External diffraction efficiency

The chromophores serve for the formation of efficient photorefractive gratings and they are photo-chemically active, probably by 2 -n 2 photochemical reactions, when triplet sensitized. The photorefractive external diffraction efficiency is highly dependent on the glass transition temperature of the composites. A low glass transition temperature favors the efficiency. 

For temperature-dependent experiments the sample is placed in a box of anodized aluminum with optical windows. In its walls it contains meandering water pipes connected to an external thermostat (Julabo Ultratemp 2000). Long-term stability experiments up to several years can be performed at room temperature. To this end the samples with inscribed holograms are stored in the dark and their diffraction efficiencies are measured at pre-defined times with a separate diode laser. 

The four-wave mixing technique serves to measure the diffraction efficiency t] during the writing process as a function of time and as a function of the strength of the external electric field. Figure 4.4 shows a schematic representation of a typical set-up employed in four-wave mixing experiments. 

Fig. 4.4 Schematic depiction of a set-up for a four-wave mixing experiment, as employed to measure diffraction efficiency as a function of the strength of an external electric field. Reading beam counterpropa-gating with writing beam (1). Diffracted beam counterpropagating with writing beam (2). Adapted from Kippelen et al.

Fig. 4.2 The structure and operation of the H-PDLC transmission gratings. Liquid crystal droplet planes are perpendicular to the glass plates. The grating spacing A is typically in the range of 0.5 — 5 pm. In the absence of external voltage the droplet directors are preferentially aligned perpendicular to the droplet planes, while above some threshold voltage they reorient along the electric field, i.e. perpendicular to the glass walls. This causes a decrease of the refractive index mismatch between the liquid crystal domains and the polymer matrix and hence the diffraction efficiency is reduced. The directions of s and p polarisations of incident optical beam are also denoted...

Figure 4.14 shows the dependence of the diffraction efficiency rjd and relaxation time Tf on the magnitude of the external electric field applied along the... 

Fig. 4.14 Diffraction efficiencies and relaxation time of the fast scattering mode as functions of amplitude of external electric held measured in the UV grating at room temperature. Solid line in the inset is a fit to linear dependence. The dashed lines are a guide for the eye. Reprinted with permission from [9]. Copyright by the American Physical Society...

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