Dirk Wortmann

Micro- and nanostructures inside sapphire by fs-laser irradiation and selective etching

Micro- and nanofabrication of cavities in the volume of sapphire was performed by femtosecond laser irradiation followed by chemical etching with aqueous solution of HF acid. Depending on the focusing conditions self-organized nanostructures or elliptical microchannels are produced. This manufacturing technique is potentially usable for photonic crystals in integrated optical elements or microfluidic devices for applications in life science, biology or chemistry.

Photodegradation of methyl orange under visible light by micro-nano hierarchical Cu2O structure fabricated by hybrid laser processing and chemical dealloying.

Micro-nano hierarchical structure on the substrate was fabricated by a hybrid approach including laser deposition, laser ablation and chemical dealloying. The structure consists of micro bumps with a width of 50 μm and a height of 100 μm, and nanoporous structures with a size of 70-150 nm on the micro bumps. XRD and XPS results confirm that these hierarchical structures were made of Cu(2)O. For use in comparison, three additional structures with feature size in milliscale, microscale, and nanoscale were also prepared respectively by the proposed methods. Under visible light, the micro-nano structure exhibited the best performance of photodegradation. It is the result of the large specific surface and the catalytic reaction driven by the cuprous oxides.

Refractive index modification using fs-laser double pulses

Buried waveguides in glass are manufactured by irradiation with femtosecond laser double pulses. The refractive index change Deltan is determined by measuring the numerical aperture NA of the waveguides and by through light microscopy. The value of Deltan shows a significant dependency on the time delay Deltat of the fs-laser double pulses. A Deltan of up to 2x10(-3) in fused silica is reached at a Deltat between 400 and 800 ps. Based on the results of the double pulse experiments the initial effects of the refractive index change are discussed, taking into account thermal effects and the formation of self trapped excitons (STE) and transient color centers and their interaction with the next laser pulse.

Fabrication and spectral tuning of standing gold infrared antennas using single fs-laser pulses

Upright standing gold monopole nanoantennas are fabricated by irradiation of thin gold films with single pulses of fs-laser radiation. The resulting antennas exhibit extinction resonances in the mid infrared spectral rage for p-polarized light under grazing incidence. Due to the free charge carriers in the surrounding gold film of the antenna, the resonance condition of the thin-wire monopole antenna can be explained by introducing image charges yielding an observable resonance wavelength of four times the antenna length. The antenna length is controlled coarsely by the focusing numerical aperture and fine by the pulse energy of the laser pulse producing the structure. An additional ultrafine tuning of the resonance wavelength with a sub-10 nm resolution is realized by an additional coating process subsequent to the laser structuring.

Ablation and cutting of carbon-fiber reinforced plastics using picosecond pulsed laser radiation with high average power

In this paper, processing of carbon-fiber reinforced plastics (CFRPs) using ps pulsed laser radiation with an average power of up to 80 W is investigated. The influence of average power, scanning speed, and repetition rate on the ablation rate and the width of heat affected zone is evaluated and discussed. Processing with a heat affected zone smaller than 5 μm and ablation rates of almost 100 mm3/min are achieved. Finally, precise cutting of 2 mm thick CFRP samples by repetitive ablation of a 350 μm groove with negligible heat affected zone is demonstrated.

Experimental and theoretical investigation on fs-laser-induced nanostructure formation on thin gold films

In this paper, the authors report on the formation of nanobumps and nanojets on thin goldfilms, induced by single fs-laser pulse irradiation. Experimental results on the structure size and shape depending on the pulse energy and the pulse duration are presented. For the first time, the process of short laser pulse nanostructuring on thin metal films was modeled by molecular dynamic simulations on the scale directly accessible in the experiments. Additionally, pump-probe experiments were performed for in-situ visualization of the structure formation.

Laser deposition and structuring of laser active planar waveguides of Er:ZBLAN, Nd:YAG and Nd:GGG for integrated waveguide lasers

Laser radiation is used both for the deposition of the laser active thin films and for the micro structuring to define wave guiding structures for the fabrication of waveguide lasers. Thin films of Er:ZBLAN (a glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF, ErF3) for green upconversion lasers (545 nm), Nd:YAG (Y3Al5O12) and Nd:GGG (Gd3Ga5O12) for infrared lasers (1064 nm) are produced. Manufacturing of the laser active waveguides by micro-structuring is done using fs laser ablation of the deposited films. The structural and optical properties of the films and the damping losses of the structured waveguides are determined in view of the design and the fabrication of compact and efficient diode pumped waveguide lasers. The resulting waveguides are polished, provided with resonator mirrors, pumped using diode lasers and characterized. Laser operation of a ridge waveguide structure grown by pulsed laser deposition and structured by fs laser ablation is demonstrated. A 1 &mgr;m thick, 100 &mgr;m wide and 3 mm long structured waveguide consisting of amorphous neodymium doped Gd3Ga5O12 has shown laser activity at 1.068 &mgr;m when pumped by a diode laser at 808 nm.

Manufacturing of periodical nanostructures by fs-laser direct writing

Sub wavelength ripples (spacing < λ/4) perpendicular to the polarisation of the laser radiation are obtained by scanning a tightly focused beam (~1μm) of femtosecond laser radiation from a Ti:Sapphire laser (τ =100fs, λ =800nm & 400nm, f=1kHz) and from a Yb:glass fiber laser (τ =400fs, λ =1045nm, f=0.1-5MHz) over the surface of various materials like amorphous Nd:Gd3Ga5O12 films 1 μm in thickness on YAG substrates, diamond, polytetrafluoroethylene, LiF, MgF2, ZBLAN, Al2O3, LiNbO3, SiO2, Si, Cu and Au. The ripple patterns extend coherently over many overlapping laser pulses and scanning tracks. Investigated are the dependence of the ripple spacing Λ on the material, the lateral distance of the laser pulses, the N.A. of the focussing optics, the repetition rate and the applied wavelength. The ripples are characterised using electron microscopy. Some possible models for the origin of the ripple growth are discussed and conditions under which these phenomena occur are contained. New results concerning the scaling of the production process using a high repetition rate laser and a fast translation stage are demonstrated. Potential applications are presented and consequences for precise nano- and microstructuring using ultra short pulsed lasers are discussed.

Investigations on processing of carbon fiber reinforced plastics using ultrashort pulsed laser radiation with high average power

In this paper processing of carbon fiber reinforced plastics (CFRP) using ultrashort pulsed laser radiation with average powers of up to 430 W is investigated. The influence of increasing the average power both by increased pulse fluence and increased repetition rate on the ablation rate and the width of heat affected zone is evaluated and discussed. In addition, the influence of pulse duration is investigated and the potential for CFRP cutting is examined.

In-volume waveguides by fs-laser direct writing in rare-earth-doped fluoride glass and phosphate glass

Refractive index modifications are fabricated in the volume of rare-earth-doped glass materials namely Er- and Pr-doped ZBLAN (a fluoride glass consisting of ZrF4, BaF2, LaF3, AlF3, NaF), an Er-doped nano-crystalline glass-ceramic and Yb- and Er-doped phosphate glass IOG. Femtosecond laser radiation (τ=500fs, λ=1045nm, f=0.1-5MHz) from an Ybfiber laser is focused with a microscope objective in the volume of the glass materials and scanned below the surface with different scan velocities and pulse energies. Non-linear absorption processes like multiphoton- and avalanche absorption lead to localized density changes and the formation of color centers. The refractive index change is localized to the focal volume of the laser radiation and therefore, a precise control of the modified volume is possible. The width of the written structures is analyzed by transmission light microscopy and additionally with the quantitative phase microscopy (QPm) software to determine the refractive index distribution perpendicular to a waveguide. Structures larger than 50μm in width are generated at high repetition rates due to heat accumulation effects. In addition, the fabricated waveguides are investigated by far-field measurements of the guided light to determine their numerical apertures. Using interference microscopy the refractive index distribution of waveguide cross-sections in phosphate glass IOG is determined. Several regions with an alternating refractive index change are observed whose size depend on the applied pulse energies and scan velocities.