Uwe Huebner

Ultralow refractive index substrates–a base for photonic crystal slab waveguides

Out of plane radiation losses in two-dimensional (2D) photonic crystal (PC) waveguides occur due to a lack of total internal reflection at the core-substrate∕superstrate boundaries. In order to minimize these losses, either a high vertical refractive index contrast or deep etching into the substrate is required [G. Boettger, C. Liguda, M. Schmidt, and M. Eich, Appl. Phys. Lett. 81, 2517 (2002)]. The maximum vertical contrast is achieved in air bridge type PC waveguides, which are inherently fragile. In this article, we introduce a concept which combines the advantages of a high vertical index contrast of an air bridge with those of a solid substrate. This approach consists of mesoporous silica as substrate material with an ultralow refractive index (n=1.14 at 1.3μm), close to that of air. Finite 2DPC line defect resonators consisting of an optical polymer as core and mesoporous silica as substrate were fabricated. Compared to ordinary substrates with higher refractive indices like silica or amorphous Tefl...

Deep-subwavelength plasmonic nanoresonators exploiting extreme coupling.

A metal-insulator-metal (MIM) waveguide is a canonical structure used in many functional plasmonic devices. Recently, research on nanoresonantors made from finite, that is, truncated, MIM waveguides attracted a considerable deal of interest motivated by the promise for many applications. However, most suggested nanoresonators do not reach a deep-subwavelength domain. With ordinary fabrication techniques the dielectric spacers usually remain fairly thick, that is, in the order of tens of nanometers. This prevents the wavevector of the guided surface plasmon polariton to strongly deviate from the light line. Here, we will show that the exploitation of an extreme coupling regime, which appears for only a few nanometers thick dielectric spacer, can lift this limitation. By taking advantage of atomic layer deposition we fabricated and characterized exemplarily deep-subwavelength perfect absorbers. Our results are fully supported by numerical simulations and analytical considerations. Our work provides impetus on many fields of nanoscience and will foster various applications in high-impact areas such as metamaterials, light harvesting, and sensing or the fabrication of quantum-plasmonic devices.

Electro-optically tunable photonic crystals

We report on electro-optical modulation with a sub-1-V sensitivity in a photonic crystal slab waveguide resonator which contains a nanostructured second-order nonlinear optical polymer. The electro-optical susceptibility in the core was induced by high electric-field poling. A square lattice of holes carrying a linear defect was transferred into the slab by electron-beam lithography and reactive ion etching, creating a photonic crystal slab-based resonator. Applying an external electric modulation voltage to electrodes leads to a linear electro-optical shift of the resonance spectrum and thus to a modulation of the transmission at a fixed wavelength based on the electronic displacement polarization in a noncentrosymmetric medium (Pockels effect). This effect is therefore inherently faster than other reported electro-optic modulation effects in nanophotonics.

Surface-enhanced Raman spectroscopy (SERS) in food analytics: Detection of vitamins B2 and B12 in cereals

Food analysis has been gaining interest throughout recent decades for different reasons: the detection of hazardous substances in food and routine investigations of food composition and vitamin/nutrient contents. Regardless of the targeted component, food analysis raises a few challenges regarding the complexity of the matrix and detecting trace amounts of substances. We report herein the results obtained regarding the simultaneous detection of two B vitamins (riboflavin, vitamin B2 and cyanocobalamin, vitamin B12) by means of SERS. SERS provides molecular fingerprint identification and high analytical sensitivity together with a low processing time and cost. All these make SERS a promising tool for the development of food analytical methods.

Optical activity in sub-wavelength metallic grids and fishnet metamaterials in the conical mount

We report on measurements of optical activity in reflection in the conical mount from two plasmonically resonant nanostructures; a sub-wavelength silver meshed grid and a fishnet metamaterial. The square-centimeter size of the materials, formed by nano-imprint lithography, allows reliable investigation of such materials by plane-wave techniques with minimal focusing. For both materials we observe strong polarization conversion (s- to p-polarization, and vice versa) in generalized ellipsometry measurements. We compared the spectra to analytical predictions using surface plasmon polariton (SPP) theory and find good agreement for the meshed grid. The spectra for the meshed grid are also well modeled using the rigorous coupled wave analysis (RCWA) technique. Simulated results for the more complicated fishnet layer showing qualitative agreement are also presented. We then probe the validity of describing the observations using homogenous parameters such as dichroism and birefringence, by examining the calculated reflection of nominally polarized incident light using simulated and measured Mueller matrices. The results show that the cross-polarization that we observe is primarily related to linear birefringence and dichroism, although circular effects are indeed present.

Multi-stencil character projection e-beam lithography: a fast and flexible way for high quality optical metamaterials

In this work we report on the strong improvement of pattern quality and significant write-time reduction using Character Projection with a multi-stencil character stage with more than 2000 apertures for the fabrication of nanomaterials and, in particular, on an optical metamaterial, which is called “Metamaterial Perfect Absorber”. The Character Projection ebeam lithography allows the transition from the time-consuming serial to a fast quasi-parallel writing method and opens the way for the fabrication of device areas which are impossible to realize with often in the R&D used SEM based Gaussian electron beam-writers. More than 150.000 times faster than the comparable Gaussian E-beam exposure, 100 times faster and with a factor of 10 improved pattern size homogeneity than the corresponding Variable Shaped E-beam exposure – these are our main results for the fabrication of optical metamaterials using a Variable Shaped E-beam with Character Projection.

UV-TRIMMING OF TWO-DIMENSIONAL PHOTONIC CRYSTAL STRUCTURES

We present a new concept to adjust the transmission properties of a two-dimensional photonic crystal by photobleaching the structure. A square lattice of 500 nm lattice constant and 300 nm hole diameter was fabricated by etching air holes into a slab waveguide consisting of a polymethylmethacrylate polymer covalently functionalized with 10 mol% of the nonlinear Disperse Red 1 chromophore. While illuminating the photonic crystal with UV light the azo molecule bonds degenerate, resulting in a considerable refractive index change and a reduced slab waveguide core layer thickness. Wavelength and polarization dependent measurements show a very large shift of the dielectric band edge of 35 nm in TE and of 27 nm in TM polarization. With our novel concept it is possible to compensate fabrication inaccuracies and to trim photonic crystal transmission properties.

Hierarchically-Designed 3D Flower-Like Composite Nanostructures as an Ultrastable, Reproducible, and Sensitive SERS Substrate

Surface-enhanced Raman spectroscopy (SERS) is an attractive tool in the analytical sciences due to its high specificity and sensitivity. Because SERS-active substrates are only available as two-dimensional arrays, the fabrication of three-dimensional (3D) nanostructures allows for an increased number of hot spots in the focus volume, thus further amplifying the SERS signal. Although a great number of fabrication strategies for powerful SERS substrates exist, the generation of 3D nanostructures with high complexity and periodicity is still challenging. For this purpose, we report an easy fabrication technique for 3D nanostructures following a bottom-up preparation protocol. Enzymatically generated silver nanoparticles (EGNPs) are prepared, and the growth of hierarchically-designed 3D flower-like silica-silver composite nanostructures is induced by applying plasma-enhanced atomic layer deposition (PE-ALD) on the EGNPs. The morphology of these nanocomposites can be varied by changes in the PE-ALD cycle number, and a flower height of up to 10 μm is found. Moreover, the metallized (e.g., silver or gold) 3D nanostructures resulting from 135 PE-ALD cycles of silica creation provide highly reproducible SERS signals across the hydrophobic surface. Within this contribution, the morphological studies, optical properties, as well as the SERS response of these metallized silica-silver composite nanostructures applying vitamin B2 as a model analyte are introduced.

Fabrication of regular patterned SERS arrays by electron beam lithography

Due to its enormous signal intensity and high fingerprint sensitivity, surface enhanced Raman spectroscopy (SERS) is a powerful technique in chemical and biological applications. Our goal is the employment of the SERS technique for (bio)analytical devices. A main feature in order to achieve a versatile applicability is to ensure a reproducible and homogeneous signal across large measuring areas. Electron beam lithography is an adequate approach to assemble such reproducible arrays. Within this contribution, the fabrication process of regular patterned nanostructures based on electron beam lithography and argon ion beam etching is described. Using the exposure of crossed gratings of lines within a resist layer, gold nanorhomb and nanorectangle arrays are produced on a quartz wafer. The patterns are periodically arranged with pitches of 200 to 400 nm and exhibit sharp edges with corner radii of less than 10 nm. The pattern dimensions in combination with the small edge radii yield a high electromagnetic field enhancement caused by plasmonic excitation. The SERS arrays were characterized by means of SEM and were tested with respect to their SERS response, especially with regard to their reproducibility.

Micrometer to 15 nm Printing of Metallic Inks with Fountain Pen Nanolithography

The field of printed electronics is continually trying to reduce the dimensions of the electrical components. Here, a method of printing metallic lines with widths as small as 15 nm and up to a few micrometers using fountain pen nanolithography (FPN) is shown. The FPN technique is based on a bent nanopipette with atomic force feedback that acts similar to a nanopen. The geometry of the nanopen allows for rapid placement accuracy of the printing tip, on any desired location, with the highest of optical sub-micrometer resolution. Using this nanopen, investigations of various inks are undertaken together with instrumental and script-tool development that allows accurate printing of multiple layers. This has led to the printing of conductive lines using inks composed of silver nanoparticles and salt solutions of silver and copper. In addition, it is shown that the method can be applied to substrates of various materials with minimal effect on the dimension of the line. The line widths are varied by using nanopens with different orifices or by tailoring the wetting properties of the ink on the substrate. Metallic interconnections of conducting lines are reported.