Single-emitter nanocrystal fluorescence in liquid crystal hosts: nonclassical light sources with definite polarization

Overview is presented on using liquid crystals in single-photon sources (SPSs) exhibiting antibunching (separation of all photons in time) with definite linear or circular polarizations. SPSs are key components for secure, long-distance quantum communication systems with quantum repeaters. If we can produce a photon with definite polarization, the efficiency of the quantum cryptography system is doubled. Both nematic and cholesteric liquid crystal (CLC) hosts were used to create definite linear or circular polarization of antibunched photons emitted by different types of single emitters (dye molecules, colloidal semiconductor nanocrystal quantum dots (NQDs) with different fluorescence wavelengths, nitrogen-vacancy (NV) color-center nanodiamonds). Definite polarization from nanocrystals doped with trivalent rare-earth ions in liquid crystal hosts at low light level is also reported. Both monomeric and glassy oligomeric liquid crystal hosts have been used. Raman scattering in monomeric liquid crystals can prevent photon antibunching at some fluorescence wavelengths. A circular polarized microcavity resonance in NQD fluorescence in a 1-D photonic bandgap CLC microcavity and spontaneous emission enhancement as well as some pitfalls on purity of single-photon emission with using photoalignment materials are discussed (photoalignment materials’ fluorescence imaging may show single-emitter fluorescence behavior with emission of antibunched light).