Holographic surface relief gratings written in azobenzene containing films were studied for the use as masters for
polymeric thin film distributed feedback (DFB) lasers. Light induced mass transport driven by E-Z isomerization in
azobenzene containing materials have shown to be attractive for all optical and one-step fabrication of periodic surface
structures with varying parameters for different optical applications. Based on new azobenzene materials and their
holographic processing deep surface relief gratings were generated with grating pitches in the range of 400 nm as
resonant structures for second order DFB lasers emitting in the VIS range. Nanoimprint techniques enabled multiple
duplications of azobenzene master gratings in UV adhesives. The replicas were coated via spin casting with thin films of
red light emitting polymer layers to form DFB thin film lasers. These active layers are guest-host-systems consisting of
an UV-light absorbing conjugated polymer as host transferring its excitation via Förster resonant energy transfer to a red
emitting conjugated polymer. Simple adjustment of grating depth via controlling of illumination time allowed it to
investigate the influence of the corrugation depth and thereby the coupling of laser light and grating on the lasing
behavior of second order DFB lasers in the red region. For this purpose multiple surface structures with different
corrugation depths of up to 130 nm were generated holographically, duplicated and coated.
We present here two approaches for the fabrication of 2D and 3D optical structures. The first one is a step-by-step
fabrication process of 3D structures using thin relief gratings (stacks of thin 1D or 2D gratings). Azobenzene containing
materials for the surface relief inscription have been used in the step-by-step procedure, where after holographic
inscription of desired relief structure and coverage with spacer layer another correlated relief structure has been written
in the next active layer etc. The method provides full flexibility of the structure type and parameters including different
gratings in different layers (hierarchical structures). A technique to produce hexagonal relief gratings of enlarged
diameter which can be used for layer-by-layer photonic structures is developed. The second approach is a multi-beam
holographic recording using special phase masks. Such mask consists of three phase gratings arranged in plane
equilateral triangle geometry with gratings vectors at 120° to each other. A simple method of fabrication of well-adjusted
mask with rather high diffraction efficiency is developed. Hexagonal 2D surface relief and 3D volume phase structures
were fabricated by a single laser beam exposure using UV or visible wavelengths (depending on the material) through
the mask. Azobenzene containing materials and photopolymers, including new specially designed one, were used as
holographic materials.
Light-induced mass transport in azobenzene functionalized polymers has been used for generation of surface relief
gratings (SRG) for different optical applications. The effect of grating formation has been ascribed to the light-induced
motion of the azobenzene chromophores involving the covalently bound polymer chains. We propose a concept of
supramolecular materials for the effective all-optical generation of surface relief structures and optical anisotropy. The
materials are based on the non-covalent interactions between charged photochromic azobenzene units and oppositely
charged polymer matrix, for example polyelectrolytes including charged alkoxysilanes. This new supramolecular
approach opens a new way for the simple, cost effective and environment friendly preparation from building blocks of a
variety of materials for the effective formation of SRG. Up to 1.65 μm deep relief gratings were inscribed onto a few
micrometers thick films of these materials. The high thermal stability of the induced structures has been explained in
terms of the network of oppositely charged ions inherent to the materials. Also 2D-structures, for example square and
hexagonal gratings, were inscribed by the successive recordings. The latter possibility was also used to generate gratings
with non-sinusoidal profiles by Fourier transform technique. A new technique to control the grating profile has been
developed based on the real-time process of grating formation in these materials. The gratings with sow-tooth like
profiles were induced by this method. The diffraction efficiencies up to 60 % in one diffraction order were achieved.
Two amorphous side-chain copolymethacrylates containing 60% azobenzene chromophores (PII, PIV) were irradiated homogeneously with a linear polarized Ar+ ion laser beam at 488 nm. From polarized UV/VIS spectra the maximum of the photoinduced dichroism was calculated (dPII=0.25, dPIV=0.35). No intensity dependence of this values was notified. PIV was irradiated under holographic conditions with a grating period of 30 μm. Two types of polarization gratings, circular orthogonal and linear orthogonal ones were inscribed. Using a microscope spectrometer polarized UV/VIS spectra were measured within the grating period with a lateral resolution of about 2 μm. The induced local dichroism was measured and the different modulation of the molecular orientation in the recorded gratings was confirmed by this direct experiment. PII was used to monitor the grows up stages of the surface relief grating, starting from the molecular photoorientation until the surface deformation. Parallel to the online observed diffraction efficiency, the offline surface profile measurements were done by AFM. The details of surface topography and the reversibility of the process are discussed and an interesting possibility of the surface structuring is demonstrated.
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