Imaging of Nanoparticle Delivery Using Terahertz Waves - Intracellular Delivery: Fundamentals and Applications - page 703

Fig. 1 Characteristic energies in electromagnetic spectrum
et al. 1991 , Son et al. 1992 ) illuminate a semiconductor, such as semi-insulating
GaAs (SI-GaAs) having a band gap of 1.45 eV, electron-hole pairs are generated
and drift to the electrodes fabricated on the semiconductor wafer (see Fig. 2 ). The
drifted charges are collected by the opposite electrodes biased with a dc voltage,
which, in turn, generate a photocurrent surge in the coplanar waveguide. The
increase in the photocurrent is proportional to the integration of the optical pulse,
and the decrease is dependent on the semiconductor carrier lifetime. The time-
varying photocurrent gives rise to the free-space radiation of electromagnetic waves
with an equivalent Lienard-Wiechert potential (Schwartz 1972 ). This generation
scheme is known as the photoconductive switching technique. Using this technique,
Fattinger et al . achieved a frequency spectrum of over 1 THz by Fourier trans-
formation of the subpicosecond electromagnetic pulse in the free space. They
referred to this radiation as the THz beams (Fattinger and Grischkowsky 1988 ).
Other generation methods using femtosecond laser interaction include techniques
using a built-in surface field in semiconductors, the photo-Dember effect, and
optical rectification. These techniques do not require the use of external voltage
bias (Zhang and Xu 2010 ).
Variations in THz pulses can occur in less than a picosecond, making them
impossible to measure with conventional electronic equipment such as oscillo-
scopes or network analyzers. The first technique to measure such high-frequency