Dark hologram enhancement

Dark HSE is typical for As$_2$S$_3$ and some other chalcogenide films. This phenomenon is caused by the recording mechanism in As$_2$S$_3$ films. Illumination of such films induces a change in the medium range order and interlayer bonding [20,21]. Thus, the illuminated film is denser and thinner, and the holographic recording induces a mechanical stress with a periodical distribution (with period $\Lambda$ of the recorded grating). In darkness, the stress field can relax, allowing self-enhancement. The dark relaxation time is several days and the HSE is of the order of 5-10. This self-enhancement effect is not a general effect and occurs only in some special cases. HSE can be used for optical recording at low light intensities with subsequent enhancement. However, this enhancement effect depends on the grating period, $\Lambda$. Therefore, the HSE of a real image (i.e., a superposition of different interference structures) will distort the image. For the depression of the HSE in real-time optical recording, the diffraction efficiency of the recorded hologram must be close to the maximum (saturation) value [2].