Broadband Terahertz Spectroscopy
121
Fig. 3 THz generation and detection using the photoconductive antenna
50 THz. However, the photocarrier’s slow response in the available materials severely
limits the high-frequency part of the spectral bandwidth [26].
The principle behind the detection using photoconductive antennas is the same
as that of the generation. An optical gate pulse and a THz pulse are overlapped
and interact simultaneously with the switch. The charge carriers generated due to the
photoexcitation by the optical gate pulse are accelerated by the bias field provided by
the electric field of the THz pulse. The induced transient photocurrent is proportional
to the THz electric field. The resulting photocurrent is coherently detected as the time
delay between the THz and the gate pulse is scanned. Thus, the entire THz waveform
is recorded in the time-domain [32, 33]. Fourier transformation of the time-domain
waveform will give the amplitude and phase spectrum (frequency domain).
2.2 Optical Rectification (OR) and Electro-optic (EO)
Sampling
In the optical rectification technique, propagation of an intense laser light through a
non-linear crystal induces a time-dependent polarization that emits an electromagnetic wave. We may envision optical rectification as a difference frequency generation between pairs of different frequency components of the exciting optical pulse
[34]. The phase-matching condition for the generation of THz radiation via OR is
given by Nahata et al. [20]
k
ω opt +
− k
ω opt
= k()
(1)
where k is the wave vector, ω opt is the angular frequency of the optical pump,
and is one particular frequency of the radiated THz wave-packet. We can depict
the terahertz radiation as the superposition of all the frequency components of the
optical pump pulse (Fig. 4).
The polarization of the emitted THz field depends on the crystal orientation and the
polarization of the excitation pulse. In this case, the spectral bandwidth, contrary to
photoconductivity, depends only on the incident pulse duration. In practice, though,
121
Fig. 3 THz generation and detection using the photoconductive antenna
50 THz. However, the photocarrier’s slow response in the available materials severely
limits the high-frequency part of the spectral bandwidth [26].
The principle behind the detection using photoconductive antennas is the same
as that of the generation. An optical gate pulse and a THz pulse are overlapped
and interact simultaneously with the switch. The charge carriers generated due to the
photoexcitation by the optical gate pulse are accelerated by the bias field provided by
the electric field of the THz pulse. The induced transient photocurrent is proportional
to the THz electric field. The resulting photocurrent is coherently detected as the time
delay between the THz and the gate pulse is scanned. Thus, the entire THz waveform
is recorded in the time-domain [32, 33]. Fourier transformation of the time-domain
waveform will give the amplitude and phase spectrum (frequency domain).
2.2 Optical Rectification (OR) and Electro-optic (EO)
Sampling
In the optical rectification technique, propagation of an intense laser light through a
non-linear crystal induces a time-dependent polarization that emits an electromagnetic wave. We may envision optical rectification as a difference frequency generation between pairs of different frequency components of the exciting optical pulse
[34]. The phase-matching condition for the generation of THz radiation via OR is
given by Nahata et al. [20]
k
ω opt +
− k
ω opt
= k()
(1)
where k is the wave vector, ω opt is the angular frequency of the optical pump,
and is one particular frequency of the radiated THz wave-packet. We can depict
the terahertz radiation as the superposition of all the frequency components of the
optical pump pulse (Fig. 4).
The polarization of the emitted THz field depends on the crystal orientation and the
polarization of the excitation pulse. In this case, the spectral bandwidth, contrary to
photoconductivity, depends only on the incident pulse duration. In practice, though,