The ablation of lithium niobate (LiNbO3), poly(tetrafluoroethylene) (PTFE, teflon), poly(methylmethacrylate) (PMMA) and polyimide (PI) by 500 fs UV excimer laser pulses at 248 nm is reported. Time-resolved measurements were carried out with pulse pairs of variable delay in the range from −200 to +200 ps. The ablation rate is very sensitive to the time delay between the two pulses, and —depending on the material and fluence—can increase or decrease for very short time delays. For LiNbO3, efficient shielding is observed within a few picoseconds. For PTFE and PMMA, and the total fluence just above threshold, the ablation rate versus time delay shows an autocorrelation type behavior with a full width at half-maximum below 400 fs, since two-photon absorption dominates the ablation process. For polyimide, excited state absorption is found to decrease the ablation rate for delay times below 30 ps.
Due to significant advantages, the trend in the field of medical technology is moving towards minimally or even non-invasive examination methods. In this respect, optical methods offer inherent benefits, as does diffuse reflectance imaging (DRI). The present study attempts to prove the suitability of DRI—when implemented alongside a suitable setup and data evaluation algorithm—to derive information from anatomically correctly scaled human capillaries (diameter: $$10\,\upmu \hbox {m}$$ 10 μ m , length: $$45\,\upmu \hbox {m}$$ 45 μ m ) by conducting extensive Monte–Carlo simulations and by verifying the findings through laboratory experiments. As a result, the method of shifted position-diffuse reflectance imaging (SP-DRI) is established by which average signal modulations of up to 5% could be generated with an illumination wavelength of $$\lambda =424\,\hbox {nm}$$ λ = 424 nm and a core diameter of the illumination fiber of $$50\,\upmu \hbox {m}$$ 50 μ m . No reference image is needed for this technique. The present study reveals that the diffuse reflectance data in combination with the SP-DRI normalization are suitable to localize human capillaries within turbid media.
Laser surgery provides clean, fast and accurate cutting of tissue. However, it is difficult to detect what kind of tissue is being cut. Therefore, a wrong cut may lead to iatrogenic damage of structures. A feedback system should automatically stop the cutting process when a nerve is reached or accidentally being cut to prevent its damage. This could increase the applicability and safety of using a laser scalpel in surgical procedures. In this study, random lasing (RL) is used to differentiate between skin, fat, muscle and nerve tissue. Among these tissue types, a special emphasis is made on the differentiation of nerve from the rest of the tissues, especially fat since nerve is covered by a fatty layer. The differentiation is done for ex-vivo tissues of a pig animal model. The results show that random lasing can be used to differentiate these tissue types also under room light conditions in open air.
Delayed ionization is found to be absent for sub-picosecond laser excitation of free C60 and C7o at 248 nm. The autocorrelation trace obtained for C~0 in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech 2 pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C6o via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.
In general, the measurement of the main three optical properties (µ a , µ s and g) in turbid media requires a very precise measurement of the total transmission (TT), the total reflection (TR) and the collimated transmission (CT). Furthermore, an inverse algorithm such as inverse adding doubling or inverse Monte-Carlo-simulations is required for the reconstruction of the optical properties. Despite many available methods, the error free measurement of the scattering coefficient or the g-factor still remains challenging. In this study, we present a way to directly calculate the scattering coefficient from the total and collimated transmission. To allow this, it can be shown that T T C T is proportional to e μ s ⋅ d for a wide range of optical properties if the sample is thick enough. Moreover, a set-up is developed and validated to measure the collimated transmission precisely.
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