  We study the influence of residual strain in InGaAs/InAlAs superlattice (SL)
on the ultrafast photocarrier dynamics under femtosecond laser excitation. We
propose and fabricate a novel-designed strained InGaAs/InAlAs SL which allows
us to obtain an ultrashort photocarrier relaxation time of $\tau\sim$ 1.5 ps
without Be-doping of the InGaAs photoconductor. We assume two dominant
mechanisms to be responsible for a sharp reduction of $\tau$: photocarriers
trapping by defect levels in InAlAs barriers and increased photocarriers
scattering at InGaAs/InAlAs interface roughness due to residual strain in the
SL. The THz time-domain spectroscopic measurements reveal an increase in both
emitted THz waveform and spectrum amplitudes with an increase of residual
strain in SL. The results might be of considerable interest for accommodating
the needs of THz pulsed spectroscopy and imaging in fundamental and applied
branches of THz science and technology.
