Iridium wire grid polarizer fabricated using atomic layer deposition

In this work, an effective multistep process toward fabrication of an iridium wire grid polarizer for UV applications involving a frequency doubling process based on ultrafast electron beam lithography and atomic layer deposition is presented. The choice of iridium as grating material is based on its good optical properties and a superior oxidation resistance. Furthermore, atomic layer deposition of iridium allows a precise adjustment of the structural parameters of the grating much better than other deposition techniques like sputtering for example. At the target wavelength of 250 nm, a transmission of about 45% and an extinction ratio of 87 are achieved

Investigation of mechanical losses of thin silicon flexures at low temperatures

The investigation of the mechanical loss of different silicon flexures in a temperature region from 5 to 300 K is presented. The flexures have been prepared by different fabrication techniques. A lowest mechanical loss of $3\times10^{-8}$ was observed for a 130 $\mu$m thick flexure at around 10 K. While the mechanical loss follows the thermoelastic predictions down to 50 K a difference can be observed at lower temperatures for different surface treatments. This surface loss will be limiting for all applications using silicon based oscillators at low temperatures. The extraction of a surface loss parameter using different results from our measurements and other references is presented. We focused on structures that are relevant for gravitational wave detectors. The surface loss parameter $\alpha_s$ = 0.5 pm was obtained. This reveals that the surface loss of silicon is significantly lower than the surface loss of fused silica.Comment: 16 pages, 7 figure

Self-organized, effective medium black silicon antireflection structures for silicon optics in the mid-infrared

Thanks to its high quality and low cost, silicon is the material of choice for optical devices operating in the mid-infrared (MIR; 2 μm to 6 μm wavelength). Unfortunately in this spectral region, the refractive index is comparably high (about 3.5) and leads to severe reflection losses of about 30% per interface. In this work, we demonstrate that self-organized, statistical Black Silicon structures, fabricated by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), can be used to effectively suppress interface reflection. More importantly, it is shown that antireflection can be achieved in an image-preserving, non-scattering way. This enables Black Silicon antireflection structures (ARS) for imaging applications in the MIR. It is demonstrated that specular transmittances of 97% can be easily achieved on both flat and curved substrates, e.g. lenses. Moreover, by a combined optical and morphological analysis of a multitude of different Black Silicon ARS, an effective medium criterion for the examined structures is derived that can also be used as a design rule for maximizing sample transmittance in a desired wavelength range. In addition, we show that the mechanical durability of the structures can be greatly enhanced by coating with hard dielectric materials like diamond-like carbon (DLC), hence enabling practical applications. Finally, the distinct advantages of statistical Black Silicon ARS over conventional AR layer stacks are discussed: simple applicability to topological substrates, absence of thermal stress and cost-effectiveness

Large area gold coated nano-needles fabricated by proximity mask aligner lithography for plasmonic AR-structures

Mask-Aligner (MA) lithography is a well-known method for the fabrication of micrometer sized structures on a substrate with a diameter up to 300 mm. In spite of a theoretical resolution below 200 nm, the minimum printable feature sized remained above 1μm due to diffraction effects and limit its utilization to advanced packaging, or MEMS fabrication. Recently, developments in the illumination system and mechanical parts (known as AMALTIH for Advanced MA LITHography) as well as mask design, have permitted to used diffractive based photo-mask, and then reach the resolution limit mentioned above. This opens the possibility to fabricate smaller structures, usually accessible only by ebeam lithography. We propose here to demonstrate a fast and robust fabrication method of large area plasmonic absorber structures based on 2D sub-micrometric (350 nm period) nano-needles in a transparent polymer on a glass substrate and coated with a 50 nm thick gold layer

Black-silicon-structured back-illuminated Ge-on-Si photodiode arrays

Backside illumination enables an increase in photoactive area and numerical aperture of Ge-on-Si photodetectors for SWIR applications. The transparency of silicon in the infrared range (lambda > 1.1 mu m) allows a nearly lossless propagation of incoming light through the Si substrate and an application of various optical microstructures on the rear side of the Si substrate. Moreover, an aluminum front contact covering the whole top area serves as a mirror which extends the optical propagation of the detectable SWIR light through the absorbing layers and hence increases the quantum efficiency. We developed back-illuminated Ge-on-Si photodiodes to apply such microstructures. Especially the usage of light trapping structures to increase the quantum efficiency of the photodiodes shows great potential. Among the different microstructures we chose black silicon (b-Si) as a promising light trapping candidate. After the fabrication, photodiodes with different configurations were evaluated. The obtained results show a strong increase of the quantum efficiency due to both, the existence of an Al mirror and the application of b-Si

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 (\u1d706=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

Differential all-optical tuning of eigenmodes in coupled microdisks

A differential all-optical resonance tuning of whispering-gallery-modes (WGMs) in a system of three coupled microdisks in a line arrangement is investigated. Utilizing the temperature induced nonlinear effects observable in coupled WGM microresonators, a tunable, narrow bandwidth, three-wavelength filter is demonstrated. Variable tuning scenarios for the signal resonances are achieved at the same sample, depending on the coupling conditions of the control signal at low control powers of up to 4 mW only

Influence of black silicon surfaces on the performance of back-contacted back silicon heterojunction solar cells

The influence of different black silicon (b-Si) front side textures prepared by inductively coupled reactive ion etching (ICP-RIE) on the performance of back-contacted back silicon heterojunction (BCB-SHJ) solar cells is investigated in detail regarding their optical performance, black silicon surface passivation and internal quantum efficiency. Under optimized conditions the effective minority carrier lifetime measured on black silicon surfaces passivated with Al2O3 can be higher than lifetimes measured for the SiO2/SiNx passivation stack used in the reference cells with standard KOH textures. However, to outperform the electrical current of silicon back-contact cells, the black silicon back-contact cell process needs to be optimized with aspect to chemical and thermal stability of the used dielectric layer combination on the cell

Coupled disk microresonators

The eigenstates of fused silica coupled disk microresonators of different configuration are investigated. The optical field distributions of the coupled disks are measured using a special scattering SNOM technique correlating the position of the tip with reflection from the excited mode. A detailed spatial and spectral analysis provides picture of evolution of the mode distribution in coupled disk microresonators when scanning through the split resonances of the coupled disk system. Applying optical pump powers in the very low milliwatt range, strong temperature induced nonlinear resonance shift of the coupled disk eigenstates is observed, leading to optical bistability. The observed effects are in agreement with simulations using a theoretical model within the scope of nonlinear coupled mode theory