Biomedical Engineering Reference
In-Depth Information
Fig. 2
Schematic of THz pulse generation. (
a
) Generation of electron-hole pairs in photoconduc-
tive switching antenna. (
b
) Waveforms of excitation laser pulse, induced photocurrent, and THz
pulse in free space
signals was a photoconductive sampling technique that is the reverse of photocon-
ductive switching. Free-space THz waves are captured by a waveguide antenna and
the induced electric fields are switched on a semiconductor such as low-temperature-
grown GaAs (LT-GaAs) by a femtosecond laser. The switching occurs in less than
a picosecond to give a dc voltage to a lock-in amplifier because the carrier lifetime
of LT-GaAs is approximately 0.5 ps (Gupta et al.
1991
). The full THz waveform
can be reconstructed by varying the optical delay between the generation and detection
femtosecond optical pulses. The detection can also be accomplished with the
electro-optic (EO) sampling technique using Pockels' effect (Valdmanis and Mourou
1986
). Some examples of EO crystals that can be used for freespace detection are
ZnTe, GaP, GaSe, etc. A bandwidth of over 30 THz was realized using this technique
(Wu and Zhang
1997
).
One example of a THz spectroscopic system is shown in Fig.
3
. This system
generates THz pulses from an InAs wafer by the photo-Dember effect and detects
them using a photoconductive switch of LT-GaAs. It operates in a reflection mode
to measure signals from an absorptive sample such as water or certain biological
materials. This system can obtain spectroscopic information from 0.1 to several
THz from a specific measurement point, and can also construct a spectroscopic image
of a sample by raster scanning. This experimental setup was utilized to produce the
measurement results shown in
Sects. 3
and
4
.
Because THz technology has tremendous potential for applications in scientific
study, security, and medicine, many generators and detectors have been developed.
Among the more notable generators are quantum cascade lasers (Kohler et al.
2002
),
THz parametric generators and oscillators (Kawase et al.
1996, 2001
), and THz
photomixers using two diode lasers (McIntosh et al.
1995
). A detector fabricated with
a microbolometer array can take live images of THz signals (Behnken et al.
2008
).
See the work by Son (
2009
) for a detailed review of generators and detectors.
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