A di-block copolymer composed of poly(ethylene oxide) and poly(azobenzene methacrylate) in the ratio of 45:55 has been synthesized. Thin films of about 400-700nm thickness have been prepared from solutions in trichloromethane by gravity settling. ATR-leaky mode spectroscopy has been used for the determination of the optical anisotropy. Homogeneous exposure caused a significant change in this anisotropy.
Holographic gratings have been recorded in these films. The in-situ measurements of the diffraction efficiency exhibit a relative maximum after a recording time of about 30 min, followed by a relative minimum before increasing again to high values of up to 40%. Especially for circular polarization high efficiencies were obtained.
Depositing this film on a foil of polyethylene the grating constant of a recorded grating could be tuned by the application of mechanical stress in a testing machine.
Polycrystalline layers of organic azo-dyes like DR1 (Disperse Red) have been prepared from the vapor phase. Due to their large electric dipole moment the orientation of the molecules may be changed or controlled by electric fields. Furthermore the capability of an optically induced TRANS-CIS transition allows an optical reorientation. In this study we will report on the optical recording of stable 2D-patterns in as grown and in electrically or optically preoriented films in the thickness range of 200-500nm.
Using electronic isolators as substrates for organic thin films one has to take care of long living surface charges which might influence or even control the film deposition. Unknown charges may be the origin of poor reproducibility. On the other side these charges may be used for controlled patterning as we will demonstrate in this paper. The described phenomenon might be of interest for electronic displays or patterned OLED's. Presented are different methods to deposit surface charges. The molecular orientation is investigated by measuring the angular dependence of the absorption coefficient.
Evaporated films of the azo-material DR 1 have been investigated. In the as-grown state partly crystalline films with low transmittance are obtained. Using homogeneous exposure transparent regions may be formed. The recording of holographic gratings in thin films (< 1 μm) of the azo dye is investigated for the case of more-dimensional
light-intensity patterns. The mechanism of the photo-isomerisation of the azo-compounds is used to form dual gratings with a refractive index grating and a surface-relief grating. The grating-formation is investigated in case of 1D-gratings first. The time dependent diffraction efficiency is discussed in a model of 2 processes with different time-constants. A material transport process is involved in the formation of relief patterns. The enhancement of the modulation-depth of the surface-relief gratings is investigated for the application of a Corona discharge and a thermal treatment after the holographic recording. 2D-gratings are formed using either a 3-beam holographic set-up or a consecutive method. The resulting light patterns are simulated. Diffraction patterns and AFM-measurements are used to confirm these simulated structures. The modulation of the surface-relief gratings can be enhanced by thermal treatment after the holographic recording.
The photo-isomerisation of azo-compounds is used to record reversible holographic volume phase gratings in films of guest-host polymers. In situ recording of the diffraction efficiency has been performed for different azo-dyes in the non-polar Poly-methylmethacrylate (PMMA) and the polar Poly-α-methyl-styrene (PαMS) as a matrix. Within an exposure energy of 100 mJ/cm2 a pronounced maximum is observed for s-polarised light. With the light pattern still on this is then reduced to a lower level. In case of the polar matrix, this process can be identified as an interaction of the dye molecules with the polymer matrix.
Volume phase hologography is used for the recording of refractive index patterns with the symmetry of a photonic crystal. As a recording material solid plates of PMMA, containing up to 10 wt% of residual monomer MMA doped with the photoinitiator titanocendichloride have been used. The mechanism for the formation of the desired refractive index pattern is the photoinduced residual polymerisation.
The 2-dimensional patterns have been recorded in 2 subsequent steps using a 2-beam holgaphic setup. In a first step the recording process has been characterized for a single volume phase holographic grating with the grating constant Λ1. The recording procedure is optimized towards a short term exposure period of the order of a few seconds, followed by self-developing period.
While the first pattern is developing the sample may be turned by an arbitrary angle Φ and a second volume phase grating with a different lattice constant Λ2 can be recorded. If e.g. an orthogonal crystalline symmetry is desired, the angle Φ will be chosen as Φ = 90°.
After recording of the second grating the sample is developed and fixed by a thermal treatment. The obtained 2-D crystals are characterized using a goniometer with 2 rotary stages by the evaluation of the diffraction patterns. The different diffracted orders may be indexed according to their (h,k) Miller´s indices.
The formation of holographic gratings in an azo-glass is investigated for the simple case of an intensity grating in the s-polarization of the recording beams. A dual grating is formed as a function of time with a refractive index grating and a relief grating. The diffraction efficiency as a function of recording time is discussed in a model of 2 consecutive processes with a phase shift. A material transport process is involved in the formation of relief patterns. A thermal erasure process at the glass transition is compared with an optical process.
The formation of dual gratings is investigated with the isotropic UV curable epoxy resin Ebecryl 600 with the photoinitiator Darocur 1173. The diffraction patterns are discussed in the simple crosslinking model and the formation of relief patterns is investigated. Furthermore the phase relation between the relief pattern and the light pattern is determined using asymmetric patterns.