Three-dimensional photofabrication with femtosecond lasers for applications in photonics and biomedicine

A status report on rapidly advancing femtosecond laser technology, three-dimensional (3D) microstructuring by multiphoton illumination technique, is given. Taking its origin from multiphoton microscopy, this technique is now becoming an important microfabrication tool. In our work we apply near-infrared Ti:sapphire femtosecond laser pulses (at 800/780 nm) for 3D material processing. When tightly focused into the volume of a photosensitive material (or photoresist), they initiate 2PP process by, for example, transferring liquid into the solid state. This allows the fabrication of any computer generated 3D structure by direct laser “recording” into the volume of photosensitive material. 2PP of photosensitive materials irradiated by femtosecond laser pulses is now considered as enabling technology for the fabrication of 3D photonic crystals and photonic crystal templates. In particular, 2PP allows one to introduce defects at any desired locations, which is crucial for the practical applications. Recently, we studied possible applications of 2PP technique in biomedicine. 2PP is a very interesting technique for the fabrication of drug delivery systems, scaffolds for tissue engineering, and medical implants. These and other biomedical applications of 2PP will be reviewed.     

Constraints on the time scale of nuclear breakup from thermal hard-photon emission

Measured hard-photon multiplicities from second-chance nucleon-nucleon collisions are used in combination with a kinetic thermal model to estimate the breakup times of excited nuclear systems produced in nucleus-nucleus reactions at intermediate energies. The obtained nuclear breakup time for the ¹²⁹Xe + ^natSn reaction at 50 A MeV is Δτ ≈ 100-300 fm/c for all reaction centralities. The lifetime of the radiating sources produced in seven other different heavy-ion reactions studied by the TAPS experiment is consistent with Δτ ≈ 100 fm/c, such relatively long thermal photon emission times do not seemingly support the interpretation of nuclear breakup as due to a fast spinodal process for the heavy nuclear systems studied.     

Neutral meson production in relativistic heavy ion collisions

The production of ⁰ and mesons has been studied in the reactions²⁰Ne +Al at 350 MeV/u and⁴⁰Ar + Ca at 1.0 GeV/u. Rapidity distributions and transverse momentum spectra have been measured and are compared to thermal distributions.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Rapid laser prototyping of plasmonic components

Renewed and growing interest in the field of surface plasmon polaritons (SPPs) comes from a rapid advance of nanostructuring technologies. In this paper, we will report on the application of two-photon polymerization (2PP) technique for the fabrication of dielectric SPP-structures, which can be used for localization, guiding, and manipulation of SPPs on a subwavelength scale. This technology is based on nonlinear absorption of near-infrared femtosecond laser pulses. Resolutions down to 100 nm (and even better) are already achievable. Characterization of these structures is performed by leakage radiation microscopy. 2PP allows the fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces. The dielectric structures on metal films are demonstrated to be very efficient for the excitation of SPPs. Using these structures, one can achieve excitation and focusing of the resulting plasmon field. PACS 42.70.Gi; 42.70.Jk; 42.82.Cr; 71.36.+c; 78.20.-e     

Deep drilling of metals by femtosecond laser pulses

Results of recent investigations on deep drilling of metals by femtosecond laser pulses are reported. At high laser fluences, well above the ablation threshold, femtosecond lasers can drill deep, high-quality holes in metals without any post-processing or special gas environment. It is shown that for high-quality drilling of metals, the following processes are important: (1) laser-induced optical breakdown of air containing metal vapor and small metal particles (debris) generated by multi-pulse femtosecond laser ablation, (2) transformation of laser pulses into light filaments, and (3) low-fluence finishing. Received: 15 November 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/2788-100, E-mail: ch@lzh.de     

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasifree production mechanism, a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.