This PDF file contains the front matter associated with SPIE Proceedings Volume 12433, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The continuously increasing interest in flexible and integrated photonics requires new strategies for device manufacturing on arbitrary complex surfaces and with lowest possible size, respectively. Terahertz (THz) technology can particularly benefit from this approach to implement compact systems for generation, detection and on-demand manipulation of THz radiation. Here we present a novel fabrication method to realize conformable metasurfaces. The flexible and versatile character of polymeric nanomembranes is combined with direct laser writing via two-photon polymerization and metal deposition to develop freestanding ultra-thin quasi-perfect plasmonic absorbers with an unprecedentedly high level of conformability. Moreover, revealing new flexible dielectric materials presenting low absorption and permittivity in the THz range, this work paves the way for the realization of ultra-thin, conformable hybrid or all-dielectric devices enhancing the application of THz technologies, and flexible/integrated photonics in general.

We give an overview of our recent progress on the design and proof-of-concept demonstration of interfacing components for short-distance optical interconnects with a particular emphasis on their fabrication through two-photon polymerization-based laser direct writing. We show mode field conversion tapers printed on single-mode optical fibers for easy and efficient interfacing to various photonic integrated circuits, circular and square planar waveguide structures with V-groove inspired alignment structures for easy coupling to fibers, microlenses and fan-out diffractive optical elements.

Surface micrograting arrays have applications ranging from diffractive optics to bioengineered surfaces. We report a versatile fabrication platform for the maskless photofabrication of these arrays based on structured polarized light and photoresponsive azopolymer films. The films are patterned using a spatial light modulator (SLM) configured as a polarization modulator. The light source is a 488 nm laser with exposure times of order 5 sec or less. Structured polarized light from the SLM is imaged onto the film, writing a 120 µm x 80 µm surface relief pattern with amplitudes and periods controllable from 25 nm to 1 µm and 700 nm- 10 µm respectively. These are stitched into larger area patterns via XY translation. The versatility is demonstrated through a variety of micrograting patterns, including diffractive optical elements, multiplexed surface grating arrays, and diffractive optically variable image devices for optical security applications. In a different application, the biocompatibility of the polymeric film can be leveraged since cellular interaction with synthetic microscale structures influences a wide array of cell responses. We demonstrate this by showing directed cell growth mediated by the micrograting array. In all examples, the surface gratings required no post-exposure processing, are stable in ambient conditions, and can be replicated using nanoimprint lithography.

We report on the fabrication of form birefringent structures using 3D laser printing technique. By using a 3D self-supporting and mechanically robust architecture, polymeric structures exhibiting the quarter- and half-wave phase retardation levels needed for applications were successfully realized. These results may enable the realization of super-compact polarization-sensitive photonic devices and elements of optical integrated networks using 3D femtosecond laser printing technique.

Measuring strain without parasitic thermal influence is vital. A temperature compensated strain sensor is fabricated in fused silica using femtosecond laser micromachining. Utilizing femtosecond laser direct writing and femtosecond irradiation followed by chemical etching, two Bragg gratings are fabricated in the bulk of a fused silica substrate. By suspending one of the Bragg gratings in a cantilever, it is mechanically isolated from the rest of the substrate. Thermal and tensile characterization showed that both Bragg gratings are sensitive to thermal changes with a sensitivity around 10.5 pm/°C, while only the non-isolated Brag grating is sensitive to strain with a sensitivity of 1.1 pm/µϵ. Hence, it is proven that the parasitic thermal influence on the strain sensor can be compensated by taking into account the response of the isolated Bragg grating.

Multi-material fibers are a promising platform for integrating nanoscale structures into macroscale photonic systems due to their unique aspect ratio: fiber cores can be kilometers long and sub-micrometric in their cross-section simultaneously. We are introducing Very Large-Scale Integration for Fibers (or, in short, VLSI-Fi) – manufacturing of integrated photonic circuits in a fiber analogous to VLSI from the microelectronics realm. VLSI-Fi starts with a thermal draw of the 3D printed preform, defining the cross-sectional geometry of the fiber, followed by the axial patterning of the fiber cores into arrays of integrated devices using a material-selective spatially coherent capillary breakup1,2 . Additional control over the photonic and electronic properties of devices is accomplished through segregation-driven control of doping. The result is a fiber-embedded integrated photonic circuit with user-defined 3D architecture providing the desired functionality. We argue that the capillary breakup of a viscous thread, nonlinear and often chaotic, becomes predictable if the axial symmetry of the thread viscosity is broken. We found that the capillary breakup of semiconducting fiber cores initiated by feeding the fiber through a spot-like liquefaction zone results in deterministic photonic and optoelectronic structures, such as gratings and spherical resonators. As a proof of concept, we demonstrate a selective breakup of a silicon core in fiber with one silicon and one vanadium core into an array of spherical silicon resonators, with a vanadium electrode flanking those resonators for electrical tuning of their resonant frequencies. Such cascaded resonators are a nontrivial example of photonic circuitry implemented in fiber using a non-CMOS approach.

We have developed a new UV-replication technology for monolithic aspherical polymer lenses. Proprietary process control allows to manufacture lenses with form errors and tolerances suitable for imaging applications, similar to those originating from optical injection molding. However, since UV-replication is carried out at room temperature, in contrast to injection molding, there is a large choice for material selection for the molds and consequently the lens masters can be manufactured much cheaper. With these low initial fabrication costs demonstrators, prototypes, small series and even products in medium volumes of complex imaging systems can now be addressed economically for the first time. Those are otherwise typically limited to spherical glass lenses with all their drawbacks compared to aspheres. Polymer-on-glass UV-replication is well known from wafer level optics. But here the supporting glass wafers remain in the final lens, severely limiting the degrees of freedom of the optical design. In addition, material shrinkage occurring during hardening limits reasonable sag heights of the lenses, so that only low-resolution imaging optics are possible. In our case, the glass substrate in the individual lenses can now be omitted and a full compensation of the shrinkage is achieved with minimum form error. Sag heights of up to 1.8mm and aspherical lens profiles on both sides of thin menisci have already been realized with additional peripheral structures for mechanical self-alignment in materials that even survive reflow-soldering. In this paper we present the benefits of our new technology and the process chain, discuss applied materials and show some demo systems so far realized.

The fabrication of Diffractive Optical Elements (DOEs) involves the structuration of material surfaces with geometries on the scale of light wavelength. Differently to standard photoresists, thin films of amorphous azobenzene-containing polymers (azopolymers) directly develop surface reliefs in the irradiated area, directly usable as phase-modulating masks, acting as DOEs. Surface structuration is the result of an intrinsically reversible light-induced material displacement, which makes azopolymer films usable as photo-transformable planar diffractive optical devices. Here we demonstrate reprogrammable and ready-to-use diffractive gratings, lenses, and holographic projectors, directly obtained in a single photo-structuration step through the projection of a grayscale holographic pattern over the azopolymer film free surface. The all-optical scheme, based on the principles of computer-generated holography, allows a simple and accurate engineering of the light pattern used for the azopolymer surface structuration. Additionally, direct pattern transferring opens to the real-time optimization of the device allowing to test and prototype its diffraction properties right during the developing of structured surfaces. The proposed approach offers a versatile, efficient, and full-optical reversible fabrication framework for DOEs, making it a promising option to overcome the demanding, burdensome, and irreversible manufacturing processes typically involved in the realization of planar diffractive optical devices.

The use of one-dimensional Airy beams in light-sheet microscopy (LSM) allows the generation of thin lightsheets without scanning, and thus have great potential for simplifying multi-photon LSM systems. However, their uni-axial acceleration limits the maximum numerical aperture of the collection objective in order to keep both the active and inactive axes within its depth of field. This issue is particularly pronounced in potential miniaturized LSM implementations, such as those for endomicroscopy or multi-photon neural imaging in freelymoving animals using head-mounted miniscopes. To address this issue, we propose a new method to generate a bi-axially accelerating static Airy light-sheet that illuminates a thin spherical shell within the field-of-view (FOV) of a collection objective. This profile can be engineered to match both the detection objective’s field curvature and FOV. Using an illumination unit that combines GRIN optics with a phase mask that is 3D-printed using 2-photon polymerization, we experimentally demonstrate the validity of the approach, and verify the analytical models.

The demand on the miniaturization of products has fostered the advancement of high-resolution fabrication technologies. Here, we demonstrate a simple and low-cost UV-LED-based lithography technique for structuring at micro- and nanoscale. The corresponding experimental setup was established with off-the-shelf components and allows for single structuring within seconds based on a standard microscope projection photolithography (MPP) process. By improving the printing quality of the structure pattern on transparent foil and employing aberration-corrected optical elements in the systems, high-quality waveguides and gratings at the micro- and nanoscale were successfully fabricated. Especially, minimum feature sizes down to 80 nm using a 100x microscope objective with NA of 1.4 were obtained. This operation-friendly, highly efficient and low-cost approach exhibits great potential in micro- and nanomanufacturing for applications in fields such as nanophotonics, integrated photonics, and micro- and nanoelectromechanical systems.

3D printing technologies have significantly advanced the manufacturing of complex objects for use in various applications. Two-photon polymerization (TPP) has offered the highest resolution (around 100 nm lateral resolution) in 3D printing by introducing a non-linear absorption process. However, the extremely low probability of two-photon absorption requires pulsed lasers with high peak intensity, the cost of which strongly limits the broad dissemination of TPP. Here we propose 3D printing using upconversion nanoparticles (UCNPs) for tunable high-resolution printing of gelatin-based hydrogel while using continuous-wave lasers. Kinetic analysis of the process has been investigated and showed that the nonlinearity of UCNPs can be tuned by the excitation intensity. It offers the possibility of adjusting the voxel size together with the printing speed. This work provides new possibilities for high-resolution additive manufacturing at optimized fabrication speed.

Integrated optics are an innovation driver for a multitude of industrial applications like autonomous driving or point-of-care diagnostics. With the increasing demand for miniaturized, low-cost optical systems, new methods for fabricating tailored graded index micro-optics are required. Additive manufacturing is a promising technology for this not only due to its high design freedom, but also because of the potential for function integration via multi-material printing or the integration into digitized process chains. In fact, additive manufacturing of optical elements has not matured yet due to the requirement to fabricate 3D objects with optical quality. In this contribution, μ-dispenser direct ink writing based on transparent photopolymers is presented that enables the production of multi-material micro-optical elements. It will be shown that the achievable printing resolution for 0D- and 1D-structures is mainly depending on the needle diameter. Mono-material spherical and cylindrical lenses with a geometric dimension in the range of a few micrometers have been successfully fabricated and characterized. The geometric shape fidelity of printed 2D-layers, which suffers from surface tension effects due to the material’s molecular cohesive forces, is optimized by proper printing strategies. In an outlook, the route towards the production of micro-optic, function-integrated 3D GRIN elements is given, by using a mixer module to realize combined and gradable extrusion of two photopolymers.

We demonstrate the fabrication of a Distributed Feedback (DFB) laser from additive manufacturing techniques only. This laser is composed of SU8 as the grating and of a perovskite thin film as the active medium. The SU8 grating is directly printed on glass with a pitch of 370 nm thanks to a patented technology developed by Hummink, while the perovskite is simply spin-coated on top of the SU8. Our approach proves to be a fast and versatile tool for the fabrication and design of DFB lasers and waveguides in general.

Freeform micro-optical arrays (FMOAs) can overcome some limitations of rotationally symmetric optical components. The manufacturability assessment for FMOA designs containing many freeform elements is demanding. We present computer-aided design (CAD) tools that analyze FMOA designs for manufacturing constraints. They 1) extract the height (Sag) and 2) local slope while highlighting areas beyond manufacturing limits, 3) extract the minimum tool radius and 4) convert CADs into ray traceable solids using NURBS for performance comparison of the actual micro-structure to the design. Critical parameters can be identified early, reducing time-consuming and costly adjustments later.

In this work, we demonstrate a fabrication strategy for micro- and nanooptical structures by integrating UV-nanoimprint lithography (NIL) towards two-photon polymerization (2PP) for cost-effective scalable production. Herein, 2PP was first employed to create the structures. These were then used as master molds for the subsequent UV-NIL process for high-throughput replication. With this fabrication strategy, various structures with dimensions from sub-micrometer to centimeter scales were produced and replicated. High-quality UV imprinting of the 2PP-produced master sample can be realized. The demonstrated fabrication strategy exhibits great potential for applications in scalable production of optical and optoelectronic devices or systems.

Nanowire array LEDs rely on an interlayer spacer dielectric to enable connection of many nanowires in parallel. Conventional solutions use spin-coatable materials such as polydimethylsiloxane (PDMS) or spin-on-glass (SOG), which are thermally and mechanically unstable. Alternatively, more stable dielectric materials such as SiO₂ or Si₃N₄ can be used, however these materials deposit conformally, leading to significant surface topology above the nanowires prior to the etch back step. In this work, we present a method for removing this surface topology by utilizing a self-planarizing photoresist layer and a plasma etch which removes photoresist and SiO₂ at the same rate. By performing this planarization process several times, surface features of height h > 1 μm can be reduced to less than 50 nm, allowing further etching of the SiO₂ to expose the tips the nanowires and allow for reliable p-contact formation. Unlike CMP, this process only involves dielectric deposition and dry etching, and places no limitations on sample size and shape, making it ideal for research settings.

Many applications across photonics and semiconductor industries require the fabrication of nanostructures with non-trivial geometries with a precision and reproducibility down to the nanometer scale. Slanted gratings and metamaterials are examples of such designs that have vast applications in Augmented Reality and LiDAR. State-of-the-art lithography techniques, such as nanoimprint lithography or UV lithography, can provide such levels of fabrication precision for high-volume production. However, a rapid in-line quality inspection method for such complex patterns is required to monitor the fabrication process, verify the sample quality, and to ensure reproducibility. Here, we demonstrate a novel technique that allows us to inspect the quality of the samples in a non-destructive and fast manner, and to extract geometrical parameters of the nanostructures over large areas, generating spatial variations maps across wafers.

Two-photon lithography technology has revolutionized the fabrication of 3D-structures from photoreactive materials by providing high precision and almost unlimited design freedom. Here, a zirconium-based metal-organic framework (MOF) based on benzophenonedicarboxylate (bzpdc) linker molecules has been micro-structured by a laser-induced chemical modification. Via a post-synthetical loading of the porous crystals with appropriate organic compounds and a subsequent femtosecond laser-induced two-photon absorption (TPA), a cross-linking of organic compounds with MOF’s bzpdc-linker is triggered. As a result, a precise functionalization of the material is achieved, leading to a permanent change in properties such as refractive index. With this work a significant step forward for 3D-tailoring material properties and manufacturing tuneable functional optical micro-devices is made.

Silicon-based single-photon avalanche diodes (SPADs) implemented in front-side illuminated arrays and imagers have often suffered from fill factor limitations. The corresponding reduced sensitivity can be sometimes traded off with longer acquisition times thanks to SPAD’s noiseless read-out. The use of SPADs can however be critically affected in many applications, especially when photon-starved, or when several photons need to be detected in coincidence. The fill factor loss can be recovered by employing microlens arrays, which are difficult to build with relatively large pitch (> 10 μm) and low native SPAD fill factor (as low as 10%). To address these challenges, we have developed several generations of refractive microlenses by photoresist reflow used to fabricate molds. These structures were used to imprint UV-curable hybrid polymer microlenses on SPAD arrays. Replications were successfully carried out on large SPAD arrays with very thin residual layers (~10 μm), as required for higher numerical aperture (NA > 0.25). Replications were also carried out for the first time in a multi-chip operation regime at the wafer reticle level. By optimizing the lens sag and residual layer thickness, concentration factors (CFs) within 15-20% of the theoretical maxima were obtained for the smaller arrays (32×32 and 512×1). The spectral response was flat above 400 nm. CF values up to 4.2 with good uniformity were measured on large 512×512 arrays with 16 μm pixel pitch and a native fill factor of 10.5%. This result was confirmed by simulations when using the actual measured lens shape. We thus demonstrated good spectral and spatial uniformity and high CF, while moving to higher NAs and larger sensor sizes with respect to previous work.