  A silicon-compatible light source is the final missing piece for completing
high-speed, low-power on-chip optical interconnects. In this paper, we present
a germanium-based light emitter that encompasses all the aspects of potential
low-threshold lasers: highly strained germanium gain medium, strain-induced
pseudo-heterostructure, and high-Q optical cavity. Our light emitting structure
presents greatly enhanced photoluminescence into cavity modes with measured
quality factors of up to 2,000. The emission wavelength is tuned over more than
400 nm with a single lithography step. We find increased optical gain in
optical cavities formed with germanium under high (>2.3%) tensile strain.
Through quantitative analysis of gain/loss mechanisms, we find that free
carrier absorption from the hole bands dominates the gain, resulting in no net
gain even from highly strained, n-type doped germanium.
