H.-J. Fuchs

High-power femtosecond Yb-doped fiber amplifier.

We report on the generation of linearly chirped parabolic pulses with 17-W average power at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. Highly efficient transmission gratings in fused silica are applied to recompress these pulses down to 80-fs with an efficiency of 60%, resulting in a peak power of 1.7 MW. Power scaling limitations given by the amplifier bandwidth are discussed.

High-average-power femtosecond fiber chirped-pulse amplification system.

Efficient generation of 76-W average power of 400-fs pulses at 75-MHz repetition rate by use of a diode-pumped ytterbium-doped double-clad fiber-based chirped-pulse amplification system is demonstrated. The key element in the system is a diffraction grating compressor consisting of highly efficient transmission gratings in fused silica, allowing recompression at this high power level.

Diffractionless propagation of light in a low-index photonic-crystal film

We experimentally demonstrate diffractionless propagation of light over 12 diffraction lengths in a two-dimensional photonic crystal film made of silicon nitride (SiNx). We show that self-guided beams may propagate for transverse electric (TE) and transverse magnetic (TM) polarized light but at slightly different frequencies. Three-dimensional calculations are used to optimize the structure for low loss and narrow beam operation in this low-index photonic crystals. Experimental and theoretical results are in good agreement.

Diffractive optical isolator made of high-efficiency dielectric gratings only

The working principle of an optical isolator made of two corrugated dielectric gratings is introduced. One grating acts as a polarizer, and the other acts as a quarter-wave plate used in conical incidence converting linearly polarized light into circularly polarized light. Global maxima of diffraction efficiency for surface-corrugated gratings with binary, sinusoidal, and pyramidal ridge shapes with dependence on the material index are identified. Regarding technological feasibility for use in the visible wavelength range, high-frequency gratings with a binary shape were realized. With these gratings, an extinction ratio of more than 40 dB for the polarizer is theoretically possible, and more than 20 dB was experimentally achieved. A good correlation between theoretically calculated efficiencies and birefringences based on rigorous methods and the experimental results is demonstrated.

High transmission and single-mode operation in low-index-contrast photonic crystal waveguide devices

We investigate photonic crystal films made of low-index glass materials. Polarization-sensitive waveguiding and single-mode operation (with losses as low as 1.7 dB/mm) have been observed in wide multimode photonic crystal waveguides after propagation of some millimeters. Furthermore, single-mode operation and transmission as high as about 50% per bend has been observed experimentally in low-index photonic crystal waveguide bends.

Highly efficient waveguide bends in photonic crystal with a low in-plane index contrast

We report on the realization and characterization of highly efficient waveguide bends in photonic crystals made of materials with a low in-plane index contrast. By applying an appropriate bend design photonic crystal bends with a transmission of app. 75 % per bend were fabricated.

Narrowband, polarization-independent free-space wave notch filter

A two-dimensional-corrugated-slab-waveguide add/drop filter providing 100% resonant reflection at 1.55 microm wavelength for both TE and TM polarizations with identical FWHM is designed. The fabricated device exhibits a reflectivity spectrum of more than 95% peak reflection for both polarizations at 1.537 microm. The coupling scheme involves the TE0 guided mode only; it is made relatively tolerant by means of a double-sided crossed grating.

Plasmonic properties of aluminum nanorings generated by double patterning

In this Letter we evaluate a technique for the efficient and flexible generation of aluminum nanorings based on double patterning and variable shaped electron beam lithography. The process is demonstrated by realizing nanorings with diameters down to 90 nm and feature sizes of 30 nm utilizing a writing speed of one ring per microsecond. Because of redepositions caused by involved etching processes, the material of the rings and, therefore, the impact on the plasmonic properties, are unknown. This issue, which is commonly encountered when metals are nanostructured, is solved by adapting a realistic simulation model that accounts for geometry details and effective material properties. Based on this model, the redepositions are quantified, the plasmonic properties are investigated, and a design tool for the very general class of nanofabrication techniques involving the etching of metals is provided.

Wire-grid polarizer for the UV spectral region

In this work we present a metal wire grid polarizer for the UV spectral region. The period of the aluminum grating is 100 nm. The grating was fabricated using a spatial frequency doubling technique. The process consist of electron-beamlithography, sputter coating and various ion-beam- and ICP etching steps. The features of the fabricated aluminum wires fulfill the theoretical demands with a height of 150 nm and a width of 35 nm. Optical parameters of the grating at 250 nm wavelength are 42% transmittance and an extinction ratio of almost 15. The optical parameters are even improved with the wavelength.

Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses.

We designed, fabricated and characterized a nano-periodical highly-efficient blazed grating for extreme-ultraviolet (XUV) radiation. The grating was optimized by the rigorous coupled-wave analysis method (RCWA) and milled into the top layer of a highly-reflective mirror for IR light. The XUV diffraction efficiency was determined to be around 20% in the range from 35.5 to 79.2 nm. The effects of the nanograting on the reflectivity of the IR light and non-linear effects introduced by the nanograting have been measured and are discussed.