Patent ID: 7443900

Claim:
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 [Z 1 -Y 1 -A 1 -Y 2 -M 1 -Y 3 -] n X (1) where Z 1 is a polymerizable group or a radical which carries a polymerizable group, Y 1 , Y 2 , Y 3 independently are chemical bonds, oxygen, sulfur, —CO—O—,—O—CO—,—O—CO—O—, —CO—N(R)— or —N(R)—CO—, A 1 is a spacer, M 1 is a mesogenic group, X is an n-valent chiral radical, R is hydrogen or C 1 -C 4 -alkyl, n is 1 to 6, and Z 1 , Y 1 , Y 2 , Y 3 , A 1 and M 1 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 Z 2 -Y 4 -A 2 -Y 5 -M 2 -Y 6 -A 3 -(-Y 7 -Z 3 ) n (III) where Z 2 , Z 3 are identical or different polymerizable groups or radicals which contain a polymerizable group, n is 0 or 1 Y 4 , Y 5 , Y 6 , Y 7 independently are chemical bonds, oxygen, sulfur, —CO—O—,—O—CO—,—O—CO—O—, —CO—N(R)— or —N(R)—CO—, A 2 , A 3 are identical or different spacers and M 2 is a mesogenic group, and at least one chiral compound of formula Ia [Z 1 -Y 1 -A 1 -Y 2 -M a -Y 3 -] n X (Ia), or of formula Ib Z 1 -Y 1 -A 1 -Y 2 -X 2 (Ib), where Z 1 , Y 1 , Y 2 , Y 3 , A 1 , X and n are as defined, M a is a divalent radical which comprises at least one heterocyclic or isocyclic ring system, and X 2 is a cholesteryl radical or a derivative thereof.