2019, Karras, Christian, Chemnitz, Mario, Heintzmann, Rainer, Schmidt, Markus A.

Nonlinear pulse propagation inside highly nonlinear media requires accurate knowledge on the temporal response function of the materials used particular in the case of liquids. Here we study the impact of deuteration on the ultrafast dynamics of toluene and nitrobenzene via all optical Kerr gating, showing substantially different electronic and molecular contributions, which was quantified by fitting a multichannel decay model to the data points. Specifically we found that deuteration imposes the time-integrated nonlinearities to reduce particular for toluene which could be caused by both reduced electronic hyperpolarizabilities as well as weaker intermolecular interactions. The results achieved reveal that deuterated organic solvents represent promising materials for infrared photonics since they offer extended infrared transmission compared to their non-deuterated counterparts while maintained strong nonlinear responses.

2018, Chen, S.-Y., Bestvater, F., Heintzmann, Rainer, Cremer, Christoph

Single molecule localization microscopy (SMLM) has been established as an important super-resolution technique for studying subcellular structures with a resolution down to a lateral scale of 10 nm. Usually samples are illuminated with a Gaussian shaped beam and consequently insufficient irradiance on the periphery of the illuminated region leads to artifacts in the reconstructed image which degrades image quality. We present a newly developed patterned illumination SMLM (piSMLM) to overcome the problem of uneven illumination by computer-generated holography. By utilizing a phase-only spatial light modulator (SLM) in combination with a modified Gerchberg-Saxton algorithm, a user-defined pattern with homogeneous and nearly speckle-free illumination is obtained. Our experimental results show that irradiance 1 to 5 kW/cm2 was achieved by using a laser with an output power of 200 mW in a region of 2000 µm2 to 500 µm2, respectively. Higher irradiance of up to 20 kW/cm2 can be reached by simply reducing the size of the region of interest (ROI). To demonstrate the application of the piSMLM, nuclear structures were imaged based on fluctuation binding-activated localization microscopy (fBALM). The super-resolution fBALM images revealed nuclear structures at a nanometer scale.Single molecule localization microscopy (SMLM) has been established as an important super-resolution technique for studying subcellular structures with a resolution down to a lateral scale of 10 nm. Usually samples are illuminated with a Gaussian shaped beam and consequently insufficient irradiance on the periphery of the illuminated region leads to artifacts in the reconstructed image which degrades image quality. We present a newly developed patterned illumination SMLM (piSMLM) to overcome the problem of uneven illumination by computer-generated holography. By utilizing a phase-only spatial light modulator (SLM) in combination with a modified Gerchberg-Saxton algorithm, a user-defined pattern with homogeneous and nearly speckle-free illumination is obtained. Our experimental results show that irradiance 1 to 5 kW/cm2 was achieved by using a laser with an output power of 200 mW in a region of 2000 µm2 to 500 µm2, respectively. Higher irradiance of up to 20 kW/cm2 can be reached by simply reducing the size of the region of interest (ROI). To demonstrate the application of the piSMLM, nuclear structures were imaged based on fluctuation binding-activated localization microscopy (fBALM). The super-resolution fBALM images revealed nuclear structures at a nanometer scale.

2019, Trägårdh, Johanna, Pikálek, Tomáš, Šerý, Mojmír, Meyer, Tobias, Popp, Jürgen, Čižmár, Tomáš

Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.Multimode fibres have recently been employed as high-resolution ultra-thin endoscopes, capable of imaging biological structures deep inside tissue in vivo. Here, we extend this technique to label-free non-linear microscopy with chemical contrast using coherent anti-Stokes Raman scattering (CARS) through a multimode fibre endoscope, which opens up new avenues for instant and in-situ diagnosis of potentially malignant tissue. We use a commercial 125 µm diameter, 0.29 NA GRIN fibre, and wavefront shaping on an SLM is used to create foci that are scanned behind the fibre facet across the sample. The chemical selectivity is demonstrated by imaging 2 µm polystyrene and 2.5 µm PMMA beads with per pixel integration time as low as 1 ms for epi-detection.

2018, Schneidewind, Henrik, Zeisberger, Matthias, Plidschun, Malte, Weidlich, Stefan, Schmidt, Markus A.

Focusing light represents one of the fundamental optical functionalities that is used in a countless number of situations. Here we introduce the concept of nano-bore optical fiber mediated light focusing that allows to efficiently focus light at micrometer distance from the fiber end face. Since the focusing effect is provided by the fundamental fiber mode, device implementation is extremely straightforward since no post-processing or nano-structuring is necessary. Far-field measurements on implemented fibers, simulations, and a dual-Gaussian beam toy model confirm the validity of the concept. Due to its unique properties such as strong light localization, a close to 100% implementation success rate, extremely high reproducibility, and its compatibility with current fiber circuitry, the concept will find application in numerous areas that demand to focus at remote distances.

2019, Elsmann, Tino, Becker, Martin, Olusoji, Olugbenga, Unger, Sonja, Wondraczek, Katrin, Aichele, Claudia, Lindner, Florian, Schwuchow, Anka, Nold, Johannes, Rothhardt, Manfred

The photosensitivity of various cerium-doped fibers has been experimentally investigated for both excimer- and femtosecond-laser illumination. The results of single-pulse, few-pulse and multi-pulse inscription of fiber-Bragg-gratings with both laser systems and the thermal aging of those gratings demonstrated the restrictions of the conventional color center model for cerium-doped fibers. To explain the short-term stability of single-pulse gratings against long-term stability of multi-pulse gratings, an extension into a two-step-model was deduced.

2019, Pikálek, Tomáš, Tragardh, Johanna, Simpson, Stephen, Čižmár, Tomáš

Multimode fibres have recently shown promise as miniature endoscopic probes. When used for non-linear microscopy, the bandwidth of the imaging system limits the ability to focus light from broadband pulsed lasers as well as the possibility of wavelength tuning during the imaging. We demonstrate that the bandwidth is limited by the dispersion of the off-axis hologram displayed on the SLM, which can be corrected for, and by the limited bandwidth of the fibre itself. The selection of the fibre is therefore crucial for these experiments. In addition, we show that a standard prism pulse compressor is sufficient for material dispersion compensation for multi-photon imaging with a fibre endoscope.

2018, Chemnitz, Mario, Gaida, Christian, Gebhardt, Martin, Stutzki, Fabian, Kobelke, Jens, Tünnermann, Andreas, Limpert, Jens, Schmidt, Markus A.

We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4). By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 1.92 μm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4). The measured results match simulations of both the generalized and a hybrid nonlinear Schrödinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity. We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics.

2018, Kirchhof, Johannes, Unger, Sonja, Dellith, Jan

The viscous behavior of fluorine-doped synthetic silica is studied using collapsing experiments with different fluorine-doped tubes on a modified chemical vapor deposition (MCVD) lathe. The principles, techniques, and evaluations of this method are the same as the ones demonstrated previously in detail with pure and doped silica. The present investigations provide information about the influence of fluorine doping up to a concentration of about 10 mol% F (3.4 wt% F) in a temperature range between 1600°C and 2000°C. Fluorine doping leads to a systematic decrease in the viscosity, combined with a decrease of the activation energy of the viscous flow and a certain increase of the pre-exponential factor. In summary, this demonstrates the weakening influence of fluorine on the glass network, similar to the incorporation of hydroxyl or chlorine.

2018, Wang, Ning, Zeisberger, Matthias, Hübner, Uwe, Schmidt, Markus A.

Here we present a novel fabrication approach that allows for the implementation of sophisticated planar nanostructures with deep subwavelength dimensions on fiber end faces by electron beam lithography. Specifically, we planarize the end faces of fiber bundles such that they are compatible with planar nanostructuring technology, with the result that fibers can be treated in the same way as typical wafers, opening up the entire field of nanotechnology for fiber optics. To demonstrate our approach, we have implemented densely-packed arrays of gold nanotrimers on the end face of 50 cm long standard single mode fibers, showing asymmetrical resonance lineshapes that arise due to the interplay of diffractive coupling of the individual timer response at infrared wavelengths that overlap with the single mode regime of typical telecommunication fibers. Refractive index sensing experiments suggest sensitivities of about 390 nm/RIU, representing the state-of-the-art for such a device type. Due to its unique capability of making optical fibers compatible with planar nanostructuring technology, we anticipate our approach to be applied in numerous fields including bioanalytics, telecommunications, nonlinear photonics, optical trapping and beam shaping.

2017, Tiess, Tobias, Becker, Martin, Rothhardt, Manfred, Bartelt, Hartmut, Jäger, Matthias

We present a fiber-integrated laser enabling independent tuning of two emission wavelengths with a synchronized pulsed emission. The discrete tuning concept comprises a theta cavity fiber laser (TCFL), a fiber Bragg grating (FBG) array as a versatile spectral filter, facilitating tailored tuning ranges, and optical gating to control the emission spectrum. A novel electrical driving scheme uniquely enables independently tunable multi-wavelength emission from a single laser oscillator. Tunable dual-wavelength emission is experimentally investigated with a ytterbium (Yb)-doped TCFL using an FBG array with 11 gratings. Over a tuning range of 25 nm, 55 wavelength pairs have been demonstrated with high signal contrast (≈ 40 dB) and narrow linewidth (< 40GHz). Based on the demands of prospective applications, pulse synchronicity is studied with a fiber-based time-delay spectrometer (TDS) simultaneously measuring the joint temporal and spectral pulse properties down to a single-pulse analysis. Accordingly, tunable and fully synchronized dual-wavelength emissions have been verified by driving the TCFL with optimized electrical gating parameters. This unique operation mode achieved in a cost-efficient fiber-integrated laser design targets novel applications e.g. in nonlinear spectroscopy and biophotonics.