S. Hvilsted

An analysis of the anisotropic and topographic gratings in a side-chain liquid crystalline azobenzene polyester

We have examined in detail the formation of surface relief structures in azobenzene polyesters formed by polarization holography with orthogonally circularly polarized laser beams. We show that it is possible to separate the contribution to the diffraction efficiency into an anisotropic part and a surface relief part by examining the polarization content of the first order diffracted beam. By studying the dynamics of the growth of the grating, we show that the gratings due to anisotropy and surface relief appear at the same time. Atomic force microscopic investigations of the film after irradiation reveal a strongly polarization dependent surface relief pattern.

Optically induced surface relief phenomena in azobenzene polymers

Azobenzene polymers and oligomers show intriguing surface relief features when irradiated with polarized laser light. We show through atomic force microscopic investigation of side-chain azobenzene polymers after irradiation through an amplitude mask that large peaks or trenches result depending on the architecture of the polymer. Extensive mass transport over long distances has been observed, paving the way for easy replication of nanostructures. We also show that it is possible to store microscopic images as topographic features in the polymers just through polarized light irradiation.

Physics and technology of optical storage in polymer thin films

Abstract We discuss different strategies for optical storage of information in polymeric films. An outline of the existing trends is given. The synthesis and characterization of side-chain azobenzene polyester films for holographic storage of information is described. A compact holographic memory card system based on polarization holography is described. A storage density of greater than 10MB/cm 2 has been achieved so far, with a potential increase to 100MB/cm 2 using multiplexing techniques and software correction. Finally the role of surface relief in azobenzene polymers on irradiation with polarized light is mentioned.

Biphotonic holographic gratings in azobenzene polyesters: Surface relief phenomena and polarization effects

Biphotonic holographic gratings have been recorded in a side-chain azobenzene liquid crystalline polyester using a blue incoherent source and a He–Ne laser. Intensity gratings and the appearance of surface relief have been observed when two linearly polarized beams from a He–Ne laser are made to interfere on a film illuminated with blue light. Polarized holographic gratings are also created with two orthogonally circularly polarized He–Ne beams. All these gratings are stable in darkness but can be erased with blue light.

High diffraction efficiency polarization gratings recorded by biphotonic holography in an azobenzene liquid crystalline polyester

High diffraction efficiencies have been achieved with polarization gratings recorded in thin films of an azobenzene side-chain liquid crystalline polyester by means of biphotonic processes. Efficiency values up to 30% have been reached after an induction period of 300 s and subsequent evolution with the sample in darkness. These values are at least two orders of magnitude higher than those previously reported for biphotonic recording. The gratings can be erased with unpolarized blue light and partial recovery of the diffraction efficiency has been observed after the erasure process when the sample is kept in darkness. Red light illumination of the erased film increases the recovered efficiency value and the recovery rate.

Propagation of Polarized Light Through Azobenzene Polyester Films

When elliptically polarized light of appropriate wavelength corresponding to trans - cis - trans isomerisation process is incident on thin films of azobenzene polyesters, a helical structure is induced. We investigate the propagation of the exciting light beam (self-induced) as well as a probe light beam outside the absorption band through the polyester films. Investigations are carried out in one amorphous and one liquid crystalline polyester. We show that amorphous polyester after irradiation behaves like classical helical material.

Simulation of erasure of photoinduced anisotropy by circularly polarized light

The temporal evolution of photoinduced birefringence is investigated on the basis of a model proposed by Pedersen and co-workers. This model is extended for the case of elliptically polarized light, and used to describe the erasure of photoinduced birefringence by circularly polarized light. It is shown that the anisotropy is not erased, only the direction of the optical axis is changed, and this is measured as erasure. The computed results are compared with measurements and show good agreement for several intensities.

Light scattering of thin azobenzene side-chain polyester layers

Light scattering properties of liquid crystalline and amorphous azobenzene side-chain polyester layers used for optical data storage were examined by means of transmissive scatterometry. Comparative experiments show that the amorphous polyester has significantly lower light scattering characteristics than the liquid crystalline polyester. The amorphous samples have negligible polarization part orthogonal to the incident beam, the liquid crystalline samples have relative high orthogonal polarization part in light scattering. The light scattering results can be used to give a lower limit for the domain size in thin liquid crystalline polyester layers being responsible for the dominant light scattering. The characteristic domain sizes obtained from the Fourier transformation of polarization microscopic pictures confirm these values.

Rewritable azobenzene polyester for polarization holographic data storage

Optical storage properties of thin azobenzene side-chain polyester films were examined by polarization holographic measurements. The new amorphous polyester film is the candidate material for the purpose of rewritable holographic memory system. Temporal formation of anisotropic and topographic gratings was studied in case of films with and without a hard protective layer. We showed that the dominant contribution to the diffraction efficiency comes from the anisotropy in case of expositions below 1 sec even for high incident intensity. The usage of the same wavelength for writing, reading and erasing was tested. The ability of azobenzene polyester for rewriting was found satisfactory after many writing-erasing cycles.

Holographic recording in thiophene-based polyester

The synthesis and optical data storage properties of a side-chain thiophene-phenyl azopolyester ThPhAzoPol is reported. The polyester is derived from diphenyl tetradecanedioate and a thiophene-based liquid crystalline diol which exhibits a short-lived enantiotropic SmA phase (Cryst 177.7 SmA 180.4 I). The polyester ThPhAzoPol exhibits amorphous (Tg, 78.6 °C), crystalline and liquid crystalline character as evidenced by differential scanning calorimetry and thermal polarising microscopy. A grainy texture, which is thermally reversible, with increasing birefringence on cooling from the isotropic melt is observed. The polyester is amenable to optical data storage, showing efficient induced anisotropy, which is stable at room temperature. Polarisation gratings can be inscribed using orthogonally linear and circularly polarised light to good effect (>1%) and surface relief gratings are formed in addition as evidenced by atomic force microscopy.