Felix Zimmermann

Laser induced nanogratings beyond fused silica - periodic nanostructures in borosilicate glasses and ULE™

We report on the ultrashort pulse laser induced formation of birefringent structures in the volume of different glasses: Borofloat 33, BK7 and ULE™. Using polarization contrast and scanning electron microscopy we could prove that this birefringence is induced by nanogratings. We were able to identify the pulse duration as a crucial process parameter for the generation of nanogratings in these glasses. The achieved birefringence in ULE is comparable to fused silica, while borosilicate glasses show much less birefringence (only about 12%). Remarkably, the period of the nanogratings is also dependent on the type of the glass, being 250 nm for ULE and only 60 nm in case of Borofloat 33.

Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses

The absorption and heat accumulation of successive ultrashort laser pulses in fused silica leads to melting of the material. We analyze the structure and formation of disruptions that occur within the trace of the molten material. We employed focused ion beam (FIB) milling to reveal the inner structure of these disruptions. The disruptions consist of several small voids which form a large cavity with a diameter of several tens of micrometer. Based on the observations, we suggest a model explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped. In addition, we analyzed the periodic and non-periodic formation of disruptions. The processing parameters strongly influence the formation of disruptions.

Ultrastable bonding of glass with femtosecond laser bursts

We report on the welding of fused silica with bursts of ultrashort laser pulses. By optimizing the burst frequency and repetition rate, we were able to achieve a breaking resistance of up to 96% of the bulk material, which is significantly higher than conventional high repetition rate laser bonding. The main reason for this stability increase is the reduced stress in the surroundings of the laser induced weld seams, which is proven by measurements of the stress-induced birefringence. A detailed analysis of the shape of the molten structures shows elongated structures in the burst regime. This can be attributed to stronger heating, which is supported by our thermodynamic simulations of the laser melting and bonding process.

Ultrashort laser pulse induced nanogratings in borosilicate glass

We report on nanogratings inscribed by repetitive femtosecond laser pulses into the bulk of borosilicate glass. The irradiation produces small nanopores (10–20 nm thick) which start to self-organize in gratings as well as elongated sheets of up to 400 nm length. A quantitative description of the grating structure and its development are obtained by a combination of focused ion beam milling, scanning electron microscopy, and small angle X-ray scattering (SAXS). The SAXS partial invariant of the thin sheets is found to correlate well with the measured optical retardance. Compared to fused silica nanogratings borosilicate glass shows a much smaller retardance due to re-annealing of pores. In addition, the nanograting period strongly deviates from the well-known λ/2n prediction. We could observe periods down to 60 nm (at an inscribing wavelength of 800 nm). This has not been observed yet in other glasses.

Evolution of hole shape and size during short and ultrashort pulse laser deep drilling

A detailed study of the influence of the pulse duration, from the femtosecond to the nanosecond regime, on the evolution of the hole shape and depth during percussion drilling in silicon is presented. Real-time backlight imaging of the hole development is obtained for holes up to 2 mm deep with aspect ratios extending to 25:1. For low pulse energies, the hole-shape and drilling characteristics are similar for femtosecond, picoseconds and nanosecond regimes. At higher pulse energies, ns-pulses exhibit slower average drilling rates but eventually reach greater final depths. The shape of these holes is however dominated by branching and large internal cavities. For ps-pulses, a cylindrical shape is maintained with frequent small bulges on the side-walls. In contrast, fs-pulses cause only a limited number of imperfections on a tapered hole shape.

Femtosecond laser written nanostructures in Ge-doped glasses

We report on nanostructures induced by femtosecond laser pulses in the bulk of Germanium-doped silica glasses. For studying structural properties of the nanostructure constituents small-angle x-ray scattering and SEM served to map pore size, filling factor and periodicity. Our results show that with increasing the Ge doping concentration, the aspect ratio (transverse to inscribing laser) of nanometric pores rises while they arrange in a smaller period in contrast to nanogratings in pristine fused silica. Consequently, higher optical retardance can be obtained demonstrating the pronounced glass decomposition due to the changing network structure.

Ultrashort pulse induced modifications in ULE - from nanograting formation to laser darkening

We report on ultrashort pulse laser induced modifications in ultra-low expansion (ULE) glass. This silicate glass has a significant fraction of TiO2 (7.5 wt%) to ensure a low thermal expansion. Ultrafast laser irradiation generates different kinds of modifications in this glass: so-called nanogratings are formed when low irradiation power is used; while high laser powers result in heat accumulation and induce local melting. In addition, for almost all laser parameters applied, the processed material tends toward darkening. With help of Small Angle X-ray Scattering (SAXS), Focused Ion Beam (FIB) milling and optical retardance measurements we analyzed the structure of laser induced nanopores which are the basic components of nanogratings. Investigation of the mechanisms responsible for laser induced darkening were conducted by multiple techniques such as FIB milling, Electron Spin Resonance (ESR) and Raman spectroscopy. We could identify the formation of hollow cavities filled with molecular oxygen surrounded by a compressed shell of glass which contains trivalent titanium. While light scattering on the cavities causes opacity, the reduction of colorless Ti4+ (d0) to blue Ti3+ (d1) is responsible for the darkening. By combining the inscription of nanogratings with laser induced darkening it is easily possible to locally tune the type of the modification by three independent degrees of freedom (retardance, orientation of optical axis, amount of darkening) rendering ULE an ideal material for future data storage applications.

Welding of transparent materials with ultrashort laser pulses

The realization of stable bonds between different glasses has attracted a lot interest in recent years. However, conventional bonding techniques are often problematic due to required thermal annealing steps which may lead to induced stress, whereas glue and other adhesives tend to degrade over time. These problems can be overcome by using ultrashort laser pulses. When focussed at the interface, the laser energy is deposited locally in the focal volume due to nonlinear absorption processes. While even single pulses can lead to the formation of bonds between transparent glass substrates, the application of high repetition rates offers an additional degree of freedom. If the time between two pulses is shorter than the time required for heat diffusion out of the focal volume, heat accumulation of successive pulses leads to localized melting at the interface. The subsequent resolidification finally yields strong and robust bonds. Using optimized processing parameters, we achieved a breaking strength up 95% of the pristine bulk material. In this paper, we will detail the experimental background and the influence of the laser parameters on the achievable breaking strength.

On the rewriting of ultrashort pulse-induced nanogratings.

In this study, we report on the erasure and rewriting of nanogratings by femtosecond laser pulses in the bulk of fused silica. To map the structural processes during rewriting, a combination of optical retardance measurement, small angle X-ray scattering, and scanning electron microscopy was used. The results reveal that already few pulses lead to erasure and formation of anisotropic structures. Repetitive rewriting favors the formation of nanoscopic pores, which increases the optical retardance of nanogratings for large pulse numbers.

Spatial and temporal temperature distribution of ultrashort pulse induced heat accumulation in glass

We report on the first direct measurements of the laser induced temperature distribution after the absorption of multiple ultrashort laser pulses at high repetition rates in borosilicate glass. To this end, we developed an in-situ micro Raman setup to determine the temperature dependent ratio between Stokes and Anti-Stokes scattering. The results indicate a critical influence of the pulse energy on the induced temperature. In borosilicate glass, the maximal temperature directly after the excitation (pulse energy of 1100 nJ, repetition rate of 1 MHz, wavelength of 1044 nm, pulse duration of 600 fs, 2000 pulses per laser spot) is more than 5000 K and rapidly cools down within several hundreds of ns.