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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Reducing the noise of fiber supercontinuum sources to its limits by exploiting cascaded soliton and wave breaking nonlinear dynamics

The low-noise and phase-coherent nonlinear transformation of a narrowband laser into a broadband supercontinuum (SC) in an optical fiber forms the basis of extremely precise applications ranging from optical frequency comb technology to ultrafast photonics and biomedical imaging. A major challenge of this process is the avoidance of incoherent nonlinear effects that amplify random quantum noise, requiring careful birefringence and dispersion engineering of the fiber. However, fundamental trade-offs exist between working in normal or anomalous dispersion regimes. Here, we combine the benefits of nonlinear dynamics in both regimes by cascading soliton compression and optical wave breaking in a hybrid fiber, formed by joining two widely available, commercial, polarization-maintaining step-index fibers exhibiting anomalous and all-normal dispersion, respectively. We experimentally demonstrate that this hybrid approach results in an ultra-low-noise fiber SC source covering the 930–2130 nm range with phase coherence near unity, spectrally resolved relative intensity noise (RIN) as low as 0.05%, and averaging 0.1% over a bandwidth of 750 nm, approaching the theoretical limits close to the pump laser noise. This corresponds to a doubling of the generated spectral bandwidth
and a decrease of RIN by up to 1 order of magnitude compared to direct pumping of the individual fibers, where modula-tional polarization instabilities play a limiting role. Owing to its simplicity and its scalability to high repetition rates, our hybrid scheme is readily applicable to various laser platforms and could enhance the performance of applications such as hyperspectral nonlinear microscopy, coherent optical communications, and photonic signal processing.

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.

Vorherige Weitere