Picosecond internal $Q$-switching mode correlates with laser diode breakdown voltage

The record picosecond power density recently achieved with a current-pumped laser diode turned our attention to a still unexplained 50-year-old phenomenon termed "internal $Q$-switching". The correlation found experimentally here between the relatively high breakdown voltage ($\sim$ 5–11 V) in a heavily doped single-heterostructure laser diode and its high-power picosecond lasing provides a means for solving the puzzle. Together with the experimental fact that picosecond lasing occurs from the $p$–$n$ junction, this implies that internal $Q$-switching is determined by the compensated layer rather than by “traditional” single-heterostructure waveguide. This finding is valid for various growth technologies independently of whether the high breakdown voltage and picosecond lasing are achieved by exact compensation of shallow donors by shallow acceptors, or by doping profile gradients.