3D printed objects was first presented by Maruo in a technology-opening article in 1997, [2] where the use of this technology to fabricate micro-optics was also envisaged. The serial writing method of this technique offered flexibility for freeform fabrication, but was limited by its throughput. [3] It took some years for the technology to mature from proof-of-principle level to additive manufacturing as a tool for efficient and reliable fabrication in the modern lab. [4][5][6] Though the first micro-optical elements were demonstrated as early as 2006, [7] the major efforts and results only started to emerge in 2010, [8,9] together with the development of hybrid organic-inorganic materials, [10,11] and rapidly accelerated with the implementation of commercial 3D lithography systems. [12][13][14] By 2020, ultrafast laser 3D printing, also known as two-photon polymerization (TPP or 2PP), multiphoton lithography (MPL), [15][16][17] or simply laser direct writing (LDW), also in literature referenced as direct laser writing (DLW), [18] ) was already an established technique for routine fabrication of diverse micro-optical single elements, stacked components, and integrated devices. [19][20][21] The latest advances in the 3D printing of free-form micro-optics are enhanced by optical grade materials of high refractive index (n) polymers, [22] high-performance hybrids, [23] and optically active [24] or pure inorganic glasses. [25] Figure 1 shows the development of the technique in terms of published papers and citations, defining an "innovator stage" of the technology followed from 2015 by the "early adopters stage." Examples of micro-optical elements fabricated using MPL and the growth in the complexity of the structures that can be achieved by this technique are also shown in Figure 1.The advances in this scientific field attracted the attention of the related laser-assisted precision additive manufacturing industry. First, in 2007 Nanoscribe GmbH and in 2008 Workshop of Photonics established companies oriented toward commercialization of this technology aimed at general wide angle application fields. While in 2013, Multiphoton Optics GmbH and Femtika UAB were established and made micro-optics a significant part of their targeted applications. Finally, in 2017, Vanguard Photonics GmbH manufactured dedicated MPL equipment for micro-lenses and wire bond production. Other companies targeting more diverse applications have continued to emerge, such as UpNano established in 2018, and focusing mostly on biomedical applications yet also offering solutionsThe field of 3D micro-optics is rapidly expanding, and essential advances in femtosecond laser direct-write 3D multi-photon lithography (MPL, also known as two-photon or multi-photon polymerization) are being made. Micro-optics realized via MPL emerged a decade ago and the field has exploded during the last five years. Impressive findings have revealed its potential for beam shaping, advanced imaging, optical sensing, integrated photonic circuits, and much more. This is suppo...

Optical waveguide segments based on geometrically unbound photonic crystal fiber (PCF) designs could be exploited as building blocks to realize miniaturized complex devices that implement advanced photonic operations. Here, we show how to fabricate optical waveguide segments with PCF designs by direct high-resolution 3D printing and how the combination of these segments can realize complex photonic devices. We demonstrate the unprecedented precision and flexibility of our method by fabricating the first-ever fiber polarizing beam splitter based on PCFs. The device was directly printed in one step on the end-face of a standard single-mode fiber and was 210 µm long, offering broadband operation in the optical telecommunications C-band. Our approach harnesses the potential of high-resolution 3D printing and of PCF designs paving the way for the development of novel miniaturized complex photonic systems, which will positively impact and advance optical telecommunications, sensor technology, and biomedical devices.

Biocompatible functional materials play a significant role in drug delivery, tissue engineering and single cell analysis. We utilized 3D printing to produce high aspect ratio polymer resist microneedles on a silicon substrate and functionalized them by iron coating. Two-photon polymerization lithography has been used for printing cylindrical, pyramidal, and conical needles from a drop cast IP-DIP resist. Experiments with cells were conducted with cylindrical microneedles with 630  15 nm in diameter with an aspect ratio of 1:10 and pitch of 12 m. The needles have been arranged in square shaped arrays with various dimensions. The iron coating of the needles was 120  15 nm thick and has isotropic magnetic behavior. The chemical composition and oxidation state were determined using energy electron loss spectroscopy, revealing a mixture of iron and Fe3O4 clusters. A biocompatibility assessment was performed through fluorescence microscopy using calcein/EthD-1 live/dead assay. The results show a very high biocompatibility of the iron coated needle arrays. This study provides a strategy to obtain electromagnetically functional microneedles that benefit from the flexibility in terms of geometry and shape of 3D printing. Potential applications are in areas like tissue engineering, single cell analysis or drug delivery.

Imaging neuronal activity with high and homogeneous spatial resolution across the field-of-view (FOV) and limited invasiveness in deep brain regions is fundamental for the progress of neuroscience, yet is a major technical challenge. We achieved this goal by correcting optical aberrations in gradient index lens-based ultrathin (< 500 μm) microendoscopes using aspheric microlenses generated through 3D-microprinting. Corrected microendoscopes had extended FOV (eFOV) with homogeneous spatial resolution for two-photon fluorescence imaging and required no modification of the optical set-up. Synthetic calcium imaging data showed that, compared to uncorrected endoscopes, eFOV-microendoscopes led to improved signal-to-noise ratio and more precise evaluation of correlated neuronal activity. We experimentally validated these predictions in awake head-fixed mice. Moreover, using eFOV-microendoscopes we demonstrated cell-specific encoding of behavioral state-dependent information in distributed functional subnetworks in a primary somatosensory thalamic nucleus. eFOV-microendoscopes are, therefore, small-crosssection ready-to-use tools for deep two-photon functional imaging with unprecedentedly high and homogeneous spatial resolution.

Stimulated Raman scattering (SRS) microscopy is a label‐free method generating images based on chemical contrast within samples, and has already shown its great potential for high‐sensitivity and fast imaging of biological specimens. The capability of SRS to collect molecular vibrational signatures in bio‐samples, coupled with the availability of powerful statistical analysis methods, allows quantitative chemical imaging of live cells with sub‐cellular resolution. This application has substantially driven the development of new SRS microscopy platforms. Indeed, in recent years, there has been a constant effort on devising configurations able to rapidly collect Raman spectra from samples over a wide vibrational spectral range, as needed for quantitative analysis by using chemometric methods. In this paper, an SRS microscope which exploits spectral shaping by a narrowband and rapidly tunable acousto‐optical tunable filter (AOTF) is presented. This microscope enables spectral scanning from the Raman fingerprint region to the Carbon‐Hydrogen (CH)‐stretch region without any modification of the optical setup. Moreover, it features also a high enough spectral resolution to allow resolving Raman peaks in the crowded fingerprint region. Finally, application of the developed SRS microscope to broadband hyperspectral imaging of biological samples over a large spectral range from 800 to 3600 cm−1, is demonstrated.