The tasks and research topics in the field of technical optics, which mainly or only marginally deal with lens design, are very diverse. One focus of lens design is first-order design. For the optimized design of complex optical systems with defined pupil and image planes, a numerical calculation of the optical thin component parameters on the basis of the paraxial equation model is required before the next steps in the optical design are started. The software PARAX was specially developed for so-called collinear design. It includes an extensive library of examples that demonstrate the functionality of the software tool. With the PARAX program, for example, start systems for zoom lenses can be found . The movements of the variator and the compensator can be visualized with the help of an integrated parameter variation. The possibility of parameter variation also enables the conversion of a classic zoom system into zoom systems with tunable lenses. The commercial programs ZEMAX(TM) and CODE V(TM) are available for further steps to optimize a found start system.
Another field of research are optical microresonators and in detail whispering gallery resonators. These are a special type optical microresonators which confine the light inside the resonator by total internal reflection. The light then propagates near the circumference of the resonator. These resonators can exhibit exceptional high q-factor and are a promising field of research to study: resonant effects, nonlinear optics, quantum optics, microlasers, microsensor and even novel light sources.
A main challenge is the fabrication of these resonators due to their high sensitivity to environment perturbations. For example, we need to achieve surface roughness of optimally below 1nm to not limit the q-factor.
Another interesting subfield are deformed microresonator. These deviate in shape from a circle which leads to a chaotic trajectory inside the cavity. This makes it an interesting object of study in the field of quantum chaos. Furthermore, some of these deformed Microresonators exhibit a strongly directional farfield. We investigate these kinds of resonators which follow the form of a shortegg by doing a Time Correlated Single Photon Counting (TCSPC) analysis which allows us to monitor the decay of the light emission. In addition, we fabricate polymer resonators which a doped with fluorescent dyes which can act as microlasers.
To fabricate optical elements at our group we use the cleanroom facilities of the ZMN, which is equipped with several different plasma etching plants. We usually fabricate structures like diffractive optical elements but in recent times we expanded our capabilities to fabricate subwavelength structures and optical microresonators. our requirements for the processes for the production of DOE are: homogeneous etch rate over a wafer, stable etch rates to control precisely the depths of the DOE, low surface roughness to reduce scattering losses and high shape fidelity.
For the fabrication of optical elements, we can use different approaches. These approaches are usually conducted using a ICP-RIE with fluorocarbon chemistry.
The standard etching of a masked substrate. Depending on the requirements we can use normal photoresist masks or metal hardmask which are a lot more durable. The resulting etching profile is binary and the etch depths is controlled by the etch time
The proportional transfer (a etch process to transfer a 3D shaped photoresist mask usually fabricated with greyscale lithography into the substrate by using a fixed selectivity near 1
2.5D etch process which can vary the flank angle and therefore achieve quasi 3D structures with different etch steps.
The Materials we primarily use are SiO2 (fused silica), Si (for IR applications) and hybrid polymers (like OrmoComp) but we are working on Si3N4 and maybe in the future TiO2.
The structuring of surfaces in the micro- and nanometre regime offers great application potential in many fields of optics. For example, diffraction gratings with specifically adapted polarization properties can be realized.
In addition to the state-of-the-art lithographic technologies of mask-based UV- and electron beam lithography, we also have two maskless direct exposure systems available for lithography. One is the MLA150 Maskless Aligner from Heidelberg Instruments and the other is a LW405 laser direct writer from Microtech.
With these two systems, different designs for the fabrication of optical elements can be realized quickly and cost-efficiently. Furthermore, it is possible to produce spherical as well as aspherical lenses by means of gray scale lithography.
For the fast patterning of large areas with nanostructures, we have another technology at our disposal. The Soft-UV Nanoimprint Lithography (Soft-UV-NIL) System from Gd nano. In this process, nanostructures are transferred over a large area into a UV-NIL photoresist on a substrate via an imprinted stamp negative. Immediately after the structures are imprinted, the UV-NIL resist is cured using a UV light source. The structures created can serve as etch masking. It is also possible to use already curved surfaces as substrate.
Focus tunable lenses in the form of membrane lenses and fluidic lenses have recently found increasingly widespread application in machine vision and cameras. However, their use in commercial systems as of now seems to be mostly limited to providing fast autofocus, although different authors have shown that they offer the potential for more lightweight and more compact optical zoom lenses. One challenge with systems containing focus tunable lenses is, that it is almost never possible to achieve aberration correction for the tunable lens itself, at least for membrane and fluidic lenses. The whole system has to compensate for the aberrations of the focus tunable lens. Therefore, our work focusses on considering the performance of the whole system from the beginning of the design process, searching the system configuration best suited for the specific limitations of the focus tunable lens, as well as investigation of different concepts of focus tunable lenses such as refractive and diffractive Alvarez-Lohmann lenses.
The exploitation of the resonant effects of sub-wavelength structures enables us to enter the nano-optical regime. Structuring diffractive optical elements (DOE) with subwavelength structures means adding an additional degree of freedom in the design of compact optical elements.
We use the Soft-UV-Nanoimprint-Lithography (NIL) as a potentially cost-effective manufacturing method to reproduce small structures on a large scale. Using both the effective medium approximation (EMA) and rigorous methods (RCWA), we designed and manufactured a binary subwavelength-structured form-birefringent diffraction grating, which acts as a polarizing beamsplitter for a wide range of incidence angles −30°… +30°.
The designing and manufacturing methods we refined and developed, potentially enable us to realize integrated compact optical measurement systems, such as common-path interferometers. Furthermore, our methods can be applied to realize other polarization-dependent DOEs and can be extended for multilevel elements.
Expansion of the flexibility and applicability for sub-micrometer patterning in large working volumes is one of the main goals in maskless holographic lithography. We explore two- and three-dimensional holographic pattern projection with the use of spatial light modulator-based (SLM-based) exposure systems.
Using a state of the art nanopositioning- and nanometrology-machine (NPMM) allows us to achieve high stitching and alignment accuracy, which results in precise fabricated structures. To reach high quality and resolution of projected patterns, we investigate algorithms for calculating computer generated holograms, which are displayed on the spatial modulator, as well as innovative methods to accelerate a calculation process.
Additionally, we investigate an integration of the exposure setup into a NPMM with a compact optical setup. This can be attained for example with the help of modified hologram calculation algorithms or by implementing transmissive light modulators.
3D light shaping has applications in illumination and imaging in optical microscopy, optical lithography and positioning of multiple micro-particles in 3D space.
We designed a multi-focal diffractive lens that generates a desired number of highly efficient axial foci with uniform intensity. By overlapping the phase structures of specific diffraction gratings, a 3D array of uniform focal spots in transverse and axial directions can also be generated. Additionally, we integrated a spiral phase into the phase structure of the multi-focal diffractive lens, and generated axial arrays of rotatable structured optical beams such as petal beams and optical ring lattices.
Based on the concept of Alvarez-Lohmann lenses, we explore a tunable multi-focal diffractive lens, which provides precise adjustment of the multiple focus positions along the propagation direction. This paves the way for simultaneous and dynamic 3D manipulations and 3D multi-plane fluorescence microscopy.
Fluorescence Imaging is a non invasive technique based on the excitation of fluorophores, which then emit light with a longer wavelength. Usually, different filters are used to separate the light of the excitation, so that only the emitted fluorescence light is used for the sample imaging.
In a Light Sheet Fluorescence Microscope (LSFM) a fluorescing sample, which can be stained or autofluorescing, is illuminated with a thin sheet of light generated, e.g. by a cylindrical lens. In combination with diffractive optical elements sophisticated light sheet distributions can be formed, for example a focused Talbot carpet for structured illumination with its self-healing properties. The emitted fluorescence light is then imaged through an orthogonally placed detection arm onto a 2D detector. By a relative movement of the sample through the set-up an image stack can be recorded and assembled to a 3D-picture of the sample.
Fluorescence lifetime imaging microscopy (FLIM) is an imaging technique which is based on the difference in the lifetimes of fluorescence effects in a sample environment, and can be used as an additional source of information about the observed sample. This is usually achieved by the excitation with a pulsed laser source and single photon detection. The imaging device is synchronized with the pulsed laser and records the photons hitting the detector at a certain time. Frequencies of MHz are used to gather enough information. The image and the additional time domain is then recorded to acquire the decay histogram, which obeys the Poisson statistics. A main application is so-called FRET (Förster Resonance energy transfer) imaging, which e.g. allows advanced monitoring of living cells in combination with a suitable fluorophore.
In our group we perform research on innovative LSFM set-ups in combination with FLIM measurements for the characterization of 3D cell cultures on one hand and the structure analysis of silicate materials by means of naturally fluorescing crystals on the other hand.
