Wafer-scale nanofabrication of sub-5 nm gaps in plasmonic metasurfaces.

In the rapidly evolving field of plasmonic metasurfaces, achieving homogeneous, reliable, and reproducible fabrication of sub-5 nm dielectric nanogaps is a significant challenge. This article presents an advanced fabrication technology that addresses this issue, capable of realizing uniform and reliable vertical nanogap metasurfaces on a whole wafer of 100 mm diameter. By leveraging fast patterning techniques, such as variable-shaped and character projection electron beam lithography (EBL), along with atomic layer deposition (ALD) for defining a few nanometer gaps with sub-nanometer precision, we have developed a flexible nanofabrication technology to achieve gaps as narrow as 2 nm in plasmonic nanoantennas.

Mid-infrared dielectric laser acceleration in a silicon dual pillar structure.

Dielectric laser accelerators use near-infrared laser pulses to accelerate electrons at dielectric structures. Driving these devices with mid-infrared light should result in relaxed requirements on the electron beam, easier fabrication, higher damage threshold, and thus higher acceleration gradients. In this paper, we demonstrate dielectric laser acceleration of electrons driven with 10 μm light in a silicon dual pillar structure.

Structured illumination ptychography and at-wavelength characterization with an EUV diffuser at 13.5 nm wavelength.

Structured illumination is essential for high-performance ptychography. Especially in the extreme ultraviolet (EUV) range, where reflective optics are prevalent, the generation of structured beams is challenging and, so far, mostly amplitude-only masks have been used. In this study, we generate a highly structured beam using a phase-shifting diffuser optimized for 13.

Antireflection Structures for VIS and NIR on Arbitrarily Shaped Fused Silica Substrates with Colloidal Polystyrene Nanosphere Lithography.

Antireflective (AR) nanostructures offer an effective, broadband alternative to conventional AR coatings that could be used even under extreme conditions. In this publication, a possible fabrication process based on colloidal polystyrene (PS) nanosphere lithography for the fabrication of such AR structures on arbitrarily shaped fused silica substrates is presented and evaluated. Special emphasis is placed on the involved manufacturing steps in order to be able to produce tailored and effective structures.

Enhancement of third harmonic generation induced by surface lattice resonances in plasmonic metasurfaces.

We investigate experimentally third harmonic generation (THG) from plasmonic metasurfaces consisting of two-dimensional rectangular lattices of centrosymmetric gold nanobars. By varying the incidence angle and the lattice period, we show how surface lattice resonances (SLRs) at the involved wavelengths are the major contributors in determining the magnitude of the nonlinear effects. A further boost on THG is observed when we excite together more than one SLR, either at the same or at different frequency.

Design of computer-generated holograms based on semi-rigorous simulations of sub-wavelength structures.

Conventional design methods for computer-generated holograms often rely on the scalar diffraction theory because the calculation effort of rigorous simulations is too high. But for sub-wavelength lateral feature sizes or large deflection angles, the performance of realized elements will show distinct deviations from the expected scalar behavior. We propose a new design method that overcomes this issue by incorporating high-speed semi-rigorous simulation techniques that allow the modeling of light propagation at an accuracy close to the rigorous methods.

Second harmonic generation under doubly resonant lattice plasmon excitation.

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation.

Optical properties of black silicon structures ALD-coated with AlO.

Atomic layer deposited (ALD) AlOcoatings were applied on black silicon (b-Si) structures. The coated nanostructures were investigated regarding their reflective and transmissive behaviour. For a systematic study of the influence of the AlOcoating, ALD coatings with a varying layer thickness were deposited on three b-Si structures with different morphologies.

Ultrafast farfield simulation of non-paraxial computer generated holograms.

The simulation of large-area diffractive optical elements (DOEs) is challenging when non-paraxial propagation and coupling effects between neighboring structures shall be considered. We developed a novel method for the farfield simulation of DOEs, especially computer-generated holograms (CGHs) with lateral feature sizes in the wavelength range. It uses a machine learning approach to predict the optical function based on geometry parameters.

Macroscopic wave-optical simulation of dielectric metasurfaces.

We propose a novel method for the wave-optical simulation of diffractive optical elements (DOEs) like metasurfaces or computer-generated holograms (CGHs). Existing techniques mostly rely on the assumption of local periodicity to predict the performance of elements. The utilization of a specially adapted finite-difference beam propagation method (BPM) allows the semi-rigorous simulation of entire DOEs within a reasonable runtime due to linear scaling with the number of grid points.

Diffuse scattering due to stochastic disturbances of 1D-gratings on the example of line edge roughness.

Diffuse scattering of optical one-dimensional gratings becomes increasingly critical as it constrains the performance, e.g., of grating spectrometers.

Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces.

Hyperspectral-grating-based imaging spectrometer systems with F/3 and covering the visual-near-infrared (420-1000 nm) spectral range are investigated for monitoring Earth's environmental changes. The systems have an entrance slit of 24 μm and a 6.5 nm spectral resolution.

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.

Rowland ghost suppression in high efficiency spectrometer gratings fabricated by e-beam lithography.

In this paper we report different methods to improve the stray light performance of binary spectrometer gratings fabricated by electron beam lithography. In particular, we report the optimization concerns about spurious stray light peaks, also known as "Rowland ghosts". As already known these Rowland ghosts arise from a non-optimized stitching process of special subareas needed in order to fabricate large area gratings.

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.

250 nm period grating transferred by proximity i-line mask-aligner lithography.

This Letter, describes a fabrication method based on a high refractive index binary phase mask combined with a suitable illumination setup, which produces a close to normal incidence illumination, to fabricate sub-micrometer diffraction gratings. The method uses the i-line (365 nm) of a mercury lamp spectrum in a mask-aligner in proximity mode, to avoid any contact between the mask and the wafer, which is normally used to produce high resolution structures. The transfer of the structure in a fused silica wafer demonstrates that mask-aligner lithography can produce high aspect ratio sub-wavelength structures without resorting to any contact between mask and wafer.

Pulse compression grating fabrication by diffractive proximity photolithography.

We report about a newly devised throughput-scalable fabrication method for high-quality periodic submicron structures. The process is demonstrated for optical transmission gratings in fused silica with a period of 800 nm (1250  lines/mm) to be used in laser pulse compression. The technology is based on an innovative advancement of i-line proximity photolithography performed in a mask aligner.

Structured Mo/Si multilayers for IR-suppression in laser-produced EUV light sources.

Laser produced plasma sources are considered attractive for high-volume extreme-ultraviolet (EUV) lithography because of their high power at the target wavelength 13.5 nm. However, besides the required EUV light, a large amount of infrared (IR) light from the CO drive laser is scattered and reflected from the plasma as well as from the EUV mirrors in the optical system.

Mode shaping in semiconductor broad area lasers by monolithically integrated phase structures.

By broadening the stripe width of the active waveguide region, it is possible to extract high optical powers from semiconductor broad area lasers. However, a weak output beam quality, optical filamentation, and high peak power densities will result, which are invoked by the amplification of higher order modes. We show an approach to influence the optical field inside the resonator by integrating optical phase structures directly into the waveguide.

Modeling electro-optical characteristics of broad area semiconductor lasers based on a quasi-stationary multimode analysis.

To increase the brightness of broad area laser diodes, it is necessary to tailor the optical properties of their waveguide region. For this purpose, there is the need for simulation tools which can predict the optical properties of the complete device and thus of the outcoupled light. In the present publication, we show a numerical method to calculate typical intensity distributions of the multimode beam inside a high-power semiconductor laser.

High numerical aperture hybrid optics for two-photon polymerization.

We report on an immersion hybrid optics specially designed for focusing ultrashort laser pulses into a polymer for direct laser writing via two-photon polymerization. The hybrid optics allows for well-corrected focusing over a large working distance range of 577 μm with a numerical aperture (NA) of 1.33 and low internal dispersion.

Improving the brightness of a multi-kilowatt single thin-disk laser by an aspherical phase front correction.

We report on results obtained with an aspherical mirror to compensate for the phase front aberrations of a cw thin-disk laser with a single disk in the resonator. A record output power of 5 kW with a beam quality suitable for laser cutting (beam propagation factor M2=9.2) has been achieved.

Advanced mask aligner lithography: new illumination system.

A new illumination system for mask aligner lithography is presented. The illumination system uses two subsequent microlens-based Köhler integrators. The second Köhler integrator is located in the Fourier plane of the first.

Advanced mask aligner lithography: fabrication of periodic patterns using pinhole array mask and Talbot effect.

The Talbot effect is utilized for micro-fabrication of periodic microstructures via proximity lithography in a mask aligner. A novel illumination system, referred to as MO Exposure Optics, allows to control the effective source shape and accordingly the angular spectrum of the illumination light. Pinhole array photomasks are employed to generate periodic high-resolution diffraction patterns by means of self-imaging.

Highly efficient three-level blazed grating in the resonance domain.

We designed, fabricated, and characterized three-level transmission gratings in the resonance domain with reduced shadowing losses based on a three-wave interference mechanism. A new technological approach allows for fabrication of homogeneous and large area multilevel gratings without spurious artifacts. To our knowledge, the measured efficiency of 86% exhibits the largest value yet reported for a multilevel transmission grating in the resonance domain close to normal incidence.