Martin Reininghaus

Fabrication and spectral tuning of standing gold infrared antennas using single fs-laser pulses

Upright standing gold monopole nanoantennas are fabricated by irradiation of thin gold films with single pulses of fs-laser radiation. The resulting antennas exhibit extinction resonances in the mid infrared spectral rage for p-polarized light under grazing incidence. Due to the free charge carriers in the surrounding gold film of the antenna, the resonance condition of the thin-wire monopole antenna can be explained by introducing image charges yielding an observable resonance wavelength of four times the antenna length. The antenna length is controlled coarsely by the focusing numerical aperture and fine by the pulse energy of the laser pulse producing the structure. An additional ultrafine tuning of the resonance wavelength with a sub-10 nm resolution is realized by an additional coating process subsequent to the laser structuring.

Effect of pulse to pulse interactions on ultra-short pulse laser drilling of steel with repetition rates up to 10 MHz.

We report on the effect of pulse to pulse interactions during percussion drilling of steel using high power ps-laser radiation with repetition rates of up to 10 MHz and high average powers up to 80 W. The ablation rate per pulse is measured as a function of the pulse repetition rate for four fluences ranging from 500 mJ/cm2 up to 1500 mJ/cm2. For every investigated fluence an abrupt increase of the ablation rate per pulse is observed at a distinctive repetition rate. The onset repetition rate for this effect is strongly dependent on the applied pulse fluence. The origin of the increase of the ablation rate is attributed to the emergence of a melt based ablation processes, as Laser Scanning Microscopy (LSM) images show the occurrence of melt ejected material surrounding the drilling holes. A semi empirical model based on classical heat conduction including heat accumulation as well as pulse-particle interactions is applied to enable quantitative conclusions on the origin of the observed data. In agreement with previous studies, the acquired data confirm the relevance of these two effects for the fundamental description of materials processing with ultra-short pulsed laser radiation at high repetition rates and high average power.

Experimental and theoretical investigation on fs-laser-induced nanostructure formation on thin gold films

In this paper, the authors report on the formation of nanobumps and nanojets on thin goldfilms, induced by single fs-laser pulse irradiation. Experimental results on the structure size and shape depending on the pulse energy and the pulse duration are presented. For the first time, the process of short laser pulse nanostructuring on thin metal films was modeled by molecular dynamic simulations on the scale directly accessible in the experiments. Additionally, pump-probe experiments were performed for in-situ visualization of the structure formation.

Heat input and accumulation for ultrashort pulse processing with high average power

Abstract Materials processing using ultrashort pulsed laser radiation with pulse durations <10 ps is known to enable very precise processing with negligible thermal load. However, even for the application of picosecond and femtosecond laser radiation, not the full amount of the absorbed energy is converted into ablation products and a distinct fraction of the absorbed energy remains as residual heat in the processed workpiece. For low average power and power densities, this heat is usually not relevant for the processing results and dissipates into the workpiece. In contrast, when higher average powers and repetition rates are applied to increase the throughput and upscale ultrashort pulse processing, this heat input becomes relevant and significantly affects the achieved processing results. In this paper, we outline the relevance of heat input for ultrashort pulse processing, starting with the heat input of a single ultrashort laser pulse. Heat accumulation during ultrashort pulse processing with high repetition rate is discussed as well as heat accumulation for materials processing using pulse bursts. In addition, the relevance of heat accumulation with multiple scanning passes and processing with multiple laser spots is shown.

Digital photonic production along the lines of industry 4.0

The context of future laser applications in modern manufacturing can be summarized by Digital Photonic Production (DPP). “From Bits to Photons to Atoms” describes the vision of DPP: Designing a component or product in the computer and creating it directly by additive or subtractive photon based processes or production-systems.

Laser fabricated nanoantennas for near-field applications

By irradiation of thin gold films with single pulses of fs-laser radiation, upright standing gold nanoantennas with lengths up to approx. 2 µm and tip diameters of less than 100 nm are produced. The antennas exhibit resonant extinction in the near infrared and can there-fore be used for optical near-field amplification with potential applications for e.g. surface enhanced IR spectroscopy. The resonance wavelength is approx. four times the antenna length and can be tuned by the processing parameters used for the fabrication of the nanoantennas. The influence of different processing parameters on the antenna length and its correlation to the observed resonance wavelength in FTIR spectra are investigated.By irradiation of thin gold films with single pulses of fs-laser radiation, upright standing gold nanoantennas with lengths up to approx. 2 µm and tip diameters of less than 100 nm are produced. The antennas exhibit resonant extinction in the near infrared and can there-fore be used for optical near-field amplification with potential applications for e.g. surface enhanced IR spectroscopy. The resonance wavelength is approx. four times the antenna length and can be tuned by the processing parameters used for the fabrication of the nanoantennas. The influence of different processing parameters on the antenna length and its correlation to the observed resonance wavelength in FTIR spectra are investigated.

The physics in applications of ultrafast lasers

High precision and high throughput material processing using ultrashort pulsed laser radiation of high average power requires a detailed understanding of the laser matter interaction on ultrafast time scales. In this paper, we will focus on energy transport mechanisms based on the two-temperature-model and the resulting ablation regimes for single pulses. Heat accumulation at high pulse repetition rates and spatial pulse overlap will be discussed. Additional, a novel nonthermal ablation mechanism for graphite and corresponding materials will be presented.

Non-thermal ablation of graphite by ultrashort pulsed fs-laser radiation

We will present results of studies on a novel fs-laser induced ablation mechanism, which in literature is denoted as “athermal” or “non-thermal” and includes no melting or evaporation processes. This non-thermal ablation process is capable for solids where the atomic bond strength normal to the surface is significantly different to those in lateral direction. One example for such a solid is given by graphite. In graphite the Carbon atoms of a single graphene layer are bond covalently and thus exhibiting a bond strength of 4 eV per atom. In contrast, the interlayer bond strength normal to the surface results of the more than two orders of magnitude weaker Van-der-Waals-interaction. Without influencing the strong layer internal sp2-bonds this ablation process only breaks the weak van-der-Waals bonds between the layers resulting in a spalation dynamic of the topmost intact graphene layers from the surface of the highly oriented pyrolytic graphite target. Maintaining the internal sp2 crystalline structure of the ablated graphene layers this non-thermal ablation process is feasible to transfer thin layer of sandwich structured solids on various substrates, conserving their mechanical and electronic properties.We will present results of studies on a novel fs-laser induced ablation mechanism, which in literature is denoted as “athermal” or “non-thermal” and includes no melting or evaporation processes. This non-thermal ablation process is capable for solids where the atomic bond strength normal to the surface is significantly different to those in lateral direction. One example for such a solid is given by graphite. In graphite the Carbon atoms of a single graphene layer are bond covalently and thus exhibiting a bond strength of 4 eV per atom. In contrast, the interlayer bond strength normal to the surface results of the more than two orders of magnitude weaker Van-der-Waals-interaction. Without influencing the strong layer internal sp2-bonds this ablation process only breaks the weak van-der-Waals bonds between the layers resulting in a spalation dynamic of the topmost intact graphene layers from the surface of the highly oriented pyrolytic graphite target. Maintaining the internal sp2 crystalline structure of ...

EUV-pump-probe microscopy of FS-Laser induced nano-structure formation

In this paper, we report on our efforts on setting up and building an EUV-pump-probe microscope. The goal is the observation of femtosecond laser induced nanostructure formation with a spatial resolution of less than 100nm and a temporal resolution of less than 1ns. As exemplary structures, we use nanojets on thin gold films and periodic surface structures (ripples) on dielectrics, but in the future, the EUV-pump-probe microscope can become a versatile tool to observe physical or biological processes with high resolution in space and time.In this paper, we report on our efforts on setting up and building an EUV-pump-probe microscope. The goal is the observation of femtosecond laser induced nanostructure formation with a spatial resolution of less than 100nm and a temporal resolution of less than 1ns. As exemplary structures, we use nanojets on thin gold films and periodic surface structures (ripples) on dielectrics, but in the future, the EUV-pump-probe microscope can become a versatile tool to observe physical or biological processes with high resolution in space and time.