Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tmdoped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.
Electron beam lithography becomes attractive also for the fabrication of large scale diffractive optical elements by the use of the character projection (CP) technique. Even in the comparable fast variable shaped beam (VSB) exposure approach for conventional electron beam writers optical nanostructures may require very long writing times exceeding 24 hours per wafer because of the high density of features, as required by e.g. sub-wavelength nanostructures. Using character projection, the writing time can be reduced by more than one order of magnitude, due to the simultaneous exposure of multiple features. The benefit of character projection increases with increasing complexity of the features and decreasing period. In this contribution we demonstrate the CP technique for a grating of hexagonal symmetry at 350nm period. The pattern is designed to provide antireflective (AR) properties, which can be adapted in their spectral and angular domain for applications from VIS to NIR by changing the feature size and the etching depth of the nanostructure. This AR nanostructure can be used on the backside of optical elements e.g. gratings, when an AR coating stack could not be applied for the reason of climatic conditions or wave front accuracy.
Gratings with binary and blazed profiles and periods in the low micron and sub-micron range define a class
of microstructures with a huge application potential. We present a mask based photolithographic fabrication
method for these demanding grating geometries. It combines the advantages of electron beam lithography and
holographic exposure, which are superior homogeneity, high resolution and pattern flexibility on one hand, and
a fast, large aerial exposure with the option for smooth profiles on the other hand. This is accomplished by the
use of an electron beam written phase mask which contains a very homogeneous pattern of diffractive features
and is used for a full-field exposure in a proximity mask aligner. The key for the beneficial use of the technology
is the proper design of the phase mask surface profile which can have a binary or multilevel geometry. Since the
patterns to be exposed are periodic, this is also the case for the phase mask which allows calculating their physical
light transmission with exact methods like rigorous coupled wave analysis. An optimization algorithm has been
developed which can find mask geometries that synthesize a desired complex aerial image in the proximity
distance of choice. Aerial images offering e.g. high resolution features, phase shifts, and tilted propagation
directions can be realized that way. This technology has been successfully used to fabricate e.g. binary gratings
of very high quality with a period of 800 nm as well as blazed gratings with a period of 3 μm.
The fabrication of complex nano-optical structures for plasmonics, photonic-crystals, or meta-materials on application
relevant areas by electron-beam lithography requires a highly parallel writing strategy. In case of periodic pattern as they
are found in most of the mentioned optical elements this can be achieved by a so called character projection writing
principle where complex exposure pattern are coded in a stencil mask and exposed with a single shot. Resulting shotcount
and writing time reductions compared to standard Variable-Shaped-Beam exposures can be in the order of
100...10000. The limitation in flexibility by using hard-coded exposure shapes can be overcome by implementing the
character projection principle with a highly precise motorized aperture stage capable of carrying several 1000 different
apertures. Examples of nano-optical elements fabricated with the new character projection principle are presented.
Fabrication of high performance gratings may significantly benefit from the use of high index materials such as Ta2O5, TiO 2 or Al2O3. However, these materials can typically not be patterned with the required quality by common etching processes. To overcome this limitation we developed novel grating fabrication technologies based on a combination of conventional lithography with Atomic-Layer-Deposition. For that the basic structure of the grating is first realized in a fused-silica substrate or a SiO2-layer. This template is then functionalized by an ALD-coating in a specific pre-defined manner. The new approach opens up a huge variety of new options for the realization of gratings whose fabrication would otherwise not be possible.
We present novel filter elements with an asymmetric angle dependent transmission based on high-contrast gratings.
Asymmetric means a different efficiency for positive and negative incidence angles. Our approach provides the realization of asymmetric direction selective filters by using blaze-like grating structures combined with subwavelength
high contrast gratings respectively grating periods in the resonance domain. We also discuss the influence of the effective medium theory on the transmission function depending on the angle of the incident light. For realization of those high contrast gratings Silicon is chosen as material with high refractive index and adequate compatibility with semiconductor fabrication.
Modern electron beam lithography is a suitable technology for the fabrication of high performance gratings for
spectroscopic applications. Due to a significant improved accuracy of the lithographic exposure the resulting gratings do
show a very high wave-front quality, low stray-light, and grating ghosts. The high resolution accessible with e-beam
writing can be used for sub-period engineering of the grating pattern in order to optimize the efficiency performance of
the devices. This is demonstrated by different examples of pure dielectric reflection and transmission gratings developed
for space missions. One is the Sentinel-4 earth observation mission; the second is the astrometry satellite GAIA of the
We report on novel concepts for reflective diffractive elements based on high-contrast gratings. To demonstrate
the possibilities for such devices reflective cavity couplers with three output ports are investigated. A diffracting
period is superposed to a highly reflective subwavelength grating in order to realize diffractive elements. This
superposition can be realized with a periodic depth, fill factor or period modulation of the reflector. Further, to
limit the total transmission of the device it is necessary to enhance its angular tolerance. We discuss different
approaches in order to realize this increased reflectivity in broad range of the angular spectrum. The contribution
focuses on the material combination silicon-silica, but the presented concepts also hold for other material
combinations with large index contrast and even for monolithic silicon structures.
We introduce concepts for direction selective transmission filters based on dielectric high-contrast gratings. The
devices act as angular bandpass filters at an incidence angle of 45° with a total transmission of 68% and a full
width at half maximum of 20°. Since the filters are based on a material combination of silicon and silicon dioxide
they provide an excellent compatibility to well established fabrication processes in semiconductor industry. The
results of measurements on fabricated samples are presented and the performance of the components is compared
to that of metallic gratings. It is found that the latter can basically provide similar filter properties, however the
feasible transmission efficiency is significantly lower than for the dielectric gratings. The presented configurations
are applicable in the field of sensors and detectors.
A novel technique for the fabrication of high resolution sub-micrometer patterns by diffractive proximity lithography in a
mask-aligner is presented. The technique is based on the use of specially designed diffractive photo-masks. It requires
some small modifications of the mask-aligner, especially for the mask illumination and the settings of the proximity gap
between mask and substrate. The huge potential of this novel technique is demonstrated at the example of structures
having lateral feature sizes in the sub-500nm range printed with mask-to-substrate distances of several ten micrometers.
One and two dimensional grating structures with submicron period have a huge number of applications in optics and
photonics. Such structures are conventionally fabricated using interference or e-beam lithography. However, both
technologies have significant drawbacks. Interference lithography is limited to rather simple geometries and the
sequential writing scheme of e-beam lithography leads to time consuming exposures for each grating. We present a novel
fabrication technique for this class of microstructures which is based on proximity lithography in a mask aligner. The
technology is capable to pattern a complete wafer within less than one minute of exposure time and offers thereby high
lateral resolution and a reliable process. Our advancements compared to standard mask aligner lithography are twofold:
First of all, we are using periodic binary phase masks instead of chromium masks to generate an aerial image of high
resolution and exceptional light efficiency at certain distances behind the mask. Second, a special mask aligner
illumination set-up is employed which allows to precisely control the incidence angles of the exposure light. This degree
of freedom allows both, to shape the aerial image (e. g. transformation of a periodic spot pattern into a chessboard
pattern) and to increase its depth of focus considerably. That way, our technology enables the fabrication of high quality
gratings with arbitrary geometry in a fast and stable wafer scale process.