Eight Inch Wafer-Scale Flexible Polarization-Dependent Color Filters with Ag-TiO2 Composite Nanowires

Electrically switchable multicolored filter using plasmonic nanograting integrated with liquid crystal for optical storage and encryption

This study proposed the synergistic merging of twisted-nematic liquid crystals (LCs) and nanograting embedded etalon structures for plasmonic structure color generation, realizing dynamic multifunctional metadevices. Metallic nanogratings and dielectric cavities were designed to provide color selectivity at visible wavelengths. Meanwhile, the polarization for the transmission of light could be actively manipulated by electrically modulating these integrated LCs. Moreover, manufacturing independent metadevices as single storage units with electrically controlled programmability and addressability facilitated secure information encoding and secretive transfer by dynamic high-contrast images. The approaches will pave the way for the development of customized optical storage devices and information encryption.

Full-color reflective filter in a large area exploiting a sandwiched metasurface

Metasurface-based color filters show great potential in imaging devices and color printing. However, it is still a great challenge to meet the high demand for large-area flexible displays with structural color filters. Here, a reflective color filter is developed with a sandwiched metasurface, where the photoresist grating, complementary silver grating and silicon nitride grating are sequentially stacked on the substrate. Analytical results show that bandpass reflective spectra can be achieved due to the combined influence of guided mode resonance and cavity resonance, and full-spectrum colors including three primary colors can be generated by merely varying the period of the metasurface. With only photolithography and deposition technology involved, large-area samples incorporating pixelated metasurfaces are easily fabricated. Metasurfaces with three periods of 540 nm, 400 nm and 320 nm are experimentally obtained having peak reflective efficiency of ∼ 60%, demonstrating red, green and blue colors as theoretical results. A stripe sample with the structural period varying from 250 nm to 550 nm is fabricated in an area of 10 mm × 30 mm, displaying full-color reflections as simulated. Finally, with metasurfaces of three structural periods, the pixelated Soochow University logo is fabricated in a larger area of ∼ 30 mm × 30 mm. Therefore, the proposed structure shows high compatible to roll-to-roll nano-imprinting for large-area flexible displays, with the photoresist film can be easily substituted by UV film in addition.

A Reversible Moisture-Responsive Plasmonic Color-Raman and Transmittance Modulation Device by Dispersing Hyaluronan-Functionalized Ag into Nanofibers

Plasmonic physical color generation, which mostly depends on selective absorption, creates unique colors by light transmission and scattering. Based on this, regulating plasmon and transparency with external stimulation is a promising approach for fabricating optical devices with enhanced visual displays; however, few studies have addressed the implementation of dual-optical modulation. In addition, developing a color response to environmental stimuli through the highly shape-sensitive plasmon depth modulation has long remained a significant challenge once the nanostructure is determined. Some stimulations also require high amounts of electricity, which can be costly. In this study, strategically designed hyaluronan-functionalized triangular silver nanoparticles (AgNPs) were embedded in polyvinyl alcohol-polyethylene nanofiber films to achieve a breakthrough in the moisture-responsive dual-optical modulation of the plasmonic color-Raman and transparency. Switchable colors that are reversible were induced in plasmon-resonance-modulation AgNPs via moisture stimulation, adjusting the expansion-tunable dielectric constant of hyaluronan-functionalized AgNPs and varying the electron density due to electron transfer. Furthermore, a moisture gradient was used to decrease the Raman scattering and increase the photoluminescence, which is a significant demonstration of smart-plasmonic evolution. This effect occurred due to the gradual transition from plasmon-driven photoluminescence quenching to photoluminescence enhancement as the interval of the Ag and hyaluronic acid molecules was increased. The transparency of the composite film was also dynamically regulated by turning moisture on/off. This occurred because of the significant difference in hygroscopic expansion between hyaluronan and the nanofibers, which generated a large variation in the total refractive index and caused changes in the surface roughness.

Nanoimprint lithography for high-throughput fabrication of metasurfaces

Metasurfaces are composed of periodic sub-wavelength nanostructures and exhibit optical properties that are not found in nature. They have been widely investigated for optical applications such as holograms, wavefront shaping, and structural color printing, however, electron-beam lithography is not suitable to produce large-area metasurfaces because of the high fabrication cost and low productivity. Although alternative optical technologies, such as holographic lithography and plasmonic lithography, can overcome these drawbacks, such methods are still constrained by the optical diffraction limit. To break through this fundamental problem, mechanical nanopatterning processes have been actively studied in many fields, with nanoimprint lithography (NIL) coming to the forefront. Since NIL replicates the nanopattern of the mold regardless of the diffraction limit, NIL can achieve sufficiently high productivity and patterning resolution, giving rise to an explosive development in the fabrication of metasurfaces. In this review, we focus on various NIL technologies for the manufacturing of metasurfaces. First, we briefly describe conventional NIL and then present various NIL methods for the scalable fabrication of metasurfaces. We also discuss recent applications of NIL in the realization of metasurfaces. Finally, we conclude with an outlook on each method and suggest perspectives for future research on the high-throughput fabrication of active metasurfaces.

Low-Loss Broadband Transverse Electric Pass Hybrid Plasmonic Fiber Polarizers Using Metallic Nanomaterials

Metals were for decades perceived as devoid of interesting optical properties that could be harnessed for optical components and devices. However, with the development of accurate nanofabrication techniques and precise control over architectural parameters, metals can be structured and characterized on the nanoscale. Metallic plasmonic nanomaterials exhibit a number of unique structural and optical properties, which offer the potential for developing new types of plasmonic devices. Here, we demonstrate a low-loss broadband polarizer based on a hybrid plasmonic fiber structure using metals as polarization-selective absorption materials. The polarization mechanism, design, fabrication, and characteristics of the plasmonic polarizers are investigated theoretically, numerically, and experimentally. The theoretical analysis predicts that the polarization-selective absorption with insensitivity to wavelength enables hybrid plasmonic fibers to function as broadband polarizers. Numerical simulations give the comparison of the polarization-selective absorption of various metallic nanomaterials (Ag, Au, In, Al, Cr) and show that aluminum is regarded as the optimum absorption material for the plasmonic polarizer. Experimental results show that through precise control over geometrical parameters, this device is capable of offering a high polarization extinction ratio (PER) of over 40 dB and a low insertion loss (IL) of less than 1.3 dB in the wavelength region of 810.1-870.0 nm. Compared with commercial birefringent-crystal-fiber polarizers, the plasmonic fiber polarizer has a better PER and IL bandwidth. These merits, combined with a compact and robust configuration, enable the plasmonic polarizer to have great potential in a broad range of applications.

Shape-Controlled and Well-Arrayed Heterogeneous Nanostructures via Melting Point Modulation at the Nanoscale

A novel method for fabricating shape-controlled and well-arrayed heterogeneous nanostructures by altering the melting point of the metal thin film at the nanoscale is proposed. Silver nanofilms (AgNFs) are transformed into silver nanoislands (AgNIs), silver nanoparticles (AgNPs), and silver nanogaps (AgNGs) that are well-ordered and repositioned inside the gold nanoholes (AuNHs) depending on the diameter of the AuNHs, the thickness of the AgNF, and the heating temperature (120-200 °C). This method demonstrates the ability to fabricate uniform, stable, and unique structures with a fast, simple, and mass-producible process. For demonstrating the diverse applicability of the developed structures, high-density AgNGs inside the AuNHs are utilized as surface-enhanced Raman spectroscopy (SERS) substrates. These AgNGs-based SERS substrates exhibit a performance enhancement, which is 1.06 × 10⁶ times greater than that of a metal film, with a relative standard deviation of 19.8%. The developed AgNP/AgNI structures are also used as nonreproducible anti-counterfeiting signs, and the anti-counterfeiting/readout system is demonstrated via image processing. Therefore, our method could play a vital role in the nanofabrication of high-demand nanostructures.

3D Layer-By-Layer Pd-Containing Nanocomposite Platforms for Enhancing the Performance of Hydrogen Sensors

Herein, a nanowelding technique is adopted to fabricate three-dimensional layer-by-layer Pd-containing nanocomposite structures with special properties. Nanowires fabricated from noble metals (Pd, Pt, Au, and Ag) were used to prepare Pd-Pd nanostructures and Pd-Au, Pd-Pt, Pd-Ag, and Pd-Pt-Au nanocomposite structures by controlling the welding temperature. The recrystallization behavior of the welded composite materials was observed and analyzed. In addition, their excellent mechanical and electrical properties were confirmed by performing 10,000 bending test cycles and measuring the resistances. Finally, flexible and wearable nanoheaters and gas sensors were fabricated using our proposed method. In comparison with conventional techniques, our proposed method can not only easily achieve sensors with a large surface area and flexibility but also improve their performance through the addition of catalyst metals. A gas sensor fabricated using the Pd-Au nanocomposites demonstrated 3.9-fold and 1.1-fold faster H₂ recovery and response, respectively, than a pure Pd-Pd gas sensor device. Moreover, the Pd-Ag nanocomposite exhibited a high sensitivity of 5.5% (better than that of other fabricated gas sensors) for 1.6% H₂ concentration. Therefore, we believe that the fabricated nanocomposites appear promising for wide applications in wearable gas sensors, flexible optical devices, and flexible catalytic devices.

Polarization tunable color filters based on all-dielectric metasurfaces on a flexible substrate

Structural color filters based on all-dielectric materials are considered to be promising alternatives to metal nanostructures due to significant advantages, such as high-quality resonance effects and low losses of Ohmic effects. We demonstrate a polarization tunable color filter based on all-dielectric metasurfaces, which is based on the arrays of asymmetric monocrystalline silicon nanoblocks on the flexible substrate. By adjusting the physical dimensions of nanoblocks, the filter can exhibit a variety of bright transmission colors. Furthermore, the designed dielectric metasurfaces are sensitive to the linear polarization direction of the incident light, thus a wide range of color images can be created by changing the polarization angles. All of the color filter including the dielectric silicon nanoblocks, the overcladding, and the flexible substrate can be delaminated from the handler substrates and the optical property is reconfigurable, which will find applications in the functional color display, polarization detection and imaging, and secured optical tag.

Polarization Selectivity of the Thin-Metal-Film Plasmon-Assisted Fiber-Optic Polarizer

The interaction between light and metallic nanostructures leads to many impressive achievements and has a wide range of applications. The thin-metal-film plasmon-assisted fiber-optic polarizer is one of the essential applications. However, the polarization mechanism and the transmitted polarization of the plasmon-assisted polarizer have given rise to controversy over the past decade. Which of the polarizations is preferentially transmitted through the polarizer? The transverse electric polarization or the transverse magnetic polarization? Here, special emphasis is placed upon the polarization mechanism and the transmitted polarization of thin-metal-film plasmon-assisted fiber polarizers. We first investigate the polarization mechanism of the polarizers theoretically and numerically. Furthermore, a novel approach is proposed to demonstrate the transmitted polarization in the plasmon-assisted fiber polarizers experimentally. We demonstrate that the polarization mechanism is based on the polarization selective absorption of the metallic material, and the transverse electric polarization is the only transmitted polarization of the metallic plasmon-assisted polarizer. Moreover, the plasmon-assisted polarizer can offer a high polarization extinction ratio (33.1 dB) and a low insertion loss (1.1 dB) at room temperature and have excellent temperature stability in the range of -48 to 82 °C. Experimental results agree well with our theoretical and numerical analyses. The findings clarify the confusion about the polarization mechanism and the transmitted polarization of metallic plasmon-assisted fiber polarizers over the past decade, providing new ground for the exploration of polarization-sensitive optical systems, with good potential applications in the fields of optical sensors, plasmonic lasers, coherent optical communications, and biosensor systems.

Nanopattern-Embedded Micropillar Structures for Security Identification

A novel method was developed for fabricating nanopatterns embedded on micropillar-structured surfaces using nanowelding technology for security identification. Commonly used substrates, that is, polyethylene films, glass wafers, Si wafers, and curved surfaces, were employed and their characteristics were evaluated. Cr was deposited onto the selected substrate to strengthen the adhesion force, and an adhesive layer of ultra-thin metal was deposited on top of the Cr layer. Lastly, nanopatterns were embedded on the substrates by nanowelding. The morphologies, cross sections, and three-dimensional (3D) images of the fabricated nanostructures were evaluated, and their crystalline structures and compositions were analyzed. Using the same method, nanopatterns embedded on micropillar-structured surfaces were fabricated for the first time as security patterns to improve security identification. The fabricated security patterns were characterized in three stages. First, micropillar structures and structural color were simply observed via optical microscopy to achieve a preliminary judgment. The appearance of structural color was due to the nanostructures fabricated on the micropillar surface. Next, the designed nanopatterns on the micropillar-structured surfaces were observed by scanning electron microscopy. Lastly, the changes in the spectral peaks were precisely observed using a spectrometer to achieve an enhanced security pattern. The fabricated security patterns can be suitable for valuable products, such as branded wines, watches, and bags. In addition, the proposed method offers a simple approach for transferring metal nanopatterns to common substrates. Moreover, the fabricated security patterns can have potential applications in semiconductor electrodes, transparent electrodes, and security identification codes.

Repeatable and metal-independent nanotransfer printing based on metal oxidation for plasmonic color filters

Many recently developed nanotransfer printing techniques have received much attention because of their simplicity and low cost. In addition, such techniques are suitable for fabricating nano/microscale sensors, optical elements, and electrical devices. However, conventional nanotransfer printing methods are time-consuming, cannot be easily used over large areas or with several different materials, and are not suitable for repeatedly transferring various materials onto the same substrate or a curved surface. Herein, a new nanotransfer printing method is introduced based on the oxidation of various metals and the formation of covalent bonds between spin- and spray-coatable adhesives and the chosen metal at low temperatures. These strong covalent bonds allow the fast transfer of the deposited materials from a polymer stamp without additional processing. A major advantage of this process is that it is metal-independent; nanowires of various metals are successfully transferred from the polymer stamp because strong covalent bonds form instantaneously between the metal and an adhesive-coated substrate. Moreover, this nanotransfer process can be used repeatedly to fabricate large-scale color filters from smaller areas of nanowires, regardless of the metal type and nanostructure orientation. Furthermore, plasmonic color filters composed of nanohole arrays can be obtained on both flat and curved surfaces.

3D chiral color prints for anti-counterfeiting

Here we demonstrate a novel anti-counterfeiting method, 3D chiral color printing, which displays a plasmonic color print under left-handed circular polarized (LCP) light, while revealing a different color image under right-handed circular polarized (RCP) light. We employ double-layer arcs as 3D chiral color pixels due to their giant difference of chiroptical response to LCP and RCP illumination. Asymmetric and symmetric plasmon coupling modes are attributed to the origin of LCP and RCP colors, respectively. Through tuning the arc lengths, a wide range of RCP and LCP colors, respectively occupying 32% and 19% of the sRGB space, are achieved. Moreover, an algorithm to hide a covert circular-polarized color image underneath an opposite handed circular-polarized color print is presented. Our 3D chiral color printing has potential applications in optical security and document authentication.

Heterogeneous Nanostructures Fabricated via Binding Energy-Controlled Nanowelding

A novel concept for fabricating heterogeneous nanostructures based on different melting temperatures is developed. Au-Ag composite cross-structures are fabricated by nanowelding technologies. During the fabrication of Au-Ag composite cross-structures, Ag nanowires transform into ordered particles decorating the Au nanowire surfaces with an increase in the welding temperature because of the different melting temperatures of Au and Ag. To compare and explain the melting temperatures, the thicknesses of Au and Ag nanowires as parameters are analyzed. Scanning electron microscopy and focused ion beam imaging are used to observe the morphologies and cross sections of the fabricated samples. The evolution of 3D nanostructures is observed by atomic force microscopy, whereas the compositions and binding energies of the nanostructures are determined by X-ray diffraction and X-ray photoelectron spectroscopies. In addition, the atomic structures are analyzed by transmission electron microscopy, and the optical properties of the fabricated nanostructures are evaluated by spectrometry. Furthermore, color filter electrodes are fabricated, and their polarization properties are evaluated by sheet resistance measurements and observing the color and brightness of light-emitting diodes. The proposed method is suitable for application in various fields such as biosensors, optics, and medicine.