Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}