Mikkel Dysseholm Mar

Fabrication of high aspect ratio TiO2 and Al2O3 nanogratings by atomic layer deposition

The authors report on the fabrication of TiO2 and Al2O3 nanostructured gratings with an aspect ratio of up to 50. The gratings were made by a combination of atomic layer deposition (ALD) and dry etch techniques. The workflow included fabrication of a Si template using deep reactive ion etching followed by ALD of TiO2 or Al2O3. Then, the template was etched away using SF6 in an inductively coupled plasma tool, which resulted in the formation of isolated ALD coatings, thereby achieving high aspect ratio grating structures. SF6 plasma removes silicon selectively without any observable influence on TiO2 or Al2O3, thus revealing high selectivity throughout the fabrication. Scanning electron microscopy was used to analyze every fabrication step. Due to nonreleased stress in the ALD coatings, the top parts of the gratings were observed to bend inward as the Si template was removed, thus resulting in a gradual change in the pitch value of the structures. The pitch on top of the gratings is 400 nm, and it graduall...

Large-scale high aspect ratio Al-doped ZnO nanopillars arrays as anisotropic metamaterials.

High aspect ratio free-standing Al-doped ZnO (AZO) nanopillars and nanotubes were fabricated using a combination of advanced reactive ion etching and atomic layer deposition (ALD) techniques. Prior to the pillar and tube fabrication, AZO layers were grown on flat silicon and glass substrates with different Al concentrations at 150-250 °C. For each temperature and Al concentration the ALD growth behavior, crystalline structure, physical, electrical and optical properties were investigated. It was found that AZO films deposited at 250 °C exhibit the most pronounced plasmonic behavior with the highest plasma frequency. During pillar fabrication, AZO conformally passivates the silicon template, which is characteristic of typical ALD growth conditions. The last step of fabrication is heavily dependent on the selective chemistry of the SF6 plasma. It was shown that silicon between AZO structures can be selectively removed with no observable influence on the ALD deposited coatings. The prepared free-standing AZO structures were characterized using Fourier transform infrared spectroscopy (FTIR). The restoration of the effective permittivities of the structures reveals that their anisotropy significantly deviates from the effective medium approximation (EMA) prognoses. It suggests that the permittivity of the AZO in tightly confined nanopillars is very different from that of flat AZO films.

Design and modeling of an all-optical frequency modulated MEMS strain sensor using nanoscale Bragg gratings

We present modeling and design of an all-optical MEMS Bragg grating (half-pitch of 125 nm) strain sensor for single-fiber distributed sensing. Low optical loss and the use of frequency modulation rather than amplitude modulation, makes this sensor better suited for distributed systems than comparable designs, e.g. Fabry-Perot and Mach-Zender. Also, multiplexing of several sensors with different period gratings, allow sensors to be connected to a single fiber, thereby minimizing cabling and simplifying readout. We show through analytical analysis and finite element modeling (FEM) that large mechanical amplification can be obtained if using an angled double beam micrometer scale MEMS structure, compared to conventional fiber Bragg grating sensors. An optimized design and fabrication process is presented.

Highly ordered transparent conductive oxide nanopillar metamaterials for mid-infrared plasmonics

Plasmonics for the mid-infrared wavelength regime offers unique applications such as molecular vibrational absorption spectroscopy for bio-sensing [1]. The quest for novel materials and structures has been continuing in the last decade [2]. Meanwhile, the fabrication of plasmonic metamaterial structures with large area in a reproducible manner is a tremendous challenge. In this report we show that highly ordered nanopillar structures made of aluminum-doped ZnO (AZO) can work as plasmonic components in the mid-infrared wavelength region.

Fabrication of deep-profile Al-doped ZnO one- and two-dimensional lattices as plasmonic elements

In this work, we report on fabrication of deep-profile one- and two-dimensional lattices made from Al-doped ZnO (AZO). AZO is considered as an alternative plasmonic material having the real part of the permittivity negative in the near infrared range. The exact position of the plasma frequency of AZO is doping concentration dependent, allowing for tuning possibilities. In addition, the thickness of the AZO film also affects its material properties. Physical vapor deposition techniques typically applied for AZO coating do not enable deep profiling of a plasmonic structure. Using the atomic layer deposition technique, a highly conformal deposition method, allows us to fabricate high-aspect ratio structures such as one-dimensional lattices with a period of 400 nm and size of the lamina of 200 nm in width and 3 μm in depth. Thus, our structures have an aspect ratio of 1:15 and are homogeneous on areas of 2×2 cm2 and more. We also produce two-dimensional arrays of circular nanopillars with similar dimensions. Instead of nanopillars hollow tubes with a wall thickness on demand from 20 nm up to a complete fill can be fabricated.