Near-field phase profile and far-field contrast modulation for micro-lens arrays at the refraction limit

We investigate the far-field pattern generation for a micro-lens array (MLA) illuminated under different conditions. Plane wave and Gaussian beam illumination are considered for an MLA with a small diameter of 27 microns and 30 microns period. At these dimensions, the optical effects are governed by diffraction and refraction and sometimes the regime is called the refraction limit. For Gaussian beam illumination, a high contrast dot pattern can be obtained in the far field according to the self-imaging theory for point source illumination and it is investigated in the simulation part. Also, we designed an interference microscopy setup to record both the phase and intensity in near field behind the MLA and also in the far field. The new instrument allows us to change illumination conditions from plane wave to point source. We then experimentally compare the near-field phase modulation and resulting far-field intensity for different conditions. For plane wave illumination, a high contrast pattern is observed in the far field. For the Gaussian beam illumination, the contrast of the far-field pattern depends on the distance of the source and MLA resulting in high contrast and a larger field of view only for particular distances depending on the interference of the Gaussian beam curved phase front and the MLA.

Light pattern generation with hybrid refractive microoptics under Gaussian beam illumination

Toralf Scharf, Maryam Yousefi

The generation of wide-angle diffraction patterns can be done in different ways using either thin diffractive optical elements with small features sizes or arrays of microoptics with large optical paths that are thick diffractive optical elements. Our aim is to create as many high contrast diffraction-limited dots in the far-field as possible with a uniform intensity distribution. As a model system, we use a sinusoidal phase grating and as a peculiarity, we introduce non-uniform illumination using a Gaussian beam illumination. By making use of the self-imaging phenomenon, a large number of peaks with uniform distribution are generated for a defined range of the phase grating thicknesses due to the sinusoidal curvature. For very high structures, the pattern distribution is not uniform and it demonstrates that very thick sinusoidal phase gratings are not suitable pattern generators. For simulation, we compare thin element approximation, fast Fourier transform beam propagation method, and the rigorous finite difference time domain method. The large-angle diffraction is considered using a high numerical aperture propagator for far-field simulation. We demonstrate that the beam propagation and the Fraunhofer approximation are not accurate enough. Also, our rigorous near-field calculation versus phase grating thickness confirms the significant influence of reflection of thick structures on the far-field distribution, especially on pattern uniformity. Finally, experiments were carried out to confirm our findings and a good agreement between the simulation and experimental far-field distributions confirms our approach. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OPTICAL SOC AMER2020

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Amazing and beautiful optical effects are present in Nature. Some examples are the iridescent colors produced by peacocks, butterflies and beetles. While the simulations of some of these structures have already been realized, only a few elements have been fabricated. We are interested to understand and to reproduce some of these amazing optical properties. Applications such as security or decorative elements in different fields like jewelery or horology can be imagined. SEM pictures of the structure of these animals show a frequent combination of periodic or pseudo-periodic elements such as interference filters, multilayer reflectors, microlenses and gratings. The different elements combine angular and spectral effects. This combination enables strong optical effects for a large range of viewing angles and spectra. Different technologies such as standard photolithography, recording of interference patterns, transfer of photoresist structures in glass substrates, replications, hot embossing and spincoating were used to fabricate a variety of optical elements. The fabricated elements were produced with polymer materials because of their ease of use, and the possibility to build multilevel structures with replication methods. A drawback is the small refractive index difference possible between polymers and compared to air. In the first part, different combinations of polymer micro- and nano-optical elements such as corrugated gratings, multilayer Bragg reflectors, microlens arrays, micro-prisms arrays and diffusers were successfully fabricated. Poly (vinyl alcohol) and poly (N-vinycarbazole) with refractive indices of 1.56 and 1.72, respectively, at a wavelength of 500nm were used to fabricate the polymer multilayer reflectors. The micro-optical elements used were: microlenses with diameter between 32 μm and 250 μm and around 20 μm height; prisms with 50 μm period and 25 μm height. The angularly dependent reflectivity of the different fabricated elements was studied. Thanks to the close combination of diffractive, refractive and reflective micro- and nano-optical elements, non-standard artificial visual color effects were produced. The fabricated elements were modeled with different tools according to the dimensions of the different optical elements. Ray tracing analysis was used in the case with micro-optical elements while the Fourier modal method permitted simulation of the interaction of the light with the periodic nano-structures. The specific effects of the variation of the different parameters were highlighted, the basic principles and the limitation of the polymer technology were identified. In the second part, the optical modeling tools and the fabrication technologies developed were used to model and fabricate polymer light emitting diodes in a distributed feedback regime. The optical properties of the different layers were modeled and the physical dimensions of elements with active conductive polymers me-LPPP and F8 were calculated, fabricated and tested. The sensitive dimensions and parameters were underlined. Polymer materials permitted rapid fabrication of complex and innovative optical elements with very simple technologies like spin-coating and replication methods. These technologies allow the combination of nano-optical elements with dimensions of the order of a hundred nanometers with micro-optical elements with dimension of the order of about ten to one hundred microns.

Institut de Microtechnique, Université de Neuchâtel2008

Diffractionless Fourier Domain Microscopy

Cédric Blatter, Theo Lasser, Rainer Leitgeb, Tilman Schmoll

Several methods have been proposed and realized to increase the lateral resolution in microscopy and ultimately to beat the actual diffraction limit as stated by Abbe. Structured illumination for example enhances the lateral resolution by virtually extending the spatial frequency support of the system transfer function. Higher spatial frequencies are in fact mapped into the range covered by the detection transfer function. The set of spatial frequencies can be further enhanced by decoupling illumination and detection and by employing concepts similar to synthetic apertures. We present a novel method for diffractionless microscopic imaging – Fourier Domain or k-Microscopy[1] (FDM). Again we decouple illumination from detection to achieve a lateral resolution that is independent of the detection numerical aperture. Similar to structured illumination we produce a set of fringes covering a band of spatial frequencies on the sample. However, instead of detecting with an area sensor we use a simple PIN detector: By tuning the spatial frequency of the illumination we obtain the Fourier coefficient from the measured total backscattered or transmitted intensity for the particular spatial frequency in the sample dimension normal to the fringe orientation. The fringe pattern is obtained from two collimated coherent beams that are superimposed. The spatial frequency of the pattern is controlled via tuning the wavelength. In order to obtain an unambiguous Fourier representation of the one dimensional sample structure it is necessary to record at least two π/2 phase shifted copies of the frequency response signal. The lateral resolution is then determined by the tuning bandwidth of the laser as well as the subtended angle and the central wavelength of the laser. This relation is in fact very similar to the concept of Fourier Domain Optical Coherence tomography, only that it acts on the transverse instead of the longitudinal resolution. We give first experimental proof of the concept in one dimension using a slowly tuning broad- bandwidth Ti:Sapph laser. We resolve the defined structure of a resolution test target. A particular feature of the method is that the reconstructed sample will appear spatially band-pass filtered due to the finite spectral support given by the tuned fringe frequencies. Hence only the edges of the test target lines are visible. We believe that the concept of FDM will greatly benefit from new developments in rapidly- wavelength tuning source technology that recently entered the field of OCT. 1. M. Geissbuehler, T. Lasser, and R. A. Leitgeb, "K- Microscopy - resolution beyond the diffraction limit," in Progress in Biomedical Optics and Imaging - Proceedings of SPIE(2008).

2011

Diffractionless Fourier Domain Microscopy

Cédric Blatter, Theo Lasser, Rainer Leitgeb, Tilman Schmoll

2011

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Institut de Microtechnique, Université de Neuchâtel2008