Oliver Henneberg

Linear viscoelastic analysis of formation and relaxation of azobenzene polymer gratings.

Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Youngs modulus by at least three orders of magnitude. Force densities of about 10(11) N/m3 were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility.

X-ray investigations of formation efficiency of buried azobenzene polymer density gratings

We present the results of time-dependent x-ray scattering measurements during the formation of a buried, modulated density structure below polymer film surface created at temperatures above the glass transition temperature TG during the thermal erasure of a holographically written surface relief grating. The surface relief gratings are initially formed on polymer films containing azobenzene side groups by exposure to a polarization holographic pattern made using light at 488 nm. The material responds to the actinic light by forming a sinusoidal surface relief pattern in a one-step process. Theoretical predictions show that the surface relief grating formation is accompanied by a density grating just below the film surface. Upon annealing above polymers glass transition temperature the surface relief grating melts while some of the density modulation in the bulk remains and a nonsinusoidal density grating is rebuilt after long term annealing at a temperature approximately 30 K above the glass transition te...

Formation of a buried density grating on thermal erasure of azobenzene polymer surface gratings

The transformation of a lateral surface relief grating inscribed on a polymer film containing azobenzene moieties into a density grating of equal spacing buried under a smooth surface was found for the polymer poly{(4-nitrophenyl)[4-[[2-(methacryloyloxy)-ethyl]ethylamino]phenyl]diazene} (pDR1M) using an X-ray scattering experiment. Annealing a polymer sample pre-inscribed with a surface grating for several hours above the glass transition temperature creates a nearly sinusoidal lateral density difference up to about 10%. The new state is stable between room temperature and the decomposition temperature of the original polymer when the formation was performed under the influence of VIS light. Growth of liquid-crystalline aggregates is proposed as a most probable explanation for the process.

Investigation of azobenzene side group orientation in polymer surface relief gratings by means of photoelectron spectroscopy

The molecular orientation of azobenzene side groups in polymer films before (nonpatterned) and after (patterned) development of a surface relief grating has been investigated by photoelectron spectroscopy using synchrotron radiation. The photoemission spectra obtained for 60–100 eV photons of a patterned and a nonpatterned surface are similar when the polarization vector of the synchrotron light is parallel to the grating vector. However, for perpendicular excitation, considerable spectral intensity differences can be observed for 9–14 eV electron binding energy. The observed changes are attributed to the formation of well-oriented azobenzenes at the surface.

X-ray reflectivity from sinusoidal surface relief gratings

The sinusoidal shape of light-induced surface relief gratings of polymers can be probed by x-ray scattering techniques. A particular approach of kinematic x-ray scattering theory is developed to interpret experimental scattering curves. The simulations demonstrate the particular sensitivity of x-ray reflectivity for very small grating amplitudes. At angles of incidence close to the critical angle of total external reflection a grating amplitude h < 2 nm already provides measurable grating peaks. In general the grating amplitude h can be measured from the envelope function over the grating peak maxima. The capability of the approach is demonstrated by simulation of the reflection curve recorded from a polymer sample with non uniform grating height.

Investigation of material flow on inscribing a polymer surface grating probing X-ray and VIS light scattering

Abstract Surface relief patterning on amorphous polymer films containing azobenzene-side chains can be performed by holographic exposure with visible light of about 488 nm at room temperature. In order to understand the dynamics of the induced material flow we performed in-situ time-resolved coherent X-ray and VIS light scattering measurements at exposure with a holographic pattern obtained using circularly polarised light of 40 mW cm −2 . The films under investigation were made from the polar azobenzene side-chain homopolymers. The efficiency of grating formation is probed by the time development of the first-order grating peak intensities. While the VIS signal increases continuously during continuous holographic exposure, the X-ray grating peak intensity reaches a maximum after about 60 s and decreases thereafter. Similar measurements during short time exposure show that the maximum of the X-ray signal depends of the pulse length of the inscribing light. An analysis of the time evolution of the VIS scattering reveals the existence of sequential elastic and plastic processes during exposure. Both experiments can be explained qualitatively by assuming the formation of a density grating in addition to the surface relief patterning. Our assumption are validated by model calculations using a finite element (FE) approach including a sinusoidal force distribution accompanied by visco-elastic flow.

X-ray and VIS light scattering from light-induced polymer gratings

Abstract Sinusoidally shaped surface relief gratings made of polymer films containing azobenzene moieties can be created by holographic illumination with laser light of about λ ≈ 500 nm. The remarkable material transport takes place at temperatures far (100 K) below the glass transition temperature of the material. As probed by visible light scattering the efficiency of grating formation crucially depends on the polarization state of the laser light and is maximal when circular polarization is used. In contrast to VIS light scattering X-ray diffraction is most sensitive for periodic surface undulations with amplitudes below 10 nm. Thus, combined in-situ X-ray and visible light scattering at CHESS were used to investigate the dynamics of surface relief grating formations upon laser illumination. The time development of grating peaks up to 9th order at laser power of P = 20 mW/cm2 could be investigated, even the onset of grating formation as a function of light polarization. A linear growth of grating amplitude was observed for all polarizations. The growth velocity is maximal using circularly polarized light but very small for s-polarized light.

Optically induced mass transport generated in near fields

In the last few years a range of techniques for opto-mechanical manipulations of organic films and small structures has been developed and significantly improved. Among these techniques a very promising candidate turned out to be the optically induced mass transport. Not only that the physical mechanisms underlying this phenomenon is not yet been fully understood, but in addition, the lateral dimensions of structures created in that way have been limited by the used light wavelength. In order to gain deeper insight into the physical fundamentals of this phenomenon and to open possibilities for applications (lithography, data storage, manipulation of molecules, ...) it is necessary to create and study reproducible, sharply defined single structures not only in a macroscopic but also in nanometer range. SNOM (Scaning Nearfield Optical Microscopy) seemed to us an intriguing method to approach this goal. We report here novel experimental results about the generation of ultra-small structures by optically driven mass transport. We have investigated different ways to generate localized mass transport in azobenzene-containing films by using focused light in far and nearfields. Thus, the dimensions of optically created structures range to 5 μm (lens focusing) and even down to 100 nm (SNOM nearfield). These experiments offer new expectations to manipulate ultra small objects on surfaces by optical means without mechanically touching them.

2D and 3D buried gratings made from azobenzene-polymer multilayer films

Surface relief gratings (SRG) made from azobenzene polymer films by holographic exposure with actinic light show remarkable density modifications in addition to the surface relief. The origin of the huge material transport is attributed to cooperative phenomena associated with the light induced trans-cis and cis-trans isomerization of azobenzene moieties and the subsequent changing of viscoelastic properties during illumination. In case of polydisperse red 1 methacrylate (pDR1m) films and using particular illumination conditions the amplitude of the density grating (DG) can be maximized whereas the amplitude of surface undulations keeps small. The capability of DG formation makes it possible to induce grating formation in the azobenzene polymer film through a thick polymer cover layer which is not affected by the actinic light. Using PMMA as cover layer the grating is located at the PMMA - pDR1m interface (interface grating) while the sample surface stayed almost flat. This concept can be used to prepare 3D mesoscopic crystals by stacking several PMMA/pDR1m bi-layers on top of each other. The interface grating is created in each bi-layer before it becomes covered by the next bi-layer. Patterning of the upper bi-layer takes place after careful positioning of the writing position with respect to the underlying one. These 3D multilayer gratings can be used as dispersive elements for optical light. Their structural performance can be probed by means of light diffraction similar to the x-ray Laue experiment.

Simultaneous X-ray and Visible Light Diffraction for the Investigation of Surface Relief and Density Grating Formation in Azobenzene Containing Polymer Films

ABSTRACT The development of surface relief and density patterns in azobenzene polymer films was studied by diffraction at two different wavelengths. We used x-ray diffraction of synchrotron radiation at 0.124 nm in combination with visible light diffraction at a wavelength of 633 nm. In contrast to visible light scattering x-ray diffraction allows the separation of a surface relief and a density grating contribution due to the different functional dependence of the scattering power. Additionally, the x-ray probe is most sensitive for the onset of the surface grating formation.