Birger Seifert

Nontrivial ambiguities for blind frequency-resolved optical gating and the problem of uniqueness

Several schemes and methods exist for frequency-resolved optical gating as a technique for the full characterization of ultrashort optical signals as complex electric fields. However, the uniqueness of the reconstructed fields has never been shown. Here we derive conditions that are sufficient for unique reconstruction of the complex pulses. Furthermore, we construct several examples of distinct pulse pairs with identical or essentially identical frequency-resolved optical-gating traces; even examples with identical spectral intensities of the pulse pairs were found.

A method for unique phase retrieval of ultrafast optical fields

A self-referencing technique for measuring amplitude and phase of ultrashort laser pulses is presented. In contrast to the other methods the relative-phase ambiguities do not appear in our method. Thus, we can characterize ultrashort pulses with well-separated frequency components. The relative-phase ambiguities can be avoided by the use of a cross-correlation technique with two independent laser pulses. Further we propose and demonstrate experimentally a new realtime phase-retrieval algorithm that reconstructs both pulses fast and uniquely.

Multilevel Gauss-Newton methods for phase retrieval problems

The phase retrieval problem is of wide interest because it appears in a number of interesting application areas in physics. Several kinds of phase retrieval problems appeared in laser optics over the past decade. In this paper we consider the numerical solution of two phase retrieval problems for an unknown smooth function f with compact support. We approximate f by a linear spline. The corresponding spline coefficients are iteratively determined by local Gauss–Newton methods, where convenient initial guesses are constructed by a multilevel strategy. We close with some numerical tests which illustrate our method.

Towards bio-silicon interfaces: formation of an ultra-thin self-hydrated artificial membrane composed of dipalmitoylphosphatidylcholine (DPPC) and chitosan deposited in high vacuum from the gas-phase.

The recent combination of nanoscale developments with biological molecules for biotechnological research has opened a wide field related to the area of biosensors. In the last years, device manufacturing for medical applications adapted the so-called bottom-up approach, from nanostructures to larger devices. Preparation and characterization of artificial biological membranes is a necessary step for the formation of nano-devices or sensors. In this paper, we describe the formation and characterization of a phospholipid bilayer (dipalmitoylphosphatidylcholine, DPPC) on a mattress of a polysaccharide (Chitosan) that keeps the membrane hydrated. The deposition of Chitosan (~25 Å) and DPPC (~60 Å) was performed from the gas phase in high vacuum onto a substrate of Si(100) covered with its native oxide layer. The layer thickness was controlled in situ using Very High Resolution Ellipsometry (VHRE). Raman spectroscopy studies show that neither Chitosan nor DPPC molecules decompose during evaporation. With VHRE and Atomic Force Microscopy we have been able to detect phase transitions in the membrane. The presence of the Chitosan interlayer as a water reservoir is essential for both DPPC bilayer formation and stability, favoring the appearance of phase transitions. Our experiments show that the proposed sample preparation from the gas phase is reproducible and provides a natural environment for the DPPC bilayer. In future work, different Chitosan thicknesses should be studied to achieve a complete and homogeneous interlayer.

Unambiguous ultrashort pulse reconstruction from double spectrograms alone

We report a spectrographic technique for amplitude and phase measurements of ultrashort laser pulses (above 10 fs). Pulse information is obtained directly from two different spectrograms, using the mathematical relations between Wigner–Ville function projections. Pulses are reconstructed rapidly and unambiguously without stagnation. This non-interferometric method is demonstrated experimentally for the successful characterization of 100 fs pulses.

Unique phase retrieval of ultrafast optical fields by double - blind correlation techniques

Several methods for the full characterization of ultrashort optical signals have been developed. There are interferometric methods like spectral phase interferometry for direct electric field reconstruction (SPIDER [1]) or measurement of the electric field by interferometric spectral trace observation (MEFISTO [2]) and spectrographic techniques such as frequency-resolved optical gating (FROG [3]) or temporal analysis of spectral components (TASC [4]). All of these methods have one thing in common. They can not recover ultrashort pulses with well-separated frequency components due to relative-phase ambiguities [5]. However, such pulses appear rather frequently for example in soliton molecules [6].

Passively Mode-locked Nd:Cr:YAG Laser pumped by a Modular Scalable Solar Light Concentrator

We demonstrate a passively mode-locked Nd:Cr:YAG laser which is solar-pumped by optical fibers. With the achievement of a higher beam quality a semiconductor saturable absorber mirror can be used to generate picosecond pulses.

Modular Scalable Solar Light Concentrator for Pumping a Nd:Cr:YAG Laser Medium

A solar-pumped Nd:Cr:YAG ceramic laser is presented that is pumped by Fresnel lens sunlight concentrators via polymer optical fibers. Focusing into the laser medium via aspheric lenses allows to improve the laser beam quality.

Surface Morphology of Vapor-Deposited Chitosan: Evidence of Solid-State Dewetting during the Formation of Biopolymer Films

Chitosan is a useful and versatile biopolymer with several industrial and biological applications. Whereas its physical and physicochemical bulk properties have been explored quite intensively in the past, there is a lack of studies regarding the morphology and growth mechanisms of thin films of this biopolymer. Of particular interest for applications in bionanotechnology are ultrathin films with thicknesses under 500 Å. Here, we present a study of thin chitosan films prepared in a dry process using physical vapor deposition and in situ ellipsometric monitoring. The prepared films were analyzed with atomic force microscopy in order to correlate surface morphology with evaporation parameters. We find that the surface morphology of our final thin films depends on both the optical thickness, i.e., measured with ellipsometry, and the deposition rate. Our work shows that ultrathin biopolymer films can undergo dewetting during film formation, even in the absence of solvents and thermal annealing.

Phase and intensity retrieval of ultrashort laser pulses with single-shot VAMPIRE

We present a single-shot (SH) self-referencing technique for measuring phase and intensity of ultrashort laser pulses (SH-VAMPIRE). The cross-correlation setup utilizes filter elements to circumvent relative-phase or direction of time ambiguities.