Advanced passive micro-optical elements

Institut de Microtechnique, Université de Neuchâtel, 2008
Thèse hors EPFL

Résumé

Micro-optical elements are optical elements having size or details in the range of the micrometer scale. The emergency of such elements is strongly linked to the development made in the fabrication techniques during the nineties to follow the still actual trend of miniaturization. From this time it has been convenient to separate micro-optical element having refractive or reflective optical design from those ones having strictly diffractive design. In the present thesis we are in between this two fields and from conventional refractive optical elements, different features of physical optics like diffraction, polarization or interference effects are added. We start by adding the birefringent property of liquid crystal polymers (LCP) to microlenses. The interest to employ LCP is linked to the fact that bulky birefringent structures of various shapes can easily be obtained by embossing. Moreover the choice of the optical axis of this material can be chosen by boundary conditions imposed by the different surfaces embedding the liquid crystal. Using this material a micro-optical device allowing the increase of the efficiency of conventional polarizer has been designed, simulated and realized. We continue by studying the influence of the aperture size on the position of the peak irradiance of microlenses. This diffractive effect of the aperture limitation is known as the focal shift which tends to decrease the real focal length obtained from paraxial geometric optics. In the frame of this thesis, we pursue the investigation and show that this effect could be advantageously used to obtain achromatic microlenses. In the frame of beam shaping applications we realized diffusers with very narrow angular distribution. These diffusers allow high energy throughputs and homogeneity distribution for highly coherent sources employed in conventional Fly's eye condensers. The consequences of coherent sources used in such beam shaper are discused in detail and rules are given to employ our diffusers in such systems. To finish, we developed and characterized a new method to fabricate concave microlenses with diffraction limited property and applied them in the realization of optical diffusers.

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https://infoscience.epfl.ch/record/164584?ln=fr

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Institut de Microtechnique, Université de Neuchâtel, 2008
Thèse hors EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/164584?ln=fr

À propos de ce résultat

Concepts associés (30)

Concepts associés

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Publications associées (12)

Publications associées

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Photonic crystals are periodic dielectric structures, where the periodicity varies in one, two or three dimensions. Analogous to the periodic potential of electrons in semiconductors, the periodic variation of the dielectric constant influences the electromagnetic properties. The energy of the light is separated in bandgaps, energy ranges in which the propagation of the light is forbidden for certain directions and energies. These properties suggest that photonic crystals may be suitable for fabrication of the components needed for integrated optics. Since the fabrication of three dimensional photonic crystals is still limited by complex fabrication problems we have studied two dimensional photonic crystals. These photonic crystals can be fabricated by standard microelectronic technology. The photonic crystals studied in this thesis consist of GaAs and InP based low index vertical waveguides with a matrix of holes etched into it. The fabrication of photonic crystals with good optical properties is an important factor for photonic crystal devices in integrated optics. In the first part we studied the optical properties of photonic crystal slabs and of Fabry-Pérot cavities. These structures were measured by the internal light source technique, which allows quantitative normalized transmission measurements. The photonic crystals were etched by three different dry etching technologies. Out-of plane scattering is described by a 2D-FDTD fit of the transmission spectra and by a semi-analytical model. The finite hole depth and the conical shape of the holes are important structural parameters. The detailed study of their effects yields an important feedback for the critical parameters of the fabrication process. Photonic crystal waveguides are another group of components in integrated optics. We measured two types of straight photonic crystal waveguides by the endfire technique. Here the light is coupled by optical fibers and ridge waveguides in the photonic crystal waveguides. The transmission spectra reveal fringes which are due to multiple interference of the light in the sample. This interference contains information about propagation losses of the waveguides and an analysis based on the Fourier transform of the transmission spectra enabled us to deduce the propagation losses of the photonic crystal waveguides. The necessity of tuning or trimming the optical properties of photonic crystals is an important issue either compensating fabrication imperfections (trimming) or controlling the optical properties on demand (tuning) for devices like filters. We have shown that it is possible to tune the optical properties of planar photonic crystal cavities by temperature. The experimental results were validated by theoretical calculations. Also we have shown that is possible to tune the optical properties of photonic crystals by infiltrating liquid crystals in the holes. Liquid crystals are a birefringent material whose optical axis can be tuned by external fields (electric, temperature, photonic source, etc.). Phase transitions exhibiting abrupt changes in the refractive index are important characteristics of liquid crystals. The holes of the photonic crystal are infiltrated by a reversible and reliable infiltration process. The infiltration efficiency and the orientation of the molecules in the holes were determined by polarization resolved internal light source measurements. We have shown that the frequency of the Fabry-Pérot resonance changes at the phase transitions of the liquid crystal.

EPFL2006

Chargement

Photonic crystals are stuctures in which the refractive index varies periodically on the wavelength scale in one, two or three dimensions. Due to interference effects, these structures can forbid light propagation over a large spectral range called "the photonic bandgap". Integrated optics is one of the potential applications of photonic crystals in which the information can be treated as a light signal that propagates through photonic crystal-based devices, such as filters, lasers, waveguides, bends and multiplexers. The photonic crystals structures studied for integrated optics applications consist of periodic arrays of holes etched through classical semiconducteur-based planar waveguides. For any practical application, the optical properties of the fabricated components have to be tuned by external means. In this thesis, we explore a possible approach to photonic crystal tuning : the infiltration of photoactive organic materials in the holes of these photonic structures. First we considered liquid crystals, which exhibit anisotropic optical properties like birefringence due to their long-range orientational order. This latter molecular organization can be modified by external factors, i.e. temperature and electric field. Nematic liquid crystals were infiltrated in the holes of the photonic crystals to tune their properties by changing the temperature and applying an electric field. Moreover, the organization of liquid crystals in the nanometric holes were studied by optical techniques. The demand for all-optical tunable devices in integrated optical circuits led us to consider the possibility of optically tuning the response of photonic crystal structures. For this purpose, we used photo-induced phase transitions of a liquid crystal blend doped with azobenzene photochromic molecules. Since the phase transitions in these mixtures depend on the concentration of the photochromic compound and on the temperature, we studied in detail the phase diagram of the blend. The last part of this thesis focused on one of the most innovative aspects in the domain of photonic crystal tuning : selective infiltration to locally modify the optical properties of the photonic structures. We used the photopolymerization of acrylate monomers by ultraviolet laser irradiation. Finally, with three clear examples, this thesis shows that organic photoactive materials are well-adapted to be combined with photonic crystals. We demonstrated that the obtained hybrid structures are very promising for integrated optics applications.

EPFL2009

Chargement

On the chromatic aberration of microlenses

Hans Peter Herzig, Toralf Scharf

The optical properties of plano-convex refractive microlenses with low Fresnel Number (typically FN < 10) are investigated. It turns out that diffraction effects at the lens aperture limit the range of the effective focal length. The upper limit of the focal length is determined by the diffraction pattern of a pinhole with equal diameter. In addition achromatic microlenses can be realized because refraction and diffraction have opposing effects on the focal length. Gaussian beam propagation method has been used for simulation. The presented results are of relevance for applications, where microlenses with small apertures and long focal lengths are used, for example, Shack Hartmann wavefront sensors or confocal microscopes. © 2006 Optical Society of America.

2006

Chargement

EPFL2006

EPFL2009