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Understanding the mechanisms and controlling
the possibilities of surface nanostructuring is of crucial interest
for both fundamental science and application perspectives.
Here, we report a direct experimental observation
of laser-induced periodic surface structures (LIPSS) formed
near a predesigned gold step edge following single-pulse
femtosecond laser irradiation. Simulation results based on a
hybrid atomistic-continuum model fully support the experimental
observations. We experimentally detect nanosized
surface features with a periodicity of ∼300 nm and heights of
a few tens of nanometers.We identify two key components of
single-pulse LIPSS formation: excitation of surface plasmon
polaritons and material reorganization. Our results lay a
solid foundation toward simple and efficient usage of light
for innovative material processing technologies.
There are a lot of photonic micro- and nano-structures in nature that consist of materials with a low refractive index and that can keep up with artificial structures concerning optical properties like scattering or coloration. This work aims to understand the photonic structures in the silver ant Cataglyphis bombycina, the blue butterfly of genus Morpho, the beetle Entimus imperialis, which shows polarization-dependent reflection, and the white beetle Cyphochilus insulanus. Furthermore, corresponding micro- and nano-structures are fabricated.
Bioinspired models with the same optical properties as the investigated structures are developed and analyzed using geometric optics and finite-difference time-domain calculations. These models are qualitatively and quantitatively compared regarding their optical properties with the original structures and fabricated by direct laser writing. To mimic potential effects of material-based disorder of the natural photonic structures, a cellulose-based resist for direct laser writing is developed and examined.
Conventional resists in direct laser writing can be replaced by a resist containing cellulose derivatives. Here, different combinations of cellulose derivatives, initiators, and solvents are examined. The best performance is observed for a combination of methacrylated cellulose acetate (MACA500), 2-Isopropyl-9H-thioxanthen-9-one (ITX), and dimethyl sulfoxide (DMSO). These resists allow for direction is attained. The achieved cross-linking enables stable three-dimensional structures and, together with the possible resolution, allows to fabricate the model inspired by the white beetle Cyphochilus insulanus in the cellulose-based resist.
The silver appearance of the Cataglyphis bombycina can be completely explained with geometric optics in the prism-shaped hairs that cover its body. The more complex structures of the other three insects use photonic crystal-like material arrangements with a varying amount of disorder. The polarization dependence of the Entimus imperialis arises from a diamond structure inside the scales of the beetle and can be mimicked with a photonic woodpile crystal. The blue butterfly of the genus Morpho and the white beetle Cyphochilus insulanus both can be reduced to disordered Bragg stacks, in which the exact properties are achieved by introducing different amounts of disorder. For Cataglyphis bombycina, Entimus imperialis, and Cyphochilus insulanus, the developed bioinspired models are fabricated using conventional resists in direct laser writing. All models show a qualitative correspondence to the optical properties of the original structures.
The cellulose-based resists enable the use of polysaccharides in direct laser writing and the concepts can be transferred to other polysaccharides, like chitin. The analysis of the different natural photonic structures and the developed bioinspired models reveal a material independence of the structures that allows the fabrication of these models in different transparent materials.
Multi-color laser excitation of diamond nitrogen vacancy centers embedded in nanophotonic structures
(2021)
Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields.
Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging
and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser
wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an
added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation
geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be
considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk.
When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results
are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries
Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, Ω-shaped microwave antenna that enables one to reliably manipulate NV spins. We found an optimal antenna design using finite integral simulations. We fabricated our antennas on low-cost, transparent glass substrate. We created highly uniform microwave fields in areas of roughly 400 × 400 μm2 while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.
The emerging field of magnonics uses spin waves and their quanta, magnons, to implement wave-based computing on the micro- and nanoscale. Multifrequency magnon networks would allow for parallel data processing within single logic elements, whereas this is not the case with conventional transistor-based electronic logic. However, a lack of experimentally proven solutions to efficiently combine and separate magnons of different frequencies has impeded the intensive use of this concept. Herein, the experimental realization of a spin-wave demultiplexer enabling frequency-dependent separation of magnonic signals in the gigahertz range is demonstrated. The device is based on 2D magnon trans- port in the form of spin-wave beams in unpatterned magnetic films. The intrinsic frequency dependence of the beam direction is exploited to realize a passive functioning obviating an external control and additional power consumption. This approach paves the way to magnonic multiplexing circuits enabling simultaneous information transport and processing.
Scattering and scattering plates have a large diversity of applications. Scattering of optical and THz electromagnetic waves can be performed
with Galois scattering plates, which had found applications in acoustics first (i.e., with sound waves in concert hall acoustics). For binary
Galois scattering plates, the single scattering entities, i.e., mesas (for a binary 1) or voids (for a binary 0), have characteristic lateral dimensions
of half the wavelength of the electromagnetic waves to be scattered. Their optimal height is a quarter of the wavelength for plates used in
reflection. Meanwhile, not too elaborate lithographic techniques allow for the implementation of Galois plates for the THz range and even
for the visible spectral range. We had reported on such scattering plates before. However, in this paper, also the mathematical concept is
described and the fabrication technologies are emphasized. In contrast to the case of scattering plates with irregular surface morphologies,
Galois plate scattering is not diffuse, but there are many scattering/diffraction orders.
Cognitive Load Theory is considered universally applicable to all kinds of learning scenarios. However, instead of a universal method for measuring cognitive load that suits different learning contexts or target groups, there is a great variety of assessment approaches. Particularly common are subjective rating scales, which even allow for measuring the three assumed types of cognitive load in a differentiated way. Although these scales have been proven to be effective for various learning tasks, they might not be an optimal fit for the learning demands of specific complex environments like technology-enhanced STEM laboratory courses. The aim of this research was therefore to examine and compare existing rating scales in terms of validity for this learning context and to identify options for adaptation, if necessary. For the present study, the two most common subjective rating scales that are known to differentiate between load types (the Cognitive Load Scale by Leppink et al. and the Naïve Rating Scale by Klepsch et al.) were slightly adapted to the context of learning through structured hands-on experimentation where elements like measurement data, experimental setups, and experimental tasks affect knowledge acquisition. N=95 engineering students performed six experiments examining basic electric circuits where they had to explore fundamental relationships between physical quantities based on observed data. Immediately after experimentation, students answered both adapted scales. Various indicators of validity, which considered the scales’ internal structure and their relation to variables like group allocation as participants were randomly assigned to two conditions with contrasting spatial arrangement of measurement data, were analyzed. For the given data set, the intended three-factorial structure could not be confirmed and most of the a priori defined subscales showed insufficient internal consistency. A multitrait-multimethod analysis suggests convergent and discriminant evidence between the scales which could not be confirmed sufficiently. The two contrasted experimental conditions were expected to result in different ratings for extraneous load, which was solely detected by one adapted scale. As a further step, two new scales were assembled based on the overall item pool and the given data set. They revealed a three-factorial structure in accordance with the three types of load and seem to be promising new tools, although their subscales for extraneous load still suffer from low reliability scores.
Spatial optical Fourier filtering is a widespread technique for in situ image or light
field processing. However, conventional fixed absorbing patterns or mechanical irises only allow
an inflexible, very restricted control. Thus, we present two electrochromic spatial filters with
ring-shaped or directional segments, which can be individually addressed and continuously tuned
in transmission resulting in up to 512 different filtering states. For realization of the electrochromic
devices, we overcome technical obstacles to realize seamless, gap-free electrochromic segments.
We describe this novel fabrication process and demonstrate the successful application in an
optical Fourier transform set-up.
We present a study of optoelectronically active Ga(As)As quantum dots (QDs) on Al-rich AlxGa1-xAs layers with Al concentrations
up to x=90%. So far, however, it has not been possible to grow optoelectronically active Ga(As)As QDs epitaxially
directly on and in between Al-rich barrier layers in the AlGaInAsSb material system. A QD morphology might appear on the
growth front, but the QD-like entities will not luminesce. Here, we use photoluminescence (PL) measurements to show that thin
Al-free capsule layers between Al-rich barrier layers and the QD layers can solve this problem; this way, the QDs become
optoelectronically active; that is, the dots become QDs. We consider antimonide QDs, that is, Ga(As)Sb QDs, either on GaAs for
comparison or on AlxGa1-xAs barriers (x >10%) with GaAs capsule layers in between. We also discuss the influence of QD
coupling both due to stress/strain from neighboring QDs and quantum-mechanically on the wavelength of the photoluminescence
peak. Due to their mere existence, the capsule layers alter the barriers by becoming part of them. Quantum dots
applications such as QD semiconductor lasers for spectroscopy or QDs as binary storage cells will profit from this additional
degree of design freedom.
Reflectance anisotropy spectroscopy (RAS), which was originally invented to monitor
epitaxial growth, can—as we have previously shown—also be used to monitor the reactive ion
etching of III/V semiconductor samples in situ and in real time, as long as the etching rate is not
too high and the abrasion at the etch front is not totally chaotic. Moreover, we have proven that—
using RAS equipment and optical Fabry-Perot oscillations due to the ever-shrinking thickness of the
uppermost etched layer—the in situ etch-depth resolution can be as good as +/-0.8 nm, employing a
Vernier-scale type measurement and evaluation procedure. Nominally, this amounts to +/-1.3 lattice
constants in our exemplary material system, AlGaAsSb, on a GaAs or GaSb substrate. In this
contribution, we show that resolutions of about +/-5.6 nm can be reliably achieved without a Vernier
scale protocol by employing thin doped layers or sharp interfaces between differently doped layers
or quantum-dot (QD) layers as etch-stop indicators. These indicator layers can either be added
to the device layer design on purpose or be part of it incidentally due to the functionality of the
device. For typical etch rates in the range of 0.7 to 1.3 nm/s (that is, about 40 to 80 nm/min), the RAS
spectrum will show a distinct change even for very thin indicator layers, which allows for the precise
termination of the etch run.