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Germanium (Ge), thanks to its CMOS compatibility and near direct bandgap configuration -140 meV offset between the conduction band states at Gamma and L - has been for long in the race for an all-group-IV laser solution. In the GeSn alloy system, such demonstration has been achieved recently [1]. For Ge, the evidences were much less apparent, in spite of the fact that by applying strain [2], a true direct bandgap configuration is expected and thus the prospect for lasing operation is valid. Here, we explored for the first time the regime where (i) we excite the strained micro bridges at an energy much below the Ge bandgap to reduce the optical loss for modes propagating in the unstrained region of the cavity, (ii) the excitation pulse is 100 ps long, a time shorter than the carrier lifetime of > 5 ns and also shorter than the thermal constant of the suspended bridges but (iii) longer than any thermalization and carrier equilibration times. Under these conditions, using <100> uniaxial loading of strain in the range of 5 %, we obtain unambiguous lasing operation near 3.65 µm at low temperatures with linewidths down to 50 GHz with (a) thresholds at carrier concentration of typically 1E18 cm-3, (b) several orders of magnitude raise of the emission efficiency under lasing and (c) spectrally single mode operation, confirming the expected mode/gain competition behaviour.
[1] S. Wirths, R. Geiger, et al. NP 2015;9(2):88-92.
[2] M.J. Süess, R. Geiger, et al. NP 2013;7(6):466-472.
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