This work improves period uniformity over larger-area gratings in a two-spherical beam interference lithography (IL) setup. Using a concave vacuum chuck, we have experimentally demonstrated the modified optical setup to reduce period variation (chirp) on a 4-inch silicon substrate. The change in the grating period is significantly reduced compared with the interference exposure on a flat silicon substrate. Wavefront aberration measurement on a curved exposed substrate showed improved grating linearity.
A novel fabrication method for apodized diffractive structures is proposed and examined experimentally and theoretically. It combines the principles of Laser Interference Lithography and a moiré effect in order to create high-quality subwavelength gratings with 2D adiabatic variation of depth. Synthesized structures can be used in optical security systems, plasmonic research and high-effective light couplers.
We present both modeling and experimental results devoted to design, fabrication and characterization of metal covered hexagonal diffraction gratings. Variation of exposition and development time allow to modify the shape of the elementary cell, leaving the depth and periodicity unchanged. The fabrication process was modeled using real parameters of the lithography bench and the photoresist, substantially improving experimental results. The high quality of metal covered gratings is confirmed by excitation of plasmonic resonances, which are in a good agreement with theoretical predictions. The described approach allows to better understand plasmonic effects in 2D periodic structures and leads to an optimized design of plasmonic sensors.
This paper presents a multi spot projection unit, used in a 3D volume measurement system employed in a heart failure monitoring device, observing the volume of a patient’s feet for symptoms of heart problems (peripheral edema - swelling of the extremities). The stereoscopic image acquisition requires a surface with enough optically detectable texture, usually not present on human skin, which can be resolved by projecting an infrared, static multi-spot optical pattern. The focus of this paper is on creating a very cost-effective, energy efficient, eye-safe projection system, realizing a strongly divergent (up to ±60°) spot pattern, using infrared LEDs and mass fabricable micro-optical elements. Two different setup were tested: a) an LED array combined with a microlens array, and b) a combination of a single LED with a microlens array and a computer generated hologram (CGH) that adds a pseudo-random spot multiplication. For approach a) the microlens array was optimized by ray-tracing. The CGH function for approach b) was found using a wave optical design algorithm (iterative Fourier-Transform Algorithm – IFTA). The micro-lens array master was fabricated by diamondturning, whereas electron-beam lithography was employed for the CGH-master. Both masters were replicated using hotembossing of PMMA. Installed in a prototype of the medical measurement device, the influence on the 3D reconstruction was measured. The proposed solutions allow installing a competitively priced product for automatic peripheral edema monitoring in chronically ill patient homes, which is of great interest for improving their quality of life and the efficiency of their treatment.
The paper demonstrates a rigorous modeling approach for 1D microstructured absorbing multi-layers for the receivers of CSP (Concentrated Solar Power), taking into account both absorption of the incident solar energy and the emissivity while considering receivers temperature. From an optimized multilayers structure achieving high absorption, the authors demonstrate that 1D sub wavelength period gratings could increase further the absorption and thus the yield of the Concentrated Solar Power system. The authors used C-method (Chandezon Method) to optimize 1D grating profile. Experimental demonstration on Silicon wafers combining writing grating and absorptive layers deposition are also presented. Experimental results are presented and absorbance enhancement of almost 2% are achieved with values of 96.5% in the visible and UV range. The results are promising for the design of future and competitive solar absorbers for CSP since the microstructuring fabrication approach can be applied to non-planar substrates such as tubes, which are the receivers of the CSP.
Functional demonstration of a wide band, wide angular width wire-grid polarizer has been made in the framework of a user project of the European project ACTMOST (Access To Micro-Optics Expertise, Services and Technologies). The polarization function relies on linear polarizers using the wire-grid polarizer principle by means of a metal grating of unusually large period, exhibiting a large extinction of the transmission of the TE polarization in the 850-nm wavelength range. This grating achieves a broadband and especially high angular aperture reflection with low loss and permits resorting to very low cost incoherent light sources for the transmitted TM polarization. This paper will describe the design, the modeling and optimization, as well as the complete technological process chain, that has been used, starting with the photoresist grating printing using phase-mask UV-based lithography to the uniform galvanic growth of a very shallow gold grating on transparent conductive layer deposited on a glass substrate. Transmission curves for both polarizations performed on the first demonstrators will be presented.
A polarization rotation is realized by subwavelength binary gratings, where the round trip phases of the smallest grating modes are fixed to the smallest possible integer numbers of 2π allowing a phase difference of π between TE and TM polarizations and almost 100% transmission. The principle is applied to a polarization transformation in the 1030 to 1064-nm wavelength range, using a segmented polarization rotating element converting a linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kinds of polarization transformers based on assembled birefringent crystals are avoided by using mass-fabrication compatible silicon-on-insulator technology on a wafer scale. It shows the general potential of microelectronic technology, concerning the batch manufacturing of wavelength-scale diffractive, grating-based elements for processing free space waves.
Functional demonstration of a wide band, wide angular width “wire-grid polarizer” has been made in the framework of a User Project of the European project ACTMOST (Access To Micro-Optics Expertise, Services and Technologies). The polarization function relies upon linear polarizers using the “wire-grid” polarizer principle by means of a metal grating of unusually large period, exhibiting a large extinction of the transmission of the TE polarization in the 850 nm wavelength range. This grating achieves a broadband and especially high angular aperture reflection with low loss and permits resorting to very low cost incoherent light sources of the transmitted TM polarization. The paper will describe the design, the modeling optimization, and the complete technological process chain which has been used: from the photoresist grating printing using phasemask UV-based lithography to the uniform galvanic growth of very shallow gold grating on transparent conductive layer deposited on a glass substrate. Transmission curves for both polarizations on the first demonstrators will be presented.
A polarization rotation is realized by subwavelength binary gratings, where the TE and TM round trip phases of the smallest grating modes are fixed to the smallest possible integer numbers of 2π that allow a straight-through phase difference of π. This results in a subwavelength grating allowing to realize a half-wave element of almost 100% transmission. The principle is applied to a polarization transformation in the 1030-1064 nm wavelength range, using a segmented polarization rotating element converting a linearly polarized incidence to a radial or azimuthal polarization distribution. The elevated costs of such kind of polarization transformers based on assembled birefringent crystals are avoided by using mass-fabrication compatible silicon on insulator technology on a wafer scale. It shows the general potential of microelectronic technology, concerning the batch manufacturing of wavelength-scale diffractive, grating based elements for processing free space waves
Computer-generated holograms (CGHs) are important for many tasks in modern optics. High efficiencies rely on an
increasing number of phase steps which usually complicates the fabrication process. In this paper, we demonstrate an
effective medium approach which uses binary subwavelength structures instead of a conventional surface relief profile to
generate a specific phase pattern. Consequently, the fabrication process is simplified to one exposure and one etching
step independent from the number of phase levels. This offers new perspectives in designing large-scale highly efficient
diffractive elements with large deviation angles. For demonstration, a highly off-axis 3-level CGH for a projection screen
application is designed, fabricated and experimentally evaluated.
Subwavelength structures open up the possibility to create an artificial index material which enables the realization of
high-efficient diffractive structures. This can be used to generate optical elements with nearly arbitrary phase profiles.
We demonstrate the realization of computer-generated holograms based on this effective medium approach. High
diffraction efficiencies can be realized by multi-phase-level modulation based on two-dimensional binary nanostructures.
The fabrication is performed by one lithographical step using a high-speed e-beam writer which allows high-resolution
patterning even on large areas. A diffractive element in the visible range is experimentally demonstrated using the
presented effective index approach.
Computer generated holograms (CGH) are used to transform an incoming light distribution into a desired output.
Recently multi plane CGHs became of interest since they allow the combination of some well known design methods for
thin CGHs with unique properties of thick holograms. Iterative methods like the iterative Fourier transform algorithm
(IFTA) require an operator that transforms a required optical function into an actual physical structure (e.g. a height
structure). Commonly the thin element approximation (TEA) is used for this purpose. Together with the angular
spectrum of plane waves (APSW) it has also been successfully used in the case of multi plane CGHs. Of course, due to
the approximations inherent in TEA, it can only be applied above a certain feature size. In this contribution we want to
give a first comparison of the TEA & ASPW approach with simulation results from the Fourier modal method (FMM)
for the example of one dimensional, pattern generating, multi plane CGH.
A new approach for the realization of highly dispersive dielectric transmission gratings is presented. By covering
conventional transmission gratings with a plane substrate a complete suppression of any reflection losses and, thus,
100% diffraction efficiency can be achieved. Theoretical design considerations, a physical investigation of the
diffraction as well as very promising experimental results are shown.
The uniform illumination of holographic screens during their recording process is commonly realized by using only the quasi-constant, inner part of the gaussian intensity profile of a very strongly expanded laser beam. This technique is characterized by a very low efficiency (about 5%, depending on the required uniformity). We present a method, which uses refractive, micro optical beamshaping elements in order to create a rectangular, extraordinarily uniform, flat-top intensity profile with minimal phase aberrations. This allows the use of about 80%-90% of the provided optical power for the illumination of the holographic screen. To ensure the required quality of the illumination wave, a spatial frequency filtering has to be applied. For certain holographic applications, requiring a combination of high beam-divergence and high optical power, conventional pinholes are either too thin or too sensitive. To solve this problem, we present a new concept of dielectric pinholes, based on optical microstructures. The combination of beamshaping elements and dielectric pinholes allowed us to extend the available parameter range during the recording of holograms.
For gaining a deeper understanding of the diffraction processes that take place in deep dielectric transmission gratings, a phenomenological explanation has been developed on the basis of a modal field, which propagates vertically through the grating region. The excitation of these modes by the incident wave, their propagation constants and how they couple to the diffraction orders determines the diffraction efficiency of the transmitted orders. The explicit modal analysis discloses the description of the highly efficient diffraction for polarized or unpolarized light by a very simple interference mechanism, which will be the subject of this paper.
Lateral intensity fluctuations of laser beams can be removed by spatial frequency filtering using a pinhole. Beams with a high divergence have a very small and narrow focus, therefore the pinhole must be both very small and thin. Conventional pinholes, based on absorption or reflection of the unwanted parts of the beam, have several limitations in this case. Their disadvantages can be overcome by a new kind of pinholes that are based on microstructures. They are characterized by a higher laser damage threshold, only limited by the bulk material, and a constant visibility of the beam, making the adjustment process of the pinhole a lot easier. The microstructures can be circular, binary gratings working mainly in a diffractive way, as we described earlier. This paper will summarize and update the results for this diffractive, dielectric pinholes and introduce a new idea of using cone-like structures, working mainly in a refractive way. We present the design, fabrication and characterization of pinholes, realized with both of these new concepts and we asses their performance and their limitations.
The design of refractive beam shaping elements can be done by the geometrical-optical approach based on ray optics. This means one has to find a map transformation, which transforms an input to a desired output distribution and can additionally be realized by an element with a continuous surface. Easy procedures to find transformations, fulfilling both of these conditions at the same time, exist only for one-dimensional and for special two-dimensional signals, e.g. separate or circular distributions. To realize completely arbitrary two-dimensional signal distributions only iterative methods, based on the wave nature of light, are applicable, e.g. the Iterative Fourier-Transform Algorithm (IFTA). However, they can not be used to design elements with a continuous surface since they usually introduce phase dislocations to the signal distribution. We present an algorithm, which uses both geometrical and wave optical methods to find a most suitable, refractive solution for two-dimensional beam shaping problems. An Iterative Mesh-Adaption Algorithm (IMA), based on the geometrical-optical domain, is used to find a map transformation to describe the energy rearrangement. An IFTA, working in the wave optical domain, is used to refine the mesh. The IFTA produces phase dislocations in the beam shaping element, but nevertheless its output can be used to change parts of the mesh. Both algorithms are used in an alternating fashion. We present the design, fabrication and characterization of beam shaping elements, realized with the help of this method.
The spatial-frequency filtering of high power laser beams with a high divergence requires pinholes with small hole diameters, small thicknesses and a high laser damage threshold. Conventional pinholes, based on absorption or reflec-tion are difficult to realize under these conditions. We present a new method for the filtering of laser beams by using dielectric pinholes, which do not absorb or reflect but rather deflect the high spatial-frequency parts of the beam. Ad-vantages of this new kind of pinholes are a significantly increased resistance to high laser energies, which is comparable to the damage threshold of the bulk form of the dielectric material, and a simplified handling because of the permanent visibility of the beam. To demonstrate the principle, we fabricated diffractive pinholes in fused silica by use of electron beam lithography with subsequent reactive ion beam etching. Finally some measurements of the filtering effect are presented.
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