Interference lithography has become a powerful platform technology for fabricating structures across nano-, micro-, and mesoscale dimensions. Although its fundamental principle has been established for decades, ongoing advances have steadily expanded its capabilities, overcoming previous limitations. It is now a key enabling technology in industrial optical manufacturing while remaining relevant for cutting-edge research. This tutorial introduces the fundamental working principles, offering students and researchers both a structured entry point and insights into more advanced aspects of the technology. Key learning outcomes include an overview of different exposure setups and strategies to achieve diverse profile geometries, including complementary processing steps. Practical guidance on implementing interference lithographic process chains is also provided. Application examples include spectroscopic gratings, imaging diffractive lenses, anti-reflective “moth-eye” structures, and other specialized optical elements, illustrating the broad range of achievable structures and their application contexts.

Spectral confocal metrology exploits the axial chromatic dispersion of optical systems for measuring surface profiles and thicknesses of partially reflective and transparent samples. Previous approaches are limited to a single axial range of chromatic focal shift, spanning from the shortest to the longest wavelength, as the depth-sensing range. Here, we present for the first time, to our knowledge, an approach in which a bifocal hybrid lens enables two independently adjustable, distinct axial regions of chromatic focal shift to serve as depth-measurement ranges. By exploiting two independently addressable axial spectral regions, the approach enables an extended overall depth-measurement range, provides control over the axial positions of the spectral foci, and permits tailoring of their relative intensities via the spectral diffraction efficiency. Consequently, the design flexibility is significantly increased, allowing adaptation to a wide range of applications. These concepts are demonstrated using simple bifocal hybrid optical designs optimized through ray-tracing simulations.

We present optical design concepts for a miniaturized multimodal endoscopic imaging system capable of targeting the same sample plane across widely separated wavelength ranges. The system uses a scanning-fiber approach and exploits the wavelength-selective imaging properties of diffractive optical elements (DOEs), allowing different diffraction orders to form images for distinct spectral bands and partially decoupling the optical design for each range. The system is designed to achieve high first-order diffraction efficiency in the UV/short visible region while simultaneously operating in the zeroth order for 1300-1700 nm. Angular emission variations of the scanning fiber are compensated, enabling a compact layout and low-loss beam delivery. Both three- and two-element designs with strongly aspheric surfaces are analyzed. Backward light propagation to the scanning fiber is evaluated theoretically, highlighting efficient collection via the fiber cladding and its potential enhancement by introducing ring-shaped lenses and reflective surfaces.

This paper presents the design and implementation of compact, fully axial wavelength-scanning systems for infrared spectroscopy. The proposed systems are based on an optical design that employs a hyperchromatic approach and a simple on-axis structure with a point detector. Two implementations are investigated: a purely refractive system combining highly and weakly dispersive elements and a hybrid system incorporating both diffractive optical elements (DOEs) and refractive components. Wavelength scanning is achieved by axially shifting an optical element, with the required displacement being significantly smaller than the corresponding axial spectral range. The hybrid spectrometer exceeds the performance of the purely refractive system, achieving a nearly constant resolution of up to 30 nm over the 1000-2200 nm wavelength range. A minimum equivalent Abbe number of 0.22 and a corresponding axial chromatic spread of 90 mm enables this characteristic. These results are achieved in a compact design, requiring an adjustment range of only 4.3 mm for the tunable optical component-approximately 1/20 of the axial chromatic spread. The findings underscore the feasibility of this axial wavelength-scanning system for NIR spectrometry, providing a compact and cost-effective alternative to existing technologies with potential applications across various scientific and industrial fields.

Multifocal diffractive optical elements (MFDOEs) form images at different scene distances using interleaved sawtooth patterns. However, single-layer MFDOEs exhibit wavelength-dependent efficiency distributions, causing chromatic variations. This work introduces multilayer MFDOEs with two materials of different dispersion characteristics being separated by a sawtooth structure, enabling efficiency achromatization across multiple diffraction orders. The focus is on selecting two materials with a wavelength-dependent refractive index difference Δn (λ), for which the refractive index contrast Δn(λ)/λ remains nearly constant. Exemplarily, the inorganic glass K-SSK3 and the polymer PS are investigated. The achromatization degree is assessed using mean absolute error (MAE) and root mean square error (RMSE). Optimal achromatic efficiency is demonstrated for orders m = 0, 1, 2. For specific geometries, simultaneous achromatization is achieved, distributing efficiency as 50% in m = 1 and 25% in m = 0, 2 over the wavelength range from 0.5 to 0.8 µm. These results advance the development of highly efficient achromatic MFDOEs.

Filter-based spectral systems are highly competitive due to their compactness, simplicity, and well-defined spectral characteristics. However, their primary drawback remains low detection efficiency. This work explores various strategies to enhance detection efficiency. While an additional row of beamsplitters can significantly improve illumination, alternative folded beam path designs—eliminating the need for beamsplitters—prove to be far more effective. Additionally, a novel approach utilizing a freeform mirror is introduced, enabling differential adjustment of detection efficiency across different spectral regions. For the first time, a comprehensive comparison of these strategies is presented.

Hybrid polymers combine the benefits of inorganic and organic material properties, offering superior thermal, mechanical, and chemical stability, making them ideal for optical applications. This study focuses on the fabrication and characterization of antireflective (AR) structures within hybrid polymers using reactive ion etching (RIE). The etching process produces nanopillars with controlled heights, achieving excellent AR performance across a broad spectral range from 450 nm to 2 µm. Optical characterization, including angle-resolved transmission and reflection measurements, shows that the structured samples maintain high transmission efficiency and reduced reflectance at varying incidence angles. Thermal stability tests reveal that the AR structures preserve their optical properties after exposure to temperatures up to 250 °C. Higher temperatures cause significant material yellowing, which is attributed to changes in the bulk material rather than damage to the structured surface. Hydrophobicity measurements show significant water repellency in structured samples, with contact angles more than twice those of unstructured layers. These findings highlight the potential of hybrid polymers with moth-eye-inspired nanostructures for high-performance, durable optical components in demanding environments.

This contribution presents the concept, optical design, implementation, and testing of a compact cross-grating spectrometer that integrates all optical functionalities of a classical Echelle spectrometer, including imaging and two-dimensional dispersion, into a single optical element. This element is a concave cross-grating formed by the superposition of two perpendicularly oriented blazed gratings on a concave surface. With reference to its well-established circular predecessor, the introduced concept can be regarded as a progression towards a Rowland-sphere spectrometer. Experimental measurements demonstrate a spectral range of 400-1100 nm with a spectral resolution exceeding 102 [λ/Δλ] (Δλ ∼ 6.2 nm @ 633.4 nm). The grating was fabricated using two-photon lithography, enabling the creation of a fine grating structure on a curved surface.

Challenges in microstructured optics require bridging meso- and nanoscale dimensions. Advancements in process chains, including lithography and replication, are essential for commercial viability. This talk explores novel approaches for scalable and efficient manufacturing. Full-text article not available; see video presentation

Multifocal diffractive optical elements (MFDOEs) as interleaved sawtooth structures with alternating heights are established as single-material layers. While the distribution of diffraction efficiencies across different orders can be selected for a specific wavelength, it is fixed for other wavelengths due to the dispersion of the material. In this work, we investigate multilayer MFDOEs consisting of two layers of sawtooth structures to enable tailored wavelength selectivity, especially the controlled efficiency distribution into specific diffraction orders for selected wavelengths or wavelength ranges. Using scalar diffraction theory, we classify material combinations into four categories based on their dispersion characteristics. This allows for the design of structures that achieve high diffraction efficiencies in either narrow or broadband spectral ranges, directing light into the zeroth, multiple positive, or negative diffraction orders. Using representative material combinations, we evaluate key quantitative measures, including maximum efficiency, wavelength of maximum efficiency, and the full width at half maximum for narrowband efficiency peaks. Broad spectral efficiency maxima are characterized by their average efficiency and root mean square error as a measure of uniformity. These parameters can be adjusted by varying the structure depths, paving the way for wavelength-selective multifocal imaging in microscopy and medical applications.

In contrast to conventional optical systems, which are optimized for wavelength-independent imaging, hyperchromats aim for strongly wavelength-dependent focal lengths. In this contribution, the design parameters of hyperchromatic two-lens optical systems were derived that provide strong axial color splitting expressed by extremely low equivalent Abbe numbers. These systems have been investigated for compositions of either pure refractive or all diffractive lenses, as well as hybrid configurations thereof. First, lens doublets made of cemented elements are considered and the variables affecting the equivalent Abbe number of the system are investigated. In particular, the influence of the focal lengths of the individual lenses and the Abbe numbers of the selected lens materials are taken into account. The best parameter-sets were determined by paraxial numerical simulations for different cemented configurations. To ensure a simple implementation, especially to avoid exotic or potentially harmful materials, only readily available inorganic standard glasses were considered. In the next phase of this investigation an air gap was inserted between the two lenses, which is an additional influence parameter on the equivalent Abbe number. Following the paraxial considerations, selected two-lens configurations were transferred to the non-paraxial domain and refined using optical design software, also taking aberrations into account. To further reduce achievable equivalent Abbe numbers, an aspherical surface was introduced to compensate for spherical aberrations. Finally, for the refractive doublets an equivalent Abbe number of 2.4 was achieved, which corresponds to only 12% of the smallest Abbe number of the selected materials. This result was even surpassed by the hybrid hyperchromat, resulting in an extraordinary minimum equivalent Abbe number of -0.6 that is more than five times smaller than the Abbe number of diffractive lenses.

To accurately model the specific detection characteristics of spectral sensors based on linear variable filters (LVFs) within an optical design tool, it is essential to consider crucial position-variable spectral properties, such as peak transmittance, central wavelength, half width, or slope steepness. In this context, we propose a straightforward approach, integrating a dynamic link library (DLL) containing all position-dependent spectral properties of the LVF into a commercial optical design software. Exemplary investigations are conducted for an LVF with a detection range of 450-850 nm. For ease of use, the measured position-, wavelength-, and angle-dependent transmission properties of the LVF have been described through a simple yet highly accurate model system. Moreover, to highlight the essential value of this simulation for specific applications, an efficiency-enhancing spectral module is simulated, which is an LVF-mirror arrangement characterized by a multiple-reflected beam path. The introduced optical design tool demonstrates its particular strength by enabling the optimization of the highest detection efficiency for either the short- or long-wavelength range. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

This study explores the design and optimization of cascaded double-hyperchromatic optical systems (i.e., 2×2 lenses), focusing on achieving an extremely linear axial spectral decomposition characterized by an exceptionally low equivalent Abbe number. The investigation involves two double hyperchromats, considering both purely refractive systems and hybrid configurations that combine refractive and diffractive elements. For purely refractive systems, alternating focal length signs of divergent and collective lenses are crucial to achieve significant axial chromatic dispersion. In hybrid systems, the position of the diffractive optical element (DOE) and the selection of focal lengths play key roles in obtaining extremely low equivalent Abbe numbers. The optimized systems demonstrate absolute equivalent Abbe numbers of 0.983 for purely refractive and 0.65 for hybrid systems—more than four times lower than the absolute Abbe number of a single diffractive element. Notably, even systems using standard materials exhibit significantly low equivalent Abbe numbers of 2.5 and 1.4 for pure refractive and hybrid configurations, respectively. These results offer promising opportunities for improving optical applications based on axial spectral decomposition, overcoming previous limitations of axial chromatic spreading.

This paper presents an advanced exposure system that, we believe, for the first time, enables annular lithography to create micro- and mesostructures on curved surfaces. The principle of ring creation is based on an axicon zoom system whereas the exposure tool exhibits several features that enhance its performance over previous models. Its optical design features a high numerical aperture, resulting in narrow ring widths and high resolution. Additionally, the ring diameter, which can be varied between 400 µm and 8 mm, remains constant along the optical axis due to telecentric imaging. Further functional components include an observation unit for alignment and monitoring, as well as an integrated autofocus system. In addition to determining the exposure ring width (∼ 10 µm), diffractive lenses on planar substrates as well as on a spherical lens with a radius of curvature of 68.8 mm were structured. The periods of the diffractive structure varied between 60 µm and 460 µm.

This study demonstrates the concept of an angle-variable compact spectral module. As a key feature, the filter-based module enables highly efficient wavelength-selective light detection by applying the reflective beam path according to the origami example. It was accomplished through inclined mirrors, which allow for different incident angles on the wavelength separating interference filters used in a robust assembly with no moving parts. To experimentally verify the concept, a wavelength range between 550 and 700 nm was detected by 11 spectral channels. These initial results showed the potential to develop easily scalable and application-tailored sensors, which can overcome conventional filter-based sensor approaches that use upright or fixed-angle illumination.

This work provides a comprehensive analysis of the maximum chromatic axial split of two-element hyperchromats, with the distance between the two lenses being a key variable. Purely refractive and diffractive systems are considered, as well as hybrid layouts combining refractive and diffractive elements. In order to achieve extreme chromatic axial splitting and accordingly a minimum equivalent Abbe number for lens combinations, a three-step procedure was used. In the first paraxial step, purely optical quantities such as focal lengths of the lenses, inter-lens distances and dispersion properties of the lenses were investigated. In the second step, which also takes place in the paraxial domain, additional geometric boundary conditions such as the radii, diameters and thicknesses of the lenses are taken into account. The results of this step serve as an input for the final optimization using optical design software, which derives practical solutions for minimum equivalent Abbe numbers with diffraction-limited image quality. As a significant result, the comparison with directly cemented lens doublets shows that the introduction of a distance between the elements allows for a much stronger chromatic decomposition for refractive, diffractive and also hybrid combinations. Quantitatively, the minimum equivalent Abbe number for refractive systems is reduced from 2.5 (without spacing) to 1.79 (with spacing). For hybrid combinations, a corresponding reduction from 0.4 to 0.29 is achieved.

Echelle-inspired cross-grating spectrometers try to combine the high performance of classical Echelle spectrometers and the small footprint of compact line-grating spectrometers. Therefore, a cross-grating is used which is a superposition of two perpendicularly oriented line gratings in a single element. Highly resolved, but overlapping, diffractions orders are created by the main grating, which are separated by the cross-disperser. This powerful approach is connected to different challenges concerning the optical design, the fabrication of the cross-grating and implementation of the device. These challenges are addressed by a compact and rigid double-pass design, which utilizes the same refractive elements for collimation of the incoming beam and focusing of the diffracted light on the detector. This contribution gives an overview on the design and focusses on the implementation of the spectrometer. This includes on one hand the mounting of the cross-grating and the refractive elements in a rigid objective group and, on the other hand, the adjustment of the objective to the entrance fiber and the 2D detector. Furthermore, the implemented and calibrated instrument allows to conduct several validating experimental tests in order to proof the working principle. The spectrometer addresses a spectral range from 400 nm to 1100 nm and reaches a resolving power of 300 with an entrance pinhole diameter of 105 μm. An even higher resolving power of more than 1000 is reached with a reduced pinhole diameter of approximately 5 μm.

Annular lithography is a recently introduced, flexible technique that has been tailored to the fabrication of rotationally symmetric optical structures in the meso and micro range. The optical concept for the exposure tool is based on a combination of axicons with movable components that create a ring-shaped light distribution with variable diameter in the image plane. This contribution demonstrates for the first time the use of gray tone exposure in annular lithography to fabricate continuous relief structures, overcoming the previous limitation using binary structures. For the controlled exposure of the continuous relief structures, the sensitivity curve of the resist, the exposure dose decreasing with increasing ring diameter, and the exposure time have to be considered. A control and simulation tool is introduced to provide radius-dependent exposure data and, furthermore, to control and iteratively improve the fabricated structures. To demonstrate the gray tone capabilities, various diffractive elements as well as refractive spherical and aspherical elements with a maximum diameter of ~6 mm and a maximum height of 4 µm are shown as examples. Profile shape measurements of fabricated elements show good agreement with the expectations.

Filter-based spectral detectors convince with their simple concept, an extremely compact and robust design and the possibility to adapt the addressed spectral range and the resolution to the individual application requirements. Unfortunately, these filter-based sensors usually suffer from low detection efficiency. In this contribution we discuss and compare different methods that allow to substantially increase the detection efficiency of filter-based spectral sensors. An initial concept is based on a wavelength-dependent redistribution of the incident light before it reaches the individual filter elements of the array. This approach allows a substantial increase in detection efficiency, but requires additional dichroic elements in the beam path. An alternative approach uses a folded beam path architecture and completely waives additional dichroic elements. This approach is not only suitable for filter-based spectral sensors, but can also be transferred to increase the efficiency of hyperspectral imaging systems.

Diffractive optical elements (DOEs) fundamentally provide the possibility to simultaneously utilize multiple orders for different imaging functions within a system. However, to take advantage of this property, it is necessary to tailor the assignment of specific wavelengths or wavelength ranges with high diffraction efficiency to specific diffraction orders. To achieve this wavelength-selective assignment to different orders, simple diffractive profile shapes are not suitable; instead, multilayer DOEs are required. In this study, we conducted theoretical, scalar investigations on the diffraction efficiency of triple-layer double-relief DOEs for the purpose of tailored wavelength selectivity. Specific materials such as nanocomposites, layer materials, and high-refractive-index liquids with strong dispersion were included, in addition to inorganic glasses, to enable wide design freedom for wavelength selectivity across multiple orders. To simultaneously account for both positive and negative orders, specific material combinations featuring intersecting or touching dispersion curves were utilized. For various material combinations, we calculated significantly different efficiency profiles for multiple orders by varying the relief depths. Further, we discuss the possibility of fine-tuning the efficiency profiles by using high-index liquids as an intermediate layer between two solid profiles, whose dispersion properties can be varied continuously or at least in small steps.