Excitation-Wavelength-Dependent Luminescence of Chemically and Physically Mixed Europium and Terbium Phosphonates: Color-Tunable Luminescence, Near-White-Light Emission, and Selective Fe3+ Detection.

Molecular lanthanide phosphonates [Ln2 (H3 tpmm)2 (H2 O)6 ] ⋅ xH2 O (Ln=Eu, EuP; Ln=Tb, TbP) were synthesized. Single-crystal X-ray diffraction confirmed that EuP has a sandwich-like dinuclear structure, in which the Eu(III) center adopts a {EuO8 } distorted dodecahedral geometry. XRPD patterns prove that TbP and EuP are isomorphous and isostructural. EuP and TbP are highly thermally stable approaching 450 °C and exhibit red- and green-light emissions from the characteristic 4 f-4 f transition of the Eu3+ and Tb3+ , respectively. Interestingly, luminescence modulation is achieved for the chemically mixed Eu/Tb phosphonate analogues, c-Eux Tb2 -x P (x=1.5, 1, 0.5), and physically mixed Eu/Tb phosphonate materials, p-yEuP : zTbP (y : z=3 : 1, 1 : 1, 1 : 3), with varying the excitation wavelength. Of particular note, near-white-light emission is also achieved for c-EuTbP, p-EuP : TbP, and p-EuP : 3TbP when excited at 365 nm. Therefore, these dinuclear molecular lanthanide phosphonates emitting excitation wavelength and Eu3+ : Tb3+ ratio dependent luminescence might be potential candidates for color-tunable luminescence materials and white-light-emitting materials. On the other hand, the bright green-light emission makes TbP to be an excellent reusable luminescence sensor for selective detection of Fe3+ with Stern-Volmer quenching constant (KSV ) of 9.66×103  M-1 and detection limit (DL) of 0.42 µM through absorption competition caused luminescence quenching effect.

Tailoring of spatial coherence in a multimode fiber by selectively exciting groups of eigenmodes.

Control of the properties of speckle patterns produced by mutual interference of light waves is important for various applications of multimode optical fibers. It has been shown previously that a high signal-to-noise ratio in a multimode fiber can be achieved by preferential excitation of lower order spatial eigenmodes in optical fiber communication. Here we demonstrate that signal spatial coherence can be tailored by changing relative contributions of the lower and higher order multimode fiber eigenmodes for the research of speckle formation and spatial coherence. It is found that higher order spatial eigenmodes are more conducive to the final speckle formation. The minimum speckle contrast occurs in the lower order spatial eigenmodes dominated regime. This work paves the way for control and manipulation of the spatial coherence of light in a multimode fiber varying from partially coherent or totally incoherent light.