Laser gain medium for solid state dye lasers

A laser gain medium, comprising:

• at least one active species adapted to be stimulated to emit laser light within a predetermined wavelength range,
• optical feedback means defining a resonator for said laser light, said feedback means comprising at least one substantially solid cholesteric layer having a substantially planar texture exhibiting selective reflection of light defined by a reflection band tuned to said predetermined wavelength range, said cholesteric layer being obtained from reactive cholesteric mixtures selected from mixtures comprising:
• a) at least one cholesteric, polymerizable monomer;
• or
• b) at least one achiral, nematic, polymerizable monomer and one chiral compound in an inert diluent;
• or
• c) at least one cholesteric, crosslinkable oligomer or polymer selected from the group comprising cholesteric cellulose derivatives, propargyl-terminated cholesteric polyesters or polycarbonates, crosslinkable oligo- or polyorgano-siloxanes;
• or
• d) crosslinkable cholesteric copolyisocyanates in a polymerizable diluent;
• or
• e) chiral nematic polyesters having flexible chains whose cholesteric phase can be frozen in by rapid cooling to below the glass transition temperature,
• werein said mixtures b) do not comprise mixtures of an achiral, nematic, polymerizable monomer having a mesogenic group comprising
• and a chiral cholesterylcarbonate and a crosslinking agent;
• wherein said monomer of mixture a) is a chiral, liquid-crystalline, polymerizable monomer of the formula I
• [Z1-Y1-A1-Y2-M1-Y3-]n X (1)
• where
• Z1 is a polymerizable group or a radical which carries a polymerizable group,
• Y1, Y2, Y3 independently are chemical bonds, oxygen, sulfur,
• —CO—O—,—O—CO—,—O—CO—O—,
• —CO—N(R)— or —N(R)—CO—,
• A1 is a spacer,
• M1 is a mesogenic group,
• X is an n-valent chiral radical,
• R is hydrogen or C1-C4-alkyl,
• n is 1 to 6,
• and Z1, Y1, Y2, Y3, A1 and M1 can be identical or different if n is greater than 1;
• wherein said cholesteric mixture b) comprises
• at least one achiral, nematic, polymerizable monomer of the formula II
• Z2-Y4-A2-Y5-M2-Y6-A3-(-Y7-Z3)n (III)
• where
• Z2, Z3 are identical or different polymerizable groups or radicals which contain a polymerizable group,
• n is 0 or 1
• Y4, Y5, Y6, Y7 independently are chemical bonds, oxygen, sulfur,
• —CO—O—,—O—CO—,—O—CO—O—,
• —CO—N(R)— or —N(R)—CO—,
• A2, A3 are identical or different spacers and
• M2 is a mesogenic group,
• and at least one chiral compound of formula Ia
• [Z1-Y1-A1-Y2-Ma-Y3-]nX (Ia),
• or of formula Ib
• Z1-Y1-A1-Y2-X2 (Ib),
• where Z1, Y1, Y2, Y3, A1, X and n are as defined, Ma is a divalent radical which comprises at least one heterocyclic or isocyclic ring system, and X2 is a cholesteryl radical or a derivative thereof.