Ordered and disordered polymer structures for photonic bandgap materials

People: Jakub Haberko*, Nicolas Müller and Frank Scheffold

A photonic crystal is a periodic structure characterized by a variation of the refractive index, which can alter the propagation of light. One of the main characteristics of such a structure is the existence of a photonic bandgap (PBG). It is the region of wavelengths where light cannot propagate inside a crystal, but is totally reflected from its surface instead. There exist a number of one-, two- and three-dimensional structures for which the existence of the PBG has been theoretically predicted. The most prominent example in one dimension is the so called Bragg reflector, consisting of a stack of layers with different thicknesses and refractive indices. Constructing a 3D photonic crystal is considerably more challenging, since such structure has to reflect certain wavelengths regardless on the direction of the incoming light.
    3D laser nanolithography is a technique which is very well suited to the fabrication of structures in question [1]. In this method a laser beam coming from a pulsed femtosecond laser is tightly focused inside a polymer photoresist. Due to high light intensity, the two-photon absorption process takes place followed by local cross-linking of the photosensitive material. The sample is moved with respect to the focal point with nanometer precision along a pre-programmed path, which results in a cross-linked pattern written inside the polymer material. As laser irradiation renders the photoresist insoluble in the developer, by dissolving the sample one can arrive at a free-standing 3D polymer structure. The resolution of the process is defined by the linear size of cross-linked volume, which is typically well below 1 μm. This is enough to manufacture photonic crystals with a bandgap in the near-infrared or even the visible range.
    In the frame of this project we have fabricated by means of the above technique so-called woodpile photonic crystals. These photonic structures consist of layers of parallel dielectric rods with rods in each layer being perpendicular to those in neighboring ones. The high quality of our structures is proven by SEM imaging as well as, most importantly, the results of optical measurements, exhibiting the presence of the photonic stop band. The scaling properties of our structures are consistent with theory. Specifically, scaling the structure up results in a red-shift of the stop-band position. Furthermore, we are able to intentionally introduce random defects into our structures. In this project we investigate how the amount of these defects affects light propagation inside the partially disordered structures [2].

Recently we have suceeded inthe mesoscale fabrication and characterization of polymeric templates for isotropic photonic materials derived from hyper-uniform point patterns using direct laser writing in a polymer photoresist [3]. We could study experimentally the microscopic structure by electron microscopy and small angle light scattering. Our aim was to demonstrate the feasibility of fabricating such random designer materials on technologically relevant length scales, a first major step towards the fabrication of three dimensional amorphous full PBG materials.

[1] Direct laser writing of three-dimensional photonic-crystal templates for telecommunica- tions, Markus Deubel, Georg von Freymann, Martin Wegener, Suresh Pereira, Kurt Busch and Costas M. Soukoulis, Nature Materials 3, 444 - 447, 2004

[2] Strong dispersive effects in the light-scattering mean free path in photonic gaps, P. D. García, R. Sapienza, L. S. Froufe-Pérez, C. López, Phys. Rev. B 79, 241109(R), 2009

[3] J. Haberko and F. Scheffold, Fabrication of three-dimensional disordered photonic materials from hyperuniform point patterns, Optics Express, Vol. 21, Issue 1, pp. 1057-1065 (2013)

* also: AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Krakow, Poland