2D Square Lattice Photonic Crystal Phosphor Films for Efficient and Excitation Polarization Insensitive Color Conversion

As a continuing effort to improve the performance of photonic crystal (PhC) phosphors—an engineered nanophotonic phosphor structure developed by the authors' group—hereby 2D PhC phosphor films are introduced. In particular, the excitation polarization dependence inherent to and suffered by the precedent 1D PhC phosphors can be eliminated using a square lattice PhC structure due to its fourfold rotational symmetry. In addition, the thickness of the high index PhC backbone layer is intentionally increased so that dual excitation resonances occur for both the transverse‐electric (TE) and transverse‐magnetic (TM) waveguide modes, thereby effectively doubling the phosphor efficiency. 2D PhC phosphor structures are implemented and examined using colloidal quantum dots (CQDs) as phosphor material. Unlike conventional impurity‐doped phosphor materials composed of micron‐sized grains, nanometer‐sized CQDs are structurally compatible with PhCs whose feature sizes are ≈100 nm. Excitation polarization–insensitive CQD fluorescence is observed approx. four times enhanced at dual resonance wavelengths for the TE‐ and TM‐guided modes. The validity of the observed results is confirmed through simulations. To eliminate the excitation polarization dependence, a two‐dimensional square lattice photonic crystal phosphor structure is suggested and characterized. The phosphor structure exhibits enhanced phosphor efficiency with no excitation polarization dependence, thus ideal with an unpolarized excitation source. In addition, it supports both the transverse‐electric and transverse‐magnetic fundamental waveguide modes, thereby effectively doubling the phosphor efficiency.