In the area of laser material processing, versatile applications for cutting glasses and transparent polymers exist. However, parasitic effects such as the creation of step-like structures appear when laser cutting inside a transparent material. To date, these structures were only described empirically. This work establishes the physical and chemical mechanisms behind the observed effects and describes the influence of process and material parameters onto the creation of step-like structures in hydrogel, Dihydroxyethylmethacrylat (HEMA). By focusing laser pulses in HEMA, reduced pulse separation distance below 50 nm and rise in pulse energy enhances the creation of unintended step-like structures. Spatial resolved Raman-spectroscopy was used to measure the laser induced chemical modification, which results into a reduced breakdown threshold. The reduction in threshold influences the position of optical breakdown for the succeeding laser pulses and consequently leads to the step-like structures. Additionally, the experimental findings were supplemented with numerical simulations of the influence of reduced damage threshold onto the position of optical breakdown.In summary, chemical material change was defined as cause of the step-like structures. Furthermore, the parameters to avoid these structures were identified.

In ophthalmic surgery fs laser pulses are used as a precise and safe cutting tool. At specific processing parameters, however, an interesting phenomenon of unintended periodic structures inside the tissue can be observed. In this study, a transparent polymer served as ocular phantom material for further investigations. A Femtosecond laser with MHzrepetition rate and pulse energies below 200 nJ was used. The size of the durable material change caused by applied fssingle pulses was measured in regard to the pulse energy. Furthermore, lines were cut inside the material with different laser spot distances and laser pulse energies. The creation and enhancement of unintended step-like structures could be related to a decrease of spot distance and further increase of pulse energy. Cutting planes inside the material resulted also in a formation of step like structures. For planes the step-like structures were formed with different orientation in the x-yplane in regard to the used line distance between two applied lines. A maximum step height of 75 µm was measured using our setup. Those periodic structures are unwanted for any application in the field of laser material processing, ophthalmology or biomedical sectioning. Hence, investigations of the parameters which trigger this phenomenon are of great interest.

One of the most frequently performed blood tests, measurement of total hemoglobin concentration, requires invasive blood sampling. We developed an optoacoustic technique for noninvasive monitoring of total hemoglobin concentration and other blood variables by probing the radial artery or other blood vessels. Recently, we designed and built a focused, wide-band, polymer-based optoacoustic transducer for blood vessel probing with high, submillimeter lateral resolution and incorporated it into a highly portable, laser diode-based optoacoustic system. The focused optoacoustic transducer combines a fiber-optic delivery system and a wide-band piezosensor. First, we experimentally measured transducer parameters (lateral resolution, sensitivity, focal length). To test the transducer capabilities in measurement of total hemoglobin concentration and other blood parameters from blood vessels, we prepared a tissue phantom simulating strongly-scattering tissues with blood vessels of different diameters, spacing, and depths. Optoacoustic signals were acquired from blood at different hemoglobin concentration and oxygenation during transducer scanning over the phantom. In vivo experiments were performed from radial arteries and peripheral veins of different size, depth, and spacing. Submillimeter lateral resolution was obtained in the in vitro and in vivo experiments. The high resolution combined with the wide-band detection of the optoacoustic waves can be used for monitoring of blood variables in blood vessels with high accuracy, sensitivity, and specificity.

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