1,721,110 research outputs found
ON THE DESIGN AND OPTIMIZATION OF WIDE ANGLE DIFFRACTIVE OPTICAL ELEMENTS
No full textDiffractive optical elements (DOEs) which consist of microstructure surface relief permit the generation of the spatial distribution of light beams by using a single element. Due to their compact size, design flexibility, and mass productivity, they are used for a variety of applications, from optical metrology to biotechnology. However, the required wide angle DOEs have been elusive due to design challenges. Conventional design approaches such as iterative Fourier transform algorithm (ITFA) fail when applied to DOEs containing very small features or nano-structures. But it is exactly the small features that are required to create high performance wide diffraction angle diffractive optics. This thesis aims to extend the range of DOEs applications by developing designing and optimization algorithms for wide angle DOEs which is far beyond the limits of scalar paraxial diffraction model such as thin element approximation (TEA).
The development of inverse design, where computational optimization techniques are used to find the geometry needed for the desired functionality, has led to the discovery of superior and non-intuitive design. Among various approaches, gradient-based optimization methods have been one of the most important techniques to obtain the optimal structure described by a huge number of design variables. These methodologies are made possible when the gradient of a merit function with respect to all design parameters efficiently enables to be calculated. Here, two approaches are considered: optimization based on the step transition perturbation approach and the adjoint-state method.
The step transition perturbation approach (STPA) is based on the evaluation of local field perturbations due to sharp surface profile transitions. When we used the positions of transition points as design parameters in DOEs, it facilitates describing an analytical solution of the gradient of diffraction efficiency with respect to the positions of transition points. The gradient-based optimization with STPA creates various one-dimensional diffractive beam splitters generating wide angle spot arrays. The results of the experimental characterization confirm that this optimization tool is valid for wide angle DOEs.
We discuss the adjoint method with rigorous electromagnetic theory, for example, rigorous coupled-wave analysis (RCWA), to optimize the DOEs with small features for generating even wider angles. Due to the adjoint method, we can compute the gradient of the objective function with respect to all design parameters efficiently even using a rigorous electromagnetic calculation. Hence, the permittivity distribution in the geometry of DOEs is used as the design variable during the optimization. This method also is able to account for application-dependent target functions while ensuring compatibility with existing fabrication processes. Thus we design the various wide angle DOEs including two-dimensional diffractive beam splitters by adjoint method. The results of the experimental characterization confirm that this optimization tool is valid in wide angle beam splitters creating a square spot array with maximal diffraction angle up to 53° from the center to diagonal edges, which is far beyond the limit of any scalar paraxial diffraction regime.NA
On the assessment of aspheric refractive microlenses
Get PDFRefractive microlenses and microlens arrays are key components for many applications such as optical data communication, laser and medical devices, or cameras. In particular, refractive micro-optics enables the miniaturization of high-tech systems but also offers novel optical functionalities. The success of this technology lies in the wafer-level fabrication technique, using the method of photoresist reflow with a subsequent pattern transfer into the substrate by reactive ion etching. Indeed, it allows the manufacturing in parallel of thousands of spherical or aspheric microlenses smaller than 1 mm. Their characterization is usually performed by measuring their surface, as this allows at the same time the evaluation of the microlens performance and feedback for fabrication process optimization. In this thesis, we assess this characterization approach to understand the fabrication process better and to improve the microlenses performance. Concretely, we first study surface form tolerancing, which is crucial to ensure the microlenses quality. However, the link between the surface form of a microlens and its performance is not straightforward, usually resulting in over-restrictive tolerances. Here, we investigate this connection for simple cases and then compare different approaches to tolerance typical micro-optical systems. Practical guidelines are proposed based on the results. Secondly, we present methods to improve surface measurements. For this, we develop an original calibration procedure that takes into account the aberrations of the imaging system. In the presented example, the accuracy is increased by a factor 7, rendering the characterization of diffraction-limited microlenses with high numerical apertures possible. Thirdly, we model the fabrication process to find correlations with the manufactured surface. Thereby, the fabrication optimization is made faster and more accurate. We validate this approach by increasing the uniformity of a large (100mm x 100mm) microlens array by a factor ~3. Finally, we evaluate another microlens characterization that consists of probing the optical functionalities in transmission and compare it to surface measurements. Particularly, we give the reasons for our doubts about its application to wafer-level microlenses. In conclusion, we show that a quantitative analysis of the microlens characterization allows for a significant improvement of the microlens quality and a better understanding of the fabrication process, resulting in lower production cost. For this reason, we believe that the results presented in this thesis will help to render wafer-level refractive micro-optics a more mature technology and build its bright future.NA
ON WAVEFRONT ABERRATIONS IN ASYMMETRIC AND MULTIPLE APERTURE OPTICAL SYSTEMS
No full textThe present work deals with monochromatic wavefront aberrations in optical systems without symmetries. The treatment begins with a class of systems characterized by misaligned spherical surfaces whose behavior is analyzed using the wavefront aberration expansion proposed in the framework of the Nodal Aberration Theory (NAT). It is derived the full field behavior of the Zernike polynomials in the Fringe indexing scheme for this class of systems. Then, the attention is focused on a more general class of asymmetric systems where the misaligned surfaces can be individually double-plane symmetric. In this case, considering aberrations up to the 4th order, it is shown that the field dependence of Zernike terms is described by general second-degree polynomials. The presence of double-plane symmetric optical surfaces induces additional perturbations to the magnitude of the field variation of primary aberrations for this class of systems. In particular, one observes that coma aberration acquires an elliptical conic shape in the field domain, while the full field variation of primary astigmatism magnitude is described by a class of surfaces that we define as â generalized Cassini surfacesË® because these are more general than the standard Cassini surfaces describing the binodal behavior of astigmatism in the class of optical systems analyzed with NAT wavefront aberration expansion.
These considerations are preliminary to the discussion of the second part of this thesis whose scope is to analyze monochromatic wavefront aberrations in a further class of systems, namely optical systems characterized by multiple apertures. In this sense, it is first introduced a general description of the wavefront aberration function in the framework of Hamiltonian Optics. This consists of a full power series expansion in the ray coordinates that provides the most general representation of optical systems without symmetries. These introductory remarks are necessary to carry out the analysis of optical systems with many apertures. Such a class of systems is well represented by light field (or plenoptic) cameras. Their general structure consists of a main objective followed by an ensemble of apertures whose function is to divide the field of view into many partitions. Each aperture defines an optical channel. The partial overlap between adjacent field of view partitions serves to extract depth information from the scene in a similar manner to stereo cameras. The wavefront aberration analysis of this class of systems is primarily based on the definition of an ensemble of base-rays playing the role of reference axis for the various channels. The wavefront error for each optical channel is described with a general power series in the ray coordinates expanded about the inherent base-ray. Finally, different approaches are expounded to calculate and visualize the evolution of the aberration behavior of the various channels of this class of optical systems.NA
Structured light generation by refractive-diffractive micro optical array illuminators
No full textStructured light generation having broad applications in different optical fields, is the topic of this thesis. Our structured light generation strategy is based on applying periodic microoptical elements at the refraction-diffraction limit, under a focused diverging source instead of a plane wave. The high contrast pattern in the far-field is achieved for certain distances between the source and periodic structure, where the self-imaging condition is satisfied. This phenomenon is the basis of our strategy to obtain a high-contrast far-field distribution. Throughout this thesis, we analytically, numerically, and experimentally examine the self-imaging condition for our light generators. We engineer the structured light using two main strategies; modifying the periodic microoptical element surface profile and modulate the source by applying an optical element in its near-field.
For surface profile modulation, we apply a sinusoidal phase grating instead of conventional periodic optical elements such as microlenses under a Gaussian beam illumination to increase the number of points in the far-field distribution. We study the far-field distribution for thin and thick sinusoidal phase gratings by comparing vectorial and scalar simulation tools for paraxial and non-paraxial diffraction angles. By properly choosing the sinusoidal phase grating thickness, large numbers of peaks with a high field of view and uniform distribution in the far-field can be achieved. We use the Two-photon Polymerization (2PP) as a rapid technique to fabricate the sinusoidal phase grating and compare the measurement results with simulations. This part of the thesis demonstrates that by properly defining the refractive-diffractive microoptical element surface profile, we can achieve even more points in far-field compared to other optical elements such as lens arrays.
In the next part, we engineer the structured light in the far-field by modifying the source near-field. By bringing a dielectric microstructure in the source near-field, a source with new optical characteristics is produced. For a dielectric microparticle, for example, a hot spot i.e a photonic nanojet (PNJ) is generated in the shadow side surface of the structure that can redistribute the dots in the far-field. We first numerically investigate the PNJ optical characteristics by changing the microsphere diameter for diverging and converging sources of low and high wavefront curvatures and compare with plane wave illumination. The PNJ shows completely different behaviors under converging and diverging illuminations when changing the particle size. In some cases, no hot spot is generated in the microparticle near-field. In this way, we can generate different sources from low to high numerical apertures.
For the experimental evaluation, we employ a high-resolution interference microscopy (HRIM) setup which is based on a Mach-Zehnder interferometer to record both the amplitude and phase. Our setup has the flexibility to work with different illumination conditions from plane wave to the Gaussian beam and also to observe both near-field and far-field distributions. We study the far-field distribution for a microlens array under a focused diverging source that is modulated by applying a microparticle in its near-field. With the microparticle in the source near-field, a PNJ is generated and for this reason, the pattern field of view in the far-field is modified.NA
On Advanced Large-Field Photolithography
No full textThe information age we live in today relies on highly integrated devices. They are fabricated
with the help of photolithography, the patterning technique at the heart of their production.
With the continuous demand for higher integration density to achieve ever growing performance
levels, not only the front-end-of-line lithography, responsible for realizing the smallest
structures, is under constant pressure for improvements. At the back-end-of-line, where ICs
are interfaced, and elements like display panels are structured, this drive can be felt as well.
The development here goes towards the processing of substrates of increasing size surpassing
the wafer-level to increase throughput, thus becoming large-field photolithography. The
current resolution requirements here are typically in the 1 &mto 2 &mfor minimumfeature
size.
An investigation in phase-space identifies two possibilities for advanced photolithographic
systems relying on different exposure mechanics. We combine a novel semiconductor laser
light source emitting in the deep ultra-violet at 193 nmwith a beam-shaping system to enable
proximity printing with sub-2 &mresolution with a proximity gap of 20 &m. The integration of
this approach with optical exposure gapmetrology and a high-precision substrate positioning
stage demonstrates the possibility for large-field exposure. In a second approach we realize a
highly integrated micro-optical multi-aperture projection lens to pattern an exposure field of
100mmby 100mm. A mechanical scanner to mount the projection lens, required to achieve
uniform exposure of the entire field, is realized as well. Projection lens and scanner are
integrated with a high-precision substrate positioning stage and a mask aligner illumination
system to demonstrate the ability for large-field photolithographic printing. The optical design
is validated by demonstrating printedminimumfeature sizes of 2 &m. Practical shortcomings
of the system are investigated and strategies to overcome these issues are presented and
discussed.NA
Phase resolved characterization of nanoscale features in optical microstructures
Get PDFIn this study, we apply interferometric microscopy to study the phase, alongside the intensity, of the light field transmitted through a wide variety of samples. Additionally, we conduct those interferometric measurements at different wavelengths within the visible spectrum, probing the spectral evolution of the optical effects that the studied samples create. Using a priori knowledge about the samples, we identify specific features and trends they imprint on the parameters of the transmitted field, thus we optimally analyze the recorded intensity and phase data. We demonstrate that phase maps include useful information that reveal the features of the samples. The wide applicability of the spectrally resolved interference microscopy is the major novelty contributed by the present work. The phase seldom recorded, therefore we first present and explain the tools that we use throughout this work, both in terms of set-up and data post-processing. Next, we show the acquired results, starting from samples with typical dimensions in the few hundreds of microns and concluding with particles of few hundreds of nanometers. More specifically, we study microlenses of high numerical aperture that are difficult to characterize with conventional methods. We propose a novel approach to extract the surface profile of such microlens, providing useful feedback for manufacturing purposes. We do that by using phase measurements at a plane that lies between the surface of the lens and its focal plane. The next sample we study is a phase element, which is naturally easier to discern in the phase domain. We localize specific points in intensity and phase profiles that are correlated to the position of the walls of the actual sample. We also demonstrate the existence of phase singularities in phase measurements that can be proved an invaluable tool in high precision characterization processes. Next, we investigate the photonic nanojet phenomenon, which is the bright spot created on the shadow side of a dielectric sphere. The creation of this bright spot is not achieved through propagation inside the material, like conventional lenses, it is the combination of the light scattered and diffracted by the sphere. Therefore, we study its spectral evolution using intensity measurements and outline the size of the sphere that indicates the crossing from the dominant refractive regime to the diffractive one; combining intensity and phase information, we attempt to identify the size at which the behavior of the dielectric spheres changes again from diffractive to scattering. In the last experiments chapter, we study individual nano-sized particles, which are either simple spheres (dielectric / metallic) or the more complex structures of the core-shell meta-atom. We show that the spectral information of their response in intensity and phase can be used to identify the particle itself (simple dielectric / metal or meta-atom) and assess its responses with the respect to the engineered one (for the meta-atom case). Those examples validate the claimed benefits of the phase exploration. Still, there is ample room for further study; we debate about those prospects in the concluding chapter of this work.OP
On design strategies for binary dielectric metasurfaces based on the Fourier modal method
No full textBinary dielectric metasurfaces are arrays of sub-wavelength structures that act as a thin layer of artificial material. They are generally lossless and relatively simple to fabricate since only a single structuring step is required. By carefully designing the metasurface, the phase, amplitude and the polarization of the incident light can be controlled at will. In practice, fabrication constraints and the limited choice of materials reduce what can be done with metasurfaces. But a wide range of functionalities can be implemented with the proper design techniques and knowledge. This thesis contributes to both.
The modes are key to understand the phenomena occurring inside a metasurface. To facilitate the analysis of the modes, the Poynting operation, which is related to the Poynting vector, is introduced. We describe how this operation can be used to reformulate the boundary condition in order to estimate the reflection and transmission coefficients with reduced knowledge on the modes involved, and to orthonormalized the modes.
The Fourier modal method, which is the method used for the rigorous simulation of metasurfaces in this work, is improved in order to facilitate the access to valuable information that can be used to better understand the phenomena occurring inside a metasurface, and to speed up the design and optimization process. This method computes the eigen-modes present in the metasurface. To better analyze them, the eigen-modes are orthonormalized using the Poynting operation. We show that most of the modes can be filter out in order to simulate a metasurface with different thicknesses in milliseconds.
From the analysis of the modes propagating in metasurfaces, two types of metasurface are identified: single-mode metasurfaces and multi-mode metasurfaces. For single-mode metasurfaces, we provide design techniques that translates the desired response into internal properties of the metasurfaces. For multi-mode metasurfaces, the concept of self-coupling mode is developed. We show that, based on this concept, the angular and spectral response of a metasurface can be interpolated safely with a few simulations even if high-Q resonances are present. For both types of metasurfaces, examples of design are provided.
Gradient-based optimization methods allow to obtain the optimal metasurface in a few iterations, but the derivative of the merit function is necessary. The adjoint method computes the functional derivative of the merit function with respect to the permettivity and permeability. We provide the equations of the adjoint method and apply them to diffractive optical elements such that they can be used in conjunction with the Fourier modal method.
This thesis contributes to design challenges for complex electromagnetic problems,and it does not only provides concepts, but also the tools to put them into operation.NA
On the Limits of Precision Glass Molding for Diffractive Optical Elements
Get PDFDiffractive optical elements (DOEs) consist of surface reliefs with dimensions in the micrometer range and nanometer precision. Two technologies dominate: Elements replicated in plastic and directly microfabricated elements in fused silica. Plastic DOEs are mostly used in mass production because they can be fabricated very cost efficient by replication technologies such as plastic injection molding and hot embossing. Glass DOEs are only used when its superior characteristics e.g. higher temperature stability, higher form accuracy due to a low reaction to humidity and stress are necessary for the specific application. Today, glass DOEs are fabricated by cleanroom technology based on direct structuring of fused silica. In this thesis we investigate the possibility to use precision glass molding to fabricate glass DOEs. Up to now, precision glass molding is used only for continuous surfaces like in aspherical lenses or freeform elements. Diffractive optical elements with more complex structures including surfacediscontinuities (steps) are not found. One reason is the lack of a suitable mold material that can withstand the high molding temperatures and can be microstructured with the necessary accuracy. One potential candidate to close this technology gap is glassy carbon. Glassy carbon is a fullerene like carbon with extreme temperature resistance and unmatched chemical inertness. The key factor of applying glassy carbon is the possibility to structure its surface with the suitable dimensions and conformity. We tested and developed microstructuring processes to overcome limitations such as process compatibility, etch selectivity, structural integrity and surface roughness. Our major objective of this work aimed to investigate the limits of precision glass molding for DOEs. Of special interest are the minimal feature size and maximal aspect ratio that can be obtained by keeping the optical quality of surfaces. We showed that precision glass can replicate features down to 800 nm and the process is stable. With this newly established processes the whole fabrication chain could be tested for the first time including lifetime tests of the stamp. For each step extensive characterization was done. Measuring the optical performance was the last step that allowed us to develop a complete guideline for the fabrication of diffractive optics with precision glass molding. Beamsplitting elements were chosen as test designs, because they allow a rigorous evaluation of the optical performance by measuring the diffraction efficiencies and uniformity distribution. As the main results we could show that precision glass molding of DOEs can reach a comparable optical performance as directly etched fused silica DOEs, which is the state-of-the-art technology. We could confirm that glassy carbon is an excellent mold material for precision glass molding of complex optical surfaces.OP
On Micro Optical Elements for Efficient Light Diffusion
Get PDFEfficient light management is one of the key issues in modern energy conversion systems, be it to collect optical power or to redistribute light generated by high power light emitters. This thesis touches mainly on the subject of efficient light redistribution for high power sources by means of refractive and reflective micro optical elements. Refractive micro optical elements have dimensions that are big enough to neglect diffraction phenomena and small enough to be still manufactured by the methods used in micro fabrication, typically above 50 micron for visible light but below or of the order of one millimeter. The advantage of this limitation that could be called the “refraction limit” is that the design and performance predictions can be based on simple methods such as ray tracing or the edge ray principles for non-imaging optics. In contrary to many studies on engineered diffusers we concentrate here on optical surface where the functional is given by concave shapes! The first part of the thesis treats the development and fabrication of one dimensional small angle diffusers for collimated high power and potentially coherent light sources. The generation of high power laser lines with a uniform intensity distribution is useful for the optimization of laser manufacturing applications such as annealing of amorphous silicon on large surfaces. This is typically needed for the fabrication of TFT’s or thin film solar cells. The one dimensional diffusers discussed in this thesis are based on an array of concave cylindrical microlenses with a typical lens width of 200 μm and a radius of curvature ranging from 300 μm to 1500 μm. In order to avoid diffraction grating effects due to the regular nature of the array a statistical variation of the lens width was introduced. The proposed fabrication process is based on isotropic etching of fused silica in hydrofluoric acid. The fabrication and design parameters were explored and their influence on the final performance determined. Extensive computer simulations based on ray tracing and diffractive beam propagation were compared with the measured performance of fabricated devices. Design rules based on an analytical model were also developed and verified. The performance under real world conditions were tested with good results for the smoothing of laser lines at the Bayrisches Laserzentrum in Erlangen, Germany. The subject of the second part are compact large angle transformers and their possible applications. A short introduction to non-imaging optics and its basic design tools are followed by development of the compound parabolic concentrator (CPC) based on work known for thermal solar concentration. This non-imaging light funnel is concentrating light and has the ability to efficiently transform the angle of an incoming bundle of rays into large angles up to the full half sphere. If inversed, the CPC works as a collimator. The novelty of the approach presented in this thesis lies in the reduced dimensions of the design and the use of the concentrator not as such but rather as an angle transformer with very high efficiency. When the dimensions of the classical solar concentrators are usually of the order of a few 10 cm or more the design developed in this thesis has dimensions of a few mm or less. Different possible applications for a compact CPC array are discussed such as LED collimation at chip level, fiber coupling with large numerical aperture and improved light management for thin film solar cells. The fabrication of a prototype of a compact dielectric filled CPC array as a proof of concept is described and first attempts at its characterization are discussed.OP
High power ultraviolet LED illumination systems: coherence properties and applications in photolithography
Get PDFMask-aligner photolithography is a technology used to replicate patterns from a mask to a photosensitive substrate. It is widely used in the fabrication of MEMS and micro-optical components, and for other applications with dimensions in the micrometer range. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the ultraviolet range with peaks at 365nm, 405nm and 435nm, the so-called g-, h- and i- lines. These lamps suffer from several disadvantages, such as a low efficiency, bulkiness, a short lifetime, and the toxicity of mercury. Finding an alternative to mercury arc lamps would be highly beneficial. In addition, specific techniques in mask-aligner technologies like Talbot lithography, multiple exposures, mask-source optimization or optical proximity correction lithography require high power sources with an increased control over the angular distribution and the spatial coherence. This is not easily done with a mercury arc lamp illumination. A method to easily measure the angular distribution and the spatial coherence in the mask-plane would be of great interest. In recent years, high power ultraviolet LEDs at the same wavelengths have appeared on the market. LEDs possess a smaller Etendue, they can be electronically driven at high frequencies and have a superior lifetime. This makes them ideal candidates to overcome the limitations of the mercury arc lamp illumination systems.
The work focuses on the development and study of a novel LED-based illumination system for mask aligner lithography. This illumination system consists of an array of 7x7 LEDs, with individual reflectors. They form a modular 250W source which can replace a 1kW mercury arc lamp. The light is collected by the reflectors and brought onto a fly¿s eye integrator with two subsequent lenses which shape and homogenize the light field. Different patterns can be created by the source, determining the angles and the spatial coherence in the mask plane. The first part of the work presents the design and a complete set of characterizations of the final prototype. The achieved irradiance uniformity in the mask-plane of a MA/BA8 Gen3 SUSS mask-aligner is within ±1.2-2%. Prints tests in proximity printing, with a gap of 30µm, demonstrate a resolution of 3.5µm.
In the second part of the thesis, the development of a method to measure the spatial coherence, which is based on a double slit approach and backed up by simulation, is described. The link between the spatial coherence length and the angular extent is made and the measurements show a good agreement with the analytical expression. A compact system is described, which enables the measurement of the spatial coherence in the mask plane, where only a little space is available. An original method is put into practice in order to measure the directional spatial coherence in two dimensions in the mask plane of a mask-aligner. This is achieved by using a circular double slits system. Prints are also made which illustrate the importance of the angular distribution and the spatial coherence.
This novel LED-based illumination system is implementable in current, commercial, mask-aligners. It enables a greater efficiency, more functionalities and a longer lifetime compared to standard mercury arc lamp illumination systems. In particular, the spatial coherence properties of the source can be precisely managed and monitored thanks to the directional spatial coherence measurement.OP
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