Broadband Terahertz Spectroscopy
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applications in detecting explosives and drugs, biomedical imaging, and as body
scanners [9].
Today, the research in THz spectroscopy has grown so diverse that it is not
possible to include all aspects in one review article. The major areas of current works
can be classified into four categories: (1) developing/improving the THz source,
detector, and optics; (2) applying time-domain and time-resolved THz spectroscopy
to molecules and materials; (3) THz imaging, and (4) non-linear THz spectroscopy.
The scope of this review is limited to pulsed broadband THz spectroscopy using
tabletop emitters and detectors based on femtosecond lasers. For other aspects of
THz spectroscopy, we recommend the readers to refer to several books and review
articles published recently and the reference therein [3, 5, 10–15]. Section 2 discusses
the popular and effective ways of ultrafast laser-based generation and detection of
broadband THz pulse. Here the focus will be more on using air-plasma-based techniques because of their advantages over the other methods. Next, in Sect. 3, THz
time-domain spectroscopy (THz-TDS) and time-resolved THz spectroscopy (TRTS)
are presented in detail. In Sect. 4, we discuss some recent representative applications
of broadband THz spectroscopy.
2 Generation and Detection of Broadband THz Pulse
One of the main research interests of THz science and technology is to improve the
ways of generation and detection of broadband THz radiation. The oldest method
is the use of photoconductive antennas for generating and detecting THz pulses
[16, 17]. Non-resonant processes such as optical rectification (OR) and the linear
electro-optic (EO) sampling are also widely used for broadband THz generation and
detection [18–20]. Recently, air-photonics have been utilized for generating intense
ultra-broadband THz light and its coherent detection [21–23].
The ultrafast laser mediated tabletop methods for the generation and detection of
broadband THz pulse discussed in this review are based on a generic scheme shown
in Fig. 2. A typical THz spectroscopy setup would consist of an ultrafast laser, a THz
source (emitter) and detector (receiver), and elements (THz optics) to modulate the
THz radiation from the source to the detector. The ultrafast laser (pulse width <100 fs)
beam is split into a pump beam and a gate beam using a beam splitter. The pump
beam excites/interacts with the THz emitter to produce a single cycle or a few-cycle
(depends on the emitter) THz pulse. The generated THz pulse is collected, collimated,
and focused onto the THz receiver using off-axis parabolic mirrors. The gate beam
travels through an optical delay line and meets the THz pulse at the THz receiver. A
delay line controls the temporal overlap of the gate pulse (fs) and the THz pulse (ps).
Here the electric field, not the intensity, of the THz pulse is detected as a function of
time. Hence, both the amplitude and the phase of the spectral components of the entire
band can be measured. It is easier for an all-optical generation and detection system
to incorporate a time-synchronized femtosecond excitation pulse in the THz-TDS
set-up, thus making simultaneous TRTS experiments also possible.
119
applications in detecting explosives and drugs, biomedical imaging, and as body
scanners [9].
Today, the research in THz spectroscopy has grown so diverse that it is not
possible to include all aspects in one review article. The major areas of current works
can be classified into four categories: (1) developing/improving the THz source,
detector, and optics; (2) applying time-domain and time-resolved THz spectroscopy
to molecules and materials; (3) THz imaging, and (4) non-linear THz spectroscopy.
The scope of this review is limited to pulsed broadband THz spectroscopy using
tabletop emitters and detectors based on femtosecond lasers. For other aspects of
THz spectroscopy, we recommend the readers to refer to several books and review
articles published recently and the reference therein [3, 5, 10–15]. Section 2 discusses
the popular and effective ways of ultrafast laser-based generation and detection of
broadband THz pulse. Here the focus will be more on using air-plasma-based techniques because of their advantages over the other methods. Next, in Sect. 3, THz
time-domain spectroscopy (THz-TDS) and time-resolved THz spectroscopy (TRTS)
are presented in detail. In Sect. 4, we discuss some recent representative applications
of broadband THz spectroscopy.
2 Generation and Detection of Broadband THz Pulse
One of the main research interests of THz science and technology is to improve the
ways of generation and detection of broadband THz radiation. The oldest method
is the use of photoconductive antennas for generating and detecting THz pulses
[16, 17]. Non-resonant processes such as optical rectification (OR) and the linear
electro-optic (EO) sampling are also widely used for broadband THz generation and
detection [18–20]. Recently, air-photonics have been utilized for generating intense
ultra-broadband THz light and its coherent detection [21–23].
The ultrafast laser mediated tabletop methods for the generation and detection of
broadband THz pulse discussed in this review are based on a generic scheme shown
in Fig. 2. A typical THz spectroscopy setup would consist of an ultrafast laser, a THz
source (emitter) and detector (receiver), and elements (THz optics) to modulate the
THz radiation from the source to the detector. The ultrafast laser (pulse width <100 fs)
beam is split into a pump beam and a gate beam using a beam splitter. The pump
beam excites/interacts with the THz emitter to produce a single cycle or a few-cycle
(depends on the emitter) THz pulse. The generated THz pulse is collected, collimated,
and focused onto the THz receiver using off-axis parabolic mirrors. The gate beam
travels through an optical delay line and meets the THz pulse at the THz receiver. A
delay line controls the temporal overlap of the gate pulse (fs) and the THz pulse (ps).
Here the electric field, not the intensity, of the THz pulse is detected as a function of
time. Hence, both the amplitude and the phase of the spectral components of the entire
band can be measured. It is easier for an all-optical generation and detection system
to incorporate a time-synchronized femtosecond excitation pulse in the THz-TDS
set-up, thus making simultaneous TRTS experiments also possible.