“…Among the candidates for guest materials one may consider here a series of water soluble proper ferroelectric crystals, like e.g. TGS [16,17], KDP [18,19] or GPI [20], efficient acoustooptic crystals with suitable elastic properties, as e.g. Cs 2 HgCl 4 or Cs 2 HgBr 4 [21,22], or efficient nonlinear optical crystal materials, such e.g.…”