Lasing in Rare-Earth-Doped Semiconductors: Hopes and Facts
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RS BULLETIN/SEPTEMBER 1999
the even terms of the crystal field split the free-ion J-multiplets into the Stark components typically by several meV for the ground State. The energy-level diagram of an Er 3+ ion in a cubic crystal field is shown in Figure 1, where the energytransfer paths relevant for SiiEr are also schematically indicated. The odd terms of the crystal field potential admix the states of opposite parity to the 4 / " configuration of the Er 3+ ion, thereby introducing a certain degree of electric-dipole strength into the otherwise forbidden intra-4/-shell transitions. This effect enhances slightly the magnetic-dipole strength of the 4 11 5/2 «-* 4 I 13/2 transition and is host- and site-dependent. There-
fore, Er-related centers of different microstructure can be fairly easily identified. 2 Note, however, that changes in the level-splitting are still in the meV ränge. Also, the change of the lifetime, frequently observed in emission from dif ferent Er-doped hosts, comes more from nonradiative relaxation than from changes in the radiative lifetime alone. 3 Such a Situation takes place for most Er 3+ -doped glasses and also in a Silicon matrix. The small effect of the crystal field and, consequently, the forbidden character of the 4115/2 «^ 4Io/z transition (pure radia tive lifetime in the 10-20 ms ränge 3 ), proved crucial for the development of Er-doped fiber amplifiers for optical Communications. A critical factor to this success is the long lifetime of the metastable 4Iu/2 State. It permits the required population Inversion under steady-state conditions of moderate-power pumping. Population inversion and high gain have been shown for various Er-doped glasses; stimulated emission can be achieved by direct pumping into the first as well as into the higher excited states.4 Compared with other materials, silica was shown to be a relatively poor host because of lower Er solubility and vulnerability to Er clustering; small additions of AI were found to effectively block Er clustering and, therefore, to improve Performance of Si0 2 :Er structures very effectively. A very long radiative lifetime indicates the need for large concentrations of active Er ions and large volumes to achieve the sufficient gain necessary for laser action.3 In a spontaneous emission mode, a typical
SiiEr* (4/11) 4
I
4
IiUL. h
4
e +h
Intermediate State
^^
2r6 + r7 + 2r8
Nonradiative recombination
r 6 +r 7 + 3r„ Non-Er-related recombinations Spin-orbit interaction
Cubic crystal field
Figure 1. Er3+ ion in Silicon crystal: energy-level diagram for cubic symmetry of the local crystal field is shown. Major energy-transfer mechanisms relevant to Er3* core excitation are also indicated. PL is photoluminescence.
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Lasing in Rare-Earth-Doped Semiconductors: Hopes and Facts
semiconductor junction doped with Er below the precipitation limit (about 1018 cm" 3 ) can thus produce no more than just a few microwatts of optical emitting power, even assuming 100% internal quantum efficiency. Therefore, the only hope of attaining l
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