Florian Hudelist

Design and fabrication of nano-structured gradient index microlenses

We present a novel fabrication technology for nano-structured graded index micro-optical components, based on the stack-and-draw method used for photonic crystal fibres. These discrete structures can be described with an effective refractive index distribution. Furthermore we present spherical nano-structured microlenses with a flat facet fabricated with this method and designed using an algorithm based on the Maxwell-Garnett mixing formula. Finally we show theoretical verification by using FDTD simulations for a nano-structured lens as well as experimental data obtained in the microwave regime.

Nanostructured elliptical gradient-index microlenses

We show theoretical and experimental characterizations of a nanostructured gradient-index lens. The elliptical lens is a nonguiding element fabricated using the mosaic method, which is widely used for the fabrication of photonic crystal fibers. For the first time we show experimental data in the optics regime that confirm the effective medium approximation for discrete mosaic structures with subwavelength feature size. This opens the door for the development of general asymmetric gradient-index materials.

Form birefringence in nanostructured micro-optical devices

We present a detailed examination of the design and expected operation of an artificially birefringent material based around the nanostructured stack-and-draw fabrication technique developed recently. The expected degree of birefringence is estimated using a Finite Difference Time Domain simulation of the physical system and is shown to be in agreement with that predicted by a second order effective medium theory treatment of the nanostructured material. The effects of finite device dimensions are studied and an estimate of the required device thickness for a half-wave retardation is made.

Design of all-glass multilayer phase gratings for cylindrical microlenses

We introduce a design method for diffractive cylindrical microlenses fabricated with a new technology similar to the fabrication of all-solid photonic crystal fibers. Unlike conventional microlenses that are fabricated with etching methods and thus have a step-index profile, the refractive index of each layer can be individually designed. We study the transmitted field of such nonperiodic lamellar phase grating. By using the field-stitching method we can suppress the effect of periodic boundary conditions of the Fourier modal method when calculating the transmitted field of nonperiodic lamellar phase elements. We suggest an algorithm to design multilayer phase elements, which act as cylindrical lenses. We show experimental and theoretical data for a diffraction-limited lens.

Analysis of crossed gratings with large periods and small feature sizes by stitching of the electromagnetic field.

We present a new algorithm that enables the analysis of large two-dimensional optical gratings with very small feature sizes using the Fourier modal method (FMM). With the conventional algorithm such structures cannot be solved because of limitations in computer memory and calculation time. By dividing the grating into several smaller subgratings and solving them sequentially, both memory requirement and calculation time can be reduced dramatically. We have calculated a grating with 32 x 32 pixels for a different number of subgratings. We show that the increased performance is directly related to the size of the subgratings. The field-stitched calculations prove to be very accurate and agree well with the predictions from the standard FMM approach.

Photonic glass: novel method for fabrication of volume 2D photonic crystals

In this paper we report on use stack and draw technique to develop volume 2D photonic crystals made of two types of soft glasses with a large difference of refractive index. Existence of partial photonic bandgap in the material is predicted and modeled.

Nanostructured graded index materials

We present a new way of describing dielectric materials with a nano-structured refractive index distribution, developed from the random medium model. The idea of treating structures with a nano-structured discrete refractive index distribution as a graded index (GRIN) material was recently introduced by Hudelist et al. [1]. This method overcomes the problems of conventional GRIN media which can only be fabricated with a monotonic, rotationally symmetric index profile.

Simulation and Optimisation Methods for Non-periodic Subwavelength Structures.

The use of nano-structured elements in the fabrication of micro-optical subwavelength components requires a fully vectorial solution to Maxwells curl equations. In this paper, we compare the results generated by two of the main methods used in the solution of the curl equations, the Fourier Modal Method (FMM) and the Finite Difference Time Domain (FDTD) method. We address the computational issues surrounding the accurate modelling of nano-structured elements (with features in the 10nm-100nm range) for a range of micro-optical elements, e.g. cylindrical lenses, photonic bandgap reflectors and polarisation dependent beamsplitters. Finally, we show the experimental verification of the nano-structured designs using microwave radiation.

Comparison of different simulation methods for multiplane computer generated holograms

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.