Laser Interference Lithography Research Articles

Phase-Shifted Distributed Feedback Lasers With Two-Layer Moiré Gratings

Single-wavelength distributed feedback (DFB) lasers are widely used in high-speed access networks; however, the device yield is generally low due to threshold degeneracy resulting from uniform periodic gratings. In this work, we propose and experimentally demonstrate a two-layer moire grating structure to provide a quarter-wavelength phase shift in DFB lasers. The grating structure can be defined easily using low-cost laser interference lithography. In experiments, 200- $\mu \text{m}$ -long DFB lasers exhibit excellent single-wavelength operating behavior (SMSR >42 dB) in terms of Bragg wavelength and high-speed modulation capability ( $\text{f}_{\mathrm {3dB}} =22$ GHz). High device yield of 86% from 320 laser dies further validated the efficacy of the proposed DFB laser design as a promising alternative to existing phase-shifted DFB laser solutions.

Modeling of flexible light extraction structure: Improved flexibility and optical efficiency for organic light-emitting diodes

Abstract Organic light-emitting diodes (OLEDs) have recently garnered significant attention due to their excellent performance. Despite intensive research efforts, OLEDs still face the challenges of low external quantum efficiency (EQE) and instability when applied to flexible displays. Herein, we developed a periodic flexible nano-grating structure (FNG) through laser interference lithography (LIL). Enhanced flexibility through the introduction of the FNG structure was modeled based on beam theory in terms of structure parameters and represented as the relative bending deflection. In addition, the structure parameters, period and thickness of the FNG structure were simultaneously optimized by computational finite-difference time-domain (FDTD) methods. The FNG structure not only enhanced the optical efficiencies of the device, but also reduced the stress of the flexible devices. Consequently, when the FNG with the largest relative bending deflection of 2.29 was applied to the OLED device, the external quantum efficiency (EQE) were higher as 2.80% compared to that of reference as 2.08%. After cyclic bending, the difference in EQE was more increased due to reduced relative bending stiffness, where the EQE of the FNG integrated device was 2.49% and that of the reference device was 0.91%.

Fabrication of Silver-Silicon Gratings for Surface Plasmon Excitation Using Nanosecond Laser Interference Lithography

We introduce a new two-step method to fabricate silver-silicon gratings that can excite surface plasmons efficiently. The grating structure was firstly created on a flat silicon substrate by single-pulse nanosecond laser interference lithography. Next, a silver layer of 50 nm was evaporated on the silicon substrate. With proper laser energy, a silver-silicon grating with a period of 1120 nm and depth of 18 nm was successfully fabricated. This grating can produce high-quality surface plasmon resonance (SPR) peaks at various incident angles. Moreover, a SPR sensor was experimentally demonstrated based on the silver-silicon grating, which had high sensitivity of 961.5 nm/RIU and figure of merit of 60. Our method is an efficient way to fabricate periodically nanostructured metals at high speed and low cost.

Subwavelength diffraction gratings with macroscopic moiré patterns generated via laser interference lithography.

We propose a simple and flexible fabrication approach based on the moiré effect of photoresist gratings for rapid synthesis of apodized structures with continuously varying depth. Minor modifications in a standard laser interference lithography setup allow creating macroscopic, visible by naked eye moiré patterns that modulate the depth of subwavelength diffraction gratings. The spatial frequency of this modulation is easily controlled in a wide range, allowing to create a quasicrystal in extreme cases. Experimental results are confirmed by a theory with clear graphical solutions and numerical modeling. The method is universal and does not depend on a specific choice of photoresist and/or substrate materials, making it a promising choice for structured light applications, optical security elements or as a basic structuring method of complex optical devices.

Tuning of polarized room-temperature thermal radiation based on nanogap plasmon resonance.

When a one-dimensional (1D) metal array is coupled to a planar metal mirror with a dielectric gap, localized plasmon resonance is excited inside the gap at a specific polarization of light in free space. Herein, we report on the completely polarized, mid-infrared thermal radiation that is released from gap plasmon resonators with a nanometer-thick dielectric. We fabricated nanogap plasmon resonators with 1D Au or Ni array of various widths (w) using laser interference lithography. An atomic layer deposition process was used to introduce a 10 nm-thick alumina gap between a 1D metal array and a planar metal mirror. It was observed that only for the Au nanogap plasmon resonators, high-amplitude absorption peaks that were attributed to gap plasmon modes with different orders appeared at discrete wavelengths in a polarization-resolved spectrum. In addition, all the pronounced peaks were gradually redshifted with increasing w. At w = 1.2-1.6 µm, the fundamental gap plasmon mode was tuned to the main wavelengths (8-9 µm) of thermal radiation at room temperature (e.g., ∼300 K), which led to polarization-selective camouflage against standard infrared thermal imaging. The results of electromagnetic simulations quantitatively agreed with the measured absorbance spectra in both peak wavelength and amplitude. We believe that these experimental efforts towards achieving radiation/absorption spectra tailored at mid-infrared wavelengths will be further exploited in thermal-radiation harnessed energy devices, spectroscopic sensors, and radiative coolers.

Effect of laser functionalization of titanium on bioactivity and biological response

This study proposes Direct Laser Interference Lithography as a highly suitable technique for functionalization titanium surfaces for biomedical applications. DLIL was employed to produce periodic patterns on titanium after two commonly used surface treatments of implantable devices (shot peening and acid etching). The biomedical potential of the proposed method was analyzed using immersion tests in a simulated body fluid solution and cell adhesion tests. After 48 h of immersion surface morphology, the chemical composition and phase structure of the apatite layers deposited on the modified titanium were analyzed. In order to analyze the kinetics of the apatite layer growth, X-Ray Photoelectron Spectroscopy measurements at different soaking times were performed. Cell adhesion tests were performed using human fetal osteoblastic cells (hFOB). The adhered cells were analyzed using confocal and scanning electron microscopies after 48 h of incubation. The formation of biomimetic apatite layers was accelerated on the titanium surface structures after DLIL modification. The periodic titanium patterns induced more uniform and direct cell growth. This effect is mainly connected with the surface properties of the DLIL-modified substrates. The formation mechanism of biomimetic apatite on the textured titanium samples, as well as the compounds created on the surface, are discussed.

Optical characterizations and thermal analyses of HfO2/SiO2 multilayered diffraction gratings for high-power continuous wave laser

Spectral beam combining technique is a simple and straightforward method to obtain a high-power laser beam. The beam combining system typically consists of three parts: fiber laser array, transform optical system and diffraction grating. Performance of the system (diffraction efficiency and laser induced damage threshold) is mainly determined by the diffraction grating. This paper describes a multilayer dielectric diffraction gating with high diffraction efficiency and low laser-induced damage threshold, which can be potentially used to obtain a high-power laser by combing laser beams. The diffraction grating is designed by coupled-wave analysis, and single-beam laser interference lithography is used to fabricate diffraction gratings. To verify the optical and thermal characteristics of the beam, we measure diffraction efficiency and the threshold for laser-induced damage. Finite element method simulation accurately represented temperature distribution in the grating during laser irradiation. This work provides integrated methods that include design of multilayer dielectric gratings, thermal analysis and device characterization using high-power laser.

A Fundamental Study of a Surface Modification on Silicon Wafer Using Direct Laser Interference Patterning with 355-nm UV Laser

The growing need for precision machining, which is difficult to achieve using conventional mechanical machining techniques, has fueled interest in laser patterning. Ultraviolet (UV) pulsed-lasers have been used in various applications, including the micro machining of polymers and metals. In this study, we investigated direct laser interference patterning of a silicon waver using a third-harmonic diode-pumped solid-state UV laser with a wavelength of 355 nm. Direct laser lithography is much more simple process compare to other submicro processing method. We have studied interference patterning for silicon wafers as a basic research for direct laser interference patterning on wafer surfaces without mask. And Finite element analysis (FEA) was performed for a 150° biprism using modeling software (COMSOL Multiphysics 5.4) to determine changes in the periodic patterns according to the focusing distance in the direct interference lithography experiment. In further study, we expect this technique to be applied to direct laser interference lithography on metals.

Enhanced optical efficiency and color purity for organic light-emitting diodes by finely optimizing parameters of nanoscale low-refractive index grid

To extract the confined waveguided light in organic light-emitting diodes (OLEDs), inserting a low refractive index (RI) periodic structure between the anode and organic layer has been widely investigated as a promising technology. However, the periodic-structure-based light extraction applied inside devices has been shown to severely distort spectrum and affect EL characteristics. In this study, a simple light extraction technology using periodic low-RI nanodot array (NDA) as internal light extraction layer has been demonstrated. The NDA was fabricated simply via laser interference lithography (LIL). The structural parameters of periodic pattern, distance, and height were easily controlled by the LIL process. From computational analysis using finite-difference time-domain (FDTD) method, the NDA with 300 nm pitch and 0.3 coverage ratio per unit cell with 60 nm height showed the highest enhancement with spectral-distortion-minimized characteristics. Through both computational and experimental systematic analysis on the structural parameters of low-RI NDA-embedded OLEDs, highly efficient OLEDs have been fabricated. Finally, as representative indicators, hexagonal and rectangular positioned NDA-embedded OLEDs showed highly improved external quantum efficiencies of 2.44 (+29.55%) and 2.77 (+57.38%), respectively. Furthermore, the disadvantage originating from the nanoscale surface roughness on the transparent conductive oxide was minimized.

Controllable Patterning of Hybrid Silicon Nanowire and Nanohole Arrays by Laser Interference Lithography

Metal‐assisted chemical etching (MACE) is a cost‐effective method to fabricate Si nanostructures including silicon nanowires (SiNWs) and silicon nanoholes (SiNHs). However, the preparation of metallic template for MACE would require complex experimental conditions including strict cleaning process and multiple steps. Herein, superlens‐enhanced laser interference lithography is applied to directly fabricate complicated metallic patterns and then MACE is used to obtain hybrid SiNW and SiNH arrays. Ag films are first deposited on Si substrates, and then a 1064 nm high power laser source is utilized to generate two‐beam interference electric fields. Because Ag molecules are very sensitive to any input energy change, they tend to break up or aggregate and form different Ag patterns which have a specific energy threshold to lower its free energy. By manipulating the distribution of input electric field, complicated metallic patterns and their corresponding Si nanostructures with feature sizes that range from tens of nanometers to several micrometers are obtained.

Prism-Based Laser Interference Lithography System for Simple Multibeam Interference Lithography

Laser interference lithography is a powerful technique of subwavelength-scale patterning. However, this technique requires a complicated optical system, the precise control and complexity of which increases exponentially with increasing number of beams. In this study, a compact prism-based laser interference lithography system using optical fibers and a prism was proposed to simplify the technique. In this system, the beam splitter and mirrors in a typical laser interference lithography system were replaced by a designed prism, and the pattern could be easily changed by only changing the prism, without other modification, irrespective of the number of beams required. In addition, because the laser and the laser interference lithography system are connected by an optical fiber, the system can be moved easily and flexibly. The fabrication of various submicroscale line patterns, holes, and dot patterns with a 270–1400 nm pitch was achieved using the proposed laser interference lithography system.

Fabrication and transfer printing of periodic Pt nanonetwork gratings

Metal nanonetworks are applied in various applications, such as biomedicine, bionic materials, optical materials, and new energy materials. Here, periodic variable-sized Pt nanonetwork gratings (PtNGs) are fabricated on the surface of a Pt/Si substrate with single pulse two-beam direct laser interference lithography. The fabricated PtNGs are transferred onto the surface of a glass substrate with polymethyl methacrylate as the transfer mediator. Exposure with different film thicknesses, contrasts, and intensity distributions of the laser interference spot is analyzed, and the formation of nanopatterns is explained. Results show that with the change in the thicknesses of the Pt film, the exposed structures present Pt nanoparticles (PtNPs), Pt gratings, and PtNGs. The morphology and the feature size of the PtNGs are influenced by intensity distributions and the contrast of the laser interference spot significantly.

Ultra-broadband reflector using double-layer subwavelength gratings

Double-layer high-contrast subwavelength gratings that are separated by a dielectric space layer are investigated to achieve ultra-broadband reflection. The reflection phase of subwavelength gratings and the propagation phase shift between two gratings are manipulated to expand reflection bandwidth by properly stacking two reflective gratings. A reflector exhibiting a 99% reflectance bandwidth of ∼ 1080 nm in the near-infrared is designed. Then this reflector is prepared using laser interference lithography and ion beam planarization, and an ultra-broadband reflection is achieved with reflectance exceeding 97% over a wavelength range of 955 nm in the near-infrared region.

High-performance boron-doped silicon micron-rod anode fabricated using a mass-producible lithography method for a lithium ion battery

Although silicon (Si) attracts great attention as a high-capacity anode material in lithium ion batteries (LiBs), a large volume being expanded during charge/discharge (de/lithiation) cycling is a significant problem resulting in a fast capacity fade. To prevent the problem, a variety of Si structures with nano/microscales have been incorporated into the anode, but such structures still have difficulties in terms of mass production. Herein, we present a new way to repetitively produce micron boron (B)-doped Si rods from Si wafer through laser interference lithography (LIL) in combination with the metal assisted chemical etching (MACE) process, enabling the mass-production of multiple Si rods at low cost. Moreover, the effects of the B-doping level of the produced Si rods on the electrochemical LiB performances are studied in detail. As a result, the lightly B-doped Si rod (~1015 atoms cm−3) anodes exhibit the highest initial capacity of 3524 mAh g−1 and cyclic performance, showing a high average Coulombic efficiency (CE) of 98.1% and a capacity fading rate (per cycle) of 0.11% during 500 cycles. It is due to the highest kinetics of de/lithiation on the surface of the lightly B-doped Si rod attributed to favorable phase transition of Si and diffusion of Li ions.

Parallel Laser Micro/Nano‐Processing for Functional Device Fabrication

AbstractPhotolithography is one of the most commonly used techniques in semiconductor manufacturing, which is the foundation for all the modern electronic device fabrication. However, deep and extreme ultraviolet lithographic systems as well as the corresponding photomasks are both relatively expensive. The fabrication methods are based on the low‐speed high‐cost electron‐beam lithography or focused‐ion‐beam etching. Therefore, a maskless high‐speed method is highly recommended for the micro/nano‐structure fabrication. Among all these maskless methods, direct laser writing (DLW) is an important and widely adopted micro‐processing technique. Based on the nonlinear exposure, the feature size can achieve down to tens of nanometers. However, the speed of DLW is a technical bottleneck. To overcome this issue, parallel DLW methods are developed, including the self‐assembly microspheres laser patterning, laser interference lithography, and multifocal array DLW. Herein, the principles, advantages, challenges, and applications of these parallel processing technologies are summarized. Nanoscale resolution for large area arbitrary periodic pattern fabrication is achieved. Meanwhile, these technologies have the unique ability to build 3D structures instead of conventional 2D patterns, which is the direction of future micro/nano‐fabrication. These techniques are widely applied to surface processing and functional device fabrication in the field of sensing, solar cells, and metamaterials.

360 nm Continuous Wave Laser-Based Contact or Non-Contact Laser Interference Nano Lithography.

LIL (laser interference lithography), which does not require a mask, is an effective way to create a wide variety of periodic patterns by exposing two laser beams on a specimen. The LIL method can be used for a wide range of nano-pattern spacing by adjusting the laser intensity, exposure time, and development time. In addition, it has been used in many studies due to its advantage with regard to the forming of nanostructures over a large area in a short period of time. However, the existing LIL technique requires demanding precision levels to align the laser beam. In this paper, we attempt to solve this problem by using a prism laser interferometer to complement the complicated beam alignment of LIL. In this study, ma-p1205 PR-coated silicon wafers were exposed to a CW laser with 360 nm wavelength using a contact-type and a non-contact prism interferometer. A rectangular triangular prism and an equilateral triangular prism of N-BK7 were used, and periods of 300 nm and 260 nm were fabricated, respectively. The MATLAB and COMSOL programs were used to compare the theoretical values and fabricated patterns.

Optimization of sputtered Au film on grating structure as SERS substrate for detection of pesticide

Here, we report an efficient approach to optimize the performance of surface enhanced Raman scattering (SERS) substrate. The geometry of the SERS substrate consists of the Au film deposited on polymer grating. The SERS substrate were fabricated by the laser interference lithography (LIL) and magnetron sputtering of Au thin films. The effect of the Au thin film prepared by magnetron sputtering at different deposition time (5-180 s) and operated pressure (3-5 mTorr). The morphology of the obtained samples was observed by field-emission scanning electron microscopy (FE-SEM). The results indicated that optimal SERS substrate with deposition time of 180 s and 3 mTorr-operated pressure was obtained. The limit of detection for methylene blue (MB) and methyl parathion were evaluated at 10-4 M and 10-2 M, respectively. Moreover, our SERS substrate shows the application of a portable Raman spectrophotometer which also promising for on-site pesticide substance detection.

Au-Coated PDMS grating for SERS substrate: Comparison of two different grating depths

Surface enhanced Raman scattering substrate was an alternative analytical tool with ultrahigh sensitivity and rapid response for chemistry, medicine and forensics science. In this work, the surface enhanced Raman scattering (SERS) substrate based on PDMS grating structure created by laser interference lithography using excimer laser modification and further 80 nm-thick Au thin film deposition by dc magnetron sputtering was proposed. We investigated the effect of the grating depth on SERS performance. The methylene blue solution of different concentration was employed to test the SERS performance using the portable Raman spectrometer. The optimal SERS performance can be optimized by fabricating the PDMS grating structure with 850 nm-period, 310 nm-depth and 0.5-filling factor.

Wide range bilayer aluminum nanowire grating sensors with robust reflective peaks

A grating-coupled surface plasmon resonance sensor based on bilayer aluminum nanowire arrays is fabricated by laser interference lithography. The device presents impressive reflective sharp peaks by lateral surface plasmon resonances even for aluminum thicknesses of merely several nanometers. Distinct reflective peaks and dramatic color shifts under different analytes are observed within a wide range of incident angle, metal thickness, and refractive index. The sensitivity of 307 nm per refractive index unit is experimentally obtained. The reflective-peak-type surface plasmon resonance sensors are suitable for practical applications because of easy fabrication, low cost, wide range, and high signal visibility.

Advanced generation of functional dual-periodic microstructured surface based on optical in-process measurement

In order to realize dual-periodic microstructured surfaces, which are required for next-generation multi-functional surfaces for various fields, multi-exposure laser interference lithography (MELIL) is recognized as one of the potential fabrication methods. However, MELIL has one critical problem that it is difficult to adjust each exposure energy for adequate multiple exposures before the development process. To solve this critical problem, we proposed an application of special optical in-process measurement, allowing us to control each exposure condition properly during its exposure process, and experimentally verified its feasibility by generating a desired dual-periodic microstructured surface using the proposed developed system with an in-process measurement unit.