Andreas Tuennermann

Sub-diffraction limited structuring of solid targets with femtosecond laser pulses

Possibilities to produce sub-diffraction limited structures in thin metal films and bulk dielectric materials using femtosecond laser pulses are investigated. The physics of ultrashort pulse laser ablation of solids is outlined. Results on the fabrication of sub-micrometer structures in 100-200 nm chrome-coated surfaces by direct ablative writing are reported. Polarization maintaining optical waveguides produced by femtosecond laser pulses inside crystalline quartz are demonstrated.

Spectral resolved dynamic localization in curved fs laser written waveguide arrays.

We investigate dynamic localization in curved femtosecond (fs) laser written waveguide arrays. The light propagation inside the array is directly observed by monitoring fluorescence of color centers induced during the fs writing process. In addition to monochromatic excitation the spectral response of the arrays is investigated by launching white light supercontinuum into the arrays.

Diffraction control in periodically curved two-dimensional waveguide arrays.

We study propagation of light beams in two-dimensional photonic lattices created by periodically curved waveguide arrays. We demonstrate that by designing the waveguide bending, one can control not only the strength and sign of the beam diffraction, but also to engineer the effective geometry and even dimensionality of the two-dimensional photonic lattice. We reveal that diffraction of different spectral components of polychromatic light can display completely different patterns in the same periodically modulated structure, e.g. one-dimensional, hexagonal, or rectangular. Our results suggest novel opportunities for efficient self-collimation, focusing, and reshaping of light beams in two-dimensional photonic structures.

Understanding the electric and magnetic response of isolated metaatoms by means of a multipolar field decomposition

We introduce a technique to decompose the scattered near field of two-dimensional arbitrary metaatoms into its multipole contributions. To this end we expand the scattered field upon plane wave illumination into cylindrical harmonics as known from Mies theory. By relating these cylindrical harmonics to the field radiated by Cartesian multipoles, the contribution of the lowest order electric and magnetic multipoles can be identified. Revealing these multipoles is essential for the design of metamaterials because they largely determine the character of light propagation. In particular, having this information at hand it is straightforward to distinguish between effects that result either from the arrangement of the metaatoms or from their particular design.

Q-switching of Yb3+-doped fiber laser using a novel micro-optical waveguide on microactuating platform light modulator.

We report a novel micro-optical waveguide (MOW) on microactuating platform (MAP) light modulator for Q-switched all-fiber laser applications. The light modulator employs a fused biconical taper (FBT) coupler, which acts as MOW, mounted on an electromechanical system, MAP, where an axial stress over the waist of FBT coupler is precisely controlled to result in modulation of output power. The modulator was implemented in a clad pumped Yb3+-doped fiber laser cavity as a Qswitching element. Q-switching was successfully achieved at the repetition rate of 18.6kHz and average pulse energy of 1.4microJ. The proposed structure can be readily applied in power scaling up of all-fiber Q-switching laser systems.

Femtosecond writing of high-quality waveguides inside phosphate glasses and crystalline media using a bifocal approach

The fabrication of waveguides inside transparent media using ultrashort laser pulses has gained a lot of interest during the past years. When these intense pulses are tightly focused inside the material a refractive index increase in the focal volume can be achieved. Low-loss waveguides and true three-dimensional integrated optical devices have been produced by this direct writing technique in silicate glasses. However, other materials like phosphate glasses or crystalline quartz show a different behavior. In this case stress is induced around the focal volume leading to a refractive index increase in the surrounding areas (inverse profile). As a consequence high quality waveguides with a mode profile matched to conventional fibers cannot be fabricated. In this presentation we demonstrate a new approach to overcome this problem. The laser beam is split by a transmission grating and simultaneously focused at two different areas. When the distance between the two foci is appropriate the desired refractive index profile is obtained. We will demonstrate high quality waveguides in crystalline quartz and in phosphate glasses produced by this technique. The influence of the processing parameters is discussed in detail.

Fiber-laser power scaling beyond the 1-kilowatt level by Nd:Yb co-doping

A new type of multi-clad rare-earth doped silica fiber was designed, prepared and tested for the power scaling of high power fiber lasers in the 1 .1 tm wavelength region. By means of a dedicated laboratory setup a maximum output power of more than 1 .300 watts with excellent spectral and beam behavior was achieved. The fundamental investigation of the energy transfer processes and of the fluorescence lifetimes of different Nd:Yb co-doped has been studied. Such fiber-lasers were tested in the laboratory at several materials (plastics, metals, glass) in the fields of material processing and micro-marking, respectively.

Waveguides produced by ultrashort laser pulses inside glasses and crystals

Using tightly focused femtosecond laser pulses waveguides are fabricated inside glasses and crystalline materials. The guiding and attenuation properties at different wavelengths as well as the micro morphology of the irradiated samples are studied. We demonstrate the fabrication of single- and multi-mode waveguides with damping losses well below 1 dB/cm in fused silica. In crystalline quartz we found that the irradiated area has become amorphous due to the absorption of the laser radiation. In this case waveguiding is observed in a stress-induced region surrounding the irradiated, amorphous area.

Single- and multimode waveguides in glasses manufactured with femtosecond laser pulses

In this paper we demonstrate the possibility to produce single- and multimode waveguides with a well-defined number of guided modes in different doped and undoped glasses using fs laser pulses. In fused silica waveguides with damping values below 0.8 dB/cm have been realized. Moreover symmetrical beamsplitters and waveguides in different doped materials have been manufactured. Based on measurements of the generated refractive index changes calculations of the near-field intensity distribution of the guided light are performed and compared to experimental results.

Cutting of optical materials by using femtosecond laser pulses

In the past years, ultrashort pulse lasers have been established as precise and universal tools for the microstructuring of solid materials. Since thermal and mechanical influences are minimized, the application of this technology is also suitable for the structuring of optical materials and opens new possibilities. In this paper, the influence of pulse duration, pulse energy (fluence) and polarization on the cutting quality for glass and silicon will be discussed. As a concrete application, the cutting and micromarking of dielectric coated mirrors for high power fiber lasers will be highlighted.