This PDF file contains the front matter associated with SPIE Proceedings Volume 11292, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.

Beam shaping of femtosecond lasers was applied in the Kerr-lensing and aberration regime to enable high-aspect-ratio filament tracks to form uniformly through the silica cladding and core waveguide of single-mode fiber (SMF28/450). One- and two-dimensional filament arrays were embedded along the waveguide to form weak to strong photonic stopbands. The filament shape enhanced transverse light scattering into narrow azimuthal radiation zones. Tailoring of chirp and 2D patterns further facilitated high-resolution (~350 pm) spectral focusing onto a CCD camera, defining a compact “Spectrometer-in-fibre” over the visible spectrum. At higher exposure, the filaments opened into narrow nano-channels (200-400 nm diameter) presenting a novel Bragg grating for refractive index sensing of the ambient environment. This lab-in-fiber technology presents a robust, flexible, and ubiquitous communication platform for nano-scale sensing across expansive networks or into tightly confined, sinuous spaces.

In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies, thereby reducing stray light and decreasing the required power levels. This study investigates the fabrication of blazed gratings with grayscale electron beam lithography and the subsequent replication of the realized 3D grating structures in a polymer material with ultraviolet nanoimprint lithography. As such, diffractive elements are realized on a waveguide sheet, with very good control over the dimensions and the profile of the printed features. Blazed gratings are designed for green light (λ= 543 nm) and a diffraction angle of 43°. Making use of a PMMA resist and by carefully optimizing the electron-beam parameters, electron dose distributions and development step, blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are achieved. Finally, a master is realized with two blazed gratings, 3 cm apart, which are replicated using ultraviolet nanoimprint lithography onto a waveguide sheet. The in- and outcoupling of an image through these two blazed gratings is shown, appearing sharp and non-distorted in the environment, and a throughput efficiency of 17.4% is confirmed.

Optical coherence tomography (OCT) has developed rapidly and is widely used in different fields such as biomedicine and optometry. The characterization and calibration of OCT systems is essential when testing the system and during normal use to ensure that there is no misalignment or distortion that could affect clinical decisions. Imaging distortion is a significant challenge for OCT systems when viewing through non-planar surfaces. Here we present a new multi-purpose plano-convex OCT phantom which is designed to be used for OCT characterization and calibration as well as to validate the post-processing algorithm for the imaging distortion of the OCT systems. A femtosecond laser direct writing technique is used to fabricate this phantom which consists of a landmark layer with radial lines at a 45-degree angular spacing inscribed at 50μm in apparent depth (AD) underneath the planar surface. Below that there are a further 8 layers of a spherical inscription pattern which has a 150μm (in AD) separation between each layer. The first spherical layer is located at 150μm (in AD) underneath the planar surface. Due to the laser power loss when travelling through the deeper layer, an increased power is applied to the deeper layers. The spherical pattern overcomes orientation issues seen with existing calibration phantoms. The landmark layer is applied so that it can easily tell the exact location when scanning which will also benefit the image distortion correction process.

We describe fabrication and properties of 3D photonic crystals with chirped lattice period using Direct Laser Write lithography technique in photoresists, and report on the optical properties of the fabricated structures. Experimental demonstration that the fabricated structures can be used as concentrators of electromagnetic radiation along the direction of propagation at optical frequencies is reported. Potential applications of chirped photonic crystal in improved optical detectors are discussed.

We report on the fabrication and near field characterization of plasmonic slot waveguides in single- and poly crystalline gold films operating at telecommunication wavelengths. Moreover, we present freestanding photonic metasurfaces with a total thickness of only 40 nm prepared by focused ion beam milling of nanovoids in a carbon film followed by thermal evaporation of gold and plasma ashing of the carbon film.

Optical tweezers are a powerful platform for nano- and micro-assembly, as they provide a non-contact and biologically friendly method for the three-dimensional manipulation of objects over a range of sizes and of varying material properties. Three-dimensional micro- and nano-scale structures that are composed of multiple materials often achieve improved performance over single-material designs. In the case of optical devices, the inclusion of both metallic and dielectric media allows for the possibility of achieving functionality which is otherwise inaccessible. Although there are many methods for fabricating small-scale three-dimensional optical devices, the majority of these approaches only deal with a single material or type of material. Thus, in order to create structures that consist of multiple materials, it is typically necessary to use a combination of methods over the course of several steps. Here we show that optical tweezers are a promising technology for the assembly of heterogeneous optical structures in a single process. We demonstrate our approach by fabricating structures using core-shell nanoparticles with metallic shells and dielectric cores as building blocks. To the best of our knowledge, these structures represent the first nanoscale, multi-material devices built using the optical tweezer platform. Furthermore, we discuss several relevant metrics regarding the assembly process such as object translation speeds, placement accuracy, and overall rates of fabrication. Currently, we have achieved lateral speeds up to 0.2 mm/s and placement repeatability down to 50 nm. We suggest future applications of this fabrication method and discuss the next steps in its evolution.

Helium and Gallium-ion beam milling in combination with a fast and reliable Sketch-and-peel technique is used to fabricate pairs of gold nanoantennas with a superior quality factor and with gap distances down to 6nm. The high fabrication quality compared to standard ion beam milling is proven by polarization-resolved dark-field spectro-microscopy of isolated nanoantennas pairs. We observe a strong coupling of the two antenna arms for both fabrication techniques, however with an outstanding quality factor for the Sketch-and-peel-produced antennas. The presented fabrication technique can pave the way to the production of plasmonic nanostructures on large spatial scales with high milling accuracy.

This Conference Presentation, "High NA silicon metalenses for wide-field imaging," was recorded at Photonics West 2020 held in San Francisco, California, United States.

Dielectric metasurfaces require the integration of large index contrast materials with accurate control over position and size for high optical efficiency. Their fabrication usually relies on well-established lithographic techniques. While lithography bears considerable advantages in terms of reproducibility and accuracy, it remains a time and cost intensive process that remains restricted in terms of materials, and difficult to scale up to large-area and non-rigid substrates. Chalcogenide glasses constitute a class of materials particularly suited for metasurfaces, thanks to their low intrinsic losses coupled with their tunable high refractive indices. The challenges associated with the nanostructuring of glassy materials has however limited their use in nanophotonics. Here, we rely on the glass characteristic viscous properties and destabilizing Van der Waals interactions, to show the rearrangement of thin chalcogenide films into well-ordered and defect free structures at the nanoscale. In particular, we show the fabrication of arrays of a variety of chalcogenides nano-objects, both continuous and isolated, with a variety of sizes and structures, and on different, rigid, flexible and stretchable substrates. Such controlled large area arrays are shown to have strong field enhancement leading to Fano resonances, which finds applications as biosensor and second harmonic generation.

We present a new method of making monodisperse solutions of highly spherical poly-crystalline Si nanoparticles and a method for optical trapping, moving and printing these nanoparticles on a substrate. Spherical Si nanoparticle solutions with particles of 130 nm and 210 nm in diameter were fabricated using combined hole-mask colloidal lithography and laser induced transfer methods. Optical properties, size distribution and crystal properties of the particles from solutions were characterized by means of optical and Raman spectrometry, SEM and TEM imaging. We manipulated these particles in solution by means of optical trapping as well as printed them onto the substrate in a controlled manner.

Wafer level imprinting using transparent UV curable materials offers high flexibility in design, scalability and low total cost of ownership. Development of materials requires a careful balancing of all relevant properties including optical function, process constraints, (thermo-) mechanical properties and stability under reliability testing. DELO developed a modular system of materials which can be modified in this wide parameter space. These materials include highly transparent materials with refractive indices starting at below 1.4 and stretching to above 1.7 and also excellent mechanical as well as reliability performance.

Nanotechnology and more generally the ability to fabricate devices with nanoscale features has developed from a history rooted in the semiconductor industry; however, biology has long been able to programmatically assemble nanoscale structures with a vast array of functions. Inspired by this, we develop a set of volumetric deposition strategies related to the mechanisms of assembly employed in biological systems. Leveraging a technology for 3D nanofabrication, Implosion Fabrication, we have explored novel methods for depositing nanomaterials relevant to optics, photonics, and plasmonics using thiol-binding, hydrophobic interactions, protein binding, and hydrogen bonds into any 3D geometry with nanoscale resolution and gradient capabilities. Through the development of these novel deposition chemistries, we create a platform by which a large variety of functional nanomaterials can be directed to assemble for the future of device manufacture.

Two-photon polymerization (2PP) is a promising technique for additive manufacturing of 2D and 3D polymer structures with tailored physical properties. It provides the possibility to create structured samples with nearly arbitrary geometries. However, the potential of this technique to achieve structures with high resolution below the diffraction limit and complex geometries with high surface quality is not yet fully exploited. In this work, polymer gratings with controllable parameters, specially designed 3D structures with circularly symmetrical curved surfaces and 3D photonic resonators coupled with linear optical waveguides were demonstrated. In the first case, different photosensitive materials were applied for the fabrication of the gratings and their influence on the structure formation was compared. Furthermore, the approach to realize high resolution for feature sizes below the diffraction limit by controlling the laser focus position with respect to the glass substrate was shown. Periodic structures with a minimum feature size of 100nm and periodic distance of 200nm were realized. For special 3D structures with a circularly symmetrical curved shape, the main challenge was to realize smooth surfaces. An investigation of relevant influences and the optimization of processing parameters on the structure formation and surface quality were performed. Additionally, the 2PP fabrication of 3D resonators and waveguides with a nanoscale gap distance in between was studied.

Planar nanophotonic circuits allow for realizing complex optional functionality by joining photonic library elements with waveguides into integrated optical circuits. The restriction to quasi-two-dimensional geometries imposes constraints for advanced photonic devices. Here I will present recent results on hybrid 2D-3D photonic integration to overcome these restrictions. I will introduce broadband coupling geometries which enable low-loss out-of-plane optical coupling. I will further present a reconfigurable photonic device architecture which allows for implementing desired optical filter functionality after planar fabrication. This way flexible and reconfigurable integrated optical circuits come within reach which provide desired output in analogy to programmable integrated electrical circuits.

Current solid-state lighting technology is built on rare-earth-based phosphors. However, their large crystal size impedes the tuning, optimization, or manipulation of emitted light. Herein we demonstrate this can be achieved combining nanophosphors and nanophotonic architectures. Rare earth emitters are central for many applications related to the generation of light because these nanomaterials feature exceptional thermal and chemical stability. Nevertheless, such stability entails an intrinsic complexity to alter their emission properties. The most common route to control the luminescence spectrum of nanophosphors is thus modifying their chemical composition, which often comes at the expense of deteriorating the performance of the emitter. In this paper, we show that the integration of rare-earth nanocrystals, i.e. nanophosphors, in devised optical environments allows a fine control over the features of the emitted light, without modifying the chemical compositio

Quantum emitters are essential for quantum optics and photonic quantum information technologies. To date, diverse quantum emitters such as single molecules, quantum dots, and color centers in diamond have been integrated onto chips by various methods which typically have complex operation. Here, our quantum emitters are colloidal CdSe/ZnS quantum dots (QDs) embedded in polymeric nanostructures. We report two approaches based on photo-polymerization for deterministically integrating quantum emitters on chips. Firstly, based on one-photon polymerization (OPP), we coupled an external excitation laser into surface ion exchanged waveguides (IEWs), the surface evanescent wave resulting in the QD-polymer ridges. In order to scale down the dimension of the QD-polymer structures, we secondly fabricated QD-polymer nano-dots on glass substrates by a direct laser writing platform (DLW) based on two-photon polymerization (TPP). A deep fabricating parameters study has been made enable us to control the dimensions of the polymer-QDs nanocomposites. Moreover, photoluminescence (PL) measurement results demonstrate the feasible and potential of our method for integrating quantum emitters onto future complex photonic chips.

This Conference Presentation, "Adhesion properties of polymeric microstructures fabricated by two-photon polymerization," was recorded at Photonics West 2020 held in San Francisco, California, United States.

Lithography is one of the key technologies employed for the fabrication of optical devices and components, which could be applied in fields as diverse as optical sensing, communication and information technologies. Microscope projection photolithography (MPP), as a low-cost, simple and flexible lithography method, lends itself for versatile applications. Its feasibility in realizing various microstructures has been verified already. However, the improvement of the quality and resolution of structures still remains challenging. Here, we present an MPP method for the controlled generation of high-quality and high-resolution 2D optical micro- and nanostructures. Particularly, an improved process chain, which significantly shortens the time from structure design to the realization to less than one day, is introduced. The structures, first designed with vector-graphics software, are printed on a commercial transparency film. Then, the film is placed into a self-developed setup, and the structure patterns are transferred onto a chromium photomask with a demagnification of 10:1, for example. The last step is to place the chromium photomask into the MPP arrangement and implement the fabrication using a microscope objective to demagnify and project structure patterns onto photoresist which is simultaneously exposed to UV light. With this process chain, periodic structures with a minimum feature size of 150 nm were realized using an objective with NA of 1.4. Furthermore, various photonic components such as micro ring resonators and arrayed waveguide gratings with high quality were generated with application potential, e. g. in sensing and monitoring.

Asymmetric (freeform) optical components have gathered a large deal of attention triggered by the substantial performance gains demonstrated in imaging and non-imaging optical systems. In the growing trend for device miniaturization and cost reduction through optical components with relevant dimensions well below the millimetre scale, freeform microlens arrays (FMLAs) are very promising albeit considerably more challenging to design, manufacture and characterize that their macroscopic counterparts. Here, we present some innovative designs, fabrication approaches and characterization strategies developed at CSEM to overcome some of the existing limitations.

Gratings, Fresnel lenses, phase masks, and waveguides are among the optical components that have been fabricated by 172 nm irradiation of various polymers through photomasks. Intensities above ~ 70 mW/cm^2 are now commercially available at 172 nm with flat Xe2 lamps. Such optical fluences are capable of precisely (< 500 nm lateral and 20 nm depth resolution) ablating a wide range of polymers, including PMMA and ABS, thereby allowing for a variety of 3D optical and biomedical components to be realized economically by dry processing.

In this talk we will present results on stimuli-responsive reversible deformations in polymeric microstructures fabricated using direct laser writing in pre-polymers (DLW-PP) technique. This microstructure behavior can be employed for micro-actuation applications in MEMS as well as designing new passive chemical sensors for microfluidic applications via hybrid (additive-subtractive) microfabrication technique. In this talk we will present our recent results in this field introducing few new concepts: a novel all-optical readout chemical sensor suitable for microfluidic applications, micromechanical components acting as flow control mechanisms based on bi-polymeric structures and solvent-sensitive micromechanical plug for microchannels.

Complex emulsion droplets consisting of hydrocarbon and fluorocarbon oils dispersed in water have been shown to exhibit iridescent structural color via interference from total internal reflection, and the color is tunable based on the size, shape, composition, and orientation of the droplets. Our study explores the structural color properties of complex emulsion droplets and their application to colorimetric chemical sensing through the use of an α- amylase responsive surfactant solution composed of γ-cyclodextrin, Triton X-100 surfactant, and Capstone FS-30 surfactant. We aim to demonstrate proof-of-concept sensitivity of biphasic oil-in-water emulsion droplets for colorimetric sensing through the correlation of reflected structural color patterns to droplet shape and size.

In recent years, structural color has burgeoned into a vibrant and dynamic field of study. Many structurally colored organisms rely on comparatively few materials to create a large diversity of optical responses and a broad array of compelling and functional features. Scientists and engineers often explore such systems as examples of sustainable, robust solutions to complex problems. By looking to existing hierarchical material systems in biological systems, we aim to determine guidelines for fabrication of material structures with pre-determined functionality and properties.

Here, we compare approaches for producing surfaces that reflect color and discuss tunable design parameters. Methods discussed include bottom up self assembly, abstracted lithographic replication of structures, and top down systematically designed surfaces. We consider existing techniques and levers available to control output, and present the components of an inverse design approach. We compare different methods on the basis of scalability, tunability, and achievable responses, and provide practical guidelines for producing bio-inspired surface structures.

Our proposed designs are constrained for realizable fabrication using additive direct laser writing techniques such as two-photon polymerization that are suitable for producing arbitrary structures with sub-wavelength resolution. These evaluated and characterized structures could eventually be adapted to roll to roll or imprint based systems for scale-up and manufacturing on a commercial scale. Here we contribute to a broader vision of systematic materials design by exploring tools for generating color.

We propose a dual step µ-3D printing (two-photon lithography) strategy, which is the 3D version of the classic lithography etching-mask strategy, to achieve selective gold metallization for the realization of reflective micro-optics. We apply this strategy to obtain right-angle prism micro-mirrors used to create dynamic counterpropagating optical traps for biological samples in a holographic optical trapping setup with a single low-NA objective. We print the prisms on standard glass coverslips to create integrated optical trapping chips. We show the automatization of alignment between prisms and masks by a computer vision software.

In this contribution, we present the fabrication of space-variant quarter- and half-wave plates, by combining 3D laser direct writing with electroplating. A negative of the space-variant wave plates was fabricated with 3D laser direct writing in a positive tone photoresist. With an electrochemical deposition process the negative structure of the wave plates was filled with gold. This process allows fabrication of a non-quadratic air filled cross-section surrounded by a metallic cladding, a so-called hollow waveguide array. The wave plates consists of about 1000 x 1500 hollow waveguides each with sub-wavelength dimensions for an operating wavelength of 1550 nm.

In volumetric additive manufacturing, an entire three-dimensional object is simultaneously solidified by irradiating a liquid photopolymer volume from multiple angles with dynamic light patterns. Though volumetric additive manufacturing has the potential to produce complex parts with a higher throughput and a wider range of printable materials than layer-by-layer additive manufacturing, the resolution limit has not been thoroughly studied. We show that a low-etendue illumination system and an integrated feedback system to accurately control the photopolymerization kinetics improve drastically the fidelity and spatial resolution. Hard and soft centimeter-scale parts are produced in less than 30 seconds with 80-µm positive features.

We report on the experimental and theoretical study of direct laser writing on dielectrics with reduced feature size by using temporally and spatially separated pulses. Features on the front surface of fused silica within the overlapping area of two laser pulses are obtained by tuning the delay time between the two pulses. The observed dependence of feature position on delays longer than the free-carrier lifetime indicates an ionization pathway initiated by self-trapped excitons. The sensitivity of feature size with the increase of laser intensity is studied by simulating the free-carrier density distribution for different temporal and spatial separation of two laser pulses. It is found that the sensitivity is influenced by the spatial and temporal separation of the two pulses.

Nowadays, no consensus concerning the definition of the resolution of 3D measuring and fabricating systems is available. In contrast, the lateral resolution for 2D is generally accepted as the minimum distance between two single points/features at which the imaging/manufacturing tool fails to resolve. To extend this approach at least to 2.5D systems, we present an appropriate approach based on the instrument transfer function. First, using a commercial DLW system, we introduce the respective basics of the concept for metrology. In a second step, we use this method to determine the 3D resolution capabilities of a STED-inspired high-speed fabrication system.

We have recently reported high-speed 3D printing of customized optical lenses using projection micro-stereolithography (PμSL) process. However, at the reported printing speed of 24.54 mm³ h^-1, it still takes hours for 3D printing an optical lens with the size of millimeter. To further increase the printing speed, we focused on the micro Continuous Liquid Interface Production (μCLIP) process to completely eliminate the time-consuming resin recoating and dwelling steps in PμSL process. While the commonly used oxygen permeable Teflon AF2400 film is found to create rather rough surface of 3D printed structure, we report the use of the home-made Polydimethylsiloxane (PDMS) membrane as the alternative oxygen permeable membrane to overcome this issue. In this study, we demonstrated the significant increased printing speed of 4.85×10³ mm³ h^-1, which corresponds to a 200-fold improvement in comparison with previously reported PμSL process. We further combined grayscale photopolymerization and meniscus coating steps in the μCLIP process to tackle the inherent speed-accuracy trade-off in 3D printing optical components. We demonstrated the fabrication of an aspherical lens with 1,6-Hexanediol diacrylate (HDDA) using μCLIP system in about 2 minutes. The printed aspherical lens shows impeccable surface finishing, with the surface roughness (RMS=13.7 nm) well below the wavelength of the visible light. It is capable of resolving Element 3 in Group 7 of 1951 USAF resolution test chart, which corresponds to the imaging resolution of 3.10 μm. With significantly improved printing speed, this work unleashes the unprecedented potential of further utilization of 3D printing techniques for rapid manufacturing of novel optical components, such as freeform optics and integrated photonic systems.

This article presents an endoscopic 3D printer using the photo-polymerization additive method. The proposed endoscopic principle allows printing 3D objects through a flexible optical image guide. The concept is first to transmit a pattern using ultraviolet light projected through the endoscopic optical system on the printing surface. A layer by layer printing is performed by moving the focusing plane along the z-axis. The endoscopic optical design is based on a Digital Micro-Mirror (DMD) projector, an image guide (a fiber bundle of 70 000 fibers) and optical lenses. It is modeled and simulated using the Zemax optical software. The projected pattern from the DMD is injected into the image guide. Then, the pattern is refocused on the printing surface, which is the transparent bottom of a vat full of resin. First, optical losses and homogeneity of the endoscopic optical system are measured. Then, photopolymer parameters of used resin (Formlab RS-F2-GPWHH-04) are experimentally evaluated. Finally, different multi-layer objects (typically 30 layers) are printed to validate the concept of 3D endoscopic fabrication. Using a 405 nm LED, an optical irradiance of 1.7 mW/cm² on the printing surface is reached. 3D parts are printed with a lateral resolution of 150 μm and a layer thickness of 100 μm on a circular printing surface of 9.54 mm diameter.

In recent years, Time-of-Flight (TOF) camera technology is widely adopted for 3D mapping and motion gestures. TOF camera generally consists of light emitting and receiving elements. The micro lens array (MLA) which had been commercialized as a lens for light projection for TOF camera could be made of a resin with a product size of 6 x 5 x 0.85 mm while the projection range was 120 degrees and the light amount unevenness was 10 % or less.
In general, a plurality of lenses in MLA are regularly arranged, and this arrangement causes light interference. Since the light amount unevenness occurred due to the interference, the unevenness could be reduced by arranging the lenses at random in three axes. The light amount unevenness was also caused by transfer errors that occurred in the molding process. The shape error of lens was controlled to nanometer-size by developing a correction process that functioned the shape error and adapted it to mold processing. Although a high refractive resin was used to widen the projection angle, it was difficult to manufacture a thin lens by injection molding because of the high viscosity. This problem was also solved by developing a heating and cooling method in the molding process. As a result, by optimizing the lens arrangement and controlling the error of the lens shape, the light amount unevenness of projection could be suppressed to 10% or less.

Reduction of unwanted light reflection from a surface of a substance is very essential for the improvement of the performance of optical and photonic devices. Anti-reflection (AR) surface textures can be created on the surface of lenses and other optical elements to reduce the intensity of surface reflections. AR textures are indispensable in numerous applications, both low and high power, and are increasingly demanded on highly curved optical components.

Nanofabrication involves the fabrication of devices at the nanometer scale. In this work, we used nanofabrication to design and fabricate nanostructures of squares and hexagons of different spatial pitch and gap width in Gallium Arsenide (GaAs). These structures have a gap of 300nm, 400nm, and pitch of 900nm, 1000nm and 1100nm. The fabrication process involves solvent cleaning, deposition of silicon oxide, soft and hard bake, photolithography and development. Both wet and dry etching were used to fabricate the expected structures. Results from scanning electron microscopy (SEM) to examine the shapes of the fabricated arrays are presented in this study. By combining dry and wet etches, we obtained the desired shapes and depth of hexagons and squares with rounded edges. We report detailed fabrication processes and their corresponding results at each step.

SU-8 photoresist has been applied to three-dimensional (3D) patterning of photonics, micro/nanoelectromechanical systems and microfluidics. SU-8 photoresist can be patterned by absorption of ultraviolet radiation using a photomask; however, diffraction effects in the bulk resist and the use of a 2D mask limits complex 3D structure design. Direct laser writing (DLW) using multiphoton absorption can produce complex, sub-diffraction limited structures in the resist, but controlling DLW in SU-8 is complicated by many competing processes. Using 100 fs pulses at 1.7 μm, we reliably develop SU-8 photoresist via femtosecond optical curing on glass substrate and avoid competition from two, three, and four photon absorption processes. We verify optical curing of the resist by developing the resist without a post-bake.

Refractive micro-optical components are typically limited in their surface profiles to focusing lenses (spherical or parabolic) and axicons (conical). For axi-symmetric lenses, the parabolic shape can be described as the distance from the axis to the power of two up to a scaling coefficient. The fabrication of three-dimensional surface reliefs in hard substrates is challenging even for such traditional optical elements like lenses and axicons. With the advent of focused ion beam milling systems with high-brightness inductively-coupled plasma sources rapid prototyping of optical devices without a constraint on the surface profile is becoming possible. Here we demonstrate the fabrication of cylindrical lenses with the profile-exponent between 1.5 and 2.5 using 200 nA of focused Xe ions to directly and rapidly carve the desired profiles. Furthermore, we fabricated the micro-optical elements directly in single-crystal lithium niobate - an exciting optoelectrical material with a high refractive index of >2.2. Due to its high inertness and resistance to most etchants, lithium niobate has the reputation of being difficult to use for the realization of photonic nanostructures, however, its gray-scale patterning by direct milling with noble and heavy Xe ions is straightforward. We analyzed the optical performance and beam-shaping properties of the fabricated microlenses by collecting series of intensity profiles at various planes and compared the experimental results with the numerical simulations. The results indicate that free-form optical elements allow enhancement of various optical properties, such as extension of the depth-of-focus and resolution improvement.