980 nm

1. 4I11/2 4I13/2 980 nm 1540 nm 4I15/2 Effect of Ge Nanocrystals on the Optical Properties of Er-Doped Silica Eugene E. Haller, University of California, Berkeley, DMR 0405472 Research Goal: To optimize the photoluminescence (PL) of erbium (Er)-doped silica by the addition of Ge nanocrystals. Er, a rare earth element, is well known for its sharp optical transitions between the states of the f-electrons. Such transitions are rather insensitive to the chemical environment because they do not occur for valence electrons. Recently, we began incorporating Ge nanocrystals into Er-doped silica films with the aim to enhance the efficiency of Er PL. Preliminary PL measurements of Ge implanted into Er-doped-silica specimens show that the presence of Ge significantly enhances the Er PL at 1540 nm. Furthermore, it has been observed that Ge acts as an antenna by increasing the absorption of the pumping laser. Ultimately, we hope to achieve a more detailed understanding of the physical mechanisms generating the PL enhancement which may lead to novel on-chip optical fiber technology. Above: PL spectra of Er and SiO2 co-sputtered onto a Si wafer before and after Ge ion implantation. The peak at 1120 nm is attributed to luminescence of the Si substrate. The 1540 nm peak is generated by the radiative transition from the first excited state to the ground state of the Er ions as shown in the inset. It can be readily seen that the presence of Ge not only increases the laser absorption (the Si substrate peak can no longer be seen) but also the Er PL is enhanced.

2. Effect of Ge Nanocrystals on the Optical Properties of Er-Doped Silica Eugene E. Haller, University of California, Berkeley, DMR-0405472 Education: Julian Guzman, a graduate student at the University of California at Berkeley, has contributed to this research project. Julian, a native of Colombia, has advanced his synthesis and characterization skills during the past year. Broader Impact: Optical signal amplification in silica fibers is of great interest to applications where long distances have to be bridged such as transoceanic communications. Er is widely used to amplify optical signals because its 1540 nm emission line lies at the minimum of the intrinsic optical absorption of silica and because the transition can be stimulated by photons. The efficiency of Erbium-Doped Fiber Amplifiers (EDFAs) is expected to increase dramatically by the addition of Ge nanocrystals to the silica. The nanocrystals possess many properties (small energy bandgap, wide size-dependent emission tunability, etc.) which should make them highly effective for this purpose. Small on-chip Er amplifiers, a major breakthrough for optical information processing, may become possible. Above: Illustration of a typical optical amplifier. It consists of an Er-doped fiber which is pumped with a laser at a wavelength of 980 nm or 1480 nm. The pump laser supplies the energy to excite Er-electrons to higher-energy states. Furthermore, the incoming optical signal stimulates the excited electrons to relax to the ground state, emitting additional in-phase photons. These photons stimulate more electrons to relax to the ground state. The incoming signal is amplified after it has passed through the Er-doped fiber.