Performance of Grating Couplers Used in the Optical Switch Configuration

Surface plasmon resonance is an effect widely used for biosensing. Biosensors based on this effect operate in different configurations, including the use of diffraction gratings as couplers. Gratings are highly tunable and are easy to integrate into a fluidic system due to their planar configuration. We discuss the optimization of plasmonic grating couplers for use in a specific sensor configuration based on the optical switch. These gratings present a sinusoidal profile with a high depth/period ratio. Their interaction with a p-polarized light beam results in two significant diffracted orders (the 0th and the -1st), which enable differential measurements cancelling noise due to common fluctuations. The gratings are fabricated by combining laser interference lithography with nanoimprinting in a process that is aligned with the challenges of low-cost mass production. The effects of different grating parameters such as the period, depth and profile are theoretically and experimentally investigated.

Real-Time Imaging of Plasmonic Concentric Circular Gratings Fabricated by Lens-Axicon Laser Interference Lithography

Concentric circular gratings are diffractive optical elements useful for polarization-independent applications in photonics and plasmonics. They are usually fabricated using a low-throughput and expensive electron beam lithography technique. In this paper, concentric circular gratings with selectable pitch values were successfully manufactured on thin films of azobenzene molecular glass using a novel laser interference lithography technique utilizing Bessel beams generated by a combined lens-axicon configuration. This innovative approach offers enhanced scalability and a simplified manufacturing process on larger surface areas compared to the previously reported techniques. Furthermore, the plasmonic characteristics of these concentric circular gratings were investigated using conventional spectrometric techniques after transferring the nanostructured patterns from azobenzene to transparent gold/epoxy thin films. In addition, the real-time imaging of surface plasmon resonance colors transmitted from the concentric circular gratings was obtained using a 45-megapixel digital camera. The results demonstrated a strong correlation between the real-time photographic technique and the spectroscopy measurements, validating the efficacy and accuracy of this approach for the colorimetric studying of surface plasmon resonance responses in thin film photonics.

Crater-like nanoelectrode arrays for electrochemical detection of dopamine release from neuronal cells

Stem cell therapy has shown great potential in treating various incurable diseases using conventional chemotherapy. Parkinson's disease (PD)-a neurodegenerative disease-has been reported to be caused by quantitative loss or abnormal functionality of dopaminergic neurons (DAnergic neurons). To date, stem cell therapies have shown some potential in treating PD throughex vivoengraftment of stem-cell-derived neurons. However, accurately identifying the differentiation and non-invasively evaluating the functionality and maturity of DAnergic neurons are formidable challenges in stem cell therapies. These strategies are important in enhancing the efficacy of stem cell therapies. In this study, we report a novel cell cultivation platform, that is, a nanocrater-like electrochemical nanoelectrode array (NCENA) for monitoring dopamine (DA) release from neurons to detect exocytotic DA release from DAnergic neurons. In particular, the developed NCENA has a nanostructure in which three-dimensional porous gold nanopillars are uniformly arranged on conductive electrodes. The developed NCENA exhibited great DA sensing capabilities with a linear range of 0.39-150μM and a limit of detection of 1.16μM. Furthermore, the nanotopographical cues provided by the NCENA are suitable for cell cultivation with enhanced cellular adhesion. Finally, we successfully analysed the functionality and maturity of differentiated neurons on the NCENA through its excellent sensing ability for exocytotic DA.

Laser Interference Lithography-A Method for the Fabrication of Controlled Periodic Structures

A microstructure determines macro functionality. A controlled periodic structure gives the surface specific functions such as controlled structural color, wettability, anti-icing/frosting, friction reduction, and hardness enhancement. Currently, there are a variety of controllable periodic structures that can be produced. Laser interference lithography (LIL) is a technique that allows for the simple, flexible, and rapid fabrication of high-resolution periodic structures over large areas without the use of masks. Different interference conditions can produce a wide range of light fields. When an LIL system is used to expose the substrate, a variety of periodic textured structures, such as periodic nanoparticles, dot arrays, hole arrays, and stripes, can be produced. The LIL technique can be used not only on flat substrates, but also on curved or partially curved substrates, taking advantage of the large depth of focus. This paper reviews the principles of LIL and discusses how the parameters, such as spatial angle, angle of incidence, wavelength, and polarization state, affect the interference light field. Applications of LIL for functional surface fabrication, such as anti-reflection, controlled structural color, surface-enhanced Raman scattering (SERS), friction reduction, superhydrophobicity, and biocellular modulation, are also presented. Finally, we present some of the challenges and problems in LIL and its applications.

Exploring SERS from complex patterns fabricated by multi-exposure laser interference lithography

Designing uniform plasmonic surfaces in a large area is highly recommended for surface-enhanced Raman scattering (SERS). As periodic morphologies exhibit uniform SERS and optical tunability, diverse fabrication methods of periodic nanostructures have been reported for SERS applications. Laser interference lithography (LIL) is one of the most versatile tools since it can rapidly fabricate periodic patterns without the usage of photomasks. Here, we explore complex interference patterns for spatially uniform SERS sensors and its cost-effective fabrication method termed multi-exposure laser interference lithography (MELIL). MELIL can produce nearly periodic profiles along every direction confirmed by mathematical background, and in virtue of periodicity, we show that highly uniform Raman scattering (relative standard deviation <6%) can also be achievable in complex geometries as the conventional hole patterns. We quantitatively characterize the Raman enhancement of the MELIL complex patterns after two different metal deposition processes, Au e-beam evaporation and Ag electroplating, which results in 0.387 × 10⁵and 1.451 × 10⁵in enhancement factor respectively. This alternative, vacuum-free electroplating method realizes an even more cost-effective process with enhanced performance. We further conduct the optical simulation for MELIL complex patterns which exhibits the broadened and shifted absorption peaks. This result supports the potential of the expanded optical tunability of the suggested process.

Enhancing Neurogenesis of Neural Stem Cells Using Homogeneous Nanohole Pattern-Modified Conductive Platform

Biocompatible platforms, wherein cells attach and grow, are important for controlling cytoskeletal dynamics and steering stem cell functions, including differentiation. Among various components, membrane integrins play a key role in focal adhesion of cells (18-20 nm in size) and are, thus, highly sensitive to the nanotopographical features of underlying substrates. Hence, it is necessary to develop a platform/technique that can provide high flexibility in controlling nanostructure sizes. We report a platform modified with homogeneous nanohole patterns, effective in guiding neurogenesis of mouse neural stem cells (mNSCs). Sizes of nanoholes were easily generated and varied using laser interference lithography (LIL), by changing the incident angles of light interference on substrates. Among three different nanohole patterns fabricated on conductive transparent electrodes, 500 nm-sized nanoholes showed the best performance for cell adhesion and spreading, based on F-actin and lamellipodia/filopodia expression. Enhanced biocompatibility and cell adhesion of these nanohole patterns ultimately resulted in the enhanced neurogenesis of mNSCs, based on the mRNAs expression level of the mNSCs marker and several neuronal markers. Therefore, platforms modified with homogeneous nanohole patterns fabricated by LIL are promising for the precise tuning of nanostructures in tissue culture platforms and useful for controlling various differentiation lineages of stem cells.

Sensitive and Reproducible Gold SERS Sensor Based on Interference Lithography and Electrophoretic Deposition

Surface-enhanced Raman spectroscopy (SERS) is a promising analytical tool due to its label-free detection ability and superior sensitivity, which enable the detection of single molecules. Since its sensitivity is highly dependent on localized surface plasmon resonance, various methods have been applied for electric field-enhanced metal nanostructures. Despite the intensive research on practical applications of SERS, fabricating a sensitive and reproducible SERS sensor using a simple and low-cost process remains a challenge. Here, we report a simple strategy to produce a large-scale gold nanoparticle array based on laser interference lithography and the electrophoretic deposition of gold nanoparticles, generated through a pulsed laser ablation in liquid process. The fabricated gold nanoparticle array produced a sensitive, reproducible SERS signal, which allowed Rhodamine 6G to be detected at a concentration as low as 10-8 M, with an enhancement factor of 1.25 × 10⁵. This advantageous fabrication strategy is expected to enable practical SERS applications.

Realizing Large-Area Arrays of Semiconducting Fullerene Nanostructures with Direct Laser Interference Patterning

We present a laser interference patterning method for the facile fabrication of large-area and high-contrast arrays of semiconducting fullerene nanostructures, which does not rely on a tedious application of sacrificial photoresists or photomasks. A solution-deposited phenyl-C₆₁-butyric acid methyl ester (PCBM) fullerene thin film is exposed to a spatially modulated illumination intensity, as realized by a two-beam laser interference. The PCBM molecules exposed to strong intensity are photochemically transformed into a low-solubility dimeric state, so that the nontransformed PCBM molecules can be selectively removed in a subsequent solution-based development step. Following brief exposure to green laser light (λ = 532 nm, t = 5 s, p = 0.17 W cm^-2) in the designed two-beam interference setup, and a 1 min development in a tuned acetone-chloroform solution, we realize well-defined and ordered PCBM nanostripe patterns with a fwhm line width of ∼200 nm and a repetition rate of ∼2.900 lines mm^-1 over a large area of 1 cm². We demonstrate that a desired high contrast is effectuated because the initial PCBM-dimer transformation rate is dependent on the square of the illumination intensity. The semiconducting functionality of the patterned fullerene is verified in a field-effect transistor experiment, where a typical PCBM nanostripe featured an electron mobility of 5.3 × 10^-3 cm² V^-1 s^-1 and an on/off ratio of 3 × 10³.

Using Laser Interference Lithography in the Fabrication of a Simplified Micro- and Nanofluidic Device for Label-free Detection

Recently, we developed a label-free detection method based on optical diffraction, and implemented it in on our fabricated micro- and nanofluidic device. This detection method is simple and useful for detecting biomolecules, but the device fabrication consists of complicated processes. In this paper, we propose a simple method for fabricating the micro- and nanofluidic device; the fabrication combines laser interference lithography with conventional photolithography. The performance of a device fabricated by the proposed method is comparable to the performance of the device in our previous study.

Plasmonic Periodic Nanodot Arrays via Laser Interference Lithography for Organic Photovoltaic Cells with >10% Efficiency

In this study, we demonstrate a viable and promising optical engineering technique enabling the development of high-performance plasmonic organic photovoltaic devices. Laser interference lithography was explored to fabricate metal nanodot (MND) arrays with elaborately controlled dot size as well as periodicity, allowing spectral overlap between the absorption range of the active layers and the surface plasmon band of MND arrays. MND arrays with ∼91 nm dot size and ∼202 nm periodicity embedded in a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer remarkably enhanced the average power conversion efficiency (PCE) from 7.52% up to 10.11%, representing one of the highest PCE and degree of enhancement (∼34.4%) levels compared to the pristine device among plasmonic organic photovoltaics reported to date. The plasmonic enhancement mechanism was investigated by both optical and electrical analyses using finite difference time domain simulation and conductive atomic force microscopy studies.

Three-dimensional ordered ZnO/Cu2O nanoheterojunctions for efficient metal-oxide solar cells

Interface modulation for broad-band light trapping and efficient carrier collection has always been the research focus in solar cells, which provides the most effective way to achieve performance enhancement. In this work, solution-processed 3D ordered ZnO/Cu2O nanoheterojunctions, consisting of patterned n-ZnO nanorod arrays (NRAs) and p-Cu2O films, are elaborately designed and fabricated for the first time. By taking advantage of nanoheterojunctions with square patterned ZnO NRAs, solar cells demonstrate the maximum current density and efficiency of 9.89 mA cm(-2) and 1.52%, which are improved by 201% and 127%, respectively, compared to that of cells without pattern. Experimental analysis and theoretical simulation confirm that this exciting result originates from a more efficient broad-band light trapping and carrier collection of the 3D ordered ZnO/Cu2O nanoheterojunctions. Such 3D ordered nanostructures will have a great potential application for low-cost and all oxide solar energy conversion. Furthermore, the methodology applied in this work can be also generalized to rational design of other efficient nanodevices and nanosystems.