120
S. Banerjee et al.
Fig. 2 Generic scheme of generation and coherent detection of broadband THz radiation. The set
up does not include detailed optical components
2.1 Photoconductive Antenna as THz Emitter and Receiver
Photoconductive antennas (PCA) or switches are DC-biased metal dipole antenna
formed on a semiconductor substrate. As demonstrated in Fig. 3, when an optical
femtosecond pulse having photon energy more than the bandgap of the substrate
shines on the gap between the two electrodes, charge carriers are generated due
to bandgap excitation. These photogenerated charge carriers get accelerated in the
presence of the DC bias field applied to the electrodes and create a transient photocurrent. An electromagnetic pulse with picoseconds temporal width is radiated when
the lifetime of this transient current is in the order of picoseconds. This temporal
width corresponds to a very broad THz band [24, 25].
The transient photocurrent generated depends on various factors, such as the
optical pulse duration, the antenna geometry, the carrier scattering rate, the recombination lifetime of the semiconductor substrate, and the applied bias. For an increase
in the spectral bandwidth, the semiconductors used to generate THz should be defectrich for a quicker fall time of the transient current [26]. Examples of such materials
are low temperature grown or ion-implanted GaAs (shown in the inset of Fig. 3.) and
silicon [27–29] (Fig. 3).
Following Auston [30], the pioneer in PCA, and Grischkowsky [31], researchers
have optimized these PCAs for the generation and detection up to 5 THz. While the
bandwidth obtained using these antennas is impressive, it is not optimal for the width
of the optical pulses used for photoexcitation. For example, for a 10-fs transformlimited optical pulse, the optimal bandwidth generated and detected should be around
S. Banerjee et al.
Fig. 2 Generic scheme of generation and coherent detection of broadband THz radiation. The set
up does not include detailed optical components
2.1 Photoconductive Antenna as THz Emitter and Receiver
Photoconductive antennas (PCA) or switches are DC-biased metal dipole antenna
formed on a semiconductor substrate. As demonstrated in Fig. 3, when an optical
femtosecond pulse having photon energy more than the bandgap of the substrate
shines on the gap between the two electrodes, charge carriers are generated due
to bandgap excitation. These photogenerated charge carriers get accelerated in the
presence of the DC bias field applied to the electrodes and create a transient photocurrent. An electromagnetic pulse with picoseconds temporal width is radiated when
the lifetime of this transient current is in the order of picoseconds. This temporal
width corresponds to a very broad THz band [24, 25].
The transient photocurrent generated depends on various factors, such as the
optical pulse duration, the antenna geometry, the carrier scattering rate, the recombination lifetime of the semiconductor substrate, and the applied bias. For an increase
in the spectral bandwidth, the semiconductors used to generate THz should be defectrich for a quicker fall time of the transient current [26]. Examples of such materials
are low temperature grown or ion-implanted GaAs (shown in the inset of Fig. 3.) and
silicon [27–29] (Fig. 3).
Following Auston [30], the pioneer in PCA, and Grischkowsky [31], researchers
have optimized these PCAs for the generation and detection up to 5 THz. While the
bandwidth obtained using these antennas is impressive, it is not optimal for the width
of the optical pulses used for photoexcitation. For example, for a 10-fs transformlimited optical pulse, the optimal bandwidth generated and detected should be around