**3. Low-molecular-weight AZs**

Because of easy preparation and high purity of low-molecular-weight compounds, the application of AZ materials for recording holograms began with them. Formation of holographic gratings in films of poly(vinyl alcohol) doped with low-molecular-weight AZ compounds was first reported by utilizing photoisomerization of AZs (Todorov et al., 1983, 1984, 1985). The holographic gratings recorded in the polymer-based samples were selfdeveloping and auto-erasing or re-writable since the thermally induced back isomerization of AZs could be easily obtained (Pham et al., 1997). To increase the formed birefringence |n2 n1| shown in Figure 2 and enhance the performance of recorded holograms, polymerdispersed AZ-containing low-molecular-weight LC compounds were also employed as recording media (Tondiglia et al., 1995). Due to the existence of low-molecular-weight LCs, the diffraction efficiency of the gratings could be electrically controlled (Sutherland et al., 1994). Moreover, dynamic holography was achieved in AZ-doped nematic LCs (Chen & Brady, 1993).

Although many research focused on the low-molecular-weight AZ–doped polymers, the stability of the induced anisotropy was low. To improve stability of the recorded gratings, molecular glasses fabricated with low-molecular-weight AZ compounds were developed (Shirota et al., 1998). This kind of materials can form stable amorphous and homogenous films without scattering of visible light (Nakano et al., 2002). Surface-relief gratings were successfully inscribed although the AZ derivatives were confined in the glassy sates. More interestingly, low-molecular-weight AZ crystals and co-crystals also showed capability of formation of surface-relief gratings (Nakano et al., 2005). It was believed that photoreaction only occurred in the surface.

For holograms recorded in low-molecular-weight AZ materials, gain effect often occurs. It was reported that diffraction efficiency of gratings recorded in calamitic LC materials with low-molecular-weight compounds was increased sharply after annealing (Stracke et al., 1999). In recording holograms using pentaalkinylbenzene derivatives, a strong gain effect was obtained by the selective growth of crystals in the non-irradiated areas (Frese et al., 2003). For smectic trisazomelamine, the gain effect was acquired accompanied by the growth of surface modulation. At elevated temperatures, the free volume available to mesogens increased and the thermal movement led to a collective orientation along the direction given by those groups pre-aligned by the recording beams. This caused an increase of the order degree in the pre-orientated domain, which contributed to the increase of diffraction efficiency of holograms.
