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Crystalline Silicon undergoes a complex phase-change dynamic of melting, amorphization, ablation and re-crystallization upon irradiation with high intensity ultra-short laser pulses [1]. The final state of such a modified surface spot depends on many factors, most notably the local fluence and the surface’s crystal orientation. In this study, we induced superficial structure and phase changes in Silicon <111> and <100> wafers using single femtosecond laser pulses (790 nm, 30 fs) for a range of different peak fluences. The resulting surface modifications were studied in great detail using a number of different techniques, including spectroscopic imaging ellipsometry (SIE), atomic force microscopy, high-resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray spectroscopy within scanning transmission electron microscopy (STEM-EDX).
Playing a pivotal role in this work, SIE provided non-destructive measurements for the calculation of the radial amorphous layer-thickness profiles of the irradiated spots using a two-layer thin-film model (Silicon dioxide and amorphous Silicon on a crystalline Silicon substrate). The measurements further allowed for the analysis of the oxide-layer modifications induced by the laser treatment. The results of the SIE-calculations were cross-checked by an in-depth material lamella via HRTEM and STEM-EDX.
Recently, the detection of molecular species in laser-induced breakdown spectroscopy (LIBS) has gained increasing interest, particularly for isotopic analysis. In LIBS of organic materials, it is predominantly CN and C2 species that are formed, and multiple mechanisms may contribute to their formation. To gain deeper insight into the formation of these species, laser-induced plasma of 13C and 15N labeled organic materials was investigated in a temporally and spatially resolved manner. LIBS on fumaric acid with a 13C labeled double bond allowed the formation mechanism of C2 to be investigated by analyzing relative signal intensities of 12C2, 12C13C, and 13C2 molecules. In the early plasma (<5 µs), the majority of C2 originates from association of completely atomized target molecules, whereas in the late plasma, the increased concentration of 13C2 is due to incomplete dissociation of the carbon double bond. The degree of this fragmentation was found to be up to 80% and to depend on the type of the atmospheric gas. Spatial distributions of C2 revealed distinct differences for plasma generated in nitrogen and argon. A study of the interaction of ablated organics with ambient nitrogen showed that the ambient nitrogen contributed mainly to CN formation. The pronounced anisotropy of the C15N to C14N ratio across the diameter of the plasma was observed in the early plasma, indicating poor initial mixing of the plasma with the ambient gas. Overall, for accurate isotope analysis of organics, LIBS in argon with relatively short integration times (<10 µs) provides the most robust results. On the other hand, if information about the original molecular structure is of interest, then experiments in nitrogen (or air) with long integration times appear to be the most promising.
Serial interconnection of CIGSe thin film solar modules involves typically glass-side laser patterning of the molybdenum layer (P1 scribe). In this paper we present a working principle of P1 film side patterning. The investigated samples were sputter-deposited onto soda-lime glass substrates. For understanding the fundamental ablation behavior, two kinds of layer systems were studied: on the one hand monolayer systems which are compressively stressed and on the other hand bilayer systems, consisting of a tensile stressed layer on the substrate and a second layer on top. The film-side ablation process was studied using a nanosecond as well as a picosecond laser source. The influence of intrinsic stress was studied by XRD. Time resolved spectroscopy reveals the formation of plasma as important driving mechanism for ablation. It is shown that by proper adaption of the sputter conditions high-quality P1 film side patterning is achieved.
Manufacturing of CIGSe thin film solar modules involves typically one laser structuring step (P1) and two mechanical structuring steps (P2 and P3) for serial interconnection. In our approach, complete laser structuring is successfully demonstrated by application of short nanosecond laser pulses (<10 ns) with a single, visible wavelength of 532 nm. The P1 and the P3 trenches are scribed by induced and direct ablation, respectively. For the P2 scribe, the thermal input of the ns laser pulses is used to transform the CIGSe absorber layer locally into a highly conductive compound to provide proper electrical interconnection. These findings promise further simplification and flexibility to thin film solar cell production.
Structuring of thin-film photovoltaic modules requires basic knowledge of the laser – thin-film interaction in order to adapt the accessible laser parameters, like wavelength, power, repetition rate and scribing speed whilst taking into account the specific material properties of the layer. We have studied the nanosecond laserablation behavior of corresponding layers (i) of silicon based thin-film solar cells with a-Si/µc-Si tandem absorber type and (ii) of back contact and absorber layer of CIGSe solar cells. The respective ablation threshold fluences were determined as integrative parameters describing the specific laser – material interaction. For the threshold determination we used two different methods and developed a new analytical approach taking into account scribing through the glass substrate as it is preferred for most structuring processes. This was done by analyzing the thin film ablation results by means of optical microscopy, profilometry, scanning electron microscopy (SEM). Moreover, we determined the incubation coefficient of the regarded material layers which allows us to predict quantitatively the influence of the spot overlap on the scribing threshold.
Paper is one of the most important materials representing and witnessing human culture particularly as a carrier medium for text and image. As soiling hampers the reception of information, paper cleaning techniques are needed. Traditional mechanical and chemical cleaning methods are used by conservator-restorers. In some cases, a classical cleaning procedure of paper objects yields unsatisfactory results or a conventional treatment is even impossible. Especially, fragile paper objects cause problems due to mechanical instabilities. Laser cleaning as a non-contact method might be a way to overcome some of the limitations of classical cleaning techniques. Laser parameters have to be chosen to achieve removal of the soiling without influencing the artwork. Any immediate as well as long-term effects causing an irreversible change of the artwork have to be avoided. At present, most laser applications are found in stone and metal conservation, while laser treatment of complex organic materials like paper is still not fully developed for application in conservators' workshops. This contribution describes recent work of pulsed laser cleaning of soiled model samples. Pure cellulose, rag paper and wood-pulp paper were mechanically soiled with pulverized charcoal in a standardized procedure to make model samples representing essential characteristics of contaminated real-world artworks. Afterwards, model samples were cleaned using short and ultrashort laser pulses in the nanosecond and femtosecond time domain, respectively. An extensive analysis of the model samples after laser treatment using an optical microscope and a multi-spectral imaging system allows a comparison of the cleaning results obtained with both laser sources.
Single-pulse femtosecond (fs) (pulse duration ~200 fs, wavelength 398 nm) and nanosecond (ns) (pulse duration 4 ns, wavelength 355 nm) laser ablation have been applied in combination with time-of-flight mass spectrometer (TOFMS) to analyze the elemental composition of the plasma plume of single-crystalline telluria (c-TeO2, grown by the balance controlled Czochralski growth method). Due to the three-order difference of the peak intensities of the ns and fs-laser pulses, significant differences were observed regarding the laser-induced species in the plasma plume. Positive singly, doubly and triply charged Te ions (Te+, Te2+, Te3+) in the form of their isotopes were observed in case of both irradiations. In case of the ns-laser ablation the TeO+ formation was negligible compared to the fs case and there was no Te trimer (Te3+) formation observed. It was found that the amplitude of Te ion signals strongly depended on the applied laser pulse energy. Singly charged oxygen ions (O+) are always present as a byproduct in both kinds of laser ablation.
Cleaning of artificially soiled paper using nanosecond, picosecond and femtosecond laser pulses
(2010)
Cleaning of cultural assets, especially fragile
organic materials like paper, is a part of the conservation
process. Laser radiation as a non-contact tool offers
prospects for that purpose. For the studies presented here,
paper model samples were prepared using three different paper
types (pure cellulose, rag paper, and wood-pulp paper).
Pure cellulose serves as reference material. Rag and woodpulp
paper represent essential characteristics of the basic
materials of real-world artworks. The papers were mechanically
soiled employing pulverized charcoal. Pure and artificially soiled paper samples were treated with laser pulses of
28 fs (800 nm wavelength) and 8–12 ns (532 nm) duration in
a multi pulse approach. Additionally, the cellulose reference
material was processed with 30 ps (532 nm) laser pulses.
Damage and cleaning thresholds of pure and soiled paper
were determined for the different laser regimes. Laser working
ranges allowing for removal of contamination and avoiding
permanent modification to the substrate were found.
The specimens prior and after laser illumination were characterized
by light-optical microscopy (OM) and scanning
electron microscopy (SEM) as well as multi spectral imaging
analysis. The work extends previous nanosecond laser
cleaning investigations on paper into the ultra-short pulse
duration domain.
Nanoaggregates formed by metal spheres of different radii and interparticle distances represent finite, deterministic, self-similar systems that efficiently concentrate optical fields and act as nanolenses. Here we verify experimentally the theoretical concept of nanolenses and explore their potential as enhancing nanostructures in surface enhanced Raman scattering (SERS). Self-similar structures formed by gold nanospheres of different sizes are generated by laser ablation from solid gold into water. These nanolenses exhibit SERS enhancement factors on the order of 109. The chemically clean preparation process provides several advantages over chemically prepared nanoaggregates and makes the stable and biocompatible gold nanolenses potent enhancing structures for various analytical and sensing applications.
Femtosecond time-resolved microscopy has been used to analyze the structural transformation dynamics (melting, ablation, and solidification phenomena) induced by single intense 130 fs laser pulses in single-crystalline (100)-indium phosphide wafers in air on a time scale from ~100 fs up to 8 ns. In the ablative regime close to the ablation threshold, transient surface reflectivity patterns are observed by fs microscopy on a ps to ns time scale as a consequence of the complex spatial density structure of the ablating material (dynamic Newton fringes). At higher fluences, exceeding six times the ablation threshold, optical breakdown causes another, more violent ablation regime, which reduces the energy deposition depth along with the time of significant material removal. As a consequence, ablation lasts longer in a ring-shaped region around the region of optical breakdown. This leads to the formation of a crater profile with a central protrusion. In the melting regime below the ablation threshold, the melting dynamics of indium phosphide has been quantified and subsequent superficial amorphization has been observed upon solidification on the ns time scale leading to amorphous layer thicknesses of the order of a few tens of nanometers.