126
S. Banerjee et al.
of the THz fields generated using these processes is their reduced focus abilities,
making it difficult to reach peak powers above 100 kV/cm [53].
Higher frequencies beams generated using laser-induced plasmas, having a spectrum of 1–7 THz, have shown to reach amplitudes of 400 kV/cm with only 30 nJ
pulse energy [47]. Photoconductive switches have also been used to scale up the THz
energies by increasing both the excitation fluence and the external bias field.
An average power of 20 W of the emitted THz has been reported using freeelectron lasers [53]. Recently, quantum cascade lasers (QCL) have gained popularity;
QCLs generally use conduction band transitions in semiconductor heterostructures
to achieve laser emission [54]. They offer CW and pulsed mode operations in the
mid-infrared regime at room temperature [55], which has been utilized to produce
high power, continuous-wave coherent radiation in the THz frequency range [54,
56, 57]. A major limitation of using QCLs is that they operate at low temperatures.
The highest cooling temperature used in THz QCL is 178 K. To introduce cryogenic
cooling in room temperature QCLs in THz devices is considered to be a major
drawback.
3 Time-Domain and Time-Resolved THz Spectroscopy
3.1 Experimental Setup
Figure 7 shows the schematic of a THz spectroscopy setup capable of performing
both THz-TDS and TRTS [58]. Here, the THz generation and detection are done by
creating a laser-induced plasma in ambient air. Other methods of THz generation
and detection can also be implemented in this setup with minimum alteration. The
primary optical source is a Ti: Sapphire amplified laser with a central wavelength
of 800 nm, 50 fs pulse duration, and a repetition rate of 1 KHz. The laser output is
split using a beam splitter. One part is used for the generation and detection of THz,
and the other part is directed to the optical parametric amplifier (OPA), where it is
used to generate optical pulses of different wavelengths for TRTS. The beam directed
towards the THz setup is again split into two using a pellicle beam splitter (R: T =
8:92), and the transmitted part is used as the pump pulse for THz generation. The
reflected beam is used as the gate beam to map the THz waveform using ABCD. The
800 nm fundamental pump beam (ω) is incident on a type-I BBO (β barium borate)
crystal of 100 μm thickness; this generates the second harmonic (2ω) at 400 nm.
The fundamental and the second harmonic is focused in the ambient air to create
an intense plasma. The plasma generates a wide frequency range of electromagnetic
radiation along with THz light. A high resistivity silicon filter is placed just after
the plasma, which allows the transmission of only the THz radiation. The THz is
then collimated using the first pair of off-axis parabolic mirrors and focused onto the
sample; the transmitted beam through the sample is again re-collimated by another
couple of parabolic mirrors and focused between a pair of electrodes for detection.
S. Banerjee et al.
of the THz fields generated using these processes is their reduced focus abilities,
making it difficult to reach peak powers above 100 kV/cm [53].
Higher frequencies beams generated using laser-induced plasmas, having a spectrum of 1–7 THz, have shown to reach amplitudes of 400 kV/cm with only 30 nJ
pulse energy [47]. Photoconductive switches have also been used to scale up the THz
energies by increasing both the excitation fluence and the external bias field.
An average power of 20 W of the emitted THz has been reported using freeelectron lasers [53]. Recently, quantum cascade lasers (QCL) have gained popularity;
QCLs generally use conduction band transitions in semiconductor heterostructures
to achieve laser emission [54]. They offer CW and pulsed mode operations in the
mid-infrared regime at room temperature [55], which has been utilized to produce
high power, continuous-wave coherent radiation in the THz frequency range [54,
56, 57]. A major limitation of using QCLs is that they operate at low temperatures.
The highest cooling temperature used in THz QCL is 178 K. To introduce cryogenic
cooling in room temperature QCLs in THz devices is considered to be a major
drawback.
3 Time-Domain and Time-Resolved THz Spectroscopy
3.1 Experimental Setup
Figure 7 shows the schematic of a THz spectroscopy setup capable of performing
both THz-TDS and TRTS [58]. Here, the THz generation and detection are done by
creating a laser-induced plasma in ambient air. Other methods of THz generation
and detection can also be implemented in this setup with minimum alteration. The
primary optical source is a Ti: Sapphire amplified laser with a central wavelength
of 800 nm, 50 fs pulse duration, and a repetition rate of 1 KHz. The laser output is
split using a beam splitter. One part is used for the generation and detection of THz,
and the other part is directed to the optical parametric amplifier (OPA), where it is
used to generate optical pulses of different wavelengths for TRTS. The beam directed
towards the THz setup is again split into two using a pellicle beam splitter (R: T =
8:92), and the transmitted part is used as the pump pulse for THz generation. The
reflected beam is used as the gate beam to map the THz waveform using ABCD. The
800 nm fundamental pump beam (ω) is incident on a type-I BBO (β barium borate)
crystal of 100 μm thickness; this generates the second harmonic (2ω) at 400 nm.
The fundamental and the second harmonic is focused in the ambient air to create
an intense plasma. The plasma generates a wide frequency range of electromagnetic
radiation along with THz light. A high resistivity silicon filter is placed just after
the plasma, which allows the transmission of only the THz radiation. The THz is
then collimated using the first pair of off-axis parabolic mirrors and focused onto the
sample; the transmitted beam through the sample is again re-collimated by another
couple of parabolic mirrors and focused between a pair of electrodes for detection.