beam mode shaper confirmed the presented concept design.

High Throughput Direct Laser Interferential Patterning of Aluminum for Fabrication of Super Hydrophobic Surfaces

This work addresses the fabrication of hydrophobic surface structures by means of direct laser interference patterning using an optical setup optimized for high throughput processing. The developed optical assembly is used to shape the laser beam intensity as well as to obtain the two sub beams required for creating the interference pattern. The resulting beam profile consists of an elongated rectangular laser spot with 5.0 mm × 0.1 mm size, which enables the optimized utilization of the laser fluence available from an ns-pulsed laser with a wavelength of 1064 nm. Depending on the pulse repetition rate applied, heating of the substrate volume generated by heat accumulation encouraged exceptionally high aspect ratios of the trench structures due to melt flow dynamic material deformation. Finally, water contact angle measurements of the produced structures permitted the demonstration of the capability of controlling the wetting angle, in which this effect does not only depend on the height of the generated surface structures but also on their morphology.

Fabrication of inclined non-symmetrical periodic micro-structures using Direct Laser Interference Patterning

The direct fabrication of microstructures, having a non-symmetrical morphology with controllable inclination, presents nowadays a challenging task. Natural examples of surfaces with inclined topographies have shown to provide anisotropic functionalities, which have attracted the interest of several researchers in the last years. This work presents a microfabrication technique for producing microstructures with a determined and controllable inclination angle using two-beam Direct Laser Interference Patterning. Polyimide foils are irradiated with a 4 ns UV (266 nm) laser source producing line-like structures with a period varying from 4.6 µm to 16.5 µm. The inclinations, retrieved by tilting the sample with respect to the optical axis of the setup, are changed from 0° to 75°, introducing a well controllable and defined inclination of the structure walls. The structuring parameters (laser fluence, number of laser pulses and interference period) as well as the inclination of the microstructures are correlated with the global tilting of the sample. As a result, a determined laser fluence and number of pulses are necessary to observe a remarkable non-symmetrical morphology of the structures. In addition, the presence of structural undercuts is reported, which opens the possibility for developing new direction-dependent properties on polymeric materials. As an example, preliminary results on light diffraction are presented, showing a similar behavior as blazed diffraction gratings.

10-million-elements-per-second printing of infrared-resonant plasmonic arrays by multiplexed laser pulses

We report on high-quality infrared (IR)-resonant plasmonic nanoantenna arrays fabricated on a thin gold film by tightly focused femtosecond (fs) laser pulses coming at submegahertz repetition rates at a printing rate of 10 million elements per second. To achieve this, the laser pulses were spatially multiplexed by fused silica diffractive optical elements into 51 identical submicrometer-sized laser spots arranged into a linear array at periodicity down to 1 μm. The demonstrated high-throughput nanopatterning modality indicates fs laser maskless microablation as an emerging robust, flexible, and competitive lithographic tool for advanced fabrication of IR-range plasmonic sensors for environmental sensing, chemosensing, and biosensing.

Dot-Matrix Hologram Rendering Algorithm and its Validation through Direct Laser Interference Patterning

The fight against forgery of valuable items demands efficient and reasonably priced solutions. A security tag featuring holographic elements for anti-counterfeiting is one of them. However, the content and colours of a diffraction image that would be seen by an observer are often counterintuitive in the design stage. Here, we propose an original algorithm based on the conical diffraction formalism, which can be used to describe the variations of a diffraction image with respect to all aspects of observation. We validate the output of the algorithm by comparing it to test holograms, which we have produced by employing direct laser interference patterning (DLIP) in electrochemically grown nickel foil. We have employed a motorized femtosecond laser system to micro-machine arrays of 65 µm × 65 µm sized diffraction gratings with a defined orientation and pitch on the order of 1 µm. Based on completed diffraction efficiency measurements, we determined optimal ablation parameters, i.e. 57.4 mJ/cm² fluence per pulse and 1100 pulses/pixel. Furthermore, we show how accurate the proposed algorithm is through measured diffraction spectra as well as captured diffraction images of test holograms produced using the obtained parameters. Finally, we showcase anti-counterfeiting tag prototypes with complex holographic effects, i.e. colour reconstruction, animation effects, and image multiplexing. The proposed algorithm can severely shorten the time between design and production of a holographic tag, especially when realizing it via a competitive origination technology—DLIP.

Single-Step Laser Plasmonic Coloration of Metal Films

Utilization of structural colors produced by nanosized optical antennas is expected to revolutionize the current display technologies based on an inkjet or a pigmentation-based color printing. Meanwhile, the versatile color-mapping strategy combining the fast single-step single-substrate fabrication cycle with low-cost scalable operation is still missing. We propose lithography-free pure optical approach based on a direct local ablative reshaping of the gold film with nanojoule (nJ)-energy femtosecond laser pulses. Plasmon-color printing at a resolution up to 2.5 × 10⁴ dots per inch satisfying the current visualization demands and data storage capacity is achieved. By controlling only the applied pulse energy, wide gamut of colors in scattering regime was reproduced via tuning the size of the printed nanovoids, which have a polarization- and shape-dependent localized plasmon-mediated scattering. Additionally, brightness of a single pixel was gradually adjusted via varying of the spacing between the printed nanovoids. The presented experimental demonstration opens a new direction toward plasmon-color printing for various applications where durability is required: low-cost cryptography, security tagging, and ultracompact optical data storage.