Observation of field-induced optical phonon amplification in semiconductor nanostructures

We have experimentally proven the Cerenkov generation of optical phonons by drifting electrons in a semiconductor. We observe an instability of the polar optical phonons in nanoscale semiconductors that occurs when electrons are accelerated to very high velocities by intense electric fields. The instability is observed when the electron drift velocity is larger than the phase velocity of optical phonons and rather resembles a "sonic-boom" for optical phonons. The effect is demonstrated in p-i-n semiconductor nanostructures by using subpicosecond Raman spectroscopy. We suggest that the observed phenomena will have enormous impact on the carrier dynamics in nanoscale semiconductor devices.