Soft X-ray varied-line-spacing gratings fabricated by near-field holography using an electron beam lithography-written phase mask.

A fabrication method comprising near-field holography (NFH) with an electron beam lithography (EBL)-written phase mask was developed to fabricate soft X-ray varied-line-spacing gratings (VLSGs). An EBL-written phase mask with an area of 52 mm × 30 mm and a central line density greater than 3000 lines mm-1 was used. The introduction of the EBL-written phase mask substantially simplified the NFH optics for pattern transfer. The characterization of the groove density distribution and diffraction efficiency of the fabricated VLSGs indicates that the EBL-NFH method is feasible and promising for achieving high-accuracy groove density distributions with corresponding image properties. Vertical stray light is suppressed in the soft X-ray spectral range.

Reducing Rowland ghosts in diffraction gratings by dynamic exposure near-field holography.

Near-field holography (NFH) combined with electron beam lithography (EBL)-written phase masks is a promising method for the rapid realization of diffraction gratings with high resolution and high accuracy in line density distribution. We demonstrate a dynamic exposure method in which the grating substrate is shifted during pattern transfer. This reduces the effects of stitching errors, resulting in the decreased intensity of the optical stray light (i.e., Rowland ghosts). We demonstrate the intensity suppression of ghosts by 60%. This illustrates the potential for dynamic NFH to suppress undesirable periodic patterns from phase masks and alleviate the stitching errors induced by EBL.

175 nm period grating fabricated by i-line proximity mask-aligner lithography.

We report the fabrication of periodic structures with a critical dimension of 90 nm on a fused silica substrate by i-line (λ=365 nm) proximity mask-aligner lithography. This realization results from the combination of the improvements of the optical system in the mask aligner (known as MO exposure optics), short-period phase-mask optimization, and the implementation of self-aligned double patterning (SADP). A 350 nm period grating is transferred into a sacrificial polymer layer and coated with an aluminum layer. The removal of the metal initially present on the horizontal surfaces and on top of the polymer grating leaves a 175 nm period grating on the wafer, which can be used as a wire grid polarizer. A computation of the efficiency is performed from the measured profile and confirms the deep-blue visible to infra-red operation range.

Fabrication of free-standing lithium niobate nanowaveguides down to 50 nm in width.

Nonlinear optical nanoscale waveguides are a compact and powerful platform for efficient wavelength conversion. The free-standing waveguide geometry opens a range of applications in microscopy for local delivery of light, where in situ wavelength conversion helps to overcome various wavelength-dependent issues, such as biological tissue damage. In this paper, we present an original patterning method for high-precision fabrication of free-standing nanoscale waveguides based on lithium niobate, a material with a strong second-order nonlinearity and a broad transparency window covering the visible and mid-infrared wavelength ranges. The fabrication process combines electron-beam lithography with ion-beam enhanced etching and produces nanowaveguides with lengths from 5 to 50 µm, widths from 50 to 1000 nm and heights from 50 to 500 nm, each with a precision of few nanometers. The fabricated nanowaveguides are tested in an optical characterization experiment showing efficient second-harmonic generation.

Encapsulation process for diffraction gratings.

Encapsulation of grating structures facilitates an improvement of the optical functionality and/or adds mechanical stability to the fragile structure. Here, we introduce novel encapsulation process of nanoscale patterns based on atomic layer deposition and micro structuring. The overall size of the encapsulated structured surface area is only restricted by the size of the available microstructuring and coating devices; thus, overcoming inherent limitations of existing bonding processes concerning cleanliness, roughness, and curvature of the components. Finally, the process is demonstrated for a transmission grating. The encapsulated grating has 97.5% transmission efficiency in the -1st diffraction order for TM-polarized light, and is being limited by the experimental grating parameters as confirmed by rigorous coupled wave analysis.

Double-sided structured mask for sub-micron resolution proximity i-line mask-aligner lithography.

Diffractive mask-aligner lithography allows printing structures that have a sub-micrometer resolution by using non-contact mode. For such a purpose, masks are often designed to operate with monochromatic linearly polarized light, which is obtained by placing a spectral filter and a polarizer in the beam path. We propose here a mask design that includes a wire-grid polarizer (WGP) on the top side of a photo-mask and a diffractive element on the bottom one to print a 350 nm period grating by using a classical mask-aligner in proximity exposure mode. Linearly polarizing locally an unpolarized incident beam is only possible by using a WGP on the top side of the mask. This configuration opens the possibility to use different linear polarization orientation on a single mask and allows to print high resolution structures with different orientation within one exposure.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation.

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.

Influence of the oxygen plasma parameters on the atomic layer deposition of titanium dioxide.

The influence of the oxygen plasma parameters on the morphology and optical properties of TiO2 thin films has been extensively analyzed in plasma enhanced atomic layer deposition (PEALD) processes. Crystalline aggregates with the anatase phase have been identified on the film surface at a low deposition temperature (down to 70 °C) under specific plasma conditions. Up to 70% surface coverage by anatase crystallites is obtained at low oxygen gas flow rates and high plasma power. The hillocks abundance is correlated with high ion flux and electron density and with the resulting enhanced ion bombardment of the surface. Altering the plasma conditions is an important parameter besides temperature to control the morphology of the titania film for specific applications such as photocatalysis or functional optical coatings. Specifically, photocatalytic titania coatings on polymer substrates could benefit of such low temperature PEALD processes with abundant anatase crystallites; whereas optical coatings require smooth, high refractive index titania as obtained with low plasma power and high oxygen flow rate.

Fabrication influences on deep-ultraviolet tungsten wire grid polarizers manufactured by double patterning.

Substantial discrepancies are commonly observed when comparing the predicted and measured optical performance of deep-ultraviolet tungsten wire grid polarizers. Particularly, the extinction ratio is strongly impaired. Therefore, we investigate major differences between assumed and actual achieved properties regarding geometry and material of the grating structure as the origin of theses discrepancies. We find an improvement potential for the extinction ratio of one order of magnitude by improving the material and a factor of four by improving the geometry. Our results allow for a purposeful revision of fabrication processes and will therefore significantly contribute to the improvement of deep-ultraviolet wire grid polarizers.

Silicon wire grid polarizer for ultraviolet applications.

We present a silicon wire grid polarizer operating down to a wavelength of 300 nm. Besides metallic grating materials, semiconductors also offer appropriate material properties to realize wire grid polarizers in the ultraviolet (UV) spectral range. The presented polarizer with a period of 140 nm was realized by means of electron beam lithography and dry etching using amorphous silicon as the grating material. At a wavelength of 365 nm, a transmission of 42% and an extinction ratio of 90 (19.5 dB) are measured. The spectral bandwidth of these polarizers in the UV-spectral range is about 100 nm.

Comparison of different simulation methods for effective medium computer generated holograms.

The arrangement of binary subwavelength structures is a promising alternative to the conventional multiheight level technique to generate computer generated holograms (CGHs). However, the current heuristic design approach leads to a slight mismatch between the target signal and experimental data. To evaluate this deviation, a diffractive beam splitter design is investigated rigorously using a finite-difference time-domain (FDTD) method. Since the use of a rigorous Maxwell-equation solver like FDTD requires a massive computational effort, an alternative scalar approach, a fast Fourier transform beam propagation method (FFT-BPM), is investigated with a substantial higher computing speed, showing still a good agreement with the FDTD simulation and experimental data. Therefore, an implementation of this scalar approach into the CGH design process offers the possibility to significantly increase the accuracy.

Second-harmonic generation in lithium niobate nanowires for local fluorescence excitation.

We study the nonlinear optical properties of lithium niobate (LiNbO(3)) nanowires (NWs) fabricated by a top-down ion beam enhanced etching method. First, we demonstrate generation and propagation of the second-harmonic (SH) light in LiNbO(3) NWs of typical rectangular cross-sections of 400 x 600 nm(2) and length from 10 to 50 µm. Then, we show local fluorescent excitation of 4',6-diamidino-2-phenylindole (DAPI) dye with the propagated SH signal in standard concentrations as for biological applications. By measuring the detected average power of the propagated fundamental harmonic (FH) and the SH signal at the output of the NWs, we directly prove the dominating role of the SH signal over possible two-photon excitation processes with the FH in the DAPI dye. We estimate that 63 ± 6 pW of the propagated SH average power is required for detectable dye excitation. Finally, we model the waveguiding of the SH light to determine the smallest NW cross-section (around 40x60 nm(2)) which is potentially able to excite fluorescence with a FH intensity below the cell damage threshold.

Coupled grating reflectors with highly angular tolerant reflectance.

We report on stacked high-contrast grating reflectors with virtually angular independent reflectance for transverse-magnetic polarized light. The investigated structure consists of two-layer pairs of amorphous silicon and silicondioxide that are designed for a wavelengths of 1550 nm. The large angular tolerance results from coupling of the two involved silicon gratings and is achieved if the modal fields in the reflectors are matched. With this approach, a reflectance of more than 96% in the entire angular spectrum is feasible. Experimentally we demonstrate a reflectance of more than 98% for incidence angles up to 60° and more than 90% up to 80°.

Angular bandpass filters based on dielectric resonant waveguide gratings.

We report on a novel concept for transmissive optical elements based on resonant waveguide gratings (RWGs), which enables the realization of direction selective filters. Hereby, the broadband reflectivity of an RWG for nearly normal incidence angles is combined with high diffractive efficiency in transmission for a specific angle of incidence. Silicon is used as material with high refractive index and good compatibility with semiconductor fabrication. By adjusting the grating parameters different transmission angles and angular widths of the transmission range are feasible. First experimental results of the introduced filters provide a high transmission up to 63% at an incidence angle of 45° with a full width at half maximum of 20°.

Durability of stochastic antireflective structures--analyses on damage thresholds and adsorbate elimination.

We fabricated stochastic antireflective structures (ARS) and analyzed their stability against high power laser irradiation and high temperature annealing. For 8 ps pulse duration and 1030 nm wavelength we experimentally determined their laser induced damage threshold to 4.9 (±0.3) J/cm(2), which is nearly as high as bulk fused silica with 5.6 (±0.3) J/cm(2). A commercial layer stack reached 2.0 (±0.2) J/cm(2). An annealing process removed adsorbed organics, as shown by XPS measurements, and significantly increased the transmission of the ARS. Because of their monolithic build the ARS endure such high temperature treatments. For more sensitive samples an UV irradiation proved to be capable. It decreased the absorbed light and reinforced the transmission.

Michelson interferometer with diffractively-coupled arm resonators in second-order Littrow configuration.

Michelson-type laser-interferometric gravitational-wave (GW) observatories employ very high light powers as well as transmissively-coupled Fabry-Perot arm resonators in order to realize high measurement sensitivities. Due to the absorption in the transmissive optics, high powers lead to thermal lensing and hence to thermal distortions of the laser beam profile, which sets a limit on the maximal light power employable in GW observatories. Here, we propose and realize a Michelson-type laser interferometer with arm resonators whose coupling components are all-reflective second-order Littrow gratings. In principle such gratings allow high finesse values of the resonators but avoid bulk transmission of the laser light and thus the corresponding thermal beam distortion. The gratings used have three diffraction orders, which leads to the creation of a second signal port. We theoretically analyze the signal response of the proposed topology and show that it is equivalent to a conventional Michelson-type interferometer. In our proof-of-principle experiment we generated phase-modulation signals inside the arm resonators and detected them simultaneously at the two signal ports. The sum signal was shown to be equivalent to a single-output-port Michelson interferometer with transmissively-coupled arm cavities, taking into account optical loss. The proposed and demonstrated topology is a possible approach for future all-reflective GW observatory designs.

Oblique incidence ellipsometric characterization and the substrate dependence of visible frequency fishnet metamaterials.

We use spectroscopic ellipsometry to investigate the angular-dependent optical modes of fishnet metamaterials fabricated by nanoimprint lithography. Spectroscopic ellipsometry is demonstrated as a fast and efficient method for metamaterial characterization and the measured polarization ratios significantly simplify the calibration procedures compared to reflectance and transmittance measurements. We show that the modes can be well identified by a combination of comparing different substrates and considering the angular dependence of the Wood's anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.

Plasmonic properties of aluminum nanorings generated by double patterning.

In this Letter we evaluate a technique for the efficient and flexible generation of aluminum nanorings based on double patterning and variable shaped electron beam lithography. The process is demonstrated by realizing nanorings with diameters down to 90 nm and feature sizes of 30 nm utilizing a writing speed of one ring per microsecond. Because of redepositions caused by involved etching processes, the material of the rings and, therefore, the impact on the plasmonic properties, are unknown. This issue, which is commonly encountered when metals are nanostructured, is solved by adapting a realistic simulation model that accounts for geometry details and effective material properties. Based on this model, the redepositions are quantified, the plasmonic properties are investigated, and a design tool for the very general class of nanofabrication techniques involving the etching of metals is provided.

Nano- and microstructuring of graphene using UV-NIL.

In this work we demonstrate for the first time the micro- and nanostructuring of graphene by means of UV-nanoimprint lithography. Exfoliated graphene on SiO(2) substrates, as well as graphene deposited by chemical vapor deposition (CVD) on polycrystalline nickel and copper, and transferred CVD graphene on dielectric substrates, were used to demonstrate that our technique is suitable for large-area patterning (2 × 2 cm(2)) of graphene on various types of substrates. The demonstrated fabrication procedure of micrometer as well as nanometer-sized graphene structures with feature sizes down to 20 nm by a wafer-scale process opens up an avenue for the low-cost and high-throughput manufacturing of graphene-based optical and electronic applications. The processed graphene films show electron mobilities of up to 4.6 × 10(3) cm(2) V (-1) s(-1), which confirms them to exhibit state-of-the-art electronic quality with respect to the current literature.

Tungsten wire grid polarizer for applications in the DUV spectral range.

In this paper, we present a broadband wire grid polarizer with a spectral working range down to a wavelength of 193 nm. Tungsten is chosen as grating material because it provides a high extinction ratio and transmission compared with other common grating materials. The fabrication of the grating with 100 nm period was accomplished using a spatial frequency doubling approach based on ultrafast electron beam lithography and a sophisticated deposition technique. At a wavelength of 193 nm, a transmission of about 44% and an extinction ratio of 20 was measured.