The development of planar-type optical amplifier is required for the complete building of the ultra-high speed network. Although the planar waveguide amplifier has been studied in many ways, the high concentration of $Er^{3+}$ ions restricts the performance of planar optical waveguide due to concentration quenching. Recently, there are many studies on polymer materials doped with $Er^{3+}$ ions in the form of complex, which can prevent clustering of $Er^{3+}$ ions. This work is concerned on the photoluminescence(PL) of the sol-gel hybrid material doped with erbium complex.
The sol-gel hybrid material which has low absorption near 1.5 $mu$m wavelength is prepared using methyl, vinyl, and phenyl substituted silicon alkoxides. And also another hybrid materials are synthesized using colloidal silica to reduce residual OH group. Several $Er^{3+}$ complexes are doped in the hybrid film. Among them, erbium tris 8-hybroxyquinoline(ErQ) shows the most intense PL. ErQ doped hybrid films also show the 1.53 $mu$m photoluminescence when pumped at a wavelength of 477 nm that is not a resonant excitation wavelength of $Er^{3+}$ ions.
Luminescence properties of the sol-gel hybrid materials doped with erbium precursor is investigated with the structural and compositional change. When we use colloidal silica instead of silicon alkoxide, it is expected that large scattering loss and concentration quenching due to the large particle size with big index difference and inhomogeneous distribution of erbium complex, respectively. When the sol-gel hybrid materials are polymerized through UV exposure, more intense photoluminescence is observed.
Photoluminescence excitation is carried out using the different wavelengths of $Ar^+$ laser to confirm the indirect excitation of $Er^{3+}$ ions. The intensity of photoluminescence at 1.53 $mu$m increases as the pumping wavelength decreases. We obtain a good correlation between the absorption and $Er^{3+}$ luminescence without any peaks at 488 or 515 nm, which indicates that the excitation of $Er^{3+}$ is dominated by absorption of the pump beam by the ligand of ErQ. ErQ doped sol-gel hybrid film shows the photoluminescence not only at 1.53 $mu$m but also 600 nm from organic ligand. Therefore, it is expected that the organic ligands absorb the pump energy and transfer to adjacent $Er^{3+}$ ions. Finally, we suggest the indirect excitation mechanism in the sol-gel hybrid materials doped with ErQ.
For the improvement of luminescence properties, we investigate the microstructural changes with UV exposure. We find that the photoinduced free radicals play important roles in large PL increase. When the hybrid film is irradiated by UV, the photoinitiators are dissociated to free radicals. These free radicals remove $O_2$ in the matrix that can reduce the population of ligand triplet state. Free radicals also remove the protons that are protonated in ErQ molecules that show poor luminescence properties. Thus, they recover the original molecular structure of ErQ and increase PL.
For the large absorption of ligand over visible wavelength, we synthesize chlorine substituted ErQ. When chlorine is substituted in 5 and 7 position of 8-hydroxyquinoline ligand, absorption peak shifts to longer wavelength. When 5,7-Cl substituted ErQ is doped into the hybrid matrix, we obtain 2.5 times higher PL compared with ErQ doped system.
Transparent mesoporous silica films are successfully prepared by spin-coating method. The x-ray diffraction patterns of the films indicate that both hexagonal and cubic mesoporous films can be formed by varying the surfactant to silicon mole ratio. The characteristics of the mesoporous silica films are studied by x-ray diffraction, thermal gravimetric analysis, SEM, TEM, and FT-IR spectroscopy.
ErQ is impregnated into the mesoporous silica films for the high doping level with homogeneous distribution. We confirmed the impregnation of ErQ into the pore of mesoporous silica film using isothermal nitrogen physisorption studies and Rutherford back-scattering method. The maximum concentration of $Er^{3+}$ is order of $10^{21} ions/cm^3$ without any concentration quenching.
