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Accurate measurement of light wavelength is critical for applications in spectroscopy, optical communication, and semiconductor manufacturing, ensuring precision and consistency of sensing, high-speed data transmission and device production. Emerging reconstructive wavemeters synergize physical systems capable for pseudo-random wavelength dependent pattern formation with computational techniques to offer a promising alternative against established methods such as frequency beating and inteferometry for high-resolution and broadband measurements in compact and cost-effective devices. In this paper, we propose a novel type of compact and affordable reconstructive wavemeter based on the disordered chip with thousands of high quality-factor whispering gallery mode microcavities as physical model and a hybrid machine learning approach utilizing boosting methods and variational autoencoders implemented as wavelength interpreter. We leverage eccentricity mode splitting obtained via controllable deformation of the spherical microresonators in order to ensure the uniqueness of the wavelength patterns up to ultra-wide (~100 nm) spectral window while guaranteeing high (~100 fm) intrinsic sensitivity. The latter allocates the proposed model right next to the ultimate reconstructive wavemeters based on integrating spheres, but with superior miniaturization options and chip-scale integrability.

In water monitoring, environmental analysis, cell culture stability, and biomedical applications, precise pH control is demanded. Traditional methods like pH strips and meters have limitations: pH strips lack precision, while electrochemical meters, though more accurate, are fragile, prone to drift, and unsuitable for small volumes. In this paper, we propose a method for optical detection of the pH value based on the multiplexed sensor with 4D microcavities fabricated with two-photon polymerization. This approach employs pH-triggered reversible variations in microresonator geometry and integrates hundreds of dual optically coupled 4D microcavities to achieve the detection limit of 0.003 pH units. The proposed solution is a clear instance for the use-case oriented application of the two-photon polymerized structures of high optical quality. With benefits of the multiplexed imaging platform, dual 4D microresonators can be integrated alongside other microresonator types for pH-corrected biochemical studies.

The per- and polyfluoroalkyl substances (PFAS) constitute a group of organofluorine chemicals treated as the emerging pollutants and currently are of particularly acute concern. These compounds have been employed intensively as surfactants over multiple decades and are already to be found in surface and ground waters at amounts sufficient to have an effect on the human health and ecosystems. Because of the carbon-fluorine bonds the PFAS have an extreme environmental persistence and their negative impact accumulates with further production and penetration into the environment. In Germany alone, more than thousands sites have been identified to be contaminated with PFAS and thus timely detection of PFAS residues is becoming a high-priority task. In this paper we report on the high performance optical detection method based on whispering gallery modes microcavities applied for the first time for detection of the PFAS contaminants in aqueous solutions. A self-sensing boosted 4D microcavity fabricated with two-photon polymerization is employed as an individual sensing unit. On example of the multiplexed imaging sensor with multiple hundreds of simultaneously interrogated microcavities we demonstrate the possibility to detect the PFAS chemicals representatives at the level of down to 1 ppb.

Thin films of molybendum disulfide grown via thermal atomic layer deposition at low temperatures, suitable for temperature sensible substrates, can be amorphous. To avoid a high temperature post treatment of the whole sample, which can cause thermal degradation of the substrate or other layers, a ultrashort pulse (usp) laser-induced transformation to crystalline layers is one of the most promising routes. In this paper we report the crystallization of amorphous MoS$_2$ layers processed with ultrashort laser pulses. The amorphous MoS$_2$ films were deposited by atomic layer deposition (ALD) and exposed to picosecond laser pulses ($\lambda = 532$ nm). The crystallization and the influence of the processing parameters on the film morphology were analyzed in detail by Raman spectroscopy and scanning electron microscopy. Furthermore, a transition of amorphous MoS$_2$ by laser annealing to self-organized patterns is demonstrated and a possible process mechanism for the ultrashort pulse laser annealing is discussed. Finally, the usp laser annealed films were compared to thermally and continuous wave (cw) laser annealed samples.

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 $\mu$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.

Laser Shock Peening (LSP) of stainless-steel 316 and 316L was studied using Ti: Sapphire laser pulses of about 2 ps duration, maximum energy of about 1mJ and pulse repetition rate of 5kHz in different liquid confinement media of Ethanol, Deionized water and separate aqueous solutions of NaCl and Glycerol. It is found that the laser fluence and/or energy attenuating mechanisms like self-focusing, filamentation, plasma breakdown in the confinement media are less significant with ps laser pulses than those with sub-ps or fs pulse durations. It is shown that the resulting surface hardness of the peened steel as a function of laser fluence depends significantly on the confinement media and the relative increase in the hardness increases monotonically with the acoustic impedance of the liquid of the confinement medium used during LSP.

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 $S_{sub}~=~1.5~\mbox{V/dec}$.

We report on the experimental observation of selective delamination of semi-transparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a by-side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on its both surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semi-transparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials.

In this paper we demonstrate experimentally that crystalline phases appear in amorphous titanium oxide upon processing with ultrafast laser pulses. Amorphous titanium thin films were produced by plasma-enhanced chemical vapor deposition (PECVD) and exposed to femtosecond laser pulses. Formation of rutile phase was confirmed by X-ray diffraction, Raman measurements and electron backscattering diffraction. A rang of processing parameters for the crystallization is reported and possible background mechanisms are discussed.

This paper presents a novel one-step method for the periodical nanopatterning and reduction of graphene oxide (GO). Self-organized periodic structures of reduced graphene oxide (rGO) appear on GO surfaces upon processing with a femtosecond laser at fluences slightly higher than the fluence needed for reduction of the GO. This indicates that the periodic pattern is formed either simultaneously with or due to the reduction of the GO. The laser-induced reduction of GO was identified by sheet resistance measurements, Raman and X-ray photoelectron spectroscopy. This fast and simple method to both reduce and periodically structure GO offers a variety of possible applications in printed and flexible electronics.

In this paper we report on the competition in metal surface hardening between the femtosecond shock peening on the one hand, and formation of laser-induced periodic surface structures (LIPSS) and surface oxidation on the other hand. Peening of the stainless steel AISI 316 due to shock loading induced by femtosecond laser ablation was successfully demonstrated. However, for some range of processing parameters, surface erosion due to LIPSS and oxidation seem to dominate over the peening effect. Strategies to increase the peening efficiency are discussed.

The physical mechanisms of the laser-induced periodic surface structures (LIPSS) formation are studied in this paper for single-pulse irradiation regimes. The change in the LIPSS period with wavelength of incident laser radiation is investigated experimentally, using a picosecond laser system, which provides 7-ps pulses in near-IR, visible, and UV spectral ranges. The experimental results are compared with predictions made under the assumption that the surface-scattered waves are involved in the LIPSS formation. Considerable disagreement suggests that hydrodynamic mechanisms can be responsible for the observed pattern periodicity.

We theoretically and experimentally investigate the flow field that emerges from a rod-like microrotor rotating about its center in a non-axisymmetric manner. A simple theoretical model is proposed that uses a superposition of two rotlets as a fundamental solution to the Stokes equation. The predictions of this model are compared to measurements of the azimuthal and radial microfluidic velocity field components that are induced by a rotor composed of fused microscopic spheres. The rotor is driven magnetically and the fluid flow is measured with the help of a probe particle fixed by an optical tweezer. We find considerable deviations of the mere azimuthal flow pattern induced by a single rotating sphere as it has been reported by Di Leonardo \textit{et al.} [Phys. Rev. Lett. 96, 134502 (2006)]. Notably, the presence of a radial velocity component that manifests itself by an oscillation of the probe particle with twice the rotor frequency is observed. These findings open up a way to discuss possible radial transport in microfluidic devices., Comment: 7 pages, 7 figures