Using two-step absorption instead of two-photon absorption in 3D laser nanoprinting reduces demands on necessary laser systems but still poses a challenge for chemistry. Photoresists consisting of a photoinitiator and a viscous monomer are hardened through photopolymerization with one or two different laser sources. Until now, the influence of the photoinitiator type on printing behavior has remained unclear, limiting the rational design of new materials. In this study, we demonstrate a direct connection between molecular design and printing performance, providing new insights that guide the targeted development of novel photoinitiators. One essential parameter in designing new photoinitiators is the laser power required to initiate photopolymerization, which varies with molecular size, regioisomeric substitution, and substituent type. Screenings in this and a previous study about two-step absorption show that, thus far, the only applicable molecules for this specific 3D printing technique are 1,2-diketones, primarily benzil derivatives. These compounds are mainly synthesized using a Sonogashira cross-coupling reaction followed by an oxidation of the resulting triple bond. The photoinitiators introduced in this work can be referred to as two-step, one-color systems, allowing 3D structures to be printed using only a single 405 nm laser source.

Two‐step‐absorption 3D laser nanoprinting allows for using compact and low‐cost continuous‐wave (cw) lasers. Therefore, it is an attractive alternative to established state‐of‐the‐art multi‐photon‐absorption 3D laser nanoprinting. Additionally, low single‐focus polymerization‐threshold laser powers pave the way for multi‐focus parallelization approaches while still using compact cw laser sources. Parallelization is further encouraged by the fact that single‐focus scanning velocities are inherently limited by the intermediate‐state lifetime of the photoresist system used. Herein, a 3D nanoprinting setup that combines an established two‐step‐absorption photoresist system with a dynamic multi‐focus approach based on binary holography and a digital micromirror device (DMD) is presented. The generated holographic patterns allow for combined beam‐splitting and beam‐steering as well as for the control of each focus’ individual intensity. Therefore, one obtains a versatile printing approach, enabling a dynamic change of the number of foci for each pattern, based on the structure to be printed. With this setup, the printing of 3D microstructures using up to 31 laser foci in parallel at a print rate of up to is achieved. Additionally, the capabilities and limits of the chosen approach are compared with rate‐equation calculations.

State-of-the-art 3D two-photon laser printing systems already use pre-compensation algorithms to reduce systematic deviations between the printed and the targeted structures. Nevertheless, the remaining deviations are often still larger than the uncontrollable or "statistical" deviations. In principle, it is straightforward to correct for systematic deviations by measuring the difference between printed structure and target and by subtracting the difference from the first target to obtain the next-iteration target. However, in reality, one faces several issues such as noise and systematic errors of the characterization measurement itself, as well as unwanted translations and rotations between the coordinate systems of the characterization setup and the printer, respectively. Two examples of printed structures requiring sub-micrometer accuracy are considered, a large 1D micro-lens array and a specific diffractive optical element. For both, the device performance before the pre-compensation workflow described herein is insufficient for the targeted application and has become sufficient after this workflow. The workflow involving optimizations using cross-correlations with confocal-optical-microscopy data is documented by an open-access program (available via GitLab). This program includes an easy-to-use graphical user interface so that other researchers can immediately profit from it. In multi-photon 3D-laser-nano printing, several systematic factors like shrinkage or stitching lead to deviations between the printing result and the required designed structure. Fortunately, these errors can be corrected by modifying the design before printing. A method is presented to measure the systematic deviations using confocal-optical microscopy and provide a flexible user-friendly program to calculate a pre-compensated design from them.image

Most light-based 3D printing methods rely on optical or chemical nonlinearities to spatially confine the polymerization reaction. In 3D micro-and nanoprinting, this nonlinearity can be provided by two-photon absorption, which describes the simultaneous absorption of two photons. To achieve comparable absorption cross sections for two-photon as in one-photon absorption, short and intense laser pulses with intensities in the range of 1 TW/cm2 are typically required. Herein, we review three emerging excitation processes that provide a quadratic nonlinearity versus intensity without relying on two-photon absorption: upconversion luminescence, two-step absorption, and triplet-triplet annihilation. We term these "(1 + 1)-photon absorption". Such processes allow for using continuous-wave lasers at much lower peak laser powers and at much lower cost than those typical for two-photon absorption. We review recent progress, describe current challenges, and outline future perspectives.

Recent studies have opened the door to a new generation of photoinitiators for 3D laser nanoprinting. Therein, the simultaneous absorption of two photons, commonly referred to as two-photon absorption, is replaced by the sequential absorption of two photons in two consecutive one-photon absorption processes. This process has been termed two-step absorption. Importantly, two-step absorption can be accomplished by inexpensive compact low-power continuous-wave blue laser diodes instead of femtosecond laser systems in the red spectral region. Red-shifting the second absorption step with respect to the first one results in an and-type optical nonlinearity based on two-color two-step absorption. Herein, alternatives are systematically explored to the few already reported one- and two-color two-step-absorption photoinitiators, including the search for photoinitiators that can be excited by one-color two-step absorption and be de-excited by a disparate laser color.

Rotationally symmetric micro-cavities with disk, ring or toroidal shape displaying whispering gallery modes (WGMs) play an essential role in modern-day photonics. Due to the reduced symmetry of such resonators compared to spheres, an exact analytical model yielding WGMs as solutions does not exist. The established WGM classification scheme based on approximated analytical solutions is generally useful but neglects a possible interaction between the different modes. In this paper, we assess the limitation of the validity of this established classification based on extensive finite element method (FEM) simulations. We investigate respective mode couplings as well as underlying selection rules based on avoided crossings of the modes’ resonance wavelengths. We propose conserved mode properties solely based on true symmetries of the underlying refractive-index distribution and deduce a novel WGM classification scheme.

High-speed high-resolution 3D printing of polymers is highly desirable for many applications, yet still technologically challenging. Today, optics-based printing is in the lead. Projection-based linear optical approaches have achieved high printing rates of around 10 6 voxels s –1 , although at voxel volumes of >100 μm 3 . Scanning-based nonlinear optical approaches have achieved voxel volumes of <1 μm 3 , but suffer from low printing speed or high cost because of the required femtosecond lasers. Here we present an approach that we refer to as light-sheet 3D laser microprinting. It combines image projection with an AND-type optical nonlinearity based on two-colour two-step absorption. The underlying photoresin is composed of 2,3-butanedione as the photoinitiator, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl as the scavenger and dipentaerythritol hexaacrylate as the multifunctional monomer. Using continuous-wave laser diodes at 440 nm wavelength for projection and a continuous-wave laser at 660 nm for the light-sheet, we achieve a peak printing rate of 7 × 10 6 voxels s –1 at a voxel volume of 0.55 μm 3 .

Fully integrated photonic molecules (PMs) made of pairs of polymeric disk-shaped whispering gallery mode (WGM) cavities are structured onto flexible substrates made from liquid crystal elastomer (LCE) using 3D laser printing [1] , [2] . By triggering a reversible change of the molecular order of the LCE, the inter-cavity gap of the PM and therefore its coupling strength is precisely and reversibly tuned using temperature as an external stimulus.

We present an all-polymeric photonic molecule from coupled whispering gallery mode cavities on a chip-scale liquid crystal elastomer substrate. The substrate’s temperature-induced actuation is used to widely and precisely tune the coupling strength.

Photonic molecules (PMs) are of great interest for, e.g., optical filters/sensors or topological and exceptional-point photonics. A key requirement for their versatile application is the tunability of the PM’s coupling strength. This important feature is realized in the here introduced widely and precisely tunable PM on an all-polymeric chip-scale platform. The PM consists of two disk-shaped whispering gallery mode cavities on a liquid crystal elastomer (LCE) substrate. The coupling strength of the PM is controlled via the contraction of the LCE under an external stimulus like local heating. We reveal the reversible (de)coupling via the analysis of laser supermodes emitted from a dye-doped PM. The tunability of the PM’s coupling strength is apparent from the pronounced mode splittings observed in single-fiber transmission spectra and is consistent with coupled-mode theory. Finally, we demonstrate the applicability of the PM as an add-drop filter with a highly controllable intensity transfer. In this light, our PM on an LCE substrate represents a novel platform system for tunably coupled photonic resonators.

Pairs of optical Whispering Gallery Mode resonators were structured onto flexible elastomer substrates. Exploiting the substrates temperature-induced contraction, tunable coupling of the two cavities was realized and variable transmission was demonstrated via 2-fiber transmission spectroscopy.

Liquid crystal elastomers (LCEs) are highly suitable materials for the fabrication of flexible photonic elements due to their ability for directional actuation induced by external stimuli. 3D laser printing (3DLP) is a well-established method to realize complex photonic architectures. In this paper, we present the technological adaptations necessary to combine the actuation-controlled flexibility of LCE with the design options inherent to 3DLP to realize a platform for tunable photonics. The role of birefringence of the LCE in the 3DLP fabrication is addressed and theoretically modelled. We demonstrate how LCEs can be used both as a flexible substrate for arrays of rigid photonic elements and as a material for tunable photonic structures itself. Flexible coupling of two optical whispering gallery mode cavities and full spectral tunability of a single cavity are presented as exemplary applications.

Liquid-Crystal-Elastomers were integrated into building blocks containing Whispering- Gallery-Mode resonators. Exploiting their directional mechanical actuation, full tunability of single cavities and flexible coupling of resonator pairs were realized and verified by fibertransmission spectroscopy.

Multiple-cation mixed-halide perovskites show high power-conversion efficiencies and recently improved stability. But, even most advanced absorber materials still suffer from instabilities of the bandgap under illumination and applied bias. Here, we employ modulation spectroscopy as a highly sensitive electro-optical measurement technique to directly reveal such instabilities. We find a reversible decrease of the absorber bandgap of up to 70 meV in complete solar cells. In situ X-ray diffraction measurements under illumination and bias confirm structural changes of the perovskite and their reversibility, which are attributed to a segregation of the halides. These processes are most strongly activated when illumination and bias are combined, which leads to a 5 times increased shift compared to that with illumination only. Since this scenario is intrinsic to the solar cell's operation conditions, it can never be avoided completely. Furthermore, the bandgap decrease is strongly enhanced, but still reversible, by high relative humidity and oxygen content supporting the strict requirement of efficient encapsulation.