High repetition rate femtosecond lasers are commonly used for fabricating laser-induced periodic surface structures (LIPSS) over large areas at high processing speeds. Industrially relevant metals, like steel, experience thermal modifications at repetition rates beyond several hundred kilohertz. In this work, we fabricate low spatial frequency LIPSS (LSFL) on steel, varying pulse repetition rates from 10 kHz to 2 MHz. The study characterizes laser-structured areas and redeposited debris using SEM and μ-Raman spectroscopy. A simple heat dissipation model identifies repetition rate ranges associated with thermal modifications. Morphological changes and debris impact functional wetting behavior, offering insights for optimizing parameters in high repetition rate femtosecond laser materials processing.
Understanding the time-dependent states of plasmonic metasurfaces under intense laser irradiation is crucial to optimize the laser processing parameters and reach specific optical properties. In this work, the physico-chemical mechanisms of laser-induced shape transformation of Ag nanoparticle ensemble embedded in thin TiO2 layers is investigated in details. The combination of high-repetition-rate pump-probe microscopy in transmission configuration with ex situ scanning electron microscopy reveals the variety of mechanisms that contribute to the reshaping of near-coalescence Ag nanoparticles with broad size and complex shape distributions into an ensemble of spherical particles. The description of the mechanisms on multiple time scales from the ultrafast electron dynamics to the much slower thermal processes triggered by the accumulation of 800 pulses at high repetition rate reveals different pulse number regimes. Owing to the high-repetition-rate data acquisition, the effect of the heat accumulation on the nanoparticle reshaping kinetics is demonstrated. The observed structural changes are discussed in the scope of the in situ transmission spectra. The presented results are intended to help improving applications of such laser-processed plasmonic metasurfaces in security printing, data storage and digital fabrication of nanoscale electronics.
One important parameter that is often not considered in the formation of laser-induced periodic surface structures (LIPSS) is the laser-induced oxidation produced when oxidation prone materials are irradiated in air environment. In this work, we characterize the response of the oxidation prone hard-coating material chromium nitride and explain the findings with finite-difference time-domain calculations. We also employ complementary surface and in-depth analytic techniques to reveal morphological, chemical and structural features of different types of surface structures and LIPSS produced on titanium-based substrates.
Nonlinear laser processing of dielectrics with ultrafast lasers has been extensively studied over the last years and successfully applied to the production of photonics and micro-fluidic devices. Still, problems related to the presence of strong optical nonlinearities make it difficult to optimize the spatial intensity distribution in the focal region (SIDFR) in some cases. Methods providing a rapid estimate of the latter, even approximately, can be of great help for optimizing processing strategies and in other applications conditioned by nonlinear propagation like spatial soliton shaping. We have developed a numerical method for estimating the SIDFR inside a dielectric material, considering nonlinear absorption, nonlinear refraction and spherical aberration for laser beams with arbitrarily shaped wavefront. It is based on a generalized adaptive fast-Fourier evolver and has been successfully tested for flat wavefronts in subsurface processing. In this work we demonstrate its applicability to complex wavefronts, like those that can be generated with spatial light modulators (SLM). For this purpose the beam wavefront is described using Zernike polynomials before being propagated inside the material for different depths, pulse parameters. The results obtained show that under certain conditions, nonlinearities can be not only controlled and pre-compensated but also exploited for producing tailored SIDFRs.
Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing
in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser
fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of
waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10-2. The
compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of
the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large
enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The
refractive index of the La-phosphate glass increases linearly with the La2O3 content (Δn per mole fraction increase of
La2O3 ≈ 5x10-3) mainly because of the relative mass of the La3+ ions. The density increase without substantial
modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing
minor variations in the (PO2)sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective
modification of the glass composition.
Heavy metal oxide (HMO) glasses include, among others, compound oxide glasses with high Bi- and/or Pb-oxide content. These materials are very atttractive for photonic applications because they show phonon energies lower than those of silicates or borates, which provides them with an extended transparency interval in the mid infrared (λ ≤ 7-8 μm). This feature makes them specially suitable for infrared transmission applications. In addition, they have high density, a relatively low glass transition temperature, and an excellent chemical and physical stability. However, they show relatively small energy band-gaps (2 - 3 eV) and large linear (no ≥ 2) and non-linear refractive indices (n2 ≈ 10-19 - 10-18 m2/W), which makes their processing with ultrashort pulses to become a challenge. This work analyses different issues inherent to the production of waveguides in these materials by direct writing with fs laser pulses. We will show, as an example, the feasibility of writing efficient waveguides in 35PbO • 35Bi2O3 • 15 Ga2O3 • 15 GeO2 HMO glass. The comparison of the behavior of this material with glasses showing lower linear and non-linear refractive indices allows to conclude that direct fs-laser waveguide writing with peak powers above the critical self-focusing threshold is feasible. Different means to control the guided mode distribution under this condition are presented.
Low coherence photorefractive holography is a wide-field technique for 3-D imaging that offers a unique mechanism to discriminate against a background of diffuse light. This provides a wide-field method to image through scattering materials that we have demonstrated may be implemented at frame rates as high as ~ 470/second. We present our recently developed low coherence photorefractive microscope and demonstrate how it may be realized using a spatially coherent broadband c.w. diode-pumped solid-state laser. This can provide real-time sectioned images of moving 3-D objects using only a simple uncooled 8-bit CCD camera. We also demonstrate a photorefractive 3-D imaging technique that exploits structured illumination and photorefractive holography to achieve a real-time wide-field sectioning microscope that may be applied to fluorescence, as well as reflected light. We also discuss issues for improving the sensitivity and spectral coverage of photorefractive holography using semi-insulating MQW devices.
When imaging through scattering media it is easiest to visualise a pulse propagating through the material. As the light propagates it is scattered away from its original trajectory and it becomes diffuse, both spatially and temporally. It is important to note that this picture is also valid for short coherence length c.w. light. In order to create a high resolution wide-field image, in an ideal case, it is necessary to select only the unscattered ballistic photons. This may be achieved using the coherence properties of the light. If the beam is initially split into an incident beam and a reference beam then only the unscattered ballistic photons will retain coherence with the reference beam. Therefore only this light will produce an interference pattern. In the wide-field case this interference of light from the object and light from a reference beam is termed holography.
We report a whole-field fluorescence imaging microscope that combines 3-D spatial resolution by optical sectioning, using structured illumination, with fluorescence lifetime imaging and spectrally-resolved imaging. We show the potential of this technique in the elimination of common artefacts in fluorescence lifetime imaging and apply it to study the dependence of the lifetime on the emission wavelength in biological tissue.
The fluorescence decay in fluorescence lifetime imaging (FLIM) is typically fitted to a multi-exponential model with discrete lifetimes. The interaction between fluorophores in heterogeneous samples (e.g. biological tissue) can, however, produce complex decay characteristics that do not correspond to such models. Although they appear to provide a better fit to fluorescence decay data than the assumption of a mono-exponential decay, the assumption of multiple discrete components is essentially arbitrary and often erroneous. The stretched exponential function (StrEF) describes fluorescence decay profiles using a continuous lifetime distribution as has been reported for tryptophan, being one of the main fluorophores in tissue. We have demonstrated that this model represents our time-domain FLIM data better than multi-exponential discrete decay components, yielding excellent contrast in tissue discrimination without compromising the goodness of fit, and it significantly decreases the required processing time. In addition, the stretched exponential decay model can provide a direct measure of the sample heterogeneity and the resulting heterogeneity map can reveal subtle tissue differences that other models fail to show.
We report high speed (~ 470 frames/s) 3-D imaging using photorefractive holography with sources of diverse temporal and spatial coherence and discuss design considerations for real-world high bit-rate imaging systems. We also propose a new real-time optical sectioning technique based on structured illumination with photorefractive holography to detect fluorescence.
As a precursor to applying fluorescence lifetime imaging (FLIM) to studies of intercellular communication in molecular immunology, we have investigated the fluorescence lifetime of enhanced green fluorescent protein (EGFP) in mixtures of water and glycerol using time-correlated single photon counting (TCSPC). We find that the EGFP lifetime decreases with increasing glycerol content. This is accounted for quantitatively by the refractive index dependence of the fluorescence lifetime as predicted by the Strickler Berg formula which relates the fluorescence lifetime to the absorption spectrum. The solvent viscosity has no influence on the fluorescence lifetime. We also discuss the refractive index dependence of the GFP fluorescence lifetime in more complex systems. The findings are particularly relevant for the interpretation of FLIM of GFP expressed in environments such as bacteria and cells.
The cleaning of silicon surfaces from submicron dust particles has been studied by means of the 'Steam Laser Cleaning' (SLC) process and compared to 'Dry Laser Cleaning' (DLC) which is used nowadays in many applications. For SLC a thin liquid layer (e.g. a water- alcohol mixture) is condensed onto the substrate, and is subsequently evaporated by irradiating the surface with a short laser pulse. The DLC process, on the other hand, only relies on the laser pulse, without application of a vapor jet. We have systematically investigated the efficiency of these two processes for the removal of well-characterized polymer, silica and alumina particles of various sizes down to 60 nm in diameter, and have also studied the influence of light wavelength and laser pulse duration for nanosecond and picosecond pulses. The results demonstrate that for the gentle cleaning of silicon wafers SLC is a very efficient method and is superior to DLC. An effect which so far has only rarely been taken into account for laser cleaning is the field enhancement under the particles, which can give rise to serious surface damage, in particular when cleaning pulses in the picosecond and femtosecond range in the DLC are applied.
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