Our current research interests are a mixture of the previous work of Prof. Florian Baron and the research group "Femtosecond Spectroscopy and Ultrafast Laser Control" at the Institute of Physics, combined with materials science and practical applications at the Institute of Materials Engineering.

Here you will find a brief introduction to each topic and a selection of relevant scientific literature.

Laser-induced periodic surface structures

The image is stylized by high-contrast greyscales. It shows laser-induced periodic surface structures in a glass surface. The viewing angle is perpendicular to the surface. The oval edge around the structures appears to be torn open with dirt and debris on the outside. The periodic structures on the inside look like parallel lamellae.Image: Bastian Zielinski

Laser induced periodic surface structures (LIPSS) usually correspond to parallel lines that are produced with many ultrashort laser pulses on virtually any material. The orientation of the structures is strongly driven by the laser polarization used and the optical properties of the material implemented. The patterning of a surface with laser induced periodic surface structures significantly changes its properties, ranging from reflectivity and wettability to friction reduction. These changes are not only based on the difference in surface profile but are also due to chemical changes during the laser processing. The application of these structures as well as their origin are part of ongoing research.

We aim to challenge the known surface modification by combining pulse-shaping technologies (see below) and controlling the chemical processing environment, developing new technical application in cooperation with the Institute of Materials Engineering. In addition, creating LIPSS with just a single laser pulse around pre-fabricated seed structures can help illuminate the underlying processes of LIPSS generation.

Temporal pulse-shaping and spatial beam-shaping of femtosecond lasers

The image shows an abstract representation of temporal pulse formation with a liquid crystal display. The image shows the process in two planes. In the foreground, laser pulses can be seen over time. A white input pulse with a Gaussian intensity curve is on the left and a longer, irregular output pulse is on the right. The output pulse has different colors in its course, which represent the different frequency components of the pulse that are shifted against each other in time.  The rear plane shows a 3D model of a liquid crystal display (LCD) in the center. The model is shown schematically on a black background and shows a row of several LCD pixels. To the left of this are just as many colored rectangles that form a row parallel to the pixels of the LCD. The rectangles each have a different color corresponding to the rainbow from red to blue. To the right of the LCD are rectangles of the same color. However, these are not in a row but shifted relative to each other, resulting in a sweeping curve. This represents the relative shift in phase between the color components as they pass through the various switched pixels of the display.Image: Experimentalphysik 3

Laser pulses in the femtosecond range occur on the same time scale as electronic processes such as excitation and relaxation in atoms and molecules. By designing the temporal distribution of energy and the instantaneous frequency of a pulse, the excitation process of a material can be manipulated. For example, researchers in Kassel found that cleverly shaped pulses can create surprisingly deep holes in the surface of dielectrics with just a single pulse by controlling the excitation dynamics (high aspect ratio nanomachining). An additional parameter is the spatial shape of the laser beam, which also influences the excitation and the resulting ablation processes. A spatial light modulator can change the relative phase of the wavefront or the intensity of the light along the spatial profile.

The combination of both techniques, temporal and spatial shaping, will open up new avenues for single or multiple pulse material processing compatible with parallel processing laser-based techniques.

Laser-induced Breakdown Spectroscopy

This is an abstract image of colored lines and lights on a black background. It is an artistic representation or analogy to laser induced breakdown spectroscopy. The lines form a series of individual explosions in which luminous parts fly off. In the three-dimensional representation, the row starts at the front left and continues to the back right. The explosions have different colors along the row. The explosions represent the plasma generated by the measurement method and the different colors represent the wavelengths generated by atoms and molecules in the plasma.Image: Лариса Лазебная –

Tightly focused ultrashort laser pulses can provide sufficiently high intensities in different materials making the generation of plasma possible. When this plasma cools down, ions and electrons recombine and emit specific light patterns that are linked to molecules and elements present in the material that fed the plasma. Laser-induced breakdown spectroscopy with femtosecond laser pulses was first demonstrated in Kassel in 2010 by the Experimental Physics Department "Femtosecond Spectroscopy and Ultrafast Laser Control". With our femtosecond laser pulses and a gated amplification system, we can avoid the blackbody radiation of the plasma and only detect the photons that originate from recombination in the "cold" plasma.

This spatially resolved spectroscopy system was used to investigate the growth of cracks in metals and, lately, to differentiate cancerous and healthy tissue in pathological samples with the help of machine learning algorithms. Future research lines include the depth-dependent analysis of laser-assisted surface chemistry for complex materials, including metallic alloys, degraded battery electrodes and different biological samples.

Nanosphere assisted field enhancement

The image is a scientific diagram with two parts. On the left side of the image, there is a 3D transparent spherical object on a flat surface that is labeled Si substrate. It is viewed from the side. White light is shining from the top onto the sphere. A red arrow and text indicate that the light onto the spehere is a shaped femtosecond irradiation beam.  Below the sphere is a brightly illuminated region and a text labels it as field enhancement.  On the right side of the image is a color coded intensity graph. The color shows simulated values of the field intensity enhanced by a nanospehere. Most of the graph is blue, denoting low field intensity. A text at the bottom says it shows the X-Z plane. A red arrow indicates that the laser beam is coming from the top. A white circle in the upper half of the graph indicates the position of the nanosphere in the simulation. Below the sphere, a region of high field intensity is shown by red colors. There are additional stray lines of brighter blue that show other scattered and diffracted light with low intensity.Image: C. Florian

In the realm of optical microscopy and laser processing, focusing light down to the minimum spot size imposes a natural constraint on achievable resolution known as the diffraction limit. To surpass this limitation, one effective approach involves leveraging near-field effects occurring around dielectric microspheres. A big advantage of near-field processing is that the field is evanescent and not propagating, therefore, resolutions smaller than the classical diffraction limit are feasible.

Our uniquely structured femtosecond laser pulses, both temporally and spatially, present promising possibilities for governing the propagation of energy in such an optical system. This control extends not only to the lateral and axial positioning of the highest intensity point but also encompasses its temporal interaction with materials.

Printing of functional liquids

The left part of the image shows a diagram of the laser printing process. It shows a laser beam, a microscope objective, a donor film and a target substrate. The laser beam is red and is directed at the donor film which is blue. the laser is focussed by an objective. Small droptlets of the film are ejected onto the target substrate. The middle part of the image shoaws a 3D representation of a droplet on the target substrate measured by AFM. It has the shape of a half sphere. The Diameter of the droplet is 20 micrometer and its height 6 micrometers. The right part of the image shows an actual photograph of a droplet which is ejected from the donor film, but still connected by a thin pillar of liquid.Image: Camilo Florian

Laser-Induced Forward Transfer (LIFT) is a well-established laser-based printing technique used for adding functional materials. In this printing process, a laser beam focuses on a donor film, initiating an interaction whose dynamics depend on the layer being processed. Specifically, when dealing with liquids, the absorbed energy leads to a microexplosion, forming a bubble that expands, collapses, and generates a thin liquid jet. This jet elongates and propels a small amount of the material onto a target substrate positioned nearby.

Our aim is to enhance LIFT capabilities by incorporating various functional fluids in combination with spatial and temporal pulse shaping techniques (as outlined below) to precisely control and optimize the transfer process.

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