sharply focus such a beam [1] makes it a versatile tool for various applications [2] such as plasmon excitation, optical trapping and laser material processing. In the field of cutting thick metal sheets with a radially polarized laser the benefits were outlined theoretically in 1999 by Niziev et al. [3]. In 2007 it was demonstrated by Meier et al. that the drilling speed of holes in mild steel with a Q-switched nanosecond laser can be increased by a factor of 1.5-4 by using azimuthally polarized laser radiation instead of linear or circular polarization [4]. Furthermore, it was experimentally shown that picosecond lasers with such polarization states are promising tools for the fabrication of micro holes [5].Axially symmetric polarized laser radiation can be either generated intra-or extra-cavity. While the latter was realized at high average powers in the multi kW range, e.g. by means of segmented half-wave plates and a multimode input beam [5], the former has been demonstrated with different approaches such as customized fibers [6], a tripleaxicon retroreflector unit [7], the so-called Giant Reflection to Zero Order (GIRO) mirror [8] or grating mirrors [9,10]. Up to now the listed intra-cavity approaches were only demonstrated in continuous wave (CW) operation. To generate pulsed beams with axially-symmetric polarization states at high average power so far only the aforementioned segmented half-wave plates were used to transform an incident linearly polarized fundamental-mode laser beam into a radially or azimuthally polarized LG01* mode. 85 W of average output power and radially polarized pulses as short as 750 fs were, for instance, achieved by means of three cascaded single-crystal fiber (SCF) amplifier stages starting from a linearly polarized seed beam with an average power of 1.5 W [11]. The polarization conversion was implemented between the second and the third amplification stage. An even higher output power was demonstrated Abstract We report on a single-stage high-power amplification of a radially polarized mode-locked laser beam in a single-crystal fiber (SCF) amplifier. The seed beam was amplified by a factor of 5.0 to an average output power of 66.3 W. The pulse duration of the amplified pulses was measured to be 909 fs at a repetition rate of 40.7 MHz, corresponding to a pulse energy of 1.63 µJ and a resulting pulse peak power of 1.58 MW. The output beam showed a very high quality of the doughnut-shaped intensity distribution and furthermore a high radial polarization purity.
We report on the first demonstration of a radially polarized passively mode-locked thin-disk oscillator. Radial polarization was achieved by the use of a novel circular grating waveguide output coupler. We showed mode-locked operation up to a maximum average output power of 13.3 W with an optical efficiency of 21.8%. The degree of radial polarization of the emitted beam was measured to be 97±1%. The laser system generated pulses with a duration of 907 fs and an energy of 316 nJ corresponding to a peak power of 0.35 MW. To the best of our knowledge, these values exceed the performance of previously reported radially polarized mode-locked oscillator systems.
We introduce a method for marker-free cell discrimination based on optical tweezers. Cancerous, non-cancerous, and drug-treated cells could be distinguished by measuring the trapping forces using holographic optical tweezers. We present trapping force measurements on different cell lines: normal pre-B lymphocyte cells (BaF3; "normal cells"), their Bcr-Abl transformed counterparts (BaF3-p185; "cancer cells") as a model for chronic myeloid leukaemia (CML) and Imatinib treated BaF3-p185 cells. The results are compared with reference measurements obtained by a commercial flow cytometry system.
We present an optically addressed non-pixelated spatial light modulator. The system is based on reversible photoalignment of a LC cell using a red light sensitive novel azobenzene photoalignment layer. It is an electrode-free device that manipulates the liquid crystal orientation and consequently the polarization via light without artifacts caused by electrodes. The capability to miniaturize the spatial light modulator allows the integration into a microscope objective. This includes a miniaturized 200 channel optical addressing system based on a VCSEL array and hybrid refractive-diffractive beam shapers. As an application example, the utilization as a microscope objective integrated analog phase contrast modulator is shown.
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