Signatures of optical phase transitions in superradiant and subradiant atomic arrays

Resonant light interacting with matter supports different phases of a polarisable medium, and optical bistability where two phases coexist. Such phases have previously been actively studied in cavities. Here, we identify signatures of optical phase transitions and optical bistability mapped onto scattered light in free-space planar arrays of cold atoms. Methods on how to explore such systems in superradiant and extreme subradiant states are proposed. The cooperativity threshold and intensity regimes for the intrinsic optical bistability, supported by resonant dipole-dipole interactions alone, are derived in several cases of interest analytically. Subradiant states require lower intensities, but stronger cooperativity for the existence of non-trivial phases than superradiant states. The transmitted light reveals phase transitions and bistability that are predicted by mean-field theory as large jumps in coherent and incoherent signals and hysteresis. In the quantum solution, traces of phase transitions are identified in enhanced quantum fluctuations of excited level populations.

Improved photon upconversion photoluminescence and intrinsic optical bistability from a rare earth co-doped lanthanum oxide phosphor via Bi3+ doping

An Er3+, Yb3+, Bi3+ co-doped La2O3 phosphor shows upconverted emission and intrinsic optical bistability properties.

Upconversion based near white light emission, intrinsic optical bistability and temperature sensing in Er3+/Tm3+/Yb3+/Li+:NaZnPO4 phosphors

NaZnPO4:Er3+/Tm3+/Yb3+/Li+ phosphors have been prepared which show UC based near white light emission, intrinsic optical bistability and temperature-dependent population re-distribution ability.