Semiconductor nonlinearity in the range of
terahertz (THz) frequency has been attracting considerable
attention due to the recent development of high-power
semiconductor-based nanodevices. However, the under-
lying physics concerning carrier dynamics in the presence
of high-field THz transients is still obscure. This paper
introduces an ultrafast, time-resolved THz pump/THz
probe approach to study semiconductor properties in a
nonlinear regime. The carrier dynamics regarding two
mechanisms, intervalley scattering and impact ionization,
was observed for doped InAs on a sub-picosecond time
scale. In addition, polaron modulation driven by intense
THz pulses was experimentally and theoretically investi-
gated. The observed polaron dynamics verifies the
interaction between energetic electrons and a phonon
field. In contrast to previous work which reported optical
phonon responses, acoustic phonon modulations were
addressed in this study. A further understanding of the
intense field interacting with solid materials will accelerate
the development of semiconductor devices.
This paper can be divided into 4 sections. Section 1
starts with the design and performance of a table-top THz
spectrometer, which has the advantages of ultra-broad
bandwidth (one order higher bandwidth compared to a
conventional ZnTe sensor) and high electric field strength
( > 100 kV/cm). Unlike the conventional THz time-
domain spectroscopy, the spectrometer integrated a novel
THz air-biased-coherent-detection (THz-ABCD) techni-
que and utilized gases as THz emitters and sensors. In
comparison with commonly used electro-optic (EO)
crystals or photoconductive (PC) dipole antennas, the
gases have the benefits of no phonon absorption as existing