Angular multiplexing with molecular glasses

containing 50 w% of the molecular glass and 50 w% PS. The growth curves of the refractive-index modulation at elevated temperatures could be fitted with equation 32. The time constant τ₁ decreased with increasing temperature, as is shown in figure 6.2. This result can be explained by an increased mobility of the azobenzene chromophores in the photochromic amorphous molecular material. The maximum refractive-index modulation stayed constant up to 65 °C and then showed a steep decrease around T_g. The slow time constant τ₂ of the decay, followed an Arrhenius law with an activation energy of 32 kJ/mol. This value is in the same order as the energies calculated for azobenzene-containing block copolymers ^[160].

6.7 Angular multiplexing with molecular glasses

For high-density data storage, several holograms should be inscribed at the same spot of the material by angular multiplexing. In order to check, if the molecular glasses are, in principle, suitable for angular multiplexing and if the azobenzene chromophores can reorient in a strongly diluted environment, thick samples of a molecular glass diluted with polycarbonate were prepared by compression molding. A large quantity of the molecular glass is needed for the preparation of thick samples. Since a relatively large amount of material 6g was present, this compound was used. The angular selectivity is determined by the thickness of the holographic gratings. Due to the strong optical absorption of the azobenzene chromophore, the photochromic low-molecular-weight compounds must be highly diluted to obtain thick samples with sufficiently low optical density at the writing wavelength, so the writing laser can penetrate the whole sample. The samples were 1.8 mm thick and of good optical quality and had an optical density of 1.4 at 488 nm.

The experiments were performed with sp-polarized writing beams. This configuration has the advantage, in addition to its high achievable refractive-index modulation, that it yields the possibility of easily suppressing scattered light with a polarizer, since the diffracted beam is polarized perpendicularly to the reading beam and scattered light. A disadvantage of sp-polarization is the large disparity of the DEs between the inscribed gratings ^[52]. This difference can reach several orders of magnitude, since the inscription of each new grating partly erases previously written holograms.

An optimized exposure scheme, however, yields the possibility to greatly reduce this variation ^[162,179]. The exposure times leading to uniform diffraction efficiencies can be determined by an iterative approach. First a series of angle-multiplexed holographic gratings are inscribed, e.g. with equal exposure times. From this experiment, the DE of the mth grating and the corresponding total energy per area that was incident on the sample before and during inscription of the mth grating, E_m, can be obtained. From these DEs as a function of exposure energy, the cumulated grating strength C at the

76 6 Amorphous low-molecular-weight compounds energy E_m can be calculated by cumulating the square-root of all DEs up to E_m. The obtained data points for the cumulated grating strength can be fitted with a polynomial: E exposure energy per sample area a_i fit parameters

Usually a polynom of sixth order is sufficient to describe the cumulated grating strength. The maximum of the cumulated grating strength is equal to the available saturation grating strength. Equal grating strengths are obtained by dividing the available saturation grating strength by the number of holograms to be multiplexed. The desired exposure schedule is then:

 N total number of holograms

E_i energy per area irradiated for the inscription of the ith grating E_n energy per area that will be required to inscribe the nth grating By rearranging equation 35 with the factors obtained from equation 34, the new exposure scheme can be calculated as:

5 1

t_n exposure time of the nth grating I₀ total intensity of the laser beams

A second set of angle-multiplexed gratings is then inscribed according to this new scheme which yields more uniform DEs, and new exposure times are

6.7 Angular multiplexing with molecular glasses 77 calculated from this inscription. After a few steps of iteration, largely uniform DEs are obtained. Thus, it was possible to reduce the variation of the DE between the first and the last inscribed grating of a series from a factor of 20 to 1.2. The writing times leading to equal diffraction efficiencies are best described by an exponential law. The writing time of the kth grating was 90 % of that for the (k+1)th grating. This description, however, is not valid for the very first and very last gratings.

Up to 19 gratings can be inscribed at the same spot, as shown in figure 6.3.

This demonstrates that light-induced reorientation of the azobenzene chromophores is possible in strongly diluted systems and that angular multiplexing is possible in molecular glasses. This is the first report of angular multiplexing in low-molecular-weight organic glasses containing azobenzene moieties. From the experiments, the maximum sensitivity of the sample was calculated as S_max = 19 cm/J and the dynamic range as M# = 0.15.

Figure 6.3. 19 angle-multiplexed plane-wave holograms inscribed in a thick sample of compound 6g blended with polycarbonate. Writing was performed with sp-polarized beams.

The exposure times were optimized as described in the text.

78 6 Amorphous low-molecular-weight compounds

79

7 Molecular glasses as blending