The formation of periodical nanostructures with femtosecond laser pulses was used to create highly efficient substrates for surface-enhanced Raman spectroscopy (SERS). We report about the structuring of silver and copper substrates and their application to the SERS of DNA (herring sperm) and protein molecules (egg albumen). The maximum enhancement factors were found on Ag substrates processed with the second harmonic generation (SHG) of a 1-kHz Ti:sapphire laser and structure periods near the SHG wavelength. In the case of copper, however, the highest enhancement was obtained with long-period ripples induced with at fundamental wavelength. This is explained by an additional significant influence of nanoparticles on the surface. Nanostructured areas in the range of 1.25 mm2 were obtained in 10 s. The surfaces were characterized by scanning electron microscopy, Fast Fourier Transform and Raman spectroscopy. Moreover, the role of the chemical modification of the metal structures is addressed. Thin oxide layers resulting from working in atmosphere which improve the biocompatibility were indicated by vibration spectra. It is expected that the detailed study of the mechanisms of laser-induced nanostructure formation will stimulate further applications of functionalized surfaces like photocatalysis, selective chemistry and nano-biology.
TiO2 is well known as a low-cost, highly active photocatalyst showing good environmental compatibility. Recently it was found that TiO2 nanotubes promise to enable for high photocatalytic activity (PCA). In our experiments, we studied the PCA and spectroscopic properties of TiO2 nanotube arrays formed by the anodization of Ti. The PCA efficiency related to the decomposition of methylene-blue was measured. To obtain reliable data, the results were calibrated by comparing with standard materials like Pilkington Activ™ which is a commercially available self cleaning glass. The studies included a search strategy for finding optimum conditions for the nanotube formation and the investigation of the relationship between PCA and annealing temperature. TiO2 nanotubes of different shapes and sizes were prepared by an anodization of Ti foil in different electrolytes, at variable applied voltages and concentrations. The photo-dissociation of methylene-blue was detected spectroscopically. For the optimized material, an enhancement factor of 2 in comparison to the standard reference material was found. Furthermore, femtosecond-laser induced photoluminescence and nonlinear absorption of the material were investigated. Possibilities for further enhancements of the PCA are discussed.
The temporal self-reconstruction of pulsed Bessel-like needle beams was studied. Arrays of nondiffracting sub-7-fs
needle beams were shaped from Ti:sapphire oscillator pulses by programming multiple axicons in a phase-only spatial
light modulator. Defined distortions in the time domain were induced by local spectral filtering. By differently shading
parts of selected sub-beams, the self-reconstruction was analyzed under variable conditions. Pulse duration maps were measured with two-dimensional second order autocorrelation based on the Shack-Hartmann sensor principle of
wavefront division. Completely distorted pulses were found to have a pulse duration of > 13 fs whereas partially
distorted sub-beams returned to pulse durations close to the initial ones. Specific applications are proposed.
For a growing number of applications in nonlinear spectroscopy, micro- and nano-machining, optical data processing, metrology or medicine, an adaptive shaping of ultrashort pulsed, ultrabroadband laser beams into propagation-invariant linear focal zones (light blades) is required. One example is the femtosecond laser high-speed large area nanostructuring with moving substrates and cylindrical optics we reported about recently. Classical microoptical systems, however, distort the temporal pulse structure of few cycle pulses by diffraction and dispersion. The temporal pulse transfer can be improved with innovative types of reflective MEMS axicons based on two integrated rectangular mirrors, tilted by a piezoelectric bending actuator. In contrast to pixelated liquid-crystal-on-silicon (LCoS) based devices, cutoff frequencies in multi-kilohertz range, a purely reflective setup and continuous profiles with larger phase shift are realized which enable for shaping extended propagation-invariant zones at a faster and more robust operation. Additionally, a fixed phase offset can be part of the structure. Here, the performance of a prototype of linear mechanically tunable MEMS axicon is demonstrated by generating a pseudo-nondiffracting line focus of variable diameter and depth extension from a femtosecond laser pulse. The temporal transfer of 6-fs pulses of a Ti:sapphire laser oscillator is characterized with spectral phase interferometry for direct electric-field reconstruction (SPIDER) and spatially resolved nonlinear autocorrelation. Spatial and temporal self-reconstruction properties were studied. The application of the flexible focus to the excitation of plasmon-polaritons and the self-organized formation of coherently linked deep sub-wavelength laser-induced periodic surface structures (LIPSS) in semiconductors and dielectrics is reported.
For an extended wavefront analysis, structured materials processing, optical information technologies, or
superresolving microscopy with ultrashort pulses, more flexible and robust techniques of beam shaping are required.
Non-Gaussian fringe-free Bessel beams ("needle beams") can be generated with programmable phase maps of
phase-only displays. Such beams behave propagation invariant over relatively extended regions with respect to their
characteristic spatio-temporal signatures. Here, we extend the concept of needle pulses towards other types of
nondiffracting fields including significantly more complex ones. It is shown that also nondiffracting light slices,
tubular beams or pixellated images can be composed from simple nondiffracting constituents of higher degree of
symmetry. With arrangements of multiple small phase axicons programmed into liquid-crystal-on-silicon spatial
light modulators, a large variety of non-conventional nondiffracting beams of even highly asymmetrically partitions
can be achieved with widely propagation invariant spectral and temporal properties. Modified Shack-Hartmann
sensors with integrated temporal sensitivity, advanced types of multichannel autocorrelators and adaptive materials
processing with variable focal spots are proposed.
The formation of laser induced periodic surface structures (LIPSS) is to a large extent of self-organizing nature and
in its early stages essentially influenced by optical scattering. The evolution of related mechanisms, however, has
still to be studied in detail and strongly depends on materials and laser parameters. Excitation with highly intense
ultrashort pulses leads to the creation of nanoripple structures with periods far below the fundamental wavelength
because of opening multiphoton excitation channels. Because of the drastically reduced spatial scale of such laser
induced periodic nanostructures (LIPNS), a particular influence of scattering is expected in this special case. Here
we report on first investigations of femtosecond-laser induced nanostructuring of sputtered titanium dioxide (TiO2)
layers in comparison to bulk material. The crucial role of the optical film quality for the morphology of the resulting
LIPNS was worked out. Typical periods of nanoripples were found to be within the range of 80-180 nm for an
excitation wavelength of 800 nm. Unlike our previously reported results on bulk TiO2, LIPNS in thin films appeared
preferentially at low pulse numbers (N=5-20). This observation was explained by a higher number of scattering
centers caused by the thin film structure and interfaces. The basic assumptions are further supported by
supplementary experiments with polished and unpolished surfaces of bulk TiO2 single crystals.
The combination of sample translation and line focusing by cylindrical optics is shown to be a convenient and highly
effective way of generating laser induced coherent periodic surface structures (LIPSS) in TiO2 over significantly
extended areas. Compared to known techniques based on a sample translation relative to a circular symmetric focus, the
approach is much less time consuming and requires only a single translation stage. The capability of the method to form
both high and low spatial frequency LIPSS (HSFL, LSFL) at the second harmonic wavelengths of a Ti:sapphire-laser
(around 400 nm) at properly chosen scanning velocity and laser pulse energies is demonstrated. Structured multi-mm2
areas with periods of 80 nm and 325 nm were obtained corresponding to distinct sets of optimized parameters.
Furthermore, the appearance of nano-bumps on 30 nm scale on the surface of the LSFL is reported. Basic technical
issues are discussed and potential applications of LIPSS in rutile-type TiO2 like superwetting, friction control, catalysis
and photovoltaic are proposed.
Fringe-resolved noncollinear autocorrelation extracts information about the pulse duration of ultrashort optical signals
from analyzing the intensity envelope of fringes. By detecting nonlinear autocorrelation functions after frequency
conversion, even an evaluation of temporal asymmetry and frequency chirp are enabled. Here we report on a modified
approach based on replacing crossed plane waves by Bessel-like beams. In comparison to the conventional method,
appropriate mathematical transforms have to be applied. The method is simple and single-shot capable and takes
advantage of specific advantages of pseudo-nondiffracting beams. First proof-of-principle experiments with few-femtosecond
pulse durations were performed and compared to simulations. In multishot operation regime, the
implementation of phase-shifting procedures by spatial light modulators promises considerable improvements of the time
resolution analogous to the known principle of phase-shift interferometry.