Jan Winter

Jan Winter
Munich University of Applied Sciences | HM · Department of Applied Sciences and Mechatronics

Master of Science (M.Sc.)

About

25
Publications
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178
Citations

Publications

Publications (25)
Article
Full-text available
In the context of current state of the art, understanding the laser ablation efficiency decrease for pulse durations exceeding the mechanical relaxation time of a few ps remains a pending research question. A heuristic approach may be used to reveal the role of effective penetration depth on ablation efficiency. Extending familiar contributions of...
Article
The ablation efficiency during laser processing strongly depends on the initial and transient reflectance of the irradiated material surface. This article reports on the transient relative change of the reflectance ΔR/R of stainless steel during and after ultrashort pulsed laser excitation (800 nm, 40 fs) by spatially resolved pump–probe reflectome...
Article
Full-text available
In this work, we investigate single-pulse laser ablation of bulk stainless steel (AISI304), aluminium (Al) and copper (Cu) and its dependence on the pulse duration. We measured the reflectivity, ablation thresholds and volumes under the variation of pulse duration and fluence. The known drop of efficiency with increasing pulse duration is confirmed...
Article
The ablation rate in double-pulse material processing is strongly influenced by the pulse separation. For pulse separations exceeding 3 ps a significant decrease in ablation volume has been observed. This was attributed to three mechanisms: rarefaction wave interaction, shielding by ablation plume and material re-deposition. Here we present careful...
Article
In recent years, high entropy alloy research has experienced increased interest and it was found that some of these materials have extraordinary properties. High entropy alloys also show an increased damage resistance to high-energy particle irradiation, mainly due to effects caused by the increased configuration entropy. So far, no detailed studie...
Presentation
Full-text available
The laser matter interaction of alloys, like industrial relevant stainless steels, with ultra-short pulsed (usp) lasers has been widely discussed. However, a new approach to alloy design, High Entropy Alloys (HEAs), is currently in the focus of many studies. HEAs show promising properties for technical applications. One of the most investigated HEA...
Article
Full-text available
An ultra-short laser pulse irradiated surface passes through a sequence of physical processes that occur over a wide temporal range, from femtoseconds to microseconds. In this paper for the first time, the complete laser ablation process for industrially relevant metals, aluminium (Al) and stainless steel (AISI 304) is tracked from the initial puls...
Conference Paper
In recent years several works have been investigating the temporal distribution of pulse energy with the aim of maximizing ablation efficiency in terms of energy specific ablation volume by applying double pulses or pulse bursts. Here we study the energy specific ablation volume of double pulses in dependency of increasing pulse separation on alumi...
Presentation
In recent years several works have been investigating the temporal distribution of pulse energy with the aim of maximizing ablation efficiency in terms of energy specific ablation volume by applying double pulses or pulse bursts. Here we study the energy specific ablation volume of double pulses in dependency of increasing pulse separation on alumi...
Presentation
State of the art ultrafast laser systems are capable of delivering fluences up to 100 J/cm^2 when the laser pulses are tightly focused in micro material processing. This fluence value is far above the optimal fluence for processing of metal samples, which ranges between 1.5 J/cm^2 and 15 J/cm^2. An attempt to perform laser processing at the optimal...
Article
Although green femtosecond lasers provide outstanding quality and wide processing windows for monolithic interconnection of the individual cells in organic photovoltaic (OPV) modules, they are hardly used in commercial applications, due to cost reasons. In this work, a process has been developed that allows the monolithic interconnection in OPV mod...
Article
Ultrashort pulsed lasers have gained widespread use in laser material processing applications, as they enable precise ablation due to their highly efficient energy deposition and low thermal damage. Recent investigations using double laser pulses for metal ablation with temporal spacing in the region of a few picoseconds (ps) have shown that the la...
Article
Full-text available
We consider expansion, break off, and flight of 10 nm molybdenum film deposited onto glass support. These events are initiated by action of subpicosecond laser pulse onto film. Approximations for two-temperature equation of state and electron–ion coupling parameter are developed. Heat conduction is unimportant because film is ultrathin and because...
Preprint
We consider expansion, break off, and flight of 10 nm molybdenum film deposited onto glass support. These events are initiated by action of subpicosecond laser pulse onto film. Approximations for two-temperature equation of state and electron--ion coupling parameter are developed. Heat conduction is unimportant because film is ultrathin and because...
Article
Full-text available
Ultrashort laser pulses are widely used for the precise structuring of semiconductors like silicon (Si). We present here, for the first time, a comparative study of experimentally obtained and numerically simulated two-dimensional ablation profiles based on parameters of commercially relevant and widely used near-infrared and diode pumped femtoseco...
Conference Paper
Ultra-short pulsed lasers offer a great potential in precise and efficient material processing. Experimental and theoretical studies on efficiency of laser material processing from metals have demonstrated a high degree of dependency on the laser pulse duration. Within these studies, the investigation of the transient energy deposition in material...
Article
Full-text available
In this paper, we present ultrafast measurements of the complex refractive index for copper up to a time delay of 20 ps with an accuracy <1% at laser fluences in the vicinity of the ablation threshold. The measured refractive index n and extinction coefficient k are supported by a simulation including the two - temperature model with an accurate de...
Article
A theoretical approach using ab initio calculations was applied to study the interaction of an ultrashort laser pulse with the metal alloy Fe0.72Cr0.18Ni0.1 (AISI 304). The electronic structure was simulated by taking into account the chemical and magnetic disorder of the alloy by the coherent potential approximation implemented in a fully relativi...
Article
Full-text available
Numerical simulation reveals the mechanism for pulse duration dependence on laser ablation efficiency

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Project (1)
Project
The direct and induced (confined) laser ablation of materials and thin film systems with ultrashort laser pulses enables high-precision material processing in a variety of industrial applications such as drilling of injection nozzles, cutting of hardened glass displays or selective patterning of thin film solar cells and OLEDs (organic light emitting diodes). From literature numerous studies are known how to optimize the ablation efficiency of direct ablation of various materials. Depending on the material, fs- or ps-pulses show a higher ablation efficiency, but there is no clear explanation for this behaviour. Our simulations and transient reflectivity measurements indicate that both the electron diffusion length, which defines the heat deposition, and the change in absorbance during the irradiation depend on the pulse duration. For the induced ablation, the irradiated material is confined by a transparent layer. The material is removed by a lift-off of an intact disc of the layer system. This enables very precise and very energy efficient laser processing far below the thermodynamic limit. Our investigations of the driving ablation mechanisms suggest that shock waves, which are generated on ultrafast time scales, initiate the material transport or removal in the nanosecond range. The shock waves could be generated in the solid and / or liquid phase by ultrafast heating and expansion, and not in the gas or plasma phase, as observed for nanosecond pulses. In order to get a better understanding of the transient behaviour during the direct and induced ablation, detailed time- and space resolved investigations of the heating, the phase transitions and the optical properties (reflection and absorption) of the irradiated material are necessary especially in the low ps time regime. These investigations are part of the project and will be performed by measuring the complex refractive index with pump-probe ellipsometry in combination with multi-physics and multiscale modelling for various materials. Thus, a well-founded model of processes on ultrafast time scales shall be developed. The model shall then be used to answer fundamental application-related questions of the direct and induced ablation, such as the question othe optimal laser parameters (pulse width, wave length, fluence, beam diameter) for most efficient processing of various materials, the problem of the maximum repetition rate, the connection between pulse duration and ablation rate, or the influence of the beam diameter on the induced laser ablation. In this way, the results of the project will be used for the optimization of industrially relevant laser processes.