Physical Review Letters - Direct-Gap Gain and Optical Absorption in Germanium Correlated to the Density of Photoexcited Carriers, Doping, and Strain
Direct-gap gain up to 850 cm-1 at 0.74 eV is measured and modeled in optically pumped Ge-on-Si layers for photoexcited carrier densities of 2.0×1020 cm-3. The gain spectra are correlated to carrier density via plasma-frequency determinations from reflection spectra. Despite significant gain, optical amplification cannot take place, because the carriers also generate pump-induced absorption of ≈7000 cm-1. Parallel studies of III–V direct-gap InGaAs layers validate our spectroscopy and modeling. Our self-consistent results contradict current explanations of lasing in Ge-on-Si cavities
Tiny Germanium Lasers should make Chips faster
Sigg’s research team, together with colleagues from the ETH Zurich and the Politecnico di Milano, has now been able to demonstrate that Germanium, under certain conditions, can function as a laser material. “Germanium lasers could make a break-through possible here, because Germanium can be readily combined with Silicon, which is the basic material from which chips are made. Silicon itself cannot emit light and can hardly be combined with any available laser material”, points out Sigg.
In their studies, which were performed at the Swiss Light Source (SLS) at PSI, the researchers investigated those properties of Germanium that are important for the generation of laser light, and compared them with those of currently available laser materials. “We stimulate the material by means of a powerful laser and simultaneously observe the changes occurring using infra-red radiation from the SLS”, elucidates the doctoral student Peter Friedli, who carried out the decisive experiments together with scientist Lee Carroll. “To do this, we used the fact that these light pulses are only 100 picoseconds (i.e. 0.1 billionths of a second) long, allowing us to follow the relevant processes in the material; that is, the behaviour of electrons at different points in time.”
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