We present the experimental investigations of different designs of resonant waveguide-grating mirrors (RWG) which
are used as intracavity folding mirror in an Yb:YAG thin-disk laser. The studied mirrors combine structured fused
silica substrates, a thin-layer waveguide (Ta2O5), a buffer layer (SiO2) and partial reflectors. The grating period was
chosen to be 510 nm to allow resonances at an angle of incidence of ~10° for TE polarization. The waveguide layer has
a thickness of 236 nm. It is followed by the buffer layer with a thickness of 580 nm and the subsequent alternating
Ta2O5/SiO2 layers. The exact coating sequence depends on the two design approaches which were investigated in this
work: either introducing different partial reflectors, i.e. stacks of quarter-wave layers on top of the waveguide while
keeping the groove depth of the grating constant, or increasing the grating depth, while keeping an identical partial
reflector. The investigation was focused on the rise of the surface temperature due to the coupling of the incident
radiation to a waveguide mode, as well as on the laser efficiency, polarization and wavelength selectivity. It is found
that, when compared to the simplest RWG design which consists of only a single Ta2O5 waveguide layer, damage
threshold as well as laser efficiency can be significantly increased, while the laser performances in terms of
polarization- and wavelength selectivity are maintained. So far, the presented RWG allow the generation of linear
polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG thin-disk laser.
Damage thresholds of 60kW/cm2 have been reached where only 63K of surface temperature increase was observed.
This shows that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized
Yb:YAG thin-disk lasers.
The high throughput and large area nanostructuring of flexible substrates by continuous roller processes has great potential for future custom applications like wire grid polarizers, antireflection films, or super-hydrophobic surfaces. For each application different material characteristics have to be considered, e.g. refractive index, hydrophobicity, or dry etch stability. Herein, we show experimental results of nanoimprint lithography resist developments focused on inkjetable and photo-curable resists suitable for high throughput production, especially roll-to-roll NIL. The inkjet deposition of the novel materials is demonstrated by the use of different state-of-the-art inkjet printheads at room temperature. A plate-to-plate process on silicon substrates was successfully implemented on a NPS300 nano patterning stepper with previously inkjet dispensed NIL resist. Furthermore, we demonstrate a throughput of 30 m min-1 in a roller NIL process on PET. Dry etching of unstructured thin films on Si wafers was performed, and it was demonstrated that the etch stability in Si is tunable to a value of 3.5:1 by a concise selection of the resist components. The surface roughness of the etched films was measured to be < 2 nm, after etching of around 100 nm of the resist films what is an essential factor for a low line edge roughness. All resists reported herein can be deposited via inkjet dispensing at room temperature, are suitable for continuous high throughput imprinting on flexible substrates, and are applicable in step-wise NIL processes with good etch resistance in dry etch processes.
A detector which can detect a broad range of explosives without false alarms is urgently needed. Vibrational spectroscopy provides specific spectral information about molecules enabling the identification of analytes by their “fingerprint” spectra. The low detection limit caused by the inherent weak Raman process can be increased by the Surface Enhanced Raman (SER) effect. This is particularly attractive because it combines low detection limits with high information content for establishing molecular identity. Based on SER spectroscopy we have constructed a modular detection system. Here, we want to show a combination of SER spectroscopy and chemometrics to distinguish between chemically similar substances. Such an approach will finally reduce the false alarm rate. It is still a challenge to determine the limit of detection of the analyte on a SER substrate or its enhancement factor. For physisorbed molecules we have applied a novel approach. By this approach the performance of plasmonic substrates and Surface Enhanced Raman Scattering (SERS) enhancement of explosives can be evaluated. Moreover, novel nanostructured substrates for surface enhanced IR absorption (SEIRA) spectroscopy will be presented. The enhancement factor and a limit of detection are estimated.
We present the design of an optimized mixed-order photonic crystal laser structure. The lasing properties of
this two-dimensional photonic crystal structure with an organic gain material are investigated theoretically and
experimentally. A feedback structure fabricated in a thin film of Ta2O5 increases both the index contrast from
the gain material as well as the optical confinement. Furthermore, by combining first order photonic crystal
structures with second order ones losses occurring at the edge of the second order structure are dramatically
reduced leading to a lower laser threshold and / or to a much smaller footprint of the laser.
We investigate circular grating resonators (CGR) with a very small footprint. Photonic devices based on circular
grating resonators are computationally designed, optimized and studied in their functionality using finite
difference time-domain (FDTD) method. A wide variety of critical quantities such as transmission and reflection,
resonant modes, resonant frequencies, and field patterns are calculated. Due to their computational size some
of these calculations have to be performed on a supercomputer (e.g. parallel Blue Gene machine). The devices
are fabricated in SOI using the computational design parameters. First they are defined by electron-beam
lithography. Then the pattern transfer is achieved by an inductively coupled reactive-ion etch process. Finally,
the devices are characterized by coupling light from a tunable laser with a tapered lensed fiber. As predicted
from the simulations the measured transmission spectra exhibit a wide range of different type of resonances with
quality factors exceeding 1000.
Circular grating resonators could lead to the development of very advanced silicon-on-insulator (SOI) based
nano-photonic devices clearly beyond state of the art in terms of functionality, size, speed, cost, and integration
density. The photonic devices based on the circular grating resonators are computationally designed and studied
in their functionality using finite-difference time-domain (FDTD) method. A wide variety of critical quantities
such as transmission and field patterns are calculated.
Due to their computational size some of these calculations have to be performed on a supercomputer like a
massive parallel Blue Gene machine. Using the computational design parameters the devices are fabricated on
SOI substrates consisting of a buried oxide layer and a 340-nm-thick device layer. The devices are defined by
electron-beam lithography and the pattern transfer is achieved in a inductively coupled reactive-ion etch process.
Then the devices are characterized by coupling light in from a tunable laser with a lensed fiber. As predicted
the measured transmission spectra exhibit a wide range of different type of resonances with Q-factors over 1000
which compares very well with the computations.