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Applicability of the whispering gallery mode sensor in the form of the multiresonator imaging device for the atmospheric-relevant parameters monitoring has been demonstrated. The sensor design and the appropriate techniques for sensing data collection have been discussed. The sensor performance has been numerically and experimentally studied on examples of the temperature variation detection and the monitoring of small spherical microparticles ( $ ) that have been used to mimic the particulate contamination of the environment. The thermal sensitivity of the sensor on the level of 8.6 pm/°C has been experimentally demonstrated. The experimental results on the distinguishing of the microparticle concentrations (0.5% and 1%) via the wavelength sweeping technique and the results on the single particle detection under illumination at the fixed wavelength have been shown.

The topic of 4D printing, that is based on 3D printing of structures capable to response on external disturbances over the time, has evolved enormous interest in science and engineering since its first statement in 2013. Besides multi-material printing originally intended to provide self-assembly function, the interest in use of smart materials capable for self-sensing and/or self-actuating rises. Here the necessity in precise 3D fabrication techniques and solutions for structural response control are current challenges that 4D printing faces. In this paper we report on design, fabrication and characterization of a first example of 4D optical microcavity that shows advanced sensing performance boosted by self-sensing in the form of interaction of the polymer matrix with sensed molecules in aqueous environment. Distinctive features of 4D microcavity spectral response for bulk refractive index variations and sensitivity enhancement for more than one order of magnitude have been experimentally observed and discussed. Possibility for parallel interrogation with a single coupling and detection unit for a set of 4D microresonators fabricated with two-photon polymerization out of quencher-doped SZ2080 material with quality factor of $4\times 10^4$ at 685 nm without any post-processing steps has been experimentally demonstrated. The proposed 4D microcavity-based sensor opens a new avenue for designing of a multiplexed 4D printed optical sensing platform with advanced performance compared to conventional optical detection techniques for broad variety of detection tasks.

For most additive manufacturing technologies, complex components require a support structure for thermal dissipation and printability. This paper shows for the laser metal deposition process, how weld spot pillars can be used as a support structure for surfaces with large overhangs produced on a three-axis system. A self-written software is used for generating machine and motion sequences, where layered construction of these structures is not mandatory. In the first step, parameters for generating different build strategies of single pillars were found. In a second step, different concepts of combining pillars are presented and compared.

Directed Energy Deposition (DED) an AM-technology that is widely known for its applications with metals was adapted for use with thermoplastics. A drop-in approach was used based on a DED setup for metal powders consisting of a Yb fiber laser with a wavelength of 1075 nm and a powder feed from a coaxial powder nozzle. The possibility of manufacturing items with simple geometries from a polyamide 12 multi-walled carbon nanotube composite starting material is described as a proof of concept. Tensile properties of samples are reported for measurements perpendicular and parallel to the generating direction, showing still a low strength compared to Bulk PA12 or SLS sintered PA12 specimens.

Laser shock peening with a femtosecond laser system was presented in this research work. The NiTi shape memory alloy was processed by the femtosecond laser shock peening (FsLSP) treatment without a protective layer in the air. Femtosecond laser shock peening is a new surface technology, which can induce an intense shock wave with low single laser pulse energy under atmospheric conditions. The surface topography, roughness, microhardness, and wear resistance were measured on the surface of NiTi alloy before and after femtosecond laser peening treatment. The results showed that the surface roughness and microhardness could be increased after femtosecond laser shock peening, which may be due to the laser ablation and micro-plastic deformation induced by the shock wave. The wear property of NiTi alloy was improved, which may be attributed to the FsLSPed surface texturing and enhancement of surface microhardness.

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

This paper discusses a digital holographic microscopy approach for comprehensive and non-destructive inspection of spatially confined structures, which are characterized by the high aspect ratio of their lateral dimensions ( ∼ 1:30) and are fabricated out of transparent materials. Two-photon polymerization is chosen as a high-resolution lithographic technology for manufacturing of corresponding structures out of positive photoresin. The proposed holographic inspection method does not require any numerical methods for aberration correction and thus demonstrates rapid phase map evaluation ( ∼ 250 ms per cycle). Applicability and performance of the method has been successfully tested on a set of samples modelled to resemble step-index structures similar to the lab-on-chip basic elements, e.g. waveguides. In terms of lateral resolution a diffraction-limited operation of the proposed imaging system is ensured while vertical accuracy of 1.61 nm has been demonstrated.

A biochemical arrayed sensor based on optical resonance technology and characterized by the dispersed positioning and properties of individual sensing units is discussed. Fabrication simplicity and reusability of the sensor as well as robustness of signal excitation and possibility for parallel probing are among the advantageous features for the implementation of optical resonance sensing for practical-oriented tasks. The arrayed sensor applicability, performance, and correspondence between the responses and analytical models are analyzed on the example of biochemical components’ monitoring. The efficient regeneration of sensing properties realized by cleaning of the reacting surface allows to perform multiple sensing tasks in a single experimental run. The sensor interrogation approach based on the dispersion of the sensing units’ spectral characteristics is discussed.

The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappears, the mobility is enhanced by an order of magnitude and the subthreshold swing is reduced from $S_{sub}~=~2.5~\mbox{V/dec}$ to $S_{sub}~=~1.5~\mbox{V/dec}$.

The ablation process of femtosecond laser pulses of iron in air and different liquids was investigated for fluences of 0.5 J / cm 2 and 2 J / cm 2 by means of femtosecond pump-probe microscopy. Measurements of the relative change in reflectivity suggest that the surrounding liquid has a significant impact on the ablation process. During the heating phase of the metal within the first picoseconds after laser beam impact, the change in reflectivity in air and liquids is similar. Afterwards, the vapor and melt expulsion in air leads to a strong decrease in reflectivity, while the change in reflectivity in the liquids shows a more complex fluence and time-dependent behavior. This behavior is suggested to be triggered by the expansion of the molten surface and chemical reactions on the picosecond timescale.

NiTi shape memory alloy was processed by femtosecond laser shock peening (FLSP) without protective coating in distilled water to modify its surface characterizations. The surface topography, microhardness, microstructure and scratch testing were studied before and after FLSP treatment. The experimental results show that FLSP with different laser scanning speeds and passes can change the surface roughness and microhardness due to laser ablation and high pressure of shock wave. The average microhardness value of the specimens FLSPed in distilled water increased. Grain refinement was observed in the surface layer of FLSPed NiTi alloy. The scratch testing revealed that FLSP process can decrease the frictional force and coefficient of friction, and it also demonstrated that the FLSP technique is beneficial to enhance the surface wear property of NiTi alloy.

The recent advances of the artificial intelligence (AI) technologies are pushing forward the development of the novel concepts for sensing data interpretation wherein the knowledge on the exact physical models for transformation of the collected raw signal into the desired parameters becomes optional. Compared to other types of sensors, the optical ones have demonstrated a rush progress as well as a great amount of innovations over the last years. Despite the main emphasis here on the utilization of the camera-based solutions [1] , the emerging AI-inspired solutions cover various optical sensing platforms, where the flexibility of the machine learning methods is combined with the highly sensitive mechanisms of the optical field interaction with the ambient environment.

This paper discusses an intelligent sensing solution based on the phenomenon of the whispering gallery modes in the optical microcavities realized within an affordable instrument configuration featuring simultaneous excitation of multiple microresonators by a single frequency laser along with parallel collection of their signal. Supplemented with a machine learning engine for complex signal interpretation the sensor demonstrates the accuracy of 10-6 for refractive index prediction and more than 98% for protein concentration classification.

Laser shock peening is a new and important surface treatment technique that can enhance the mechanical properties of metal materials. Normally, the nanosecond laser with pulse-width between 5 ns and 20 ns is used to induce a high-pressure shock wave that can generate plastic deformation in the top layer of metals. The femtosecond laser shock peening in the air has been studied recently, which can induce higher pressure shock wave than that of traditional nanosecond laser shock peening in a very short time. The NiTi alloy is processed by femtosecond laser shock peening, then a nanoindentation device is used to measure its surface hardness and residual stress. The hardness results of NiTi alloy before and after treatment show that the femtosecond laser shock peening can increase the hardness of NiTi alloy, which also shows that the femtosecond laser can be used to perform laser shock peening on NiTi alloy without coating.

Many solutions exist for optical simulations. Many of them are isolated solutions, are complex to use due to their wide range of applications, or are difficult to adapt to new problems. Therefore, there is a need for easy-to-use, flexibly adaptable program packages. For this reason, we present here a programming library for the simulation of optical problems, which is based on geometrical optics due to its high flexibility. Special attention was paid to a flexible adaptability to different problems and an easy usability. The program package is freely available as an open-source project implemented in C++ and can be downloaded from the GitHub platform (Weigel, GitHub, 2021).

The resonance properties of a layer on the surface of a spherical microresonator are investigated using geometrical optics. The resonance condition for these shell-guided modes is formulated. Shell-guided modes are located in the layer and are bounded by the inner and outer surfaces of the layer. It is shown that the resonance condition depends---similarly to the case of a homogeneous resonator---on the difference of the phase propagation of a ray crossing the layer and the corresponding phase propagation along the associated caustic. It is shown that the layer can support resonances only in a certain thickness range that depends, among other factors, on the resonance order and the wave number. A straightforward explanation is given for these limitations. Comparison with solutions from wave theory shows excellent agreement. A simple analytical solution is presented to calculate the sensitivity of the resonator to the refractive index and the thickness of the layer as well as to the refractive index of the environment.

High strength steel has been used in the aviation industry and automotive body structural applications to reduce its mass through a reduction in thickness. Therefore, it is very important to enhance its mechanical property, such as microhardness. In the present research, the high strength steel samples were treated by laser shock peening (LSP) with different laser pulse energy and laser pulse width. The microhardness and residual stress were measured to compare the difference between laser energy of 3 J with 10 ns and 5 J with 20 ns. The results in the study show that the surface LSP treatment can increase the microhardness and the compressive residual stress can be found when the samples were tested by hole drilling testing.

Es wird eine nichtinvasive optische Messtechnik für die Lebensmittelanalytik vorgestellt, die eine zuverlässige Bestimmung des Absolutdrucks in Getränkeflaschen mit karbonisiertem Inhalt ermöglicht. Die Messtechnik nutzt eine durchstimmbare Laserdiode mit einer Emissionswellenlänge um 2004 nm für die Aufzeichnung von drei bis vier Absorptionslinien von CO\(_2\) und wertet die zum Absolutdruck proportionale Druckverbreiterung der Linien aus. Mit der entwickelten Messmethode konnte bei Feldmessungen an versiegelten Softdrinkflaschen aus PET schließlich eine Standardabweichung wiederholter Absolutdruckmessungen bis 5,5 bar von unter 50 mbar erreicht werden. A non-invasive optical measurement technique for food analysis is presented, which allows for a reliable determination of the absolute pressure in beverage bottles with carbonated contents. The method uses a tunable laser diode with an emission wavelength around 2004 nm to record three to four absorption lines of CO\(_2\) and evaluates the pressure broadening of the lines proportional to the absolute pressure. With the developed measuring method, a standard deviation of repeated absolute pressure measurements of up to 5.5 bar of less than 50 mbar could finally be achieved in field measurements on sealed soft drink bottles made of PET.

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

This paper describes how two-photon polymerization was used to generate biomimetic nanostructures with angle-insensitive coloration inspired by the blue butterflies of Morpho. Less angle dependence was achieved by engineering the structures with a certain degree of disorder, which delimited them from classical photonic crystals. Variations in the processing parameters enabled the color hue to be controlled. In this context, blue, green, yellow, and brown structures were demonstrated. Reflection spectra of the structures were simulated and studied experimentally in a broad range of incident angles. Additionally, a molding technique was performed as a potential scale-up strategy. The application of such biomimetic structures is discussed.

A possible application of the dispersed optical microresonator based sensor in aerospace-related tasks is discussed. Configuration of the proposed sensor as a set of spherical glass microparticles together with its working principle is discussed. Sensing capabilities have been verified on examples of detection of temperature variations together with monitoring of the small microparticles adsorption. Thermal sensitivity is determined on the level of 8.6 pm/°C. The possibility for detection of the adsorption of polesterene microparticles (0.9 and 3.27 µ m size) in frequency sweeping and spectrally unresolved illumination schemes have been shown.

In the present study, laser metal deposition (LMD) was used to produce compositionally graded refractory high-entropy alloys (HEAs) for screening purposes by in-situ alloying of elemental powder blends. A compositional gradient from Ti25Zr50Nb0Ta25 to Ti25Zr0Nb50Ta25 is obtained by incrementally substituting Zr powder with Nb powder. A suitable strategy was developed to process the powder blend despite several challenges such as the high melting points of the refractory elements and the large differences in melting points among them. The influence of the LMD process on the final chemical composition was analyzed in detail and the LMD process was optimized to obtain a well-defined compositional gradient. Microstructures, textures, chemical compositions and mechanical properties were characterized using SEM, EBSD, EDX, and microhardness testing, respectively. Compositions between Ti25Zr0Nb50Ta25 and Ti25Zr25Nb25Ta25 were found to be single-phase bcc solid solutions with a coarse grain microstructure. Increasing the Zr to Nb ratio beyond the equiatomic composition results in finer and harder multiphase microstructures. The results shown in the present study clearly show for the first time that LMD is a suitable processing tool to screen HEAs over a range of chemical compositions.

In this study, we use a hybrid mode-locked external cavity diode laser with subsequent amplification and pulse compression. The system provides laser pulses of 440 fs width (assuming a sech² pulse shape) and 160 mW average output power at a repetition rate of 383.1 MHz. The laser oscillator consists of a double quantum well laser diode with a gain segment of 1080 μm length and an absorber element of 80 μm lengths. The chip’s back facet is covered with a high reflective coating, the front facet with an anti-reflective coating. The resonator itself is operated in a collimated geometry and folded by two dielectric mirrors. The used output coupler provides a transmission of 20 percent, which is coupled into a tapered amplifier. Two Faraday isolators are used to decouple the laser and the amplifier from any back reflections. Subsequently, the pulses are compressed using a single pass Martinez type pulse compressor. Experiments on Two-Photon Polymerization were conducted on a conventional setup consisting of a 2D galvo-scanner system with an attached microscope objective. The oil immersion objective (NA =1.4) focusses the light pulses through a cover glass into a droplet of the photosensitive material. Process monitoring can be achieved by observing the image on a camera placed behind a semi-transparent mirror in front of the galvo-scanner. Using this experimental setup, test structures that consist of free-hanging lines supported by cuboids were produced. In addition, a procedure for automated linewidth measurements is outlined and used for analyzation of the generated structures. This work shows that mode-locked diode lasers can be used for the fabrication of microstructures by Two-Photon Polymerization. Typically used Ti:Sapphire or fiber lasers can be replaced by mode-locked diode lasers for Two-Photon- Polymerization. This allows for much cheaper Two-Photon-Polymerization systems and therefore, may open this field for more application-based research groups.

A design and application of distributed microresonator-based systems for biochemical sensing are discussed. Two different configurations of the distributed sensors based on integration of the spherical glass microresonators and fabrication of the toroidal microresonators via two-photon polymerization have been demonstrated. The performance of both configurations for biochemical sensing has been analysed.

In this Letter, the authors present the construction of three-dimensional microstructures by two-photon polymerisation induced by ultrashort pulses of a mode-locked diode laser. The ultrafast light source is based on a diode laser with segmented metallisation to realise a waveguide integrated saturable absorber. It is subsequently amplified and compressed resulting in ultrashort laser pulses of 440 fs length and average output power of 160 mW at a fundamental repetition rate of 383.1 MHz. These pulses are coupled into a customised two-photon polymerisation setup. A series of suspended lines were fabricated between support cuboids for testing the process behaviour. A 3D structure with complex features was polymerised to demonstrate the high potential for mode-locked diode lasers in the field of direct laser writing.

The influence of a deformable mirror on spatial light modulation in ultrafast lasers processing is demonstrated. The deformable mirror was integrated into an optical setup which contains an additional lens for generating a nearly linear focus shift in the focal plane behind the f-theta lens. The deformation of the mirror surface can be described by the Zernike terms Defocus, Astigmatism, and a combination of both, resulting in a cylindric lens behavior. The influence of the mirror surface deformation in this optical setup on the intensity distribution in the focal plane was simulated. From the simulation results, the caustic in the focal plane was calculated. The simulation results were compared to experiments using a picosecond laser with a maximum pulse energy of about 60 $��$J. We demonstrate that the initial astigmatism of the raw beam can be reduced using the deformable mirror. High linearity in the focus shift ($R^2 >98$%) and the generation of elliptical/ line intensity distributions are shown. Line intensity distribution was used to demonstrate slit drilling application in thin metal foils.

We study the effect of laser shock peening (LSP) without protective coating on the surface mechanical property of NiTi alloy. The Vickers microhardness and wear resistance are measured to determine the mechanical property of NiTi samples treated with different LSP parameters (3 J with 10 ns and 5 J with 20 ns). From the electron backscatter diffraction (EBSD) analysis, it can be found that the laser shock peening does not induce obvious grain refinement in the surface region of NiTi alloy. Both compressive and tensile residual stress in the top layer are determined using the hole drilling method. The results show that the LSP treatment without a protective coating increases the roughness and enhances the surface mechanical properties of NiTi alloy.

Refractory elements have high melting points and are difficult to melt and cast. In this study it is successfully demonstrated for the first time that laser metal deposition can be used to produce TiZrNbHfTa high-entropy alloy from a blend of elemental powders by in-situ alloying. Columnar specimens with a height of 10 mm and a diameter of 3 mm were deposited with a pulsed Nd:YAG laser. The built-up specimen has near-equiatomic composition, nearly uniform grain size, equiaxed grain shape, is bcc single phase and exhibits a high hardness of 509 HV0.2.

In this paper we present the results of a study on the impact of a thin reflective film between the substrate and photoresin on the two-photon polymerization procedure. We have proposed a model for the elementary polymerization volume (voxel) formation for the introduced case and carried out simulations to examine the influence of the refractive indexes relation, layer thickness, roughness, and polymerization depth on the polymerization performance. The experiments on fabrication of 2D and 2.5D structures have shown the benefit of the proposed configuration for the substrate/photoresin interface localization as well as for the distortion-free fabrication.

Laser shock peening is a surface treatment technology inducing high-pressure shock waves on metallic materials and structures. In this study, the life cycle performance improvement in high-strength steel by nanosecond laser shock peening is investigated. It is found that microstructures formed by laser shock peening lead to higher microhardness, corrosion resistance, and fatigue life, which are significantly beneficial for preventing life cycle failure of mega-scale engineering structures in critical environments. The residual stress is also measured on the surface of samples, which shows that the compressive residual stress can be found in the treated area.

Independent of the geometrical complexity of a laser metal deposition component, the fundamental geometrical element is given by basic weld seams most of the times. These weld seams are deposited on a surface area and therefore not suitable to generate lattice structures. In this study, it is successfully demonstrated that basic spot welds can be used to achieve a vertical columnar built-up of pillars. Tilting of these pillars was used to merge them or to form forks. In that way, laser metal deposition of a lattice structure was implemented.