## Article

Published Published online: 25 September 2021 , doi: 10.37188/lam.2021.022
The application of femtosecond laser-induced periodic surface texturing has significant potential in medicine, optics, tribology, and biology, among other areas. However, when irradiated by a large intense laser spot, the periodic structures usually exhibit an uncontrollable regularity, forming bifurcated patterns, thus limiting their widespread application. Irregularity originates from numerous independent branching seeds. The usual solution to this problem is to utilize the quasi-direct laser writing technique, that is, by limiting the laser beam size (diameter of <10 wavelengths) and scanning the beam or samples using 2D translation stages. Herein, we demonstrate an optical localization-induced nonlinear competition mechanism to solve this problem, which occurs at a fluence nearly one order of magnitude below the ablation threshold. Owing to the low intrinsic absorption of silicon and ultralow applied fluence, this mechanism ensures the self-selection of a single seed to initiate an array of bifurcated-free gratings under stationary irradiation with a large laser spot (diameter >100 wavelengths). Surprisingly, some unconventional complex patterns, such as radial, annular, and spiral gratings, can also be easily produced by structured light fields with unprecedented regularity. Their diameters reach up to >500 μm. Moreover, we can artificially control the initial seeding structure to further improve the regularity of the gratings, defined by dispersion in the ripple orientation angle in their 2D Fourier transform. As a result, the regularity in our experiments produced by a large laser spot is even higher than that scanned by a tiny beam. Controllable and highly regular ripples are beneficial to the structural coloring effects because they arise from the light diffraction by subwavelength gratings.
Published Published online: 01 August 2021 , doi: 10.37188/lam.2021.019

Electromagnetically induced transparency (EIT) is a coherent optical process that provides a narrow transparent peak within a broad absorption line in an atomic medium. All-dielectric metasurface analogues of EIT have enabled new developments in the nanophotonics field for obtaining smaller, more effective slow-light devices and highly sensitive detectors without a quantum approach. However, the dynamic control of the EIT response of all-dielectric metasurfaces has been rarely reported hitherto for the near-infrared (N-IR) region, although a broader range of applications will be enabled by a reconfigurable EIT system. In this study, we realise a chalcogenide (germanium antimony telluride, GST) metasurface, which possesses a dynamically tunable EIT response by optically driving the amorphous-crystalline phase change in the GST medium. Only a few tens of nanometres thick, the nanostructured GST film exhibits Mie resonances that are spectrally modified via laser-induced phase transitions, offering a high relative modulation contrast of 80% in the N-IR region. Moreover, an extreme dispersion that results in the ‘slow light’ behaviour is observed within this transparency ‘window’. Furthermore, the group delay of the N-IR beam switches reversibly under the phase transition. The measurement is consistent with both numerical simulation results and phenomenological modelling. Our work facilitates the development of new types of compact ultrafast N-IR holograms, filtering, and ultrasensitive detectors.

Published Published online: 01 June 2021 , doi: 10.37188/lam.2021.018

Optical scatterometry is one of the most important metrology techniques for process monitoring in high-volume semiconductor manufacturing. By comparing measured signatures to modelled ones, scatterometry indirectly retrieves the dimensions of nanostructures and, hence, solves an inverse problem. However, the increasing design complexity of modern semiconductor devices makes modelling of the structures ever more difficult and requires a multitude of parameters. Such large parameter spaces typically cause ambiguities in the reconstruction process, thereby complicating the solution of the inherently ill-posed inverse problem further. An effective means of regularisation consists of systematically maximising the information content provided by the optical sensor. With this in mind, we combined the classical techniques of white-light interferometry, Mueller polarimetry, and Fourier scatterometry into one apparatus, allowing for the acquisition of fully angle- and wavelength-resolved Mueller matrices. The large amount of uncorrelated measurement data improve the robustness of the reconstruction in the case of complex multi-parameter problems by increasing the overall sensitivity and reducing cross-correlations. In this study, we discuss the sensor concept and introduce the measurement strategy, calibration routine, and numerical post-processing steps. We verify the practical feasibility of our method by reconstructing the profile parameters of a sub-wavelength silicon line grating. All necessary simulations are based on the rigorous coupled-wave analysis method. Additional measurements performed using a scanning electron microscope and an atomic force microscope confirm the accuracy of the reconstruction results, and hence, the real-world applicability of the proposed sensor concept.

Published Published online: 01 April 2021 , doi: 10.37188/lam.2021.015
Many challenging measurement tasks in production simultaneously have high requirements for accuracy, measurement field size, lateral sampling, and measurement time. In this paper, we provide an overview of the current state of the art in digital holography for surface topography measurements and present three applications from completely different productions with no alternative to digital holography; we describe the HoloTop sensor family, which has been designed specifically for industrial use, and present the most recent results achieved in real-life industrial applications. All applications address measurement tasks that could not be solved until now, either by optical or tactile means. We start with a description of the first-ever inline integration of a digital holographic measurement system that inspects precision turned parts for the automotive industry. We proceed by presenting measurements performed with a compact sensor that can be placed inside a tooling machine and operated fully wirelessly. In this case, the tool holder was used to position the sensor directly. Integration into a tooling machine places high demands on both robustness and reliability. Finally, the quality control of electronic interconnectors such as microbumps with the highest demand for accuracy and repeatability is demonstrated. All of these applications illustrate the major advantages of digital holographic systems: it is possible to measure a relatively large field of view with interferometric precision and very short acquisition times. Additionally, both reflective and matt surfaces can be measured simultaneously. We end this publication with an assessment of the future potential of this technology and the necessary development steps involved.
et al.
Published Published online: 01 April 2021 , doi: 10.37188/lam.2021.016
In this study, the effect of triple-cycling heat treatment on the microstructure, phase, and compression behaviour of directed energy deposited (DED) Ti-7Mo alloy was investigated with a focus on a non-equilibrium to equilibrium microstructure transition. As a result of thermal accumulation, in situ cycling, and rapid solidification, the as-deposited sample presents a continuous gradient microstructure with α-Ti in the top region and α+β in the bottom region. After the triple-cycling heat treatment, the α+β Ti at the bottom region, which is non-equilibrium, changes to a state of equilibrium near α-Ti. Meanwhile, the microstructure becomes more uniform throughout the entire sample. The morphology of the α-Ti phase changes from acicular to a short rode-like shape with increases in the number of dimensions. In terms of the mechanical properties, both the microhardness and compression properties were improved, particularly with respect to the fracture characteristics. The heat-treated sample possesses a much higher ductility than the brittle fractural behaviour. This work provides new insights into the microstructure and property optimisation and homogenisation of DED-processed Ti-based components with cycling heat treatment.
Published Published online: 01 February 2021 , doi: 10.37188/lam.2021.009
Virtual instruments provide task-specific uncertainty evaluation in surface and dimensional metrology. We demonstrate the first virtual coherence scanning interferometer that can accurately predict the results from measurements of surfaces with complex topography using a specific real instrument. The virtual instrument is powered by physical models derived from first principles, including surface-scattering models, three-dimensional imaging theory, and error-generation models. By incorporating the influences of various error sources directly into the interferogram before reconstructing the surface, the virtual instrument works in the same manner as a real instrument. To enhance the fidelity of the virtual measurement, the experimentally determined three-dimensional transfer function of a specific instrument configuration is used to characterise the virtual instrument. Finally, we demonstrate the experimental validation of the virtual instrument, followed by virtual measurements and error predictions for several typical surfaces that are within the validity regime of the physical models.
Published Published online: 01 January 2021 , doi: 10.37188/lam.2021.006
Future quantum technology relies crucially on building quantum networks with high fidelity. To achieve this challenging goal, it is of utmost importance to connect individual quantum systems such that their emitted single photons overlap with the highest possible degree of coherence. This requires perfect mode overlap of the emitted light from different emitters, which necessitates the use of single-mode fibres. Here, we present an advanced manufacturing approach to accomplish this task. We combined 3D printed complex micro-optics, such as hemispherical and Weierstrass solid immersion lenses, as well as total internal reflection solid immersion lenses, on top of individual indium arsenide quantum dots with 3D printed optics on single-mode fibres and compared their key features. We observed a systematic increase in the collection efficiency under variations of the lens geometry from roughly 2 for hemispheric solid immersion lenses up to a maximum of greater than 9 for the total internal reflection geometry. Furthermore, the temperature-induced stress was estimated for these particular lens dimensions and results to be approximately 5 meV. Interestingly, the use of solid immersion lenses further increased the localisation accuracy of the emitters to less than 1 nm when acquiring micro-photoluminescence maps. Furthermore, we show that the single-photon character of the source is preserved after device fabrication, reaching a \begin{document}$g^{(2)} (0)$\end{document} value of approximately 0.19 under pulsed optical excitation. The printed lens device can be further joined with an optical fibre and permanently fixed.This integrated system can be cooled by dipping into liquid helium using a Stirling cryocooler or by a closed-cycle helium cryostat without the necessity for optical windows, as all access is through the integrated single-mode fibre. We identify the ideal optical designs and present experiments that demonstrate excellent high-rate single-photon emission.