Enhancing second harmonic generation in gold nanoring resonators filled with lithium niobate.

Plasmonic nanorings provide the unique advantage of a pronounced plasmonic field enhancement inside their core. If filled with a polarizable medium, it may significantly enhance its optical effects. Here, we demonstrate this proposition by filling gold nanorings with lithium niobate. The generated second harmonic signal is compared to the signal originating from an unpatterned lithium niobate surface. Measurements and simulation confirm an enhancement of about 20. Applications requiring nanoscopic localized light sources like fluorescence spectroscopy or quantum communication will benefit from our findings.

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.

Top-up fabrication of gold nanorings.

A simple method for the fabrication of gold nanorings with reliable material composition and reproducible morphology that relies on a combination of top-down and bottom-up techniques is presented. Here, lithographically defined hole arrays are used as templates for the deposition of gold nanoparticles. The resulting gold nanoparticle rings are joined by electroless deposition of gold, thereby leading to the formation of gold nanoring arrays that show plasmonic resonance in their optical spectra at theoretically predicted wavelengths. In addition, the gold nanorings can be released from the template by extensive rinsing with water, thereby enabling their further functionalization or assembly and the re-use of the template for the preparation of more nanorings.

Circular dichroism from chiral nanomaterial fabricated by on-edge lithography.

A novel-shaped plasmonic chiral nanomaterial exhibiting circular dichroism in the near-infrared spectral range is presented. Applying on-edge lithography, a large area with these nanostructures is efficiently covered. This fabrication method offers tunability of the operation bandwidth by tailoring the chiral shape.

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.

Antireflective "moth-eye" structures on tunable optical silicone membranes.

Flexible silicone membranes are key components for tunable optical lenses. The elastic operation of the membranes impedes the use of classical layer systems for an antireflective (AR) effect. To overcome this limitation, we equipped optical elastomer membranes with "moth-eye" structures directly in the flexible silicone substrate. The manufacturing of the AR structures in the flexible membrane includes a mastering process based on block copolymer micelle nanolithography followed by a replication method. We investigate the performance of the resulting AR structures under strain of up to 20% membrane expansion. A significant transmittance enhancement of up to 2.5% is achieved over the entire visible spectrum, which means that more than half of the surface reflection losses are compensated by the AR structures.

Antireflective subwavelength structures on microlens arrays-comparison of various manufacturing techniques.

Antireflective subwavelength structures (ARS) resembling nanostructures found on the cornea of night-active insects reduce the reflection of light by providing a gradual change in the refractive index at the interface. These artificial ARS have mainly been fabricated by a combination of conventional lithography and reactive ion etching, which constrains their application to planar substrates. We report on the fabrication of ARS using three different techniques including bottom-up and top-down methods as well as their combination on microlens arrays (MLAs) made of fused silica. The optical performance of the resulting ARS on the MLAs is as good as ARS fabricated on planar substrates with increased transmission of up to 96% at certain wavelengths.

Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses.

We designed, fabricated and characterized a nano-periodical highly-efficient blazed grating for extreme-ultraviolet (XUV) radiation. The grating was optimized by the rigorous coupled-wave analysis method (RCWA) and milled into the top layer of a highly-reflective mirror for IR light. The XUV diffraction efficiency was determined to be around 20% in the range from 35.5 to 79.2 nm. The effects of the nanograting on the reflectivity of the IR light and non-linear effects introduced by the nanograting have been measured and are discussed.

Simulating different manufactured antireflective sub-wavelength structures considering the influence of local topographic variations.

Laterally structured antireflective sub-wavelength structures show unique properties with respect to broadband performance, damage threshold and thermal stability. Thus they are superior to classical layer based antireflective coatings for a number of applications. Dependent on the selected fabrication technology the local topography of the periodic structure may deviate from the perfect repetition of a sub-wavelength unit cell. We used rigorous coupled-wave analysis (RCWA) to simulate the efficiency losses due to scattering effects based on height and displacement variations between the individual protuberances. In these simulations we chose conical and Super-Gaussian shapes to approximate the real profile of fabricated structures. The simulation results are in accordance with the experimentally determined optical properties of sub-wavelength structures over a broad wavelength range. Especially the transmittance reduction in the deep-UV could be ascribed to these variations in the sub-wavelength structures.

Tailored antireflective biomimetic nanostructures for UV applications.

Antireflective surfaces composed of biomimetic sub-wavelength structures that employ the 'moth eye principle' for reflectance reduction are highly desirable in many optical applications such as solar cells, photodetectors and laser optics. We report an efficient approach for the fabrication of antireflective surfaces based on a two-step process consisting of gold nanoparticle mask generation by micellar block copolymer nanolithography and a multi-step reactive ion etching process. Depending on the RIE process parameters nanostructured surfaces with tailored antireflective properties can easily be fabricated that show optimum performance for specific applications.