Institute of Applied Physics

The IAP was founded on June 2nd, 1961 as part of the faculty of natural sciences. Today about 56 employees research and teach in four divisions focusing on Biomedical Photonics, Laser Physics, Nonlinear Optics and Microwave Physics.

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Vorherige Weitere

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Inverse-Designed Narrowband THz Radiator for Ultrarelativistic Electrons

THz radiation finds various applications in science and technology. Pump–probe experiments at free-electron lasers typically rely on THz radiation generated by optical rectification of ultrafast laser pulses in electro-optic crystals. A compact and cost-efficient alternative is offered by the Smith–Purcell effect: a charged particle beam passes a periodic structure and generates synchronous radiation. Here, we employ the technique of photonic inverse design to optimize a structure for Smith–Purcell radiation at a single wavelength from ultrarelativistic electrons. The resulting design is highly resonant and emits narrowbandly. Experiments with a 3D-printed model for a wavelength of 900 μm show coherent enhancement. The versatility of inverse design offers a simple adaption of the structure to other electron energies or radiation wavelengths. This approach could advance beam-based THz generation for a wide range of applications.

Corneal absorption spectra in the deep UV range

Significance: Refractive surgery in ophthalmology uses pulsed lasers at 193, 210, or 213 nm.
The reason is that most molecular constituents of cornea absorb strongly in this wavelength
range. Precise refractive surgery via ablation requires an accurate knowledge of the absorption
coefficient at the relevant wavelengths. Yet, the absorption coefficients of corneal tissue reported
in literature vary by almost an order of magnitude; moreover, they were measured mostly at the
wavelengths mentioned earlier.
Aim: By measuring the corneal absorption coefficient of intact eyeballs stored at different
environmental conditions, prepared by following different procedures, and as a function of postmortem
time, we determine the absorption coefficient for the entire wavelength range between
185 and 250 nm for as close as possible to in-vivo conditions.
Approach:We use a specially designed UVellipsometer to measure refractive index and absorption
coefficient. Specifically, we investigate the temporal evolution of refractive index and
absorption coefficient after enucleation of the eyeballs under different environmental conditions
and preparation procedures.
Results: Our measurements provide accurate values for refractive index as well as absorption
coefficient of cornea in the wavelength range between 185 and 250 nm. We find that the absorption
coefficient decreases with time and that neither storage conditions nor preparation procedures
but a continuous degeneration of the cornea is responsible for the observed time evolution.
We use the measured time evolution to extrapolate refractive index and absorption coefficient to
in-vivo conditions.
Conclusion: Our measurements of the close to in-vivo absorption coefficient of cornea between
185 and 250 nm allow for a better understanding and modeling of refractive cornea surgery, also
at other than the three commonly used wavelengths. In the future, this may be relevant when new
pulsed laser sources with other wavelengths become available.

A quantitative interspecies comparison of the respiratory mucociliary clearance mechanism

Collectively coordinated ciliary activity propels the airway mucus, which lines the luminal surface of the vertebrate respiratory system, in cranial direction. Our contemporary understanding on how the quantitative characteristics of the metachronal wave field determines the resulting mucociliary transport is still limited, partly due to the sparse availability of quantitative observational data. We employed high-speed video reflection microscopy to image and quantitatively characterize the metachronal wave field as well as the mucociliary transport in excised bovine, porcine, ovine, lapine, turkey and ostrich samples. Image processing techniques were used to determine the ciliary beating frequency (CBF), the velocity and wavelength of the metachronal wave and the mucociliary transport velocity. The transport direction was found to strongly correlate with the mean wave propagation direction in all six species. The CBF yielded similar values (10–15 Hz) for all six species. Birds were found to exhibit higher transport speeds (130–260 μm/s) than mammals (20–80 μm/s). While the average transport direction significantly deviates from the tracheal long axis in mammals, no significant deviation was found in birds. The metachronal waves were found to propagate at about 4–8 times the speed of mucociliary transport in mammals, whereas in birds they propagate at about the transport speed. The mucociliary transport in birds is fast and roughly follows the TLA, whereas the transport is slower and proceeds along a left-handed spiral in mammals. The longer wavelengths and the lower ratio between the metachronal wave speed and the mucociliary transport speed provide evidence that the mucociliary clearance mechanism operates differently in birds than in mammals.

Toward Real-Time Giga-Voxel Optoacoustic/Photoacoustic Microscopy: GPU-Accelerated Fourier Reconstruction with Quasi-3D Implementation

We propose a GPU-accelerated implementation of frequency-domain synthetic aperture
focusing technique (SAFT) employing truncated regularized inverse k-space interpolation. Our
implementation achieves sub-1s reconstruction time for data sizes of up to 100 M voxels, providing
more than a tenfold decrease in reconstruction time as compared to CPU-based SAFT. We provide an
empirical model that can be used to predict the execution time of quasi-3D reconstruction for any
data size given the specifications of the computing system.

Computer-automated design of mode-locked fiber lasers

We automate the mode-locked fiber laser design process using a modified genetic algorithm and an intuitive optimization loss function to control highly accurate polarization-resolved simulations of laser start-up dynamics without user interaction. We reconstruct both the cavity designs and output pulse characteristics of experimentally demonstrated Yb-fiber all-normal dispersion, dispersion-managed, and wavelength-tuneable all-anomalous dispersion Tm-fiber femtosecond lasers with exceptional accuracy using minimal prior knowledge, and show that our method can be used to predict new cavity designs and novel mode locking states that meet target pulse requirements. Our approach is directly applicable to a broad range of mode locking regimes, wavelengths, pulse energies, and repetition rates, requires no training or knowledge of the loss function gradients, and is scalable for use on supercomputers and inexpensive desktop computers.

Vorherige Weitere