100
5 High-Field Terahertz Time-Domain Spectroscopy …
was positioned at the sample focus and used to measure the average THz power.
With the THz pump beam optically chopped at 5 Hz, the peak-to-peak voltage V
obtained from the pyroelectric detector was read off from an oscilloscope, and is
plotted against θ WGP in Fig. 5.6d. The pyroelectric detector has a calibrated voltage
responsivity R V = 2.44 × 10
5 VW
−1 for 632 nm light chopped at f c = 5 Hz, with
the stated absorption of the BL coating at 1 THz being α BL = 0.08 of that at 632 nm.
The THz pulse energy is therefore obtained from the voltage measurements using
W =
V
R V α BL t F N p f c
,
(5.15)
where t F = 0.80 accounts for the transmission of a THz low-pass filter (TYDEX
LPF8.8-30) placed in front of the pyroelectric detector, and N p = 100 is the number
of pulses hitting the pyroelectric detector in one on/off cycle of the optical chopper.
Using Eqs. 5.14 and 5.15 the THz electric field strength can be calculated from the
measured voltage, and is also plotted in Fig. 5.6c. This therefore allows the THz electric field strength to be varied between 8 and 279 kV cm
−1 , with two additional higher
field strengths available, 308 kV cm
−1 when WGP1 is removed, and 373 kV cm
−1
when both WGPs are removed.
5.3 A Test Case of Nonlinear THz Transmission: Indium
Antimonide
In many experimental setups, a test case or material is a valuable resource; a comparison of a well-known material response from other systems to that obtained from the
system in question can either be used for calibration purposes, or to explore the effectiveness of the experiment. For THz systems employing pulses with large electric
field strengths, a test material can be found in the form of indium antimonide (InSb).
InSb is a direct-gap III-V semiconductor with a zinc-blende crystal structure, and
exhibits both a narrow bandgap of ∼0.17 eV [55, 56] and a high electron mobility of
∼7 × 10
4 cm
2 V
−1 s
−1 [57] at 300 K. The low bandgap leads to a high intrinsic carrier
concentration, on the order of 10
16 cm
−3 at room temperature, which coupled with
the high carrier mobility results in a large absorption coefficient in the THz region.
Previous investigations of InSb using THz-TDS with high electric field strength
THz pulses have been performed by Hoffmann et al. [8], who observed a reduction
in the absorption of THz radiation with increasing electric field strength in both ntype InSb and GaAs. This effect was attributed to electron heating by the intense
THz pulses and strong intervalley scattering: the energy supplied to the electrons by
the electric field of the THz pulse is sufficient for many of them to scatter out of
the lowest energy conduction band and into side valleys, wherein they have reduced
mobilities, and since the free-carrier absorption is linked to the carrier concentration
and mobility, the THz absorption is reduced. The THz absorption in both InSb and
5 High-Field Terahertz Time-Domain Spectroscopy …
was positioned at the sample focus and used to measure the average THz power.
With the THz pump beam optically chopped at 5 Hz, the peak-to-peak voltage V
obtained from the pyroelectric detector was read off from an oscilloscope, and is
plotted against θ WGP in Fig. 5.6d. The pyroelectric detector has a calibrated voltage
responsivity R V = 2.44 × 10
5 VW
−1 for 632 nm light chopped at f c = 5 Hz, with
the stated absorption of the BL coating at 1 THz being α BL = 0.08 of that at 632 nm.
The THz pulse energy is therefore obtained from the voltage measurements using
W =
V
R V α BL t F N p f c
,
(5.15)
where t F = 0.80 accounts for the transmission of a THz low-pass filter (TYDEX
LPF8.8-30) placed in front of the pyroelectric detector, and N p = 100 is the number
of pulses hitting the pyroelectric detector in one on/off cycle of the optical chopper.
Using Eqs. 5.14 and 5.15 the THz electric field strength can be calculated from the
measured voltage, and is also plotted in Fig. 5.6c. This therefore allows the THz electric field strength to be varied between 8 and 279 kV cm
−1 , with two additional higher
field strengths available, 308 kV cm
−1 when WGP1 is removed, and 373 kV cm
−1
when both WGPs are removed.
5.3 A Test Case of Nonlinear THz Transmission: Indium
Antimonide
In many experimental setups, a test case or material is a valuable resource; a comparison of a well-known material response from other systems to that obtained from the
system in question can either be used for calibration purposes, or to explore the effectiveness of the experiment. For THz systems employing pulses with large electric
field strengths, a test material can be found in the form of indium antimonide (InSb).
InSb is a direct-gap III-V semiconductor with a zinc-blende crystal structure, and
exhibits both a narrow bandgap of ∼0.17 eV [55, 56] and a high electron mobility of
∼7 × 10
4 cm
2 V
−1 s
−1 [57] at 300 K. The low bandgap leads to a high intrinsic carrier
concentration, on the order of 10
16 cm
−3 at room temperature, which coupled with
the high carrier mobility results in a large absorption coefficient in the THz region.
Previous investigations of InSb using THz-TDS with high electric field strength
THz pulses have been performed by Hoffmann et al. [8], who observed a reduction
in the absorption of THz radiation with increasing electric field strength in both ntype InSb and GaAs. This effect was attributed to electron heating by the intense
THz pulses and strong intervalley scattering: the energy supplied to the electrons by
the electric field of the THz pulse is sufficient for many of them to scatter out of
the lowest energy conduction band and into side valleys, wherein they have reduced
mobilities, and since the free-carrier absorption is linked to the carrier concentration
and mobility, the THz absorption is reduced. The THz absorption in both InSb and