Jens Gottmann

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.

Sub-wavelength ripple formation on various materials induced by tightly focused femtosecond laser radiation

Sub-wavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (~1μm) of femtosecond laser radiation (tp = 100fs, λ = 800nm & 400nm) over the surface of various materials. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing Δ on the material. New results concerning the dependence of the spacing on the wavelength are presented. Some possible models for ripple growth are discussed and conditions under which these phenomena occur are contained. In opposition to the classical ripple theory, the observed ripple spacing is dependent on the material, giving indication to understand the processes during the sub-wavelength ripple formation by femtosecond laser radiation.

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.

Time dynamics of burst-train filamentation assisted femtosecond laser machining in glasses.

Bursts of femtosecond laser pulses with a repetition rate of f = 38.5MHz were created using a purpose-built optical resonator. Single Ti:Sapphire laser pulses, trapped inside a resonator and released into controllable burst profiles by computer generated trigger delays to a fast Pockels cell switch, drove filamentation-assisted laser machining of high aspect ratio holes deep into transparent glasses. The time dynamics of the hole formation and ablation plume physics on 2-ns to 400-ms time scales were examined in time-resolved side-view images recorded with an intensified-CCD camera during the laser machining process. Transient effects of photoluminescence and ablation plume emissions confirm the build-up of heat accumulation effects during the burst train, the formation of laser-generated filaments and plume-shielding effects inside the deeply etched vias. The small time interval between the pulses in the present burst train enabled a more gentle modification in the laser interaction volume that mitigated shock-induced microcracks compared with single pulses.

Dynamical detection of optical phase changes during micro-welding of glass with ultra-short laser radiation

The accelerating developments in micro- and nanotechnology require faster and more precise tools for application and diagnostics. A new ultra-fast diagnostics is presented as an advancement of a conventional phase microscopy method (Iatia QPm™). In contrast to the conventional method using one CCD to detect three object planes, three CCDs detect these planes separately and simultaneously. The measurement technique named TQPm has been analyzed and validated by measuring the optical phase of a commercial fiber. This novel visualization technique affords reliable quantitative time-resolved measurements of the optical phase, the transient refractive index or the dynamical geometry changes. The structural and optical modifications of welded glass have been observed coaxially in situ during melting and welding by TQPm. Technical glass plates (Schott D263) have been welded using an ultra-short pulsed laser. By the use of femtosecond laser radiation (tp = 350 fs, frep = 1 MHz) focused by a microscope objective (2ω0 ≈ 4 µm) at the interface, multi-photon absorption is the dominant phenomenon. This causes heat accumulation and thereby glass melting and welding.

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.

Fabrication of erbium-doped planar waveguides by pulsed-laser deposition and laser micromachining

Laser radiation is used both for the deposition of dielectric Er:BaTiO3 thin films and for material removal to generate wave guiding structures for photonic applications. Pulsed laser deposition with KrF excimer laser radiation (wavelength 248 nm, pulsed duration 20 ns) is used to grow dense, transparent amorphous or crystalline erbium doped BaTiO3 thin films. Visible emission due to up-conversion luminescence (wavelength 528 nm and 548 nm) under excitation with diode laser radiation at a wavelength of 975 nm is investigated as a function of the erbium concentration and structural film properties. The dielectric films are micro machined to form optical wave guiding structures using Nd:YAG laser radiation (wavelength 532 nm, pulsed duration 40 ps) and Ti:sapphire laser radiation (wavelength 810 nm, pulse duration 63 - 150 fs) by scanning the focused laser beam relatively to the sample.

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.

In-situ diagnostics on fs-laser-induced modification of glasses for selective etching

In-situ observation of the in-volume modification of glasses by focused ultra-short pulsed laser radiation with an interferometer microscope allows for the spatially resolved measurement of the transient optical path difference (OPD) in the surrounding of the laser-induced modification. By the relation of refractive index and temperature an estimation of temperature during modification process is possible. The absorption of the laser radiation is measured and is, together with the estimation of processing temperature during modification, a first step towards a process model for the induced modifications of the transparent material.

Beam shaping of laser diode radiation by waveguides with arbitrary cladding geometry written with fs-laser radiation

Waveguides with arbitrary cross sections are written in the volume of Al(2)O(3)-crystals using tightly focused femtosecond laser radiation. Utilizing a scanning system with large numerical aperture, complex cladding geometries are realized with a precision around 0.5 µm and a scanning speed up to 100 mm/s. Individual beam and mode shaping of laser diode radiation is demonstrated by varying the design of the waveguide cladding. The influence of the writing parameters on the waveguide properties are investigated resulting in a numerical aperture of the waveguides in the range of 0.1. This direct laser writing technique enables optical devices which could possibly replace bulky beam shaping setups with an integrated solution.